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 — answers 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.
39 questions in this domain, each with an explanation and source.