Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
The split
Most machine wiring divides into two: the power circuit that carries the load current (to motors, heaters), and the control circuit that decides when the power circuit switches — buttons, relays, PLC outputs, contactor coils.
Why separate them
The control circuit usually runs at a lower, safer voltage (from a control transformer or DC supply), so operators interact with low voltage while the contactor switches the real power. It also keeps the logic clear: contacts in series form 'must all be made' conditions; parallel branches form alternatives.
A contactor is the bridge — a small control signal energises its coil to switch a big load. Diagnosing a machine means knowing which circuit you're in: a dead control circuit can leave the power side fully live.
On site
Start/stop, interlocks, and sequencing all live in the control circuit. Trace it as logic: follow the path that must be complete for the coil to energise. The power circuit is then simply 'contactor closed, load runs'.
Safety first
A dead control circuit doesn't mean the panel is safe — the power side and incoming supply can be fully live. Prove dead before working.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Contactor has voltage at the coil but won't pull in
You measure the rated control voltage (e.g. 24V) across the coil terminals, but the contactor refuses to energise — no clunk, no pull-in, contacts stay open.
Related definitions
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Transformer
Transfers electrical energy between circuits by magnetic coupling, stepping voltage up or down.
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.