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Here is another take on the Thermo-Fan Controller using different component than those mentioned previously.
In many areas of South Africa temperatures soar to 40 degrees celsius during the summer
months. With Global Warming, we might all have the same problem in a few years from now.
New equipment is designed with built in thermo-fans and is fairly well protected against abnormal heat. Older equipment and home brew stuff may appreciate the acquisition of a thermal controller to cool things down. The project described is to ensure that your equipment is never allowed to exceed a preset temperature. It is extremely sensitive, and can easily be set so that a few degrees change is enough to activate the fan/fans.
The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap op-amp
as the amplifier. The circuit is designed to operate with 12 volt computer fans. These fans typically draw about 200 mA when running, so a small power transistor will be fine as the switch. A BD140 (1A, 6.5W) was used, but almost anything you have to hand will work just as well.
All diodes are 1N4004 or similar, and Q1 must be on a small heat-sink or may be mounted
to the chassis. Make sure it is properly insulated, and use thermal grease. Maximum dissipation will be about 2 watt, but Q1 will overheat very quickly if there is no heat-sink.
The temperature is set with VR1. Operate the piece of equipment to be fitted with the thermo-fan until the normal temperature is reached, then adjust VR1 until the fan starts.
Back off very slowly until the fan stops again. Any increase over the normal temperature will start the fan, and promptly bring the temperature back down again.
It will not matter if the voltage is a little higher than 12V, as the fan will work fine as long as voltage is kept below about 14V. If the voltage is higher than 14V, R5 and R6 will have to be changed.
To calculate the resistor values for R5 and R6 you will need to check the current that your fan draws. Connect the fan to a 12 V supply, and measure the current. Calculate the resistance of R6 with
R6 = (+ve - 12) / I Where +ve is your supply voltage, and I is the measured fan current.
Select the closest resistor value larger than calculated. You will also need to work out the power:
P6 = (+ve - 12)2 / R6
Now you can work out the value for R5 (1/2 will be OK) - here, use the next smaller value if an odd resistor value is calculated.
R5 = (+ve - 10) / 0.01 Where +ve is your supply voltage.
Having worked these out, you can adapt the circuit to any voltage, as long as it is less than
30V and 12V or more. Any other combination is quite acceptable, including the use of fans in series/parallel (current will be double thou) or anything else that you might want to do.
This circuit is very flexible and I am convinced that there are many other uses for this sensitive thermal controller in amateur radio.
