Electrical and Controls
Core concepts
Know basic electrical theory: Ohm's law, series and parallel circuits, AC power, and the difference between line and low voltage. Understand motor types (PSC, ECM), contactors, relays, transformers, capacitors, and defrost boards. Be able to read a ladder diagram and a manufacturer's wiring diagram, and trace a call for heat or cool from thermostat through safety switches to the load. Understand thermostats (conventional vs. communicating), sensors, and basic building automation concepts. Know lockout/tagout and meter safety.
Traps in questions
- 24V control circuits are commonly misdiagnosed; a blown low-voltage fuse or failed transformer can mimic a thermostat failure.
- Capacitors fail and cause motor overheating or failure to start; always discharge before testing.
- Series vs. parallel: safety switches are typically wired in series with the control; a parallel circuit has the same voltage across each branch.
- ECM motors are constant-torque or constant-airflow and behave differently from PSC motors when static pressure changes.
- Confusing line voltage (120/240V) with control voltage (24V) leads to dangerous assumptions.
Worked example / rule of thumb
A 24V transformer rated 40 VA can supply about 1.6 A of control current (40 ÷ 24 ≈ 1.67 A). If the connected load (contactor, gas valve, board) approaches that, add a relay or upgrade the transformer rather than overloading it.
Study guidance
Trace circuits on a ladder diagram and practice identifying opens, shorts, and grounds with a multimeter.