Interview Question Prep
The questions a controls interview actually asks, with what a good answer contains.
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1. Explain a seal-in circuit, and why you need one.
A good answer: A start button is momentary. The coil latches through its own auxiliary contact so it stays energised after you release the button, and a normally-closed stop breaks the rung. Draw it if there is a whiteboard.
Where people lose it: Saying 'it holds the motor on' without explaining that the contact is the coil's own is the answer of someone who memorised it.
2. What is the difference between a normally-open and a normally-closed contact in ladder?
A good answer: NO passes power when the bit is 1; NC passes when the bit is 0. Then add the thing that matters: a hardwired stop button is wired NC in the field so a broken wire stops the motor, and the ladder contact is examined NO. Field state and program symbol are not the same thing.
Where people lose it: Most candidates only answer the program half and miss the field-wiring half, which is the safety-relevant part.
3. Describe the PLC scan cycle.
A good answer: Read inputs to the image table, execute logic top to bottom, write outputs, then housekeeping and comms. Say why it matters: an output set on rung 1 and reset on rung 50 ends the scan reset, and rung order determines behaviour.
4. Difference between a timer and a counter, and which timer types do you know?
A good answer: TON energises after a preset time while the input stays true; TOF holds after the input goes false; retentive timers keep accumulated value through an interruption. Give a plant example for each.
5. How do you handle an output that must survive a power cycle?
A good answer: Retentive memory or a latch in a retained area, and then talk about whether it should survive. Most machines should restart in a safe known state, not resume mid-sequence.
The one that decides it
Nearly every controls interview ends with a fault-finding question. They are not testing whether you know the answer, they are testing whether you measure or guess. Say where you would put the meter, and in what order.
How to use it
- 1Read a topic in Study mode first, including the Where people lose it notes.
- 2Switch to Quiz mode and answer out loud before revealing anything. Out loud matters; it is a different skill from knowing.
- 3Spend most of your time on Commissioning and fault finding. That is the section that decides the outcome.
- 4Pick one project you can defend under follow-up questions, and rehearse it until it is boring.
Next, try these
Thank You Email
The follow-up after an interview, short enough that it gets read.
Elevator Pitch
Thirty seconds on who you are, built around what you have actually commissioned.
ATS Resume Checker
Paste your resume and see what an applicant tracking system reads, and what it throws away.
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