Electronic Controls, Sensors and Inverter/LED Appliances
What this domain tests
Electronic Controls, Sensors and Inverter/LED Appliances covers PCBs, sensors, relays and triacs, switch-mode supplies, inverter air conditioners and LED drivers.
Core rules
- Common sensors include NTC thermistors (resistance falls as temperature rises), thermocouples, IR sensors, float switches, pressure switches and door switches. A sensor is tested against its temperature-resistance table, not by a simple continuity beep.
- A microcontroller reads sensors through an ADC and drives relays, triacs or optocouplers. An SMPS provides low-voltage DC; check its output before blaming the control board.
- An inverter AC rectifies mains, filters it, and uses an IPM to produce variable-frequency, variable-voltage drive to the compressor. The compressor speed follows demand, so current and pressure vary normally.
- An LED lamp uses a constant-current driver and a series string. One open LED can kill the string; a failed electrolytic capacitor causes flicker.
Worked example
An inverter AC displays a room-temperature sensor error. With the unit isolated, the NTC is disconnected and measured. At about 25 °C it should read near 10 kΩ; the meter shows open circuit. Reasoning: the sensor or its harness is open. Check the plug and harness first, then replace the sensor with the specified type. Clear the error and confirm the displayed temperature tracks a thermometer.
Traps that cost marks
Probing a live control board; assuming every sensor is a simple switch; fitting an LED driver with the wrong output current; ignoring error codes; and handling boards without ESD protection.