Lab 09: Hold-to-Start Safety
Application Scenario
A food plant uses a large industrial mixer with blades strong enough to injure anyone who is reaching into the bowl. During cleaning, workers lean against the control panel, and a START button can be pressed by accident with an elbow, a hose, or a tool.
To prevent an accidental start, the machine uses hold-to-start (also called deliberate-action start):
- The operator must press and hold START for 3 seconds. While the button is held, an ARMING lamp lights to show that the start request is being counted.
- If the button is released before 3 seconds, nothing happens, and the next attempt must start counting from zero.
- After 3 seconds, the mixer starts and keeps running after the button is released.
A quick bump cannot start the mixer, but a deliberate 3-second press can. The same idea is used on shredders, balers, industrial washers, and the "hold to confirm" buttons on many operator touch screens.
In this lab, R1 is the ARMING lamp and G1 shows the mixer RUNNING.
Learning Outcomes
After this lab you will be able to:
- Energize a timer coil directly from a push button, so the timer runs only while the button is held.
- Use a timed contact to start a relay, and a seal-in to keep it running after the timer resets.
- Compare this lab with Lab 08: same timer, different wiring, very different behavior.
Required Components
| Device | Type | Role in this lab |
|---|---|---|
| PB1 | Push button, N.O. | START (hold for 3 s) |
| PB2 | Push button, N.C. | STOP |
| TR1 | On-delay timer | Hold time (set to 3 s) |
| CR1 | Control relay | Mixer motor contactor (simulated) |
| R1 | Red lamp | ARMING (START is being held) |
| G1 | Green lamp | Mixer RUNNING |
Background
Lab 08 vs. Lab 09. Both labs use the same on-delay timer, TR1. The difference is what keeps the timer coil energized:
| Lab 08: Pre-start warning | Lab 09: Hold-to-start | |
|---|---|---|
| TR1 coil is fed from | The CR1 seal-in node (latched) | PB1 directly (not latched) |
| Operator presses START and releases | Timer keeps counting | Timer resets to zero |
| What the delay means | "Wait before starting" | "Prove you mean it" |
In this lab, the TR1 coil is connected right after PB1. The coil is energized only while PB1 is held. If PB1 is released early, the coil loses power and the on-delay timer resets immediately (remember Lab 08).
Handing off to a seal-in. When TR1 times out, its N.O.T.C. contact energizes CR1. CR1's own seal-in contact is connected in parallel with that timed contact. When the operator releases PB1, TR1 resets and its contact opens, but the seal-in keeps CR1 running. The timer only has to hold the circuit for the instant it takes CR1 to seal in.
Setting the timer: Set TR1 to 3 seconds before you power the circuit.
Timing Diagram
The diagram shows two attempts. In the first, the operator releases PB1 too early. In the second, the operator holds PB1 for the full 3 seconds.
- First attempt: PB1 is released after about 2 seconds. TR1 resets before timing out, so TR1A never closes, and the mixer does not start. R1 turns OFF when PB1 is released.
- Second attempt: PB1 is held. At time out, TR1A closes and CR1 seals in; TR1B opens and R1 turns OFF. When PB1 is released, TR1 resets, but CR1 stays ON through its seal-in.
- STOP: CR1 drops out, and the mixer stops.
Functional Requirements
- While PB1 is held, R1 (ARMING) is ON.
- If PB1 is released before 3 seconds, the mixer does not start, and R1 turns OFF.
- If PB1 is held for 3 seconds, the mixer starts (CR1, G1 ON), R1 turns OFF, and the mixer keeps running after PB1 is released.
- Pressing PB2 stops the mixer. While PB2 is held, PB1 cannot arm or start the mixer.
- After a power outage, the mixer 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 5 grid lines.
Step 1 — Rung 1 (grid line 1): the hold timer
- Starting from L1, draw PB2 (N.C.), then PB1 (N.O.), then the TR1 coil (pins 2 and 7). Connect pin 7 to L2.
- Mark the nodes: between PB2 and PB1 is N2; between PB1 and the TR1 coil is N3.
Check: There is no seal-in on this rung. The TR1 coil has power only while PB1 is held.
Step 2 — Rung 2 (grid line 2): ARMING lamp
- From node N3, draw a vertical line down to grid line 2.
- Draw a TR1B N.C.T.O. contact (pins 8 and 5), then R1. Connect R1 to L2.
- Mark the node between TR1B and R1 as N4.
Step 3 — Rung 3 (grid line 3): mixer motor contactor
- From node N2, draw a vertical line down to grid line 3. This rung takes its power from N2, after the STOP button.
- Draw a TR1A N.O.T.C. contact (pins 1 and 3), then the CR1 coil (pins 2 and 7). Connect pin 7 to L2.
- Mark the node between TR1A and the CR1 coil as N5.
Step 4 — Rung 4 (grid line 4): seal-in
- Under TR1A, draw a CR1A N.O. contact (pins 1 and 3).
- Connect pin 1 up to N2 and pin 3 up to N5.
Check: CR1A is in parallel with TR1A, not with PB1. It takes over from the timed contact as soon as CR1 energizes.
Step 5 — Rung 5 (grid line 5): RUNNING lamp
- Starting from L1, draw a CR1B N.O. contact (pins 8 and 6), then G1. Connect G1 to L2.
- Mark the node between CR1B and G1 as N6.
Step 6 — Trace your drawing with a pencil
- PB1 is held for 1 second. Which coil is energized? Is R1 ON? Is CR1 ON?
- PB1 is released at 2 seconds. What happens to the TR1 coil and its contacts? Does CR1 ever energize?
- PB1 is held for 4 seconds. Trace the path to the CR1 coil at time out. Then PB1 is released: trace the path that keeps CR1 energized.
- The mixer is running and PB2 is pressed. Is there any path left to the CR1 coil?
- Suppose a student connected the seal-in CR1A in parallel with PB1 (N2 to N3) instead. What would happen when PB1 is tapped quickly once?
Step 7 — Fill in the contact allocation table
| Device | Part | Pins | Used in Rung# |
|---|---|---|---|
| TR1 | Coil | 2–7 | |
| TR1 | TR1B N.C.T.O. | 8–5 | |
| TR1 | TR1A N.O.T.C. | 1–3 | |
| CR1 | Coil | 2–7 | |
| CR1 | CR1A N.O. | 1–3 | |
| CR1 | CR1B N.O. | 8–6 |
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 (TR1:3, CR1:3). The N3 row is done for you.
| Node | Terminals on this node | Number of terminals |
|---|---|---|
| L1 | ||
| N2 | ||
| N3 | PB1:2, TR1:2, TR1:8 | 3 |
| N4 | ||
| N5 | ||
| N6 | ||
| L2 |
Hint: N5 connects TR1:3 and two CR1 terminals. Be careful: TR1:3 and CR1:3 are both on N5 in this lab, but they are on different devices.
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
- No seal-in on the timer coil. The TR1 coil must lose power the moment PB1 is released, so the timer resets.
- The seal-in goes around the timed contact, not around PB1. It holds CR1 after the timer resets.
- Feed the motor rung from N2. Then PB2 stops the mixer directly and also blocks arming.
- Where a coil gets its power decides how a timer behaves. Fed from a latch (Lab 08), it keeps timing. Fed from a button (Lab 09), it times only while the button is held.
Wiring Procedure
- Power OFF the trainer. Set TR1 to 3 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 TR1:1 should read near 0 Ω, and high while PB2 is pressed (TR1:1 is on N2).
- TR1:1 to TR1:2 should read high, and near 0 Ω while PB1 is pressed.
- TR1:8 to TR1:5 should read near 0 Ω (N.C.T.O. closed).
- TR1:3 to CR1:2 should read near 0 Ω (both on N5).
- TR1:1 to TR1:3 should read high, and should stay high while PB1 is pressed (the timer is not powered).
- 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 | All OFF | ☐ |
| 2 | Tap PB1 quickly | R1 flashes briefly; mixer does not start | ☐ |
| 3 | Hold PB1 for about 2 s, then release | R1 ON while held, then OFF; mixer does not start | ☐ |
| 4 | Hold PB1 and time it with a stopwatch | After about 3 s: R1 OFF, CR1 ON, G1 ON. Measured: ______ s | ☐ |
| 5 | Release PB1 | Mixer keeps running (G1 ON) | ☐ |
| 6 | Press PB2 | CR1 OFF, G1 OFF | ☐ |
| 7 | Hold PB2, then hold PB1 for 5 s | Nothing happens (R1 stays OFF) | ☐ |
| 8 | Hold PB1 for 2 s, release, then hold again for 2 s | Mixer does not start (timer restarted from zero) | ☐ |
| 9 | Start the mixer, then switch power OFF and back ON | Mixer does not restart | ☐ |
Troubleshooting
| Symptom | What to check |
|---|---|
| Mixer starts on a quick tap | CR1 is fed by an instantaneous path. Is CR1A across TR1A (N2–N5), not across PB1? Is TR1A on 1–3? |
| Mixer starts, but stops when PB1 is released | Seal-in CR1A missing or open. Check CR1:1 on N2 and CR1:3 on N5 |
| Mixer never starts; R1 stays ON while held | TR1 is not timing out: check TR1 setting, and TR1:7 to L2 |
| R1 never lights | Is R1 fed from N3 through TR1B 8–5? |
| R1 stays ON after the mixer starts, while PB1 is still held | TR1B wired to 8–6 or bypassed |
| Timer keeps counting after PB1 is released | TR1 coil has a seal-in or is fed from N2. It must be fed only through PB1 |
Review Questions
- In Lab 08, the TR1 coil was fed from a seal-in node. In this lab, it is fed directly from PB1. Explain how this one difference changes what the delay means.
- Why is CR1A connected in parallel with TR1A, and not in parallel with PB1?
- After the mixer starts, the operator keeps holding PB1 for 10 more seconds. Describe what TR1, R1, and CR1 do while PB1 is held, and when it is released.
- A hold-to-start button reduces accidental starts. Name one situation where it would not be enough, and what additional safety device would be needed.
- A supervisor asks to change the hold time from 3 s to 10 s "for extra safety." What are the disadvantages of a very long hold time?
Applying This Circuit Pattern
The signal must stay present for a time pattern: feed a timer coil directly from a signal, and use the timed contact to act. Short signals are ignored; only a signal that lasts the full time gets through. The same circuit is used to filter out nuisance signals:
| Application | Signal that must stay present | Action after the delay |
|---|---|---|
| Hold-to-start on mixers, shredders, balers | START button held | Start the machine |
| Tank level alarm | High-level float switch closed (ignores splashing) | Sound the alarm |
| Low-pressure trip on a compressor | Low-pressure switch open (ignores short dips at start-up) | Trip the compressor |
| Jam detection on a conveyor | Photo-eye blocked (ignores normal boxes passing) | Stop the conveyor and raise a jam alarm |
| Door or gate "held open" alarm | Door switch open | Sound a warning |
Design question for the future: A photo-eye (N.O. contact, closes when blocked) watches a conveyor. A box normally blocks it for less than 2 seconds. If it stays blocked for 4 seconds, there is a jam, and the conveyor must stop and a JAM lamp must light and stay lit until a RESET button is pressed. Using the ideas from this lab and Lab 02, sketch the ladder diagram.
Extension
In the current circuit, if the operator presses and holds PB1 while the mixer is already running, R1 (ARMING) lights for 3 seconds even though nothing will happen. Change the circuit so that R1 never lights while the mixer is running. Hint: G1 could be moved in parallel with the CR1 coil to free a CR1 contact set.