Lab 10: Post-Stop Cooling Fan
Application Scenario
A paint-curing oven on a production line has electric heaters and a circulation fan. When the heaters are switched off, the heating elements are still glowing hot. If the fan stopped at the same moment, the trapped heat could warp the fan blades, damage the elements, and overheat the oven walls.
The fan must therefore:
- start together with the heaters, and
- keep running for a set cool-down time after the heaters are switched off.
The same requirement appears on large motors with separate cooling blowers, variable-frequency drive cabinets, industrial lasers, projector lamps, and diesel generators. The device that provides the extra running time is an off-delay timer.
In this lab, R1 shows the HEATERS ON and G1 shows the FAN running. A real oven might cool down for 10–15 minutes; in the lab we use 10 seconds.
Learning Outcomes
After this lab you will be able to:
- Explain how an off-delay timer with a trigger input (TR2) behaves.
- Wire TR2's coil and its trigger terminals correctly.
- Compare on-delay (Labs 08–09) and off-delay timing on a timing diagram.
- Explain what happens to an off-delay timer when control power is lost.
Required Components
| Device | Type | Role in this lab |
|---|---|---|
| PB1 | Push button, N.O. | HEAT ON (start) |
| PB2 | Push button, N.C. | HEAT OFF (stop) |
| CR1 | Control relay | Heater contactor (simulated) |
| TR2 | Off-delay timer | Fan cool-down time (set to 10 s) |
| R1 | Red lamp | HEATERS ON |
| G1 | Green lamp | FAN running |
Background
How TR2 (off-delay) works. TR2 has three sets of terminals:
| TR2 terminals | Pins | What they do |
|---|---|---|
| Coil (power supply) | 2–10 (pin 10 to L2) | Must stay powered the whole time. This powers the timer; it does not start the timing. |
| Trigger | 5–6 | Controls the timer. A voltage-free (dry) contact connected between pins 5 and 6 turns the timer ON and OFF. Pins 5 and 6 connect only to that contact — never to L1 or L2. |
| Contacts | TR2A: 1 (C), 3 (N.O.), 4 (N.C.) TR2B: 11 (C), 9 (N.O.), 8 (N.C.) |
Change state when the timer is active. |
With the coil powered:
- Trigger closes: the contacts change state immediately (N.O. closes, N.C. opens).
- Trigger opens: the contacts stay changed for the set time, then return to normal.
- Trigger closes again during the timing: the timing is cancelled; the contacts stay changed, and the full delay starts over the next time the trigger opens.
Because the delay happens only on the way off, this type is called off-delay (or delay-on-break, or TOF in a PLC). Its timed contacts are named N.O.T.O. (normally open, timed open) and N.C.T.C. (normally closed, timed closed).
Important: TR2's pin numbers are different from TR1's. The coil is 2–10 (not 2–7), and the second contact set is 11-9-8. Always check the pin table for the device you are wiring.
Why not use the coil to start the timing? If the coil lost power, the timer would lose its supply and could not keep time. That is why TR2 keeps its coil powered and uses a separate trigger input. It also means that if the whole control panel loses power, TR2's contacts drop out immediately and the fan stops (see Review Question 4).
Setting the timer: Set TR2 to 10 seconds before you power the circuit.
Timing Diagram
- At start, CR1 seals in and the heaters turn ON. CR1B closes the TR2 trigger, so TR2A closes immediately and the fan starts.
- At stop, CR1 drops out and the heaters turn OFF. The trigger opens, but TR2A stays closed and the fan keeps running.
- At time out, 10 s later, TR2A opens and the fan stops.
Compare this with the on-delay diagram in Lab 08: there, the delay was at the start of the ON period. Here, the delay is at the end.
Functional Requirements
- Pressing PB1 turns the heaters ON (CR1, R1) and starts the fan (G1) at the same time.
- Pressing PB2 turns the heaters OFF immediately. The fan keeps running for 10 seconds, then stops.
- If PB1 is pressed again during the cool-down, the heaters turn ON again and the fan simply keeps running.
- 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. This design uses 6 grid lines.
Step 1 — Rung 1 (grid line 1): heater contactor
- Starting from L1, draw PB2 (N.C.), then PB1 (N.O.), then the CR1 coil (pins 2 and 7). Connect pin 7 to L2.
- 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
- 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): HEATERS ON lamp
- From node N3, draw a vertical line down to grid line 3.
- Draw R1 and connect it to L2. R1 is now in parallel with the CR1 coil.
Step 4 — Rung 4 (grid line 4): TR2 power (coil)
- Starting from L1, draw the TR2 coil (pin 2 on the left, pin 10 on the right). Connect pin 10 to L2.
- There is nothing in series with this coil. TR2 is powered whenever the panel has power.
Step 5 — Rung 5 (grid line 5): TR2 trigger
- On grid line 5, draw a small box labeled TR2 trigger near the right side. Mark its two terminals 5 (upper) and 6 (lower).
- Draw a CR1B N.O. contact (pins 8 and 6) to the left of the box.
- Connect CR1B pin 8 to TR2:5 and CR1B pin 6 to TR2:6, forming a closed loop. This rung does not touch L1 or L2.
- Mark the wire from CR1:8 to TR2:5 as N6, and the wire from CR1:6 to TR2:6 as N4.
Check: The trigger is a signal input, not a load. TR2 supplies its own signal voltage on pin 5; CR1B simply connects pin 5 to pin 6 when CR1 is ON.
Step 6 — Rung 6 (grid line 6): FAN
- Starting from L1, draw a TR2A N.O.T.O. contact (pins 1 and 3), then G1. Connect G1 to L2.
- Mark the node between TR2A and G1 as N5.
Step 7 — Trace your drawing with a pencil
- Power is ON and no button has been pressed. Is the TR2 coil energized? Is the trigger closed? Is TR2A open or closed?
- PB1 is pressed. Trace what happens to CR1, CR1B, the TR2 trigger, TR2A, and G1.
- PB2 is pressed at t = 0. What are R1 and G1 doing at t = 5 s? At t = 12 s?
- At t = 5 s during the cool-down, PB1 is pressed again. What happens to the fan? What happens when PB2 is pressed later?
- A student connects the fan G1 through a CR1 contact instead of TR2A. What changes?
Step 8 — Fill in the contact allocation table
| Device | Part | Pins | Used in Rung# |
|---|---|---|---|
| CR1 | Coil | 2–7 | |
| CR1 | CR1A N.O. | 1–3 | |
| CR1 | CR1B N.O. | 8–6 | |
| TR2 | Coil | 2–10 | |
| TR2 | Trigger | 5–6 | |
| TR2 | TR2A N.O.T.O. | 1–3 |
From Ladder Diagram to Wiring Table
Step 1 — List every terminal on each node
Write down every terminal that touches each node, with the device name. The N4 row is done for you.
| Node | Terminals on this node | Number of terminals |
|---|---|---|
| L1 | ||
| N2 | ||
| N3 | ||
| N4 | CR1:6, TR2:6 | 2 |
| N5 | ||
| N6 | ||
| L2 |
Hint: The trigger loop (N4 and N6) is a separate little circuit: CR1:8 ↔ TR2:5 and CR1:6 ↔ TR2:6. Neither node includes L1 or L2.
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.
Step 3 — Fill in the wiring table on the worksheet
Check: For this lab, you should have 15 wires in total.
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
- TR2's coil is always powered. Connect pin 2 to L1 and pin 10 to L2, with nothing in between.
- The trigger starts and stops the timing. Wire CR1B across pins 5 and 6 (CR1:8 → TR2:5, CR1:6 → TR2:6). Pins 5 and 6 are never connected to L1 or L2; a contact between pins 5 and 6 is the ON/OFF signal.
- Use a relay contact, not the button, as the trigger. The trigger must stay closed for as long as the heaters are ON, so it must follow the latched relay (CR1B), not the momentary PB1.
- TR2 pin numbers are different from TR1: coil 2–10, contacts 1-3-4 and 11-9-8.
Wiring Procedure
- Power OFF the trainer. Set TR2 to 10 seconds.
- Wire the circuit from your approved wiring table, one node at a time. Mark each wire as you install it.
- 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 TR2:2 and L1 to TR2:1 should read near 0 Ω.
- TR2:10 to L2 should read near 0 Ω.
- CR1:8 to TR2:5 and CR1:6 to TR2:6 should read near 0 Ω (nodes N6 and N4).
- L1 to CR1:8 should not read near 0 Ω (the trigger loop is isolated from L1).
- 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.
- Ask your lab partner to do a buddy check, then show the instructor.
- Power ON and complete the test table below.
Test & Acceptance
| # | Test | Expected result | ✓ |
|---|---|---|---|
| 1 | Power ON, do nothing | R1 OFF, G1 OFF | ☐ |
| 2 | Press and release PB1 | R1 ON and G1 ON at the same time | ☐ |
| 3 | Press PB2 and start a stopwatch | R1 OFF immediately; G1 stays ON, then OFF after about 10 s. Measured: ______ s | ☐ |
| 4 | Press PB1; press PB2; after about 5 s press PB1 again | R1 back ON; G1 never goes OFF | ☐ |
| 5 | Press PB2 and time the cool-down | G1 stays ON for a full 10 s after this stop. Measured: ______ s | ☐ |
| 6 | Hold PB2, then press PB1 | Nothing turns ON | ☐ |
| 7 | Heat ON, then press PB2, then switch power OFF during the cool-down | G1 goes OFF immediately | ☐ |
| 8 | Switch power back ON | Nothing restarts; G1 stays OFF | ☐ |
Troubleshooting
| Symptom | What to check |
|---|---|
| Fan never runs | Is the TR2 coil powered (TR2:2 to L1, TR2:10 to L2)? Is CR1B wired across the trigger (CR1:8 to TR2:5, CR1:6 to TR2:6)? |
| Fan stops at the same time as the heaters | G1 is driven by a CR1 contact, or TR2 is set to 0 s |
| Fan runs all the time, even at power-up | G1 is on TR2:4 (N.C.) instead of TR2:3, or the trigger pins 5 and 6 are jumpered together |
| Timer misbehaves or the control fuse blows | TR2:5 or TR2:6 is connected to L1 or L2. The trigger loop must be isolated |
| Fan stops about 10 s after START even though the heaters are still ON | The trigger is driven by PB1 instead of CR1B |
| Timer does not respond at all | Check the TR2 coil pins: 2 and 10, not 2 and 7 |
Review Questions
- Describe what TR2's N.O.T.O. contact does when the trigger closes, and when the trigger opens.
- Why is the trigger driven by CR1B instead of PB1?
- Draw the timing diagram for: PB1 at t = 0 s, PB2 at t = 4 s, PB1 at t = 8 s, PB2 at t = 12 s. Show R1 and G1 until t = 25 s.
- During the cool-down, the whole building loses power. What happens to the fan? Why? What could a real oven use to keep the fan running during a power outage?
- Compare an on-delay timer and an off-delay timer: for each, when does the delay happen, and name one application.
Applying This Circuit Pattern
The run-on after stop pattern: a signal triggers an off-delay timer, and the timer's N.O.T.O. contact keeps a second device running for a while after the signal ends.
| Application | Trigger (main device) | Keeps running after the stop |
|---|---|---|
| Curing or drying oven | Heaters | Circulation fan |
| Large motor or VFD cabinet | Motor running | Cooling blower |
| Bathroom or restroom exhaust | Light switch | Exhaust fan |
| Stairwell or parking-garage lighting | Motion sensor | Lights |
| Conveyor line | Feed conveyor | Discharge conveyor (clears the belt) |
| Industrial dust collector | Saw or grinder | Collector fan (clears the ducts) |
Design question for the future: Look at the conveyor line in Lab 07. Using an off-delay timer, how could you make Conveyor 2 keep running for 15 seconds after Conveyor 1 is stopped, so that the belt is cleared automatically? Which relay contact would drive the trigger?
Extension: SAFE TO OPEN Lamp
Operators must not open the oven door while it is hot. Add lamp G2 as a SAFE TO OPEN indicator: G2 is ON only when the heaters are OFF and the cool-down has finished. Use TR2's second contact set (TR2B: common 11, N.O. 9, N.C. 8). Which pin do you use, and why does G2 light at power-up? Connect G2:2 to L2.