Control board output devices: relays, triacs and transistor drivers, and how loads kill them

Relays: the board's logic energizes a small coil (often 12 V DC) through a driver transistor; the coil pulls contacts closed that carry the line-voltage load. Used for heavy or inductive loads such as heaters, compressors and motors. A click proves only that the coil moved.

Triacs: solid-state AC switches used for smaller AC loads such as water valves, drain pumps, dispenser motors, door locks and fans. A small gate signal (usually through an opto-isolator) turns the triac on; it turns off when the AC current falls to zero. An RC snubber across the triac helps it turn off cleanly with inductive loads.

Transistor and MOSFET drivers switch low-voltage DC loads (DC fans, LEDs, stepper and BLDC fan motors, some valves). They often have no visible fuse.

Inrush is what kills outputs: motors, transformers, solenoids and capacitive loads draw several times their running current at switch-on. Contacts arc and can weld; semiconductor junctions overheat. A shorted or low-resistance load does the same thing every time it is switched.

Failure modes: relay contacts either burn open (load never runs) or weld shut (load never stops). Triacs most often fail shorted, either fully or in one half of the AC wave; open-circuit triac failures are less common. A shorted output keeps the load energized whenever the appliance has power.