Air Conditioner Basics Part I. Electrical Control

Understanding Electrical Components in Air Conditioning

This installment describes when and how each electrical component of an air conditioning system is electrically powered on.

The Role of the Thermostat 

The master controlling device is the thermostat. Depending on the switch setting on the thermostat and/or the temperature of the space being monitored by the thermostat's thermometer, the thermostat will send electrical current to one or more switching devices to turn on one or more of the air conditioner's components.

The Function of Switching Devices

Note in the following diagram how the switching devices (relays and contactor) are connected between the line-level voltage source and the components to be turned on. The relays and contactor respectively receive low voltage power from the thermostat. This low voltage is produced when a transformer reduces the line-level voltage.

Controlling the Indoor Fan

When the FAN switch on the thermostat is set to ON, but the room temperature is still lower than the set cooling temperature, the thermometer powers on only the relay for the indoor fan, and the relay passes line-level power to the indoor fan to turn it on.

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Cooling Activation

When the FAN switch is set to AUTO and the room temperature rises above the set cooling temperature, the thermometer powers on the relays for both the indoor and outdoor fans, as well as the contactor. As a result, line-level power is passed to the indoor fan, the outdoor fan, and the compressor, turning all three components on.