Definition
A low-current electrical circuit used to operate a switching device — such as a relay, contactor, or solenoid — which in turn carries the higher current needed to power a load. The control circuit does not supply the working current itself; it only commands the device that does.
Plain English
A small circuit that tells a bigger circuit when to switch on or off. The pilot's switch is in the small circuit, and the heavy power flows through the bigger one.
Context Anchor
Seen in aircraft electrical-system descriptions, wiring diagrams, and troubleshooting of items such as lights, pumps, motors, and warning systems.
Derivation
From Latin 'circuitus,' meaning 'a going around.' A circuit is a closed loop electricity travels around. A 'control' circuit is the loop used to command something, rather than to do the heavy work itself.
Why Pilots Care
Many cockpit switches do not directly carry the load they appear to operate. A small switch on the panel commands a relay elsewhere, which switches the real current. Knowing this helps in troubleshooting — a failed system may be a problem in the control circuit, the relay, or the power circuit, and they are diagnosed differently.
Analogy
Like a doorbell button. Pressing the button completes a small circuit that triggers the chime mechanism — the button itself doesn't carry the power that rings the bell.
Intuition Check
A control circuit is not always the circuit carrying the main power to the equipment. It may only carry the command that makes another circuit or device operate.
Example Sentence 1
When the pilot moves the starter switch, the control circuit energizes the starter relay, which then sends battery power to the starter motor.
Example Sentence 2
The landing-light control circuit uses a cockpit switch to energize a relay that supplies full battery power to the lamps.