In many areas of the world, keeping an amplifier cool is no big deal, because of generally low average temperatures. In Sydney (Australia), we can guarantee at least a few days every year when the temperature will be over 40 degrees C, and we are not alone in this. With Global Warming, we might all have the same problem in a few years.
The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. I designed this circuit to use 12V computer fans, as these are now very easy to get cheaply. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch. I used a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.
As can be seen, the circuit is very simple, and needs only a 12V single supply. This can be obtained from a small transformer, which need be rated at no more than 5VA or so. The supply does require regulation for the sensor if you want it to be accurate, but a simple zener regulator is sufficient for normal operation.
All diodes are 1N4004 or similar, and Q1 must be on a small heatsink – or may be mounted to the amplifier chassis. Make sure it is properly insulated, and use thermal grease. Maximum dissipation will be about 2 W, but it will overheat very quickly if there is no heatsink.
R7 has been added to ensure that Q1 turns off when the opamp’s output is high. Most opamps can’t reach the supply rail, and the voltage is usually about 1V less than the +ve supply. Some opamps may have a lower maximum voltage, and R7 will ensure that Q1 can be turned off completely. The same change has been made to the alternative version shown below.
The temperature is set with VR1. Operate the amp until the normal temperature is reached, then adjust VR1 until the fan starts. Then 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.
You can test the circuit without the amp, using a diode (or diodes) out in the air. Adjust as above, then hold the diode between your fingers – the fan should start up almost immediately, and stop again when you release the diode. Just the heat from your fingers is enough to operate the circuit. I tested the circuit with 3 standard 1N4004 diodes in parallel, and even without device selection I could hold any one of them and make the fan start.
I do not recommend that this be used from any preamp supply, as the motor noise will almost certainly cause problems. For this reason, I also suggest that you keep the switching transistor and fan leads will away from signal circuits to prevent noise.
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 you have more than this, use a larger resistor in place of R6 to limit the voltage. This will also help filter noise. Remember that the maximum recommended voltage for the uA741 opamp is +30V, and this must not be exceeded. If the voltage is greater than 12V, R5 will also need to be changed.
These fans will not run at less than about 6V, so don’t even attempt it. You might be able to use 5V fans, but these are usually only very small and do not provide much airflow.
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