Electronic Temperature Control Circuit

For a few experiments such as solar air heaters, I needed a automatic turning on and off of a fan. Probably the simplest way to do this is with a bimetal snap disc thermal sensor.

A bimetal snap disc thermal sensor is basically a switch that closes when a specific high temperature is reached and opens when a specific lower temperature is reached. That way, by connecting in a fan in series with the switch, the fan would turn on when the high temperature is reached and off then the lower temperature is reached. They are also referred to as snap disk controllers and snap switches.

They are be found on places such as ebay. PexSupply.com is also one source amongst many.

The following are the simplest circuits that can be used, one using AC and one using DC. WARNING: Do not do this unless you are familiar with electronics.

Simplest fan (or other device) control circuit

The above AC circuit wired up in a box.

The above box was then inserted into the top of my outdoor to indoor airflow solar air heater as shown in the following photos.

Notice the snap disc sensor on the end of the white wire, extended into the hot area of the solar air heater.

The fan mounted in place and plugged in and everything ready to go.

The following videos give step-by-step details for making the AC circuit. The first video introduces the snap disc sensor, tests a prototype circuit and shows how to determine the size of fuse needed for your fan or blower. It's especially useful for anyone not familiar with electronics.

This next video wires up the actual circuit in a safe manner (safety is imporant here since it's being plugged into the household wiring), assembles it all in a nice box, puts it in the actual solar air heater and does a quick test.

Adding a thermostat for added control based on room temperature

If your room is already hot enough then you don't want your solar air heater to provide more hot air. This can be controlled by adding a thermostat to the circuit. The thermostat is basically another switch that opens when the room is hot enough and closes when the room needs more heat. The following diagram has the themostat added.

Simplest fan (or other device) control circuit

Safer use of household power

The snap disc sensor is located in the solar air heater. Using the above schematic, the sensor has 120VAC across it. It may be preferable to have lower voltage wiring in this area and for that the following schematic can be used. All other circuitry can be housed in a box that is mounted to the blower/fan enclosure and so all wiring there can have the higher 120 volts AC while only lower voltage 24 volts AC wires would extend out to the location of the snap disc sensor.

Circuit with low voltage across snap disc sensor

Adding an hour meter

For performance measurement purposes it's often handy to add an hour meter to the circuit so you can see how long your solar air heater has run for. An example is shown below on the left of an hour meter from Grainger, part no. 2PAW9, for $23.77. In the photo on the right the hour meter is mounted on the solar air heater input vent for convenience. Click here for details of this particular solar air heater.

The hour meter would go on the AC side of the circuit anywhere where these would be no power while the snap disc has turned off the fan. See the circuit on the right below. (This particular one also has a manual switch thrown in.)

Examples in solar air heaters

At the time the projects below were done, the only snap disc sensor I could find in town was a cheap one for around $5.00CDN that worked the opposite way desired. It opens when the higher temperature is reached and closes when the lower temperature is reached. So a relay was added to reverse this, and power had to be supplied to the relay. The resulting circuit was still simple, but not as simple as it could have been. WARNING: The following are some photos containing electronics arrangments that are extremely ugly. :-)

The following is from my mini can solar air heater.

Fan and electronics.

The following is from my outdoor to indoor airflow solar air heater.

The following is from my indoor to indoor airflow solar air heater.