Lab 00: Lab Requirements

Photo

Figure 4.1-1. 24 V DC, 2 A power supply adapter with DC-jack-to-screw-terminal adapter

What It Is

The power supply adapter plugs into a standard 120 V AC wall outlet and converts it to 24 V DC, the control voltage for your entire panel. A small DC-jack-to-screw-terminal adapter turns the round DC plug into two screw terminals, marked + and −, so the supply can be wired to the terminal blocks.

24 V DC is the most common control voltage in modern industrial panels. PLCs, sensors, pilot lamps, and many relays in real machines run on the same voltage you use in this lab.

Specifications

How It Works

• Inside the adapter, a switching power supply converts 120 V AC to a steady 24 V DC and electrically isolates the output from the wall outlet.
• The + terminal becomes L1 (+24 V) and the − terminal becomes L2 (0 V / GND) on your panel.
• If the load draws too much current or the output is short-circuited, the built-in protection shuts down the output, and the power LED goes out. The adapter is protecting itself and your panel: it is telling you that something in your circuit is wrong.

Is 2 A Enough?

Yes, with a good margin. The largest load on the panel is CR1: its 24 V AC coil draws about 0.5 A when operated on 24 V DC (Section 4.11). Every other coil and pilot lamp draws only a small current; each lamp, for example, draws about 12 mA (Section 4.9). Even with every coil and lamp on the panel energized at the same time, the total current stays around 1 A or less, about half of what the supply can deliver.

Wiring the Power Supply to the Panel

Figure 4.1-2. Connecting the power supply to L1 and L2

Wire Color Code for the Whole Panel

The same colors are used in every lab so that anyone can recognize a wire's job at a glance:

Checking the Power Supply with a Multimeter

1. Before connecting the adapter to the panel, plug it into the wall outlet. The power LED should light.
2. Set the meter to DC volts. Put the red probe on the adapter's + terminal and the black probe on the − terminal. The reading should be about +24 V.
3. If the reading is −24 V, the probes are reversed. Check which terminal is actually + before wiring it to L1.
4. Unplug the adapter, wire it to the panel as shown in Figure 4.1-2, then plug it in and measure between any L1 terminal and any L2 terminal. You should read about +24 V again.

Precautions and Common Mistakes

• Unplug before you wire. "Power OFF" in this lab means the adapter is unplugged from the wall outlet. Unplug it before adding, removing, or moving any wire.
• Reversed polarity. + must go to L1 and − to L2. Relays and timers with built-in LEDs are polarity-sensitive and may not operate correctly when reversed. Always check with the meter first.
• The power LED went out. Unplug the adapter immediately. Find and fix the short circuit (most often a rung with contacts but no load, or L1 wired to a green-yellow terminal) before you plug it back in. Do not keep plugging it in to "see if it works now."
• Loose adapter terminals. The screws on the DC-jack adapter are small. Tighten them with the Klein 32581 and tug-test each wire.
• DC plug not fully inserted. A partly inserted plug can cause the panel to flicker on and off. Push it in all the way.
• Never open the adapter case. The inside is connected to 120 V AC.

Photo

Figure 4.2-1. Slotted aluminum DIN rails with mounting screws

What It Is

A DIN rail is a standardized metal rail onto which everything in a control panel snaps. The rail used in this lab is the 35 mm "top-hat" rail (also called TS35), defined by the international standard EN 60715. Because the size is standardized, relays, timers, terminal blocks, circuit breakers, and power supplies from any manufacturer fit the same rail.

DIN rails make an industrial panel easy to build and service. A failed device can be unsnapped and replaced in seconds, and new devices can be added without drilling new holes.

Specifications

Figure 4.2-2. Cross-section of the 35 mm top-hat DIN rail

How Devices Mount on the Rail

Every DIN-rail device has a fixed hook on its bottom edge and a spring latch on its top edge. The hook grips the lower lip of the rail, and the latch snaps over the upper lip.

Figure 4.2-3. Mounting and removing a device on the DIN rail

1. Hook the bottom. Tilt the top of the device away from the rail and hook its bottom edge onto the lower lip of the rail.
2. Press the top. Rotate the device toward the rail and press the top until the spring latch clicks over the upper lip. Pull gently on the device: it must not come off.
3. To remove. Insert a flat screwdriver into the latch tab at the top, pull the latch up to release it, tilt the top of the device away from the rail, and lift the bottom off the lower lip. The small slotted bit of the Klein 32581 works well for terminal blocks.

This method applies to the terminal blocks, the ground terminals, and the relay and timer socket bases. The socket bases are snapped directly onto the rail; no hold-down clips are used on this panel.

Which DIN Rails Are Part of L2

The rails are aluminum, so they conduct electricity. Every green-yellow CGT10U ground terminal clamps onto its rail with a metal foot, and all CGT10U terminals on the panel are connected together as L2 (GND). As a result:

• The bottom (8 in) rail and the right (4 in) rail carry CGT10U terminals, so these two rails are at L2.
• The top (12 in) rail holds only the relay and timer socket bases. It has no CGT10U, so it is not connected to any bus.

Warning: Never let a bare L1 or L3 wire, a loose strand, or a dropped screw touch the bottom or right rail. Because those rails are L2, this is a direct short circuit, and the power supply LED will go out. Treat the top rail the same way: bare copper should never touch any metal rail.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity:

A beep between the rail and L1 or L3 means something is touching the rail. Find it before you plug in the power supply.

Precautions and Common Mistakes

• Device not fully latched. A device that is only hooked at one edge can fall off when you press a button or pull a wire. Always listen for the click and the pull test.
• Prying too hard. Pull the latch only until it releases. Forcing it can break the plastic latch spring.
• Gaps between terminal blocks. Slide the terminal blocks tightly together. A gap lets them tilt, making the jumpers hard to insert.
• Loose rail. If the rail moves, tighten its mounting screws before wiring. A loose rail strains every wire connected to it.

Photo

Figure 4.3-1. NSYTRV22BL (blue)

Figure 4.3-2. NSYTRV22 (grey)

What It Is

A feed-through terminal block is a small insulated block that snaps onto the DIN rail and has two screw-clamp connection points, one on each side. A wire clamped on one side is electrically connected to a wire clamped on the other side. Terminal blocks give every wire in the panel a neat, labeled, and removable connection point, so wires never need to be spliced or twisted together.

On your panel, every push-button contact, every selector-switch contact, and every pilot-lamp terminal is permanently wired to its own terminal block. In the labs, you connect your circuit to these terminal blocks, never directly to the buttons, switches, or lamps.

Specifications

The blue and grey models are identical except for color. The color tells you what the terminal is used for.

How It Works

Figure 4.3-3. Inside a feed-through terminal block

• Each connection point has a screw and a metal clamp. Tightening the screw raises the clamp and presses the wire against the copper current bar.
• The current bar runs through the block, so the two points on one terminal block are always connected.
• Neighboring terminal blocks are insulated from each other. Placing two terminals side by side on the rail does not connect them.
• The only way to connect neighboring terminal blocks together is with a terminal jumper. On your panel, the jumper legs are clamped in the screw clamps of the L1 and L3 terminals (Section 4.4).

Color Code on Your Panel

Figure 4.3-4. Color layout of the main terminal strip (the complete panel layout is given in Section 5)

Figure 4.3-5. Color layout of the pilot-lamp terminal strip (4 in rail)

Memory tip: a single blue terminal is always pin :1 of a device, and a grey terminal is always pin :2. A group of four blue terminals joined by jumpers is always a power bus.

How to Connect a Wire

1. Unplug the power supply.
2. Strip about 5/16 in (8 mm) of insulation from the 18 AWG solid wire (Section 2.7).
3. Using the Klein 32581 with the 1/8 in slotted bit, turn the screw counterclockwise until the clamp is fully open.
4. Push the bare copper straight into the wire opening until the insulation reaches the terminal body.
5. Turn the screw clockwise and tighten it firmly.
6. Tug the wire. It must not move, and no bare copper should be visible outside the terminal.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity:

To check that a wire is really clamped, touch one probe to the terminal screw and the other to the bare copper at the far end of the wire. It must beep.

Precautions and Common Mistakes

• Wire inserted beside the clamp. If the clamp is not fully opened, the wire can slide past it. The screw feels tight, but the wire has no connection. Always open the clamp fully and tug-test.
• Clamping the insulation. If the strip length is too short, the clamp squeezes the insulation instead of the copper. The wire looks connected, but does not conduct.
• Too much bare copper. If the strip length is too long, bare copper protrudes from the terminal and can touch a neighboring terminal or the DIN rail (L2).
• Assuming neighboring terminals are connected. They are not, unless a jumper joins them.
• Wrong screwdriver or over-tightening. Use the Klein 32581, not the 7314. Firm is enough; too much force strips the screw or cracks the housing.

Photo

Figure 4.4-1. Dinkle DSS2.5N-02P two-pole jumper

What It Is

A terminal jumper (also called a bridge or shorting bar) is a small insulated handle with two metal legs. It connects two neighboring terminal blocks electrically, without any wire. Jumpers are the standard way to build a power bus on a terminal strip: a row of terminals that all carry the same signal.

In an industrial panel, jumpers are used to distribute +24 V, 0 V, or a common signal to multiple terminals simultaneously. They are faster to install than wires, always the right length, and easy to see and check.

Specifications

This jumper was designed to press into the center jumper slot of Dinkle terminal blocks. Its legs are too short to reach the current bar through the jumper slot of the Linergy TR terminals, so on your panel the legs are inserted into the screw clamps instead, just like a wire.

How It Works

Figure 4.4-2. Three jumpers in the lower clamps join four terminals into one bus

• Each jumper leg is inserted into a screw clamp. When the screw is tightened, the clamp presses the leg against the terminal's copper current bar, exactly as it would press a wire.
• All three jumpers go in the lower clamps:
  • Jumper A: terminals 1–2
  • Jumper C: terminals 3–4
  • Jumper B: terminals 2–3, stacked on top of A and C. The lower clamps of terminals 2 and 3 each hold two jumper legs.
• The chain A → B → C connects all four terminals. They are now on one bus.
• Because every jumper is in the lower clamps, all four upper clamps are free for control wires.
• The L1 and L3 buses are each built this way. The green-yellow CGT10U terminals at the ends of each group are not jumpered; they keep L1 and L3 separated from L2 and from each other.

Where to Connect Wires on a Jumpered Bus

Never put a control wire in a clamp that holds a jumper leg. A clamp holding two different conductors grips each of them less firmly. If the control wire is ever removed, the jumper leg can loosen with it, breaking the bus.

Installing and Removing a Jumper

The jumpers are installed during panel assembly (Section 5). You will not normally move them during the labs.

1. Unplug the power supply.
2. Open the clamps. Using the Klein 32581, fully open the lower clamps at all four terminals.
3. Insert A and C. Slide jumper A into the lower clamps of terminals 1–2 and jumper C into the lower clamps of terminals 3–4, until each red handle touches the terminal bodies.
4. Insert B on top. Slide jumper B into the lower clamps of terminals 2–3, on top of the legs of A and C.
5. Tighten all four lower clamps firmly. The clamps of terminals 2 and 3 now hold two legs each.
6. Tug each jumper. None of them may move.
7. To remove: loosen the clamp screws completely, pull out jumper B first, then A and C. Pull each jumper straight out; do not rock it sideways, or the legs can bend.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity:

Measuring from terminal 1 to terminal 4 tests all three jumpers at once. If it does not beep, measure 1–2, 2–3, and 3–4 separately to find the jumper whose clamp screw is loose.

Precautions and Common Mistakes

• Control wire in a jumper clamp. Only the +24 V power wire may share a jumper clamp. All control wires go to the upper clamps.
• Loose clamp screw. A jumper leg that is not clamped tightly may conduct only sometimes, causing a circuit that works one minute and fails the next. Tug-test every jumper.
• Never jump two different signals. A jumper between an L1 terminal and a push-button, lamp, or L3 terminal permanently connects those points. A jumper from L1 to L2 is a direct short circuit.
• Never use a jumper to "bypass" a contact or button while testing. Use the multimeter instead.
• Bent legs. Do not reuse a jumper with bent legs. It will not clamp properly.

Photo

Figure 4.5-1. Connectwell CGT10U ground terminal

What It Is

The CGT10U is a ground terminal block. It looks like a feed-through terminal block, but it is green-yellow, the international color for a grounding conductor, and it has a metal mounting foot that clamps directly onto the metal DIN rail. Any wire clamped in a CGT10U is electrically connected to the rail.

On your panel, the green-yellow terminals are the L2 (0 V / GND) connection points. All CGT10U terminals on the panel are connected together, so any green-yellow terminal can be used as L2.

Specifications

How It Works

Figure 4.5-2. Inside the CGT10U: the wire is connected to the DIN rail

• Like a feed-through terminal, the CGT10U has two screw clamps joined by a metal bar.
• Unlike a feed-through terminal, the metal bar is also joined to the metal foot. When the terminal snaps onto the rail, the foot grips the rail and makes an electrical connection.
• As a result, every wire in a CGT10U, the rail it is mounted on, and every other CGT10U on the same rail are all the same point: L2.
• The panel has two rails with CGT10U terminals: the bottom (8 in) terminal-block rail and the right (4 in) lamp rail. They are separate rails, so one black L2 bonding wire connects a CGT10U on the bottom rail to a CGT10U on the right rail. This wire is what makes all CGT10U terminals on the panel one L2.
• The top (12 in) relay and timer rail has no CGT10U and is not connected to L2.

Where the CGT10U Terminals Are on Your Panel

• At both ends of the L1 bus (four jumpered blue terminals).
• At both ends of the L3 bus.
• At both ends of the pilot-lamp terminal strip.

Because they sit between the terminal groups, the CGT10U terminals also act as separators: they keep the L1 bus, the push-button terminals, and the L3 bus apart from one another.

Figure 4.5-3. The green-yellow CGT10U terminals on the main terminal strip (same layout as Figure 4.3-4)

What Connects to L2

Nothing else ever connects to a green-yellow terminal. In a ladder diagram, L2 is the right rail; every wire that ends at the right rail ends at a CGT10U.

How to Connect a Wire

Connect a wire exactly as you would to a Linergy TR terminal (Section 4.3): unplug the power supply, strip 5/16 in (8 mm), open the clamp fully with the Klein 32581, insert the wire, tighten firmly, and tug-test. A CGT10U clamp is large enough for two wires: twist the two stripped ends together first, insert them together, tighten, then tug each wire separately and check each one with the multimeter (Section 4.3). If the nearest CGT10U is full, use any other green-yellow terminal; they are all L2.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity:

A beep between L2 and L1 is a short circuit. Do not plug in the power supply until you find it.

Green-Yellow in the Real World

In industrial panels, green-yellow terminals are normally used for the protective earth (PE) conductor, which connects the metal enclosure and DIN rails to the building's ground for safety. Many 24 V DC control systems also connect their 0 V line to PE at one point. On this low-voltage training panel, the green-yellow terminals are used as the 0 V (L2) connection points, which is why the DIN rail is at L2 (Section 4.2).

Precautions and Common Mistakes

• Never connect L1, L3, or any control wire to a green-yellow terminal. The wire would be connected to every L2 point on the panel and to the DIN rail. Connecting L1 there is a direct short circuit, and the power supply LED will go out.
• Only return (negative) wires go to L2: the negative (−) output of the power supply, the negative (−) terminal of every relay and timer coil, and the negative (−) terminal of every pilot lamp. All of these wires are black.
• Terminal not fully latched. If the metal foot is not clamped firmly onto the rail, the CGT10U loses its connection to the rail and to the other L2 terminals. Listen for the click and pull-test (Section 4.2).
• Two wires inserted separately. When two wires share a CGT10U clamp, twist their stripped ends together before inserting them. Two loose ends side by side can leave one of them unclamped. Tug-test each wire.
• Missing or loose L2 bonding wire. If the black wire between the bottom rail and the right rail is loose, the lamp returns lose their connection to L2 and the lamps will not light, even though everything else works.

Photo

Figure 4.6-1. Push-button set 1: twin push button with indicator (22 mm)

What It Is

This is a twin push button: two momentary push buttons, a green START button, and a red STOP button, in one 22 mm panel-mount housing. Twin push buttons are common on machine control panels because a single compact device provides the operator with both START and STOP in one place.

The symbols I (ON) and O (OFF) are the international (IEC 60417) power symbols. You see them on machines, power supplies, and power strips around the world.

Specifications

How It Works

Figure 4.6-2. Contact states of PB1 (N.O.) and PB2 (N.C.)

• Each button pushes the plunger of its own contact block, which is behind the panel. Inside the block, a spring-loaded bridge connects or disconnects the two contact terminals.
• PB1 (N.O.): The bridge is held away from the contacts. Pressing the green button pushes the bridge onto the contacts, and PB1:1–PB1:2 conducts only while the button is held.
• PB2 (N.C.): The bridge rests on the contacts. Pressing the red button pushes the bridge away, and PB2:1–PB2:2 opens only while the button is held.
• Both buttons are momentary: a spring returns the button and its contact to the normal state as soon as you let go. To keep a machine running after START is released, the circuit needs a seal-in (holding) contact from a relay wired in parallel with PB1 (Section 4.11).

Why STOP is N.C.: if a wire to PB2 breaks or comes loose, the circuit opens, and the machine stops. A broken STOP circuit failsafe (Section 3.5).

Ladder Diagram Symbols

	PB1: N.O. push button (START). The bar is above the contacts.
	PB2: N.C. push button (STOP). The bar is below, touching the contacts.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity and measure at the terminal strip:

If a reading is wrong, check the wires between the button and the terminal strip before you blame the button. A loose wire is far more common than a failed contact block.

Precautions and Common Mistakes

• Never wire the amber indicator. It is a 220 V lamp. It will not light on 24 V, and 220 V is never used in this lab.
• Swapping PB1 and PB2. PB1 (green) is N.O. for START; PB2 (red) is N.C. for STOP. Using PB2 as the START button causes the circuit to run as soon as power is applied.
• Expecting the circuit to stay on. PB1 is momentary. If your circuit turns off when you release PB1, the seal-in contact is missing or wired incorrectly.
• Wiring at the button instead of the terminal strip. In the labs, always connect to PB1:1, PB1:2, PB2:1, and PB2:2 on the terminal strip.

Photo

Figure 4.7-1. Push-button set 2: Taiss START/STOP station box

What It Is

A push-button station is an enclosed box that holds one or more push buttons. The wires exit the box through a cable gland that grips the cable and seals out dust and water. In a factory, a station box is mounted next to the machine, often far from the main control panel, so the operator can start and stop the machine from their work station.

Specifications

How It Works

PB3 and PB4 work exactly like PB1 and PB2 (Section 4.6):

• PB3 (N.O.): PB3:1–PB3:2 conducts only while the green START button is held.
• PB4 (N.C.): PB4:1–PB4:2 conducts until the red STOP button is pressed, and opens only while it is held.
• Both buttons are momentary and return to their normal state as soon as you let go.

The difference is not in how the buttons work but in where they are used: set 1 represents the buttons on the main control panel, and set 2 represents a second operator station out on the machine.

Why Two Push-Button Sets? Two-Location Control

Many machines can be started and stopped from more than one place, for example, from the main panel and from a station at the far end of a conveyor. This is called multi-location (two-station) control. In the later labs you will wire both sets into one circuit:

Figure 4.7-2. Two-location start/stop control

• STOP buttons (N.C.) are wired in series. Pressing either STOP button opens the rung and stops the machine. The machine can run only when both STOP buttons are released.
• START buttons (N.O.) are wired in parallel. Pressing either START button closes the rung and starts the machine.
• The seal-in contact (CR1 N.O.) is in parallel with the START buttons, so the machine keeps running after the START button is released.

Rule to remember: STOP in series, START in parallel. The same rule works for any number of stations.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity and measure at the terminal strip:

If a reading is wrong, check the wires inside the station box and at the terminal strip. Because the cable runs through the gland, pulling on it can loosen a wire inside the box.

Precautions and Common Mistakes

• STOP buttons in parallel. If PB2 and PB4 are wired in parallel, pressing only one STOP button does not stop the machine, because the other one is still conducting. STOP buttons must always be in series.
• START buttons in series. If PB1 and PB3 are wired in series, the machine starts only when both are pressed simultaneously.
• Swapping PB3 and PB4. PB3 (green) is N.O. for START; PB4 (red) is N.C. for STOP.
• Pulling on the station cable. Do not carry the station box by its cable or let it hang from the cable.
• Wiring at the button instead of the terminal strip. In the labs, always connect to PB3:1, PB3:2, PB4:1, and PB4:2 on the terminal strip.

Photo

Figure 4.8-1. Front: short lever

Figure 4.8-2. Side: two contact blocks

Figure 4.8-3. Back: terminals 3 and 4

What It Is

A selector switch is operated by turning a lever or knob instead of pressing a button. SSW1 is a 3-position selector switch with a short lever: LEFT, CENTER (OFF), and RIGHT. Behind the panel, two green contact blocks are clipped onto the switch. One closes when the lever is turned left (SWL), and the other when it is turned right (SWR).

In industry, selector switches are used wherever the operator chooses a mode or a direction: HAND–OFF–AUTO pump control, FORWARD–OFF–REVERSE for a conveyor or hoist, or selecting which of two machines to run.

Specifications

How It Works

Figure 4.8-4. SSW1 positions and contact states

• Turning the lever rotates a cam inside the switch. The cam pushes the plunger of one contact block, closing that block's N.O. contact.
• CENTER (OFF) is the only position the lever stays in on its own. Both contacts are open.
• LEFT closes SWL only while you hold the lever to the left. When you let go, a spring returns the lever to CENTER, and SWL opens again.
• RIGHT works the same way for SWR.
• SWL and SWR cannot be closed simultaneously. This makes SSW1 useful for choosing between two actions that cannot occur simultaneously, such as FORWARD and REVERSE.

Because both end positions are momentary, SSW1 behaves like two N.O. push buttons that cannot be pressed together. To keep a circuit running after the lever returns to CENTER, use a seal-in contact, just as with a START push button.

Contact Table

Terminals 3 and 4 of each contact block are pre-wired to the terminal strip: SWL:1 and SWR:1 on single blue terminals, SWL:2 and SWR:2 on grey terminals (Section 4.3). In the labs, always connect to the terminal strip, not to the contact blocks.

Ladder Diagram Symbol

Figure 4.8-5. SSW1 in a ladder diagram, with its target table

A selector switch is drawn with its individual contacts joined by a dashed line indicating "operated by the same lever." The target table next to it shows which contact is closed (X) in each position. In a ladder diagram, SWL and SWR usually appear in different rungs.

Checking with a Multimeter

With the power supply unplugged, set the meter to continuity and measure at the terminal strip:

If SWL and SWR are swapped (SWR beeps when the lever is turned left), the two contact blocks are wired to the wrong terminals. Report it to the instructor.

Precautions and Common Mistakes

• Expecting the lever to stay. LEFT and RIGHT are momentary. If your circuit stops when you release the lever, you need a seal-in contact.
• Confusing SWL and SWR. Check the contact table before you wire. SWL is LEFT, SWR is RIGHT.
• Forcing the lever. Turn the lever only until it stops. Forcing it past the end position can break the cam.
• Loose contact blocks. The contact blocks are held by a clip on the mounting base. If a block is loose, its plunger may not reach the cam, and the contact will not close.

Photo

Figure 4.9-1. R1, R2: Dayton 22NY46

Figure 4.9-2. G1, G2: Dayton 22NY45 (green)

What It Is

A pilot lamp (also called an indicator light) shows the operator what the machine is doing. By industry convention, green usually means "running" or "safe to proceed," and red means "stopped," "fault," or "warning." Your panel has four LED pilot lamps: R1 and R2, and G1 and G2 (green).

Note: R1 and R2 have an amber lens. In this course, they are used as red indicators (stopped, fault, or warning) and are always referred to as R1 and R2 in ladder diagrams and wiring tables.

Specifications

What the numbers mean for you: each lamp draws only about 12 mA, so all four lamps together draw less than 50 mA from the 24 V supply. The LED will last far longer than this course: 25,000 hours is almost three years of continuous operation.

How It Works

• Inside the lamp housing is an LED with a built-in current-limiting circuit, so the lamp can be connected directly to 24 V without an extra resistor.
• The lamp is rated for 24 V AC or DC. It lights whenever 24 V appears between its two leads.
• A lamp is a load, just like a relay coil. In a ladder diagram, it is always the last element on the right side of a rung, connected directly to L2.

Terminals and Wiring

The lamps are pre-wired to the pilot-lamp terminal strip on the right (4 in) rail when the panel is built (Figure 4.3-5):

• Both lamp leads are connected to the right-side clamps of the lamp's two terminals. Where a lamp has a red lead, the red lead goes to :1 (+) and the black lead to :2 (−).
• Every :2 terminal is also pre-wired to L2 with a black wire on the right side, next to the lamp lead.

In the labs, you therefore make only one connection per lamp: a wire from the contact that controls the lamp to the left-side clamp of its :1 terminal. You never add a wire to a :2 terminal.

Ladder Diagram Symbol and Example

A pilot lamp is drawn as a circle with rays. The letter inside shows the color: R for R1/R2, G for G1/G2.

Figure 4.9-3. Pressing PB1 turns on G1 (the same example as Figure 3-2)

Checking a Lamp

A multimeter cannot easily test an LED lamp with a built-in current limiter, so test it with power instead. Because the :2 terminal is already connected to L2, one wire is enough. With the power supply unplugged, wire:

• L1 → G1:1 (left clamp)

After instructor approval, plug in the power supply. G1 should light. Unplug, then move the wire to R1:1, R2:1, and G2:1 in turn. If all four lamps stay dark, check the L2 bonding wire between the bottom rail and the lamp rail (Section 4.5). If only one lamp stays dark, check its pre-wired leads and its :2-to-L2 wire.

Precautions and Common Mistakes

• A lamp in series with a coil. Every lamp needs its own full 24 V. Never wire a lamp in series with a relay coil or another lamp. To show that a relay is on, use one of the relay's N.O. contacts to turn on the lamp in a separate rung.
• Adding a wire to a :2 terminal. Every :2 terminal is already L2. A lab wire on :2 is not needed, and a wire from L1 or L3 to a :2 terminal is a direct short circuit.
• Using the right-side clamps. The right-side clamps hold the lamp leads. Connect your lab wires to the left-side clamp of :1 only.
• Pulling on the lamp leads. The thin leads can break inside the lamp body. Do not use them to hold or move the lamp.

Photo

Figure 4.10-1. 8-pin socket (Dayton 5X852N)

Figure 4.10-2. 11-pin socket (Dayton 6X156N)

Figure 4.10-3. 14-pin socket (PYF14A)

What It Is

A socket base is the part of a plug-in relay or timer that stays on the DIN rail. The socket has a screw terminal for every pin of the device. You wire the socket once, and the relay or timer simply plugs in. If a relay fails, a technician pulls it out and plugs in a new one in seconds, without touching a single wire. This is why plug-in relays are used in so many industrial panels.

On your panel, all six socket bases are snapped onto the top (12 in) rail, in this order from left to right: CR1, CR2, TR1, TR2, TR3, TR4.

Specifications

All sockets mount on a 35 mm DIN rail and use screw terminals. Every rating is far above the 24 V DC, low-current signals on your panel.

How It Works

• Inside the socket, each pin opening holds a spring contact. When the device is plugged in, each pin is gripped by its contact.
• Each spring contact is connected to one screw terminal on the outside of the socket.
• The number printed next to each screw terminal matches the device pin number. Socket terminal 3 connects to pin 3 of the relay, socket terminal 7 connects to pin 7, and so on. The pin tables in Sections 4.11 to 4.17, therefore, apply directly to the socket terminals.

Tip: The screw terminals are not arranged in number order around the socket. Always read the number printed next to each screw before you connect a wire. Never count terminals to find a number.

Which Device Goes in Which Socket

In every lab, the coil + terminal is wired with an orange wire and the coil − terminal with a black wire to L2 (Section 4.1).

Plugging In and Removing a Device

1. Unplug the power supply. Never plug in or pull out a relay or timer while the panel is powered.
2. Octal (8-pin and 11-pin) devices: the center post has a key (a raised rib). Turn the device until the key lines up with the slot in the socket's center hole.
3. 14-pin (MY4) device: the pins are arranged so the relay fits only one way. Line up the pins with the openings before pressing.
4. Press the device straight down until it is fully seated. Do not rock it or force it at an angle.
5. To remove: grip the device body and pull it straight up, rocking it gently from side to side if it is tight. Never pry it out with a screwdriver.

Connecting a Wire to a Socket Terminal

1. Unplug the power supply.
2. Strip about 5/16 in (8 mm) of the 18 AWG solid wire.
3. Using the Klein Tools 7314 (#1 combo tip), loosen the terminal screw until the pressure plate is open.
4. Insert the bare copper under the pressure plate, not under the screw head.
5. Tighten firmly and tug the wire. It must not move.

Screwdriver: the socket terminals use combination-head screws. Always use the Klein Tools 7314. The small Klein 32581 bits do not fit these screw heads and will strip them.

Checking with a Multimeter

With the power supply unplugged and the device plugged in, set the meter to continuity:

If an N.C. contact does not beep, the device is probably not fully seated. Unplug the power supply, press the device firmly into the socket, and measure again.

Precautions and Common Mistakes

• Counting terminals instead of reading numbers. Socket terminals are not in numerical order. Read the printed number every time.
• Wrong device in the socket. TR1 and TR3 use the same 8-pin socket as CR1, and TR2 and TR4 use the same 11-pin socket. A timer plugged into the wrong position will make every circuit that uses it behave incorrectly. The sockets are not labeled; follow the panel layout. From left to right on the top rail, the order is CR1, CR2, TR1, TR2, TR3, TR4.
• Timer trigger terminals. On the 11-pin sockets of TR2 and TR4, terminals 5 and 6 are the trigger input. Never connect them to L1, L2, or L3 (Section 2.4).
• Wire under the screw head. The wire must go under the pressure plate. A wire wrapped under the screw head can work loose.
• Plugging or unplugging with power on. This can arc the pins and damage the socket contacts.

Photo

Figure 4.11-1. CR1: Omron MKS2PIN AC24 relay

What It Is

CR1 is a general-purpose power relay with an 8-pin round (octal) base. It has two contact sets, CR1A and CR1B. This contact form is called DPDT (Double Pole, Double Throw):

• Double Pole: two separate contact sets that switch at the same time.
• Double Throw: each set has a common pin that connects to either the N.O. or N.C. pin.

The model name tells you its features:

Note: CR1 has a 24 V AC coil, but on your panel, it is operated from the 24 V DC supply. It works reliably, but the coil gets warm when it remains energized for extended periods. See Why CR1 Gets Warm below.

Specifications (from the Omron MKS datasheet, AC24 model)

Why CR1 Gets Warm

An AC relay coil is designed to work with alternating current. When AC flows through a coil, the coil's inductance opposes the changing current. This opposition, called inductive reactance, limits the coil current to about 96 mA, even though the copper wire itself has only 48.4 Ω of resistance.

With DC, the current does not change, so there is no inductive reactance. Only the wire resistance limits the current:

I = V ÷ R = 24 V ÷ 48.4 Ω ≈ 0.5 A     P = V × I ≈ 24 V × 0.5 A ≈ 12 W

That is about five times the current and the power the coil sees on AC. The extra power turns into heat, so the relay body becomes warm, and eventually hot, if the coil stays energized for a long time. This is a good example of why the coil voltage and type (AC or DC) printed on a relay must match the control circuit in real equipment.

• Do not leave CR1 energized longer than needed. Turn the circuit off (or unplug the power supply) when you finish testing.
• Do not touch the relay cover after it has been energized for an extended period.
• If CR1 gets too hot to touch, unplug the power supply and tell the instructor.

Key Points from the Datasheet

• N.C. contacts are rated at half the current of N.O. contacts (5 A vs. 10 A). In industry, a heavy load is always switched with an N.O. contact.
• Inductive loads reduce the rating (7 A instead of 10 A), because a motor or solenoid causes an arc when the contact opens.
• Electrical life is 50 times shorter than mechanical life. The contacts wear out from arcing, not from moving. The small 24 V lamp and coil loads on your panel are very gentle on the contacts.
• The coil and the contacts are isolated by 2,500 V AC. This is what lets a 24 V control circuit safely switch a 120 V or 240 V load in a real machine.
• Mounting direction. Omron recommends mounting the relay with the side that has the printed wiring diagram facing down.
• Test button rules (see below): use it only for testing, use an insulated tool, and make sure it is released before the circuit is powered.

How It Works

1. Coil OFF (no voltage between pins 2 and 7): a spring holds the armature in its rest position. Common pin 1 is connected to pin 4 (N.C.), and common pin 8 is connected to pin 5 (N.C.).
2. Coil ON (24 V between pins 2 and 7): the coil becomes an electromagnet, pulling the armature in. Both contact sets switch simultaneously: pin 1 moves to pin 3 (N.O.), and pin 8 moves to pin 6 (N.O.). The LED lights and the mechanical indicator move.
3. Coil OFF again: the spring returns the armature, and all contacts return to their normal state.

The coils CR1A and CR1B are electrically isolated from one another, so they can be used in two completely different rungs.

Pin Diagram

Figure 4.11-2. CR1 pin diagram (coil de-energized) and 8-pin socket terminal layout. In the drawing, terminal 2 (coil +) is shown in red and terminal 7 (coil −) in dark grey; the real socket has no color marks.

Contact Table

Memory tip: the two contact sets are mirror images. CR1A uses pins 1-3-4, and CR1B uses 8-6-5 (subtract each CR1A pin number from 9). The coil uses the two pins left over, 2 and 7, which are the two outer terminals of the socket's bottom row.

Coil polarity: the AC coil itself does not care which way DC is applied, but always wire pin 2 as + and pin 7 as − so that every relay and timer on the panel follows the same rule.

Screwdriver: tighten the CR1 socket terminals with the Klein Tools 7314 (#1 combo tip). The small Klein 32581 bits do not fit these screw heads and will strip them.

Lockable Test Button

The MKS2PIN has a two-way test button on the front of the cover, with a red latching lever (red identifies the AC-coil version). The test button moves the contacts by hand without energizing the coil, allowing a technician to test the circuit that the relay controls.

In this course, never use the test button. Before every lab, check that the latching lever is in the normal position. A relay left in the locked test position behaves as if its coil were always ON, and your circuit will not work correctly. Omron also warns that pulling the button too hard can skip the momentary position and go straight to the locked position.

Typical Uses in This Course

• Seal-in (holding) contact: a CR1 N.O. contact wired in parallel with the START button keeps the relay energized after the button is released.
• Master control relay (MCR): a CR1 N.O. contact feeds the controlled bus L3.
• Status lamps: the N.O. side of one set lights a green RUN lamp while the N.C. side lights a red STOP lamp.
• Dry contact for a timer trigger: a CR1 contact wired directly across TR2 or TR4 terminals 5 and 6 (for example, CR1:8 → TR2:5 and CR1:6 → TR2:6).

Because CR1 has only two contact sets, a circuit that requires more than two contacts from the same relay should use CR2, which has four contact sets (Section 4.12).

Checking CR1 with a Multimeter

With the power supply unplugged and CR1 plugged into its socket, measure at the socket terminals:

If checks 2 or 4 beep, the test button is probably locked. Return the latching lever to the normal position.

Then test the relay with power. With the power supply unplugged, wire:

• L1 → CR1:2 (orange) and CR1:7 → L2 (black): energizes the coil
• L1 → CR1:1 and CR1:3 → G1:1: lights G1 through the CR1A N.O. contact (G1:2 is already connected to L2)

After instructor approval, plug in the power supply. You should hear a click, the LED and the mechanical indicator should show that the relay is operated, and G1 should light. Unplug, move the wire from CR1:3 to CR1:4 (N.C.), and repeat: G1 should now stay off while the coil is energized. Keep each test short because the coil heats up while energized.

Precautions and Common Mistakes

• Leaving CR1 energized. On 24 V DC, the AC coil draws about 0.5 A and gets warm. Do not leave a circuit running unattended.
• Test button left locked. Check the latching lever before every lab. Never use the test button to "help" a circuit work.
• Mixing pins from two contact sets. A contact is always the common pin plus one pin from the same set. Pins 1 and 6, for example, are not a contact: they belong to different sets and never connect.
• Using N.C. when N.O. is needed. Pins 1-3 and 1-4 are easy to swap. Always check your wiring table against the contact table above.
• Counting socket terminals. The socket terminals are not numbered (Figure 4.11-2). Read the number next to each screw.
• Relay not fully seated. Line up the key on the center post and push the relay straight down until it is fully seated (Section 4.10).

Photo

Figure 4.12-1. CR2: RONE HH54PL DC24V relay (MY4 type)

What It Is

CR2 is a miniature control relay with a 14-pin plug-in base. It has four contact sets, CR2A, CR2B, CR2C, and CR2D. This contact form is called 4PDT (four-pole, double-throw). The HH54P is the same size and pin layout as the widely used MY4 relay, so relays and sockets of this type are interchangeable between many manufacturers.

The model name tells you its features:

CR1 or CR2?

Specifications (from the HH52/HH53/HH54 series datasheet, HH54P-LC DC24V)

Key Points from the Datasheet

• Low coil current. The DC24V coil draws only 37 mA (0.9 W). CR2 can remain energized for extended periods without overheating, unlike CR1.
• The coil picks up at 18 V (75% of 24 V) and drops out below 2.4 V (10% of 24 V). A loose connection that makes the voltage sag can make CR2 chatter or fail to pick up.
• Do not exceed 26.4 V (110%) continuously on the coil.
• Lower contact rating than CR1. Each CR2 contact is rated 3 A resistive, and much less for inductive loads (0.3 A at 220 V AC). In industry, a relay like CR2 switches signals and other relay coils, not motors.
• The LED is polarity-sensitive. Pin 14 is coil + and pin 13 is coil −. Reversed, the green LED will not light.
• Very long mechanical life (100 million operations). Contact wear from the small 24 V signals on your panel is negligible.

How It Works

1. Coil OFF (no voltage between pins 14 and 13): each common pin rests on its N.C. pin: 9–1, 10–2, 11–3, and 12–4.
2. Coil ON (24 V between pins 14 and 13): the armature pulls in, and all four contact sets switch at the same time, within 20 ms: 9–5, 10–6, 11–7, and 12–8 close. The green LED lights.
3. Coil OFF again: all four sets return to their N.C. positions within 20 ms.

The coil and the four contact sets are electrically isolated from one another, so each contact set can be used in a different rung.

Pin Diagram

Figure 4.12-2. CR2 pin diagram (coil de-energized) and 14-pin socket terminal layout. In the drawing, terminal 14 (coil +) is shown in red and terminal 13 (coil −) in dark grey; the real socket has no color marks.

Contact Table

Memory tip: each contact set is a group of three pin numbers that differ by 4. The common is in the 9–12 row, the N.O. pin is 4 less (5–8), and the N.C. pin is 8 less (1–4). For example, CR2C: common 11, N.O. 11 − 4 = 7, N.C. 11 − 8 = 3. The coil uses the last two numbers, 14 (+) and 13 (−).

Screwdriver: tighten the CR2 socket terminals with the Klein Tools 7314 (#1 combo tip). The small Klein 32581 bits do not fit these screw heads and will strip them.

Typical Uses in This Course

• One command, many actions: a single CR2 coil can seal itself in (CR2A), interlock another relay (CR2B), turn on a RUN lamp (CR2C), and turn off a STOP lamp (CR2D) at the same time.
• Interlocking: a CR2 N.C. contact in series with another relay's coil prevents both from being ON together, for example, FORWARD and REVERSE.
• Sequencing with timers: CR2 contacts start timers, feed L3, or pass a signal from one stage of a sequence to the next.

Checking CR2 with a Multimeter

With the power supply unplugged and CR2 plugged into its socket, measure at the socket terminals:

Then test the relay with power. With the power supply unplugged, wire:

• L1 → CR2:14 (orange) and CR2:13 → L2 (black): energizes the coil
• L1 → CR2:9 and CR2:5 → G1:1: lights G1 through the CR2A N.O. contact (G1:2 is already connected to L2)

After instructor approval, plug in the power supply. You should hear a click, the green LED on CR2 should light, and G1 should light. Unplug, move the two contact wires to the next set (10 and 6, then 11 and 7, then 12 and 8), and repeat to check all four N.O. contacts.

Precautions and Common Mistakes

• Coil wired backward. Pin 14 is +, pin 13 is −. With the polarity reversed, the LED does not light, which makes troubleshooting confusing.
• Mixing pins from two contact sets. A contact is always the common (9–12) plus a pin from the same set. Pins 9 and 6, for example, belong to different sets and never connect.
• Confusing N.O. and N.C. The N.O. pins are 5–8, and the N.C. pins are 1–4. On the socket, terminal 4 is in the top row, away from 1–3, so always read the number. Check the contact table every time.
• Counting socket terminals. The terminals are not in number order (Figure 4.12-2). Read the number next to each screw.
• Forcing the relay into the socket. The 14 pins fit only one way. Line them up before pressing straight down (Section 4.10).
• Using CR2 for heavy loads. On a real machine, switch motors and heaters with a contactor or a relay rated for the load, not with CR2's 3 A contacts.

What Is a Timer Relay?

A timer relay (time delay relay) is a relay with a built-in electronic timer. Like a control relay, it has output contacts that switch other circuits. The difference is when the contacts switch: instead of switching the instant the coil is energized, a timer relay switches its contacts after a preset time, or for a preset time, depending on its function.

Timer relays are everywhere in industry: a horn sounds for 10 seconds before a conveyor starts, a cooling fan keeps running for 2 minutes after a motor stops, a pump fills a tank for exactly 30 seconds, or a warning light flashes while a machine is in a fault condition.

The Four Timer Relays on Your Panel

All four timers operate on 24 V AC/DC and have DPDT output contacts rated at 10 A at 240 V AC/30 V DC.

Two Inputs: Supply and Trigger

This is the most important idea in this section. A timer relay can have two different inputs, and you must know which one starts the timing.

• Supply-started timers (TR1, TR3, and TR4 functions A, B, E, F): timing begins when the supply is applied. Removing the supply resets the timer.
• Trigger-started timers (TR2 and TR4 functions C, D, G, H, J, K): the supply must stay ON at all times. The trigger contact across 5-6 starts and controls the timing. If the supply is removed, the timer cannot time, and the output drops out immediately.

The trigger must be a dry contact. The Macromatic instructions for the TD-7 timer (the design behind TR4) state: "This Trigger must be a dry-type contact (applying voltage to the pins could damage the unit)." Never connect terminal 5 or 6 to L1, L2, L3, or any other wire. Wire only the two terminals of one contact to them, for example, CR1:8 → TR2:5 and CR1:6 → TR2:6.

The Output Contacts

Every timer on your panel has two contact sets that follow the same a–b N.O. / a–c N.C. convention as the control relays (Section 3.3). Both sets always switch together; at the moment, the timer function says so.

These timers have no instantaneous contacts; every contact is timed. If a circuit needs a contact that switches the instant the timer is energized, use a CR1 or CR2 contact controlled by the same rung.

Timer Contact Symbols

In a ladder diagram, a timed contact is drawn as a normal contact with an arrow. The arrow shows which direction of switching is delayed.

Symbol	Name	Behavior	Used With
	N.O.T.C. (Normally Open, Timed Closed)	Closes after the preset time; opens immediately when the timer is reset.	On-Delay (TR1)
	N.C.T.O. (Normally Closed, Timed Open)	Opens after the preset time; closes immediately when the timer is reset.	On-Delay (TR1)
	N.O.T.O. (Normally Open, Timed Open)	Closes immediately when triggered; opens after the preset time once the trigger is removed.	Off-Delay (TR2)
	N.C.T.C. (Normally Closed, Timed Closed)	Opens immediately when triggered; closes after the preset time once the trigger is removed.	Off-Delay (TR2)

For interval, flasher, and other functions (TR3 and several TR4 functions), the contacts are drawn as ordinary N.O. and N.C. contacts labeled with the timer name, and the function is described in the timing diagram.

How to Read a Timing Diagram

A timing diagram shows how the inputs and outputs of a timer change over time. It is the fastest way to understand what a timer function does.

Figure 4.13-1. Example timing diagram (On-Delay)

• Time runs from left to right. Each horizontal line is one signal.
• High = ON / CLOSED, low = OFF / OPEN. For the supply and the trigger, high means voltage is applied, or the trigger contact is closed. For an output contact, high means the contact is closed.
• Read the vertical dashed lines as events. In Figure 4.13-1: at ① the supply turns on, and timing starts; at ②, one preset time t later, the N.O. contact closes, and the N.C. contact opens; at ③ the supply turns off, and both contacts return to normal immediately.
• The N.O. and N.C. contacts of the same set are always opposite. When one is high, the other is low.

Each timer tab (Sections 4.14 to 4.17) has its own timing diagram.

Setting the Time

Good practice: when a lab asks for a specific time (for example, 5 seconds), set the knob, then measure the actual delay with a stopwatch and adjust. Record the measured time, not the dial setting.

Reset

• All four timers use time-limit reset: when the supply is removed (or, for triggered functions, when the function ends), the timer returns to its starting state, ready for the next cycle.
• The timer needs a short reset time (about 0.1 s for TR4) with the supply off before it can start a new, full timing cycle. Turning the supply off and on very quickly may not reset it.
• For triggered functions, the trigger contact must stay closed for at least the minimum control pulse (0.1 s for TR4) to be recognized.

Precautions and Common Mistakes

• Voltage on the trigger terminals. Terminals 5 and 6 accept only a dry contact. Never connect them to L1, L2, L3, or any other circuit.
• Supply switched off on a triggered timer. TR2 and the triggered TR4 functions require the supply to be ON at all times. Switching the supply with the same contact that should trigger the timer is the most common timer wiring mistake.
• Trusting the dial. The knob scale is approximate. Measure the actual time.
• Changing TR4 settings while powered. The TR4 function cannot be changed while the supply is on. Unplug the power supply, set the function and time, then power up.
• Expecting an instantaneous contact. Timer contacts are all timed. Use a CR1 or CR2 contact for an instant response.
• Wrong pins on the 11-pin base. On TR2 and TR4, set B is 11-9-8, and the supply is 10, not 7 as on the 8-pin timers.

Photo

Figure 4.14-1. TR1: Dayton 24EP09

Figure 4.14-2. Wiring diagram printed on the case (bottom view of the pins)

What It Is

TR1 is an On-Delay timer, the most common timer function in industry. When its supply is turned on, TR1 waits for the preset time, and only then do its contacts switch. When the supply is turned off, the contacts return to normal immediately.

Think of it as "delay ON, instant OFF." It is used whenever something must wait before it starts: a warning horn before a conveyor moves, a motor that starts a few seconds after another one, or a lamp that turns on only if a condition lasts longer than a set time.

Specifications

How It Works

Figure 4.14-3. TR1 On-Delay timing diagram

1. ① Supply ON: timing starts. The contacts do not move yet.
2. ② Time out: after the preset time t, both contact sets switch. The N.O. contacts (1-3, 8-6) close, and the N.C. contacts (1-4, 8-5) open. They stay switched as long as the supply stays on.
3. ③ Supply OFF: the contacts return to normal immediately, and the timer resets.
4. ④–⑤ Supply ON for less than t: the timer resets before it times out, so the contacts never switch. The next time the supply is applied, timing starts again from zero.

In a ladder diagram, TR1's N.O. contacts are drawn as N.O.T.C. (normally open, timed closed) and its N.C. contacts as N.C.T.O. (normally closed, timed open) (Section 4.13).

Pin Diagram

Figure 4.14-4. TR1 pin diagram and 8-pin socket terminal layout. In the drawing, terminal 2 (+) is shown in red and terminal 7 (−) in dark grey; the real socket has no color marks.

Contact Table

TR1 uses exactly the same pin numbers as CR1 (Section 4.11). The only difference is when the contacts switch.

Application Example: Conveyor Start-Up Warning

Many plants require a warning before a conveyor starts, so that nobody is touching the belt. With TR1:

• The operator presses START. A control relay seals in, turns on a warning lamp or horn, and energizes TR1.
• After the preset time (for example, 5 seconds), TR1's N.O.T.C. contact closes and starts the conveyor motor, and its N.C.T.O. contact turns the warning off.
• Pressing STOP drops out the control relay, which removes TR1's supply. The conveyor stops immediately, and TR1 is reset for the next start.

Checking TR1

With the power supply unplugged, set the knob to about 5 seconds and wire:

• L1 → TR1:2 (orange) and TR1:7 → L2 (black): supply
• L1 → TR1:1 and TR1:3 → G1:1: G1 shows the N.O.T.C. contact
• L1 → TR1:8 and TR1:5 → R1:1: R1 shows the N.C.T.O. contact

After instructor approval, plug in the power supply and start a stopwatch. R1 should be on at first; after about 5 seconds, R1 should turn off, and G1 should turn on. Unplug the power supply: G1 should turn off and R1 should turn back on immediately. Record the measured delay and compare it with the knob setting.

Precautions and Common Mistakes

• Expecting the output to stay on after the supply is removed. TR1 resets instantly when its supply goes off. If the load must stay on, the circuit needs a seal-in contact.
• Interrupting the supply during timing. Any interruption, even a short one, resets the timer to zero. A loose supply wire can make TR1 appear never to time out.
• Trusting the dial. The scale is approximate, especially near the minimum mark. Measure the actual delay.
• Confusing TR1 with CR1. TR1 and CR1 use the same pin numbers but plug into different sockets (the top rail order is CR1, CR2, TR1, TR2, TR3, TR4).

Photo

Figure 4.15-1. TR2: Dayton 24EN86

Figure 4.15-2. Wiring diagram printed on the case (bottom view of the pins)

What It Is

TR2 is an Off-Delay timer: "instant ON, delay OFF." Its contacts switch immediately when the trigger closes, and they stay switched for the preset time after the trigger opens.

Off-delay timers keep something running for a while after the command to stop: a cooling fan that runs for 60 seconds after a motor stops, a stairway light that stays on after the switch is released, or a conveyor that keeps moving until the last part has cleared.

TR2 has two inputs. The supply (2 and 10) must be ON all the time. The trigger (5 and 6) is what controls the timing, and it must be a dry contact wired directly across 5 and 6 (Section 4.13).

Specifications

How It Works

Figure 4.15-3. TR2 Off-Delay timing diagram

1. ① Supply ON: TR2 is powered and ready. Nothing else happens yet.
2. ② Trigger closes: the contacts switch immediately. The N.O. contacts (1-3, 11-9) close and the N.C. contacts (1-4, 11-8) open.
3. ③ Trigger opens: timing starts. The contacts stay switched.
4. ④ Time out: after the preset time t, the contacts return to normal.
5. ⑤–⑧ Trigger re-closed during timing: the trigger closes at ⑤ and opens at ⑥, so timing starts. At ⑦, it closes again before the time is up: the timing is canceled, and the contacts simply stay switched. At ⑧, the trigger opens again, and the timing resets to zero.
6. ⑨ Time out: one full time t after the last opening of the trigger, the contacts return to normal.

In a ladder diagram, TR2's N.O. contacts are drawn as N.O.T.O. (normally open, timed open) and its N.C. contacts as N.C.T.C. (normally closed, timed closed) (Section 4.13).

Wiring the Trigger

Figure 4.15-4. Wiring the TR2 supply and trigger

• The supply is wired straight from L1 (or L3) to TR2:2 and from TR2:10 to L2, so it is ON whenever the panel is powered.
• The trigger is a relay contact (or a push button) with its two terminals wired only to TR2:5 and TR2:6. In the example, CR1's N.O. contact is used: CR1:8 → TR2:5 and CR1:6 → TR2:6.
• When CR1 energizes, its contact closes the trigger, and TR2 switches immediately. When CR1 drops out, the trigger opens, and TR2 begins its off-delay.

Pin Diagram

Figure 4.15-5. TR2 pin diagram and 11-pin socket terminal layout. In the drawing, terminal 2 (+) is shown in red and terminal 10 (−) in dark grey; the real socket has no color marks.

Contact Table

Pin 7 is not used on TR2. Set A uses the same pins as the 8-pin timers (1-3-4), but set B is 11-9-8, and the supply is 10, not 7.

Application Example: Motor Cooling Fan

• A control relay, CR1, runs a motor. A CR1 N.O. contact is wired across TR2's trigger (5-6).
• A TR2 N.O.T.O. contact runs the cooling fan. As soon as the motor starts, the fan starts too.
• When the operator presses STOP, the motor stops and CR1 drops out, opening the trigger. The fan runs for the preset time (for example, 60 seconds) and then stops automatically.

Checking TR2

With the power supply unplugged, set the knob to about 5 seconds and wire:

• L1 → TR2:2 (orange) and TR2:10 → L2 (black): supply, always ON
• PB1:1 → TR2:5 and PB1:2 → TR2:6: PB1 (N.O.) is the trigger. Do not connect PB1 to L1.
• L1 → TR2:1 and TR2:3 → G1:1: G1 shows the N.O.T.O. contact

After instructor approval, plug in the power supply. Press and hold PB1: G1 should turn on immediately. Release PB1 and start the stopwatch: G1 should stay on for about 5 seconds, then turn off. Then try pressing PB1 again before the time runs out, and watch the timer restart.

Precautions and Common Mistakes

• Voltage on 5 or 6. Never connect terminal 5 or 6 to L1, L2, L3, or any wire other than the two terminals of the trigger contact. This can damage the timer.
• Switching the supply instead of the trigger. If the supply (2) is wired through the controlling contact, TR2 loses power the moment the contact opens and drops out immediately, with no off-delay. The supply must be ON at all times.
• Using pin 7 or set B as 8-6-5. TR2 is an 11-pin timer: supply is 10, and set B is 11-9-8.
• Expecting timing to start when the trigger closes. The off-delay starts when the trigger opens.
• Trusting the dial. Measure the actual delay with a stopwatch.

Photo

Figure 4.16-1. TR3: Dayton 24EN71

Figure 4.16-2. Wiring diagram printed on the case (bottom view of the pins)

What It Is

TR3 is an Interval-On timer: "instant ON, timed OFF, while powered." When its supply is turned on, its contacts switch immediately and stay switched for the preset time; then they return to normal even though the supply is still on.

Interval timers are used when an output must run for a fixed duration after a start signal: a pump that fills a tank for 30 seconds, a lubrication valve that opens for 10 seconds at start-up, or a horn that sounds for a few seconds and then stops on its own.

Compare with TR1: TR1 (On-Delay) waits t, then switches. TR3 (Interval-On) switches first, then switches back after t. In both, removing the supply resets the timer.

Specifications

Reading the minute scale: the knob scale is in minutes, not seconds. For example, 0.1 min = 6 s, 0.5 min = 30 s, and 1.2 min = 72 s. Convert before you set the knob: seconds = minutes × 60.

How It Works

Figure 4.16-3. TR3 Interval-On timing diagram

1. ① Supply ON: both contact sets switch immediately. The N.O. contacts (1-3, 8-6) close, and the N.C. contacts (1-4, 8-5) open. Timing starts.
2. ② Time out: after the preset time t, the contacts return to their normal state, while the supply remains on.
3. ③ Supply OFF: nothing changes at the contacts; the timer resets and is ready for the next cycle.
4. ④–⑤ Supply removed early: if the supply is turned off before t is reached, the contacts return to normal immediately. The interval is cut short.

To run another interval, the supply must be turned off and back on. Leaving the supply on does not repeat the interval.

Pin Diagram

Figure 4.16-4. TR3 pin diagram and 8-pin socket terminal layout. In the drawing, terminal 2 (+) is shown in red and terminal 7 (−) in dark grey; the real socket has no color marks.

Contact Table

TR3 uses the same pin numbers as CR1 and TR1. In a ladder diagram, its contacts are drawn as ordinary N.O. and N.C. contacts labeled TR3, and the timing diagram explains the interval behavior (Section 4.13).

Application Example: Timed Tank Filling

• The operator presses START. A control relay seals in and energizes TR3.
• TR3's N.O. contact opens the fill valve (a lamp on your panel) immediately.
• After the preset time (for example, 30 seconds = 0.5 min), TR3's contact opens, and the valve closes automatically, even though the control relay remains energized.
• Pressing STOP drops out the control relay and removes TR3's supply, which ends the fill early if needed and resets TR3 for the next batch.

Checking TR3

With the power supply unplugged, set the knob to about 0.1 min (6 s) and wire:

• L1 → TR3:2 (orange) and TR3:7 → L2 (black): supply
• L1 → TR3:1 and TR3:3 → G1:1: G1 shows the N.O. contact

After instructor approval, plug in the power supply and start a stopwatch. G1 should turn on immediately and turn off automatically after about 6 seconds, while the supply stays on. Unplug and plug in again to repeat the interval.

Precautions and Common Mistakes

• Reading the scale in seconds. TR3's scale is in minutes. Setting "1" gives 60 seconds, not 1 second.
• Expecting the output to repeat. The interval runs once per power-up. The supply must be removed and reapplied for another interval.
• Confusing TR3 with TR1. TR1 delays the ON; TR3 delays the OFF while powered. Check the timing diagram before you choose.
• A flickering supply. Any interruption of the supply resets TR3 and restarts the interval on the next power-up.
• Trusting the dial. Measure the actual interval with a stopwatch.

Photo

Figure 4.17-1. TR4: Dayton 5DRA3A

Figure 4.17-2. Wiring diagram (bottom view of the pins); the trigger is dashed because only some functions use it

What It Is

TR4 is a multi-function, multi-range digital timer. One unit can be set to any of 10 timing functions and any delay from 0.05 seconds to 999 hours. Multi-function timers are popular in industry because one spare timer on the shelf can replace many single-function timers.

In the labs, you will use TR4 in three functions: A – On Delay, B – Interval On, and C – Off Delay. The other seven functions are described here so that you recognize them in the field.

Specifications

Setting TR4

Figure 4.17-3. TR4 push-button thumbwheels

TR4 has five push-button thumbwheels. Each press of a thumbwheel's button advances it to the next value.

Delay time = (3-digit TIME SETTING) × (TIME RANGE unit) 

Set TR4 with the power OFF. The function cannot be changed while the supply is on. Unplug the power supply, set the FUNCTION, TIME SETTING, and TIME RANGE, then power up. Always read all five windows back before you start the lab.

The 10 Functions

For the triggered functions (C, D, G, H, J, K), the supply (2 and 10) must be ON all the time, and the trigger contact is wired directly across 5 and 6, exactly as for TR2 (Section 4.15).

Functions Used in the Labs

Figure 4.17-4. Function A – On Delay (works like TR1, Section 4.14)

Figure 4.17-5. Function B – Interval On (works like TR3, Section 4.16)

Figure 4.17-6. Function C – Off Delay (works like TR2, Section 4.15)

Because TR4 can do what TR1, TR2, and TR3 do, it is often used in the labs as a second On-Delay, Off-Delay, or Interval timer when a circuit needs two timers of the same kind.

Other Functions (for Reference)

In these diagrams, the supply is ON the whole time for the triggered functions.

Figure 4.17-7. D – One Shot: extra triggers during timing are ignored.

Figure 4.17-8. E – Flasher (OFF 1st). Example: a warning beacon that flashes while a fault exists.

Figure 4.17-9. F – Flasher (ON 1st): the same as E, but the output starts ON.

Figure 4.17-10. G – On/Off Delay: delayed both when the trigger closes and when it opens.

Figure 4.17-11. H – 1 Shot Falling Edge: the pulse starts when the trigger opens.

Figure 4.17-12. J – Watchdog: the output stays on while triggers keep arriving; it drops out when they stop for longer than t. Example: detecting that a conveyor sensor has stopped pulsing.

Figure 4.17-13. K – Triggered On Delay: like On Delay, but started by the trigger instead of the supply.

Pin Diagram

Figure 4.17-14. TR4 pin diagram and 11-pin socket terminal layout. In the drawing, terminal 2 (+) is shown in red and terminal 10 (−) in dark grey; the real socket has no color marks.

Contact Table

TR4 has the same pin layout as TR2. Pin 7 is not used.

Checking TR4

With the power supply unplugged, set TR4 to A, 050, 0.1s (On Delay, 5.0 s) and wire:

• L1 → TR4:2 (orange) and TR4:10 → L2 (black): supply
• L1 → TR4:1 and TR4:3 → G1:1: G1 shows the N.O. contact

After instructor approval, plug in the power supply and start a stopwatch: G1 should turn on after 5.0 seconds. Then unplug, change the function to B, and repeat: G1 should turn on immediately and turn off after 5.0 seconds. Compare how close TR4's digital setting is to the measured time with the knob timers.

Precautions and Common Mistakes

• Voltage on 5 or 6. The trigger accepts only a dry contact. The manufacturer warns that applying voltage to these pins could damage the unit.
• Changing settings with power on. Set the function and time with the power supply unplugged.
• Wrong time unit. 050 means 5 seconds at 0.1s, 50 seconds at S, and 50 minutes at M. Always check the TIME RANGE window.
• Wrong function letter. A (On Delay) and K (Triggered On Delay) look similar on paper, but are started differently. B and D also differ: B is started by the supply, D by the trigger.
• Triggered the function with the supply switched. For C, D, G, H, J, and K, the supply must stay ON; only the trigger across 5-6 is switched.
• Very short settings. Use at least 50 ms. Settings below this are not reliable.

Figure 5.3-1. Rail positions and edge margins (inches)

Figure 5.3-2. Metric version: 500 × 300 mm board with 300, 200, and 100 mm rails

The metric version uses round numbers and is not an exact conversion of the inch version. Pick one system for your board and use only that system; the values in parentheses in the table below are for the metric board.