Electronic Thermostat & Cooling Fan Driver

new.gif (1957 bytes)  3 Jan 05 - Added resistor value calculator to the form.

I originally built this unit to control the temperature in my father's orchid box. It is equally suited to speeding the cooling of battery packs after use. The design is built around the TC622 integrated circuit which is a programmable solid state temperature sensor designed to replace mechanical thermostat switches. The only components required in addition to the TC622 are a resistor to set the switching temperature and a MOS FET to switch the load. The units were built on to 2 separate pieces of veroboard so they fitted into the box well. The circuits and layouts were (click for full size):

Thermostat Schematic Thermostat Boards

For the original application I wanted to be able to vary the fan speed. This meant building a variable power supply for the unit, which was run from a mains transformer with a 15v output. The power supply allowed the power to the fan to be altered between 6v (the lowest voltage it would run consistently) and 12v (it's nominal voltage). As the TC622 will operate between 4.5v and 18v, this variable voltage could be used for the whole circuit & not just the fan.

The only calculation to be made when using the TC622 is the value of the resistor (R) connected between the +ve supply and the TSET pin. The formula is:

R = 0.5997 x T2.1312

The value of R is in Ohms, and the value of T is the switching temperature in Kelvin (C + 273.15).

For my application I wanted the be able to vary the switching temperature between 10C (283.15K) and 32C (305.37K). The value of R therefore had to vary between 100850 Ohms (0.5997 x (283.15)2.1312) and 118450 Ohms. This was achieved with a 100k fixed resistor and a 22k potentiometer. This gives the range of cut-off temperatures with the lower limit at 9C and the upper at 36C.

If you want to use the circuit at one temperature a fixed resistor is all that is necessary. The following table can be used to determine the value of R required for the range of temperatures shown:

C 10 15 20 25 30 35 40
F 50 59 68 77 86 95 104
R (Ohms) 100849 104682 108592 112577 116639 120777 124992

Type the lowest and highest temperatures (in C) for the switching range and then click the

Lowest Temperature = C
Highest Temperature = C

Fixed Resistor Needed = Ω
Variable Resistor Needed = Ω

I actually connected the TC622 to the veroboard by a piece of cable. This allows the sensor to be sited where most convenient. Please note that I experienced problems when using a piece of cable 1m long. I did not managed to find the reason, but the circuit worked correctly with a different piece of cable about 0.1m long. The Power Supply above is rated for 1.5A which is the limit of the system when it is used. This is not normally a problem as most small 12v fans from computers & the like only draw about 0.1A. The FET is capable of driving 9A, but is limited to 75W.

It is important if you intend to bolt the device to the case or a case mounted heatsink that you insulate the device from the case. This is best achieved using the readily available greaseless semiconductor insulators. Some of these feature in-built bushes to prevent the mounting bolt touching the device, whilst some require the use of a plastic 'top hat" bush to ensure insulation. Also commonly used are mica insulators which require the using of a thermal paste (grease) to ensure a reasonable thermal bond. These insulators provide a good thermal bond to the case whilst ensuring electrical isolation.

The Cooling Fan Driver. The following design has been specifically created for cooling of motor battery packs. Whilst the temperature is above the threshold the fan runs; when the temperature drops below the limit the buzzer sounds letting you know the pack is ready for charging.

Cooling Schematic Cooling Veroboard Layout

The sensor would need to be placed in contact with the pack to read it's temperature properly. This could be as simple as bolting the TC622 to the top (outside) of the box and the pack on top of it. If the fan is mounted in a tube with the TC622 on the top of a ledge inside, as the following sketch shows, the airflow over the pack will be greater and cooling more rapid.

Cooling Tube Sketch

If the Gate of the IRF630 is connected to pin 1 (OUT) instead of pin 2 (OUT), power would be supplied when the temperature dropped below the threshold. This would allow the circuit to operate a heater (with the FET driving a relay) to keep the temperature up in, for example, a workshop. If you want to know any more information or have a question please ask me. The following design shows a unit that is selectable (using a jumper) between heating and cooling.

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