what tellz the contactor to tur o the ac


What Tells the Contactor to Turn on the AC


Imagine a scorching summer day, and all you can think about is finding solace in the cool, refreshing breeze of your air conditioning. But have you ever wondered how your AC magically knows when to turn on and start cooling your space? The answer lies in a little device called a contactor. The contactor is responsible for controlling the flow of electricity to your air conditioning unit, ensuring it turns on and off at the right time. In this article, we will delve into the intricacies of what tells the contactor to turn on the AC, exploring the components and processes at play.

The Role of the Contactor

The contactor is an essential component in the electrical system of an air conditioning unit. Its primary function is to make or break the electrical circuit that powers the compressor and fan motor. When the contactor is energized, it closes its contacts, allowing electricity to flow and turn on the air conditioner. On the other hand, when the contactor is de-energized, it opens the contacts, interrupting the electrical supply and turning off the AC.

The contactor is like the gatekeeper of your air conditioner, determining whether it is time to let the electricity flow and activate the system or to cut off the power and bring everything to a halt. But what tells the contactor when to perform these actions? Let's find out!

The Thermostat

One of the key components that communicate with the contactor is the thermostat. The thermostat is a device that monitors the temperature of your indoor space and sends signals to the contactor based on the set temperature and the current room temperature. It acts as the control center, instructing the contactor when to turn on or off the AC.

When the temperature in your room rises above the set temperature on the thermostat, it sends a signal to the contactor, telling it to close the contacts and allow electricity to flow. This energizes the compressor and fan motor, starting the cooling process. Once the desired temperature is reached, the thermostat sends another signal to the contactor, instructing it to open the contacts and disconnect the power, thus turning off the AC.

It's fascinating how a small device like the thermostat can precisely regulate the temperature in your space by communicating with the contactor, ensuring optimal comfort and energy efficiency.

Pressure Switches

While the thermostat plays a crucial role in determining when to turn on and off the AC based on temperature, pressure switches come into play to ensure the system operates within safe limits. Pressure switches are sensors that monitor the pressure levels in the refrigerant lines of your air conditioning unit.

There are two types of pressure switches: the low-pressure switch and the high-pressure switch. The low-pressure switch protects the AC from running with insufficient refrigerant, which can cause damage to the compressor. If the pressure drops below a predetermined level, the low-pressure switch sends a signal to the contactor, interrupting the flow of electricity and preventing the AC from turning on.

Similarly, the high-pressure switch safeguards the AC from operating under excessively high pressures, which can lead to mechanical failures. If the pressure exceeds a safe limit, the high-pressure switch communicates with the contactor, cutting off the power and shutting down the AC.

Pressure switches work hand in hand with the contactor, ensuring the safety and longevity of your air conditioning system by monitoring and controlling the pressure levels.

Overload Relay

Another noteworthy component that cooperates with the contactor is the overload relay. The overload relay is a protective device designed to safeguard the compressor and motor from overheating due to excessive currents.

Think of the overload relay as a safety net. It continuously monitors the electrical current flowing through the system. If it detects a current higher than the rated limit, it triggers the contactor to open its contacts, cutting off the power supply and preventing damage to the AC's crucial components.

The overload relay is like the guardian of your air conditioner, preventing potential burnout and ensuring the system remains in optimal working condition. Without its intervention, high currents could cause irreparable harm to your AC, leading to costly repairs or even necessitating the replacement of the entire unit.

Time Delay Relay

When you switch off your air conditioner, you might have noticed that it takes a few moments before the outdoor unit shuts down completely. This delay in turning off the AC is due to a device called the time delay relay.

The time delay relay enables a pause or delay in shutting down the outdoor unit after receiving a signal from the thermostat or another control device. This delay is intentional and serves various purposes. For instance, it allows any remaining heat to dissipate from the system, preventing short cycling or rapid on-off cycles that can put strain on the compressor.

Moreover, the time delay relay helps protect the compressor from potential damages that may occur due to power surges or voltage fluctuations. By incorporating a delay in shutting down, the relay ensures that the compressor starts from rest, minimizing the risk of damage and prolonging its lifespan.


In conclusion, the contactor in your air conditioning unit plays a vital role in regulating the flow of electricity and controlling the activation and deactivation of the system. The contactor is guided by various components, including the thermostat, pressure switches, overload relay, and time delay relay. The thermostat communicates with the contactor based on the temperature settings, while the pressure switches monitor the pressure levels to ensure safe operation. The overload relay protects the compressor and motor from overheating, while the time delay relay allows for a pause in shutting down to prevent potential damages.

Understanding what tells the contactor to turn on the AC gives us insight into the intricate mechanisms at play behind the scenes. It highlights how different components work together harmoniously to provide us with the comfort and convenience we crave during summer's scorching days. So, the next time you enjoy the blissful coolness of your air conditioner, you can appreciate the contactor's role and the factors that influence its actions.


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