Controls & Electrical
15% of this examThe 24-volt control world and the motors it commands: how the transformer feeds the thermostat, what each terminal letter does, classic capacitor failure symptoms, and just enough Ohm's law to check a circuit. Smallest domain, fastest points — every fact here is memorizable.
Core concepts
The transformer makes the control circuit
A small transformer steps line voltage down to 24 volts AC for the control circuit. The thermostat is just a switch panel routing that 24V to loads: the compressor contactor coil, the blower relay, the reversing valve solenoid. Low voltage means lighter wire and safer service.
Know the terminal letters
R is the 24V hot feed, C the common return to the transformer, Y the compressor contactor (cooling call), G the indoor blower relay, W the heat call. A cooling call closes R-to-Y and R-to-G. Terminal-matching questions are free points.
Hums but won't start = capacitor
A PSC motor needs its run capacitor to develop starting torque. With a failed capacitor the motor hums and stalls, but runs if you spin the shaft — torque is only missing at startup. That symptom pattern means capacitor, not contactor, thermostat, or charge.
Ohm's law settles current questions
I = E ÷ R. A 12-ohm contactor coil across 24V draws 2 amps. Expect one straight plug-in calculation; write the formula first and the distractors fall away.
Key facts to know cold
Where it lives in your books
The real exam is open book. Knowing which book and which tab answer this domain is worth as much as memorizing it.
Lookup strategy
- · These are memory questions — the books won't rescue you in time. Drill the terminal letters and the capacitor symptom until they're reflexes.
- · If a troubleshooting stem confirms the motor has power (it hums), eliminate every upstream answer (thermostat, contactor) before reading further.
Reading isn't learning. Retrieval is.
49 questions in this domain, each with an explanation and source.