Temperature Controlled Relay Switch Circuit

In this article we discuss a simple temperature controlled relay circuit which can be switched ON/OFF depending on the temperature on its temperature sensor. Meaning, when the temperature on its temperature sensor rises above a set threshold point, the relay switches ON, and when the temperature drops below the threshold, the relay switches OFF.

Any load connected across the relay contacts thus also switches ON/OFF depending on the temperature level of the sensor.

Using a Thermistor

Clearly for this temperature switch relay circuit to work we require some sort of electronic sensor whose characteristics change with temperature.

You will find a couple of primary types of components that exhibit ideal temperature sensing qualities: semiconductor junctions and thermistors.

For this circuit we have decided to work with a thermistor.

Thermistors are exclusive varieties of resistors constructed from heavy metal oxides and built to possess highly varying "resistance versus temperature" properties.

Circuit Description

The following figure exhibits our simple temperature controlled relay switch circuit, which even with its ease-of-use has the ability of providing precise and dependable results over an extensive range of temperatures.

The preset, RV1 enables the working temperature to be established, and is tweaked to make sure that below the preferred cut-off point, the voltage across base of Q2 is adequate to allow it to be in the switched on condition.

As the thermistor temperature increases, the voltage around the base of Q1, which is extracted from the resistive divider network comprising of R1,R2, RV1 and TH1, starts falling, ultimately permitting Q1 to turn off. The moment Q1 turns off the voltage around the base of Q2 goes up and the relay is switched on.

The diode, D1 delivers temperature compensation to the a pair of transistors, and C1 functions like a back EMF preventer for the relay. As indicated in the diagram, the circuit temperature range could be anywhere around 35 to 100 degrees centigrade.

As long as the relay is in an inactive condition, the current draw of the circuit is very little at around 2 to 3mA. Therefore, the circuit could be powered from either battery packs,or a tiny DC power sources.

Construction

A straightforward circuit which is explained here might be constructed over a tiny PCB. When constructed, the circuit must be two times inspected for proper positioning of diodes, transistors and capacitors, and only after that should be powered ON.

The thermistors possess no built-in polarity, and could therefore be hooked up without worrying about its direction.

How to Set Up

To set up the circuit, adjust the preset RV1 to roughly at the midway, and then slightly heat up the thermistor using some sort of external heating source, for example you can take a light bulb, or a soldering iron in close proximity to the thermistor.

Within a few seconds you should find the the relay clicking ON and any kind of device connected to the relay contacts could be seen switched on.

Next, as soon as the heat source moved away from the thermistor, the thermistor will start cooling down, until its temperature decreases below the trip-point and the relay switches OFF, turning off the connected load.

Applications

The relay output of this temperature controlled relay circuit could be attached to either an alarm device, or any kind of heating system, or cooling system.

For example the circuit could be used for switching ON a cooling fan as soon as the temperature is detected to be beyond a high threshold. It could used for cooling heatsinks or for cooling semiconductor devices in inverters.

While picking out a appropriate location for the temperature sensor, be cautious to stay away from putting it in a situation where it will likely be exposed to external heating agents.