Lab 08: Pre-Start Warning Delay

Application Scenario

A long conveyor in a distribution center runs through areas that the operator cannot see from the control station. Someone could be standing on a crossover, reaching into the belt to clear a jam, or leaning on a guard. If the conveyor starts without warning, that person can be pulled in.

Conveyor safety standards (such as ASME B20.1) call for an audible or visible warning before a conveyor starts when the whole conveyor is not visible from the starting point. The usual solution is a pre-start warning:

1. The operator presses START.
2. A horn sounds and a beacon flashes for a few seconds. The conveyor does not move yet.
3. When the warning time is over, the warning stops and the conveyor starts.
4. Pressing STOP at any time, including during the warning, cancels everything.

The device that measures the warning time is an on-delay timer. In this lab, R1 represents the warning horn and beacon, and G1 represents the conveyor running.

Learning Outcomes

After this lab you will be able to:

• Explain how an on-delay timer (TR1) behaves when its coil is energized and de-energized.
• Read and draw timed contact symbols: N.O.T.C. (normally open, timed closed) and N.C.T.O. (normally closed, timed open).
• Read a timing diagram and use it to predict how a circuit behaves.
• Set a timer and measure the actual delay with a stopwatch.

Required Components

Background

What an on-delay timer does. An on-delay timer looks like a relay, with a coil and contact sets, but its contacts do not move right away:

• Coil energized: the timer starts counting. Its contacts remain in their normal state until the set time has passed, after which they change state.
• Coil de-energized: the contacts return to normal immediately, and the timer resets to zero.
• If the coil is de-energized before the set time is reached, the contacts do not change, and the next start resets the timer to zero.

Because the delay happens only on the way on, this type is called on-delay (or delay-on-make, or TON in a PLC).

Timed contact names.

TR1's coil is pins 2–7 (pin 7 to L2), just like CR1. Its pin numbers are the same as CR1's, so always write the device name with the pin: TR1:3 and CR1:3 are different terminals.

Timing Diagram

A timing diagram shows each signal as a line: high means ON (pressed, energized, or closed), and low means OFF. Read it from left to right, like a clock.

• At START, CR1 seals in, and the TR1 coil is energized. R1 (warning) turns ON at once, because TR1B (8–5) is still closed.
• At time out (5 s later), TR1A (1–3) closes and starts the conveyor (CR2, G1). At the same moment, TR1B (8–5) opens, and R1 turns OFF.
• At STOP, CR1 drops, the TR1 coil loses power, and both TR1 contacts immediately return to normal.

Functional Requirements

1. Pressing PB1 immediately turns the warning (R1) ON. The conveyor does not start yet.
2. After 5 seconds, the conveyor starts (CR2, G1 ON) and the warning turns OFF.
3. Pressing PB2 at any time stops everything. If PB2 is pressed during the warning, the conveyor never starts, and the next START begins a full 5-second warning again.
4. Pressing PB1 again while the conveyor is running has no effect.
5. After a power outage, nothing restarts 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(s). This design uses 6 grid lines.

Step 1 — Rung 1 (grid line 1): start request relay

1. Starting from L1, draw PB2 (N.C.), then PB1 (N.O.), then the CR1 coil (pins 2 and 7). Connect pin 7 to L2.
2. 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

1. Under PB1, draw a CR1A N.O. contact (pins 1 and 3). Connect pin 1 to N2 and pin 3 to N3.

Step 3 — Rung 3 (grid line 3): the timer coil

1. From node N3, draw a vertical line down to grid line 3.
2. On grid line 3, draw the TR1 coil (pins 2 and 7) and connect pin 7 to L2.

Step 4 — Rung 4 (grid line 4): warning beacon

1. From node N3, continue the vertical line down to grid line 4.
2. Draw a TR1B N.C.T.O. contact (pins 8 and 5), then R1. Connect R1 to L2.
3. Mark the node between TR1B and R1 as N5.

Step 5 — Rung 5 (grid line 5): conveyor motor contactor

1. Starting from L1, draw a TR1A N.O.T.C. contact (pins 1 and 3), then the CR2 coil (pins 14 and 13). Connect pin 13 to L2.
2. Mark the node between TR1A and the CR2 coil as N4.

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

1. Starting from L1, draw a CR2A N.O. contact (pins 9 and 5), then G1. Connect G1 to L2.
2. Mark the node between CR2A and G1 as N6.

Step 7 — Trace your drawing with a pencil

1. PB1 has just been pressed (time = 1 s). Which coils are energized? Is R1 ON? Is CR2 ON?
2. Time = 6 s. Which TR1 contacts have changed state? What are R1 and CR2 doing now?
3. The conveyor is running, and PB2 is pressed. Trace what happens to CR1, TR1, TR1A, and CR2, in that order.
4. PB2 is pressed at time = 3 s, during the warning. Does the conveyor ever start? If PB1 is pressed again right away, how long is the warning?
5. Rung 5 starts at L1, not at N3. Is the conveyor still stopped by PB2? Explain.

Step 8 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

Step 1 — List every terminal on each node

Write down every terminal that touches each node. Always write the device name with the pin number (TR1:3, not just 3). The N2 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. 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

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. A timer coil can be connected in parallel with a relay coil. Both then start and stop together, and no extra contact is needed.
2. Use the timed N.O. contact to start the load and the timed N.C. contact to end the warning. One timer, two contact sets, two jobs.
3. An on-delay timer resets when its coil loses power. STOP resets the warning automatically.
4. TR1 and CR1 have the same pin numbers. Label every wire end with the device name.
5. Set the timer before powering up, and never adjust the setting dial while touching live terminals.

Wiring Procedure

1. Power OFF the trainer. Set TR1 to 5 seconds.
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 CR1:1 should read near 0 Ω, and high while PB2 is pressed.
   • CR1:1 to CR1:2 should read high, and near 0 Ω while PB1 is pressed.
   • CR1:2 to TR1:2 and CR1:2 to TR1:8 should both read near 0 Ω (all on N3).
   • TR1:8 to TR1:5 should read near 0 Ω (N.C.T.O. is closed with the timer OFF).
   • TR1:1 to TR1:3 should read high (N.O.T.C. is open with the timer OFF).
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below. Use a stopwatch (a phone is fine) for tests 3 and 6.

Test & Acceptance

Troubleshooting

Review Questions

1. Describe what an on-delay timer's N.O.T.C. contact does when the coil is energized, and when the coil is de-energized.
2. Why is the TR1 coil connected in parallel with the CR1 coil? What would happen if the TR1 coil were connected directly to L1 (with no PB1 or PB2 in its path)?
3. Why must the warning restart from zero if STOP is pressed during the warning? Think about the person who might be near the conveyor.
4. Rung 5 (CR2) gets its power from L1, not from N3. Explain why PB2 still stops the conveyor. Which design would you consider safer, and why?
5. Sketch the timing diagram for this sequence: PB1 pressed at t = 0 s, PB2 pressed at t = 3 s, PB1 pressed again at t = 4 s. Show CR1, TR1A, R1, and G1 until t = 12 s.

Applying This Circuit Pattern

The command → timed permission pattern: a command starts a timer, and a timed contact starts the load. A second timed contact (the opposite type) can end whatever happens during the waiting time.

Design question for the future: Three pumps in a pump station must start one at a time, 5 seconds apart, after a single START press. With on-delay timers, how many timers do you need? What starts each timer?

Extension

Some plants require the beacon to stay ON the whole time the conveyor is moving, not just during the warning. CR1B (8–6) is still free. Without changing Rungs 1–5, how would you add a path so that R1 is ON during the warning and while the conveyor runs? Draw it, wire it, and test it.