Lab 05: Jog / Run Selection

Application Scenario

A packaging machine feeds a roll of plastic film through a series of rollers. When a new roll is loaded, the operator must thread the film through the rollers by moving the machine a few inches at a time. This is JOG mode: the machine moves only while the button is held. Once the film is threaded, the operator presses RUN, and the machine runs continuously for production.

Jog/run control is used on printing presses, packaging machines, lathes, mixers, and any machine that must be positioned by hand before production starts.

• PB1: RUN (starts and keeps running)
• PB3: JOG (runs only while held)
• PB2: STOP (stops everything)
• G1: motor RUNNING (in either mode)
• R1: motor STOPPED

Learning Outcomes

After this lab you will be able to:

• Explain why a JOG button must never be connected into a seal-in circuit.
• Use one relay to remember the RUN command and a second relay to drive the motor.
• Take power for a rung from a node on another rung, so that one STOP button controls the whole machine.
• Read the pin numbers of CR2, a relay with four contact sets.

Required Components

Background

Why not just add a JOG button to Lab 02? Suppose PB3 were wired in parallel with PB1 on the CR1 rung. Pressing PB3 would energize CR1, the seal-in contact would close, and the relay would latch. Releasing PB3 would not stop the machine, so the "jog" turns into a "run". On a real machine, the operator's hand may be near the rollers while jogging, so an unexpected latch can cause a serious injury.

The solution: two relays with two different jobs.

• CR1 remembers the RUN command. It has the seal-in contact, exactly as in Lab 02.
• CR2 drives the motor. It has no seal-in. It is energized either by a CR1 contact (RUN mode) or directly by the JOG button (JOG mode).

Because the JOG button feeds CR2 directly and never touches CR1, it can never latch.

Meet CR2. CR2 has four contact sets. In this lab you will use only CR2A and the coil:

Functional Requirements

1. Pressing PB1 starts the motor, and the motor keeps running after PB1 is released.
2. Holding PB3 runs the motor. Releasing PB3 stops it immediately. JOG must never latch.
3. Pressing PB2 stops the motor in either mode. While PB2 is held, neither RUN nor JOG can start the motor.
4. G1 is ON whenever the motor runs, in either mode. R1 is ON whenever it is stopped.
5. After a power outage, the motor does not restart by itself.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. L1 (+24 V) is the left rail and L2 (GND) is the right rail. Use pencil. This design uses 6 grid lines.

Step 1 — Rung 1 (grid line 1): RUN command relay

1. Starting from L1, draw PB2 (N.C.), the STOP button. Label pins 1 and 2.
2. To the right of PB2, draw PB1 (N.O.), the RUN button. Label pins 1 and 2.
3. At the right end, draw the CR1 coil. Label pins 2 and 7. Connect pin 7 to L2.
4. Mark the nodes: between PB2 and PB1 is N2; between PB1 and the CR1 coil is N3.

Step 2 — Rung 2 (grid line 2): seal-in for CR1

1. On grid line 2, under PB1, draw a CR1A N.O. contact. Label pins 1 and 3.
2. Connect pin 1 up to N2 and pin 3 up to N3.

Step 3 — Rung 3 (grid line 3): motor contactor, RUN path

1. From node N2, draw a vertical line down to grid line 3. This rung takes its power from N2, not from L1.
2. On grid line 3, draw a CR1B N.O. contact. Label pins 8 and 6.
3. At the right end, draw the CR2 coil. Label pin 14 on the left and pin 13 on the right. Connect pin 13 to L2.
4. Mark the node between CR1B and the CR2 coil as N4.

Step 4 — Rung 4 (grid line 4): JOG path

1. On grid line 4, under CR1B, draw PB3 (N.O.), the JOG button. Label pins 1 and 2.
2. Connect the left end of PB3 up to N2, and the right end up to N4.

Step 5 — Rung 5 (grid line 5): RUNNING lamp

1. Starting from L1, draw a CR2A N.O. contact. Label pins 9 and 5.
2. Draw G1 at the right end and connect it to L2. Mark the node between them as N5.

Step 6 — Rung 6 (grid line 6): STOPPED lamp

1. Starting from L1, draw a CR2A N.C. contact (with a slash). Label pins 9 and 1.
2. Draw R1 at the right end and connect it to L2. Mark the node between them as N6.

Step 7 — Trace your drawing with a pencil

1. PB3 is held. Trace the path to the CR2 coil. Does the path pass through the CR1 coil or any CR1 contact?
2. PB3 is released. Is there any path left to the CR2 coil?
3. PB1 is pressed and released. Trace the path that keeps CR1 energized, and the path that keeps CR2 energized.
4. The motor is running in RUN mode and PB2 is pressed. Is there any path left to the CR1 coil? To the CR2 coil?
5. PB2 is held and PB3 is pressed. Is there any path to the CR2 coil? (There must not be.)

If any answer is wrong, fix the drawing before you continue.

Step 8 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

Step 1 — List every terminal on each node

Look at each node dot on your drawing and write down every terminal that touches it. The N3 row is done for you.

Step 2 — Chain the terminals of each node together

A node with k terminals needs k − 1 wires, connected in a chain: first to second, second to third, and so on. Keep no more than two wires under any terminal screw. Spread the GND return wires across the panel's L2 terminals.

Step 3 — Fill in the wiring table on the worksheet

Write one row per wire: Rung#, Wire#, From, To, and Purpose. In the Purpose column, include the node number in parentheses, for example: STOP to RUN (N2).

Step 4 — Get instructor approval

Show your ladder diagram, contact allocation table, node table, and wiring table to the instructor. Do not start wiring until the worksheet is stamped.

Key Wiring Points

1. Separate "memory" from "output." CR1 remembers the RUN command. CR2 drives the motor. Only CR1 has a seal-in contact.
2. The JOG button feeds the output relay directly. It must never be connected to the CR1 coil or to any node of the seal-in circuit (N3).
3. One STOP for the whole machine. Rungs 3 and 4 take their power from N2, after the STOP button, so PB2 stops both RUN and JOG. If they started from L1, PB2 could not stop a JOG.
4. N2 has five terminals. Chain them carefully, and verify each one with a meter before power-up.
5. Do not mix up relay pin numbers. CR2's coil is 14–13, and CR2A's common is pin 9.
6. The RUNNING lamp follows the motor contactor (CR2), not the command relay (CR1), so it lights in both RUN and JOG modes.

Wiring Procedure

1. Power OFF the trainer (or unplug the power cord).
2. Wire the circuit from your approved wiring table, one node at a time. Mark each wire as you install it.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR1:8 should read near 0 Ω, and open while PB2 is pressed (CR1:8 is on node N2).
   • CR1:1 to CR1:2 should read high, and near 0 Ω while PB1 is pressed.
   • CR1:8 to CR2:14 should read high, and near 0 Ω while PB3 is pressed.
   • CR1:2 to CR2:14 should read high even while PB3 is pressed (JOG must not reach the CR1 coil).
   • CR2:9 to CR2:1 should read near 0 Ω; CR2:9 to CR2:5 should read high.
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Why can't PB3 simply be wired in parallel with PB1 on the CR1 rung? Describe what the operator would experience.
2. Rungs 3 and 4 start at node N2, not at L1. What would change if they started at L1? Why is this dangerous?
3. The machine is running in RUN mode and the operator presses and releases JOG. What happens? Explain using the ladder diagram.
4. Which relay "remembers" the RUN command, and which relay drives the motor? Why does industry separate these two jobs?
5. G1 is driven by a CR2 contact, not a CR1 contact. What would G1 show during JOG if it were driven by CR1 instead?

Applying This Circuit Pattern

This lab introduces the command relay + output relay pattern:

• A command relay (CR1) latches and remembers a request.
• An output relay (CR2) drives the load. It has no seal-in of its own.
• The output relay can be fed by several sources in parallel, each with its own behavior: latched, momentary, or automatic.

Use this pattern whenever one load must respond to more than one kind of command:

Design question for the future: A sump pump must also start automatically when a float switch (N.O., closes when the water is high) detects high water. Where would you connect the float switch on this ladder diagram? What problem would appear if the float switch were active all the time, even during maintenance?

Extension

Add PB4 as a second STOP button at the JOG station. Where must it go so that it stops both RUN and JOG? Which wires would change?