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 project described is not intended to allow you to skimp on the heatsink, but to ensure that the amp is never allowed to exceed a preset temperature. Class-A amplifiers and very high power amps will benefit the most, but the thermal controller principle described can be applied to almost anything.
It is extremely sensitive, and can easily be set so that a few degrees change is enough to turn on a fan, and reduce the power (or turn it off again) as the amp cools.
As of 02 Jan 2000, there is a second version, designed to operate directly from the amp's +ve supply rail, and is specifically for amps that use a supply voltage greater than 30V. The circuit is almost identical, using one additional low cost transistor and an extra resistor.
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.
Resistor Values For R5 and R8
You will need to check the current that your fan draws to calculate the resistance. Connect it to a 12V supply, and measure the current. Your supply voltage must be less than 30V (otherwise see below). Calculate the resistance of R8 with ...
R8 = (+ve -12) / I Where +ve is your supply voltage, and I is the measured fan currentSelect the closest resistor value larger than calculated. You will also need to work out the power:
PR8 = (+ve - 12)² / R8Now you can work out the value for R5 (1/2W will be OK here) - here, use the next smaller value if an odd resistor value is calculated. The zener current is nominally 10mA, so ...
R5 = (+ve - 10) / 0.01 Where +ve is your supply voltage.For example, if you have a 25V supply available, and your fan draws 200mA at 12V ...
R8 = (25 - 12) / 0.2 = 13 / 0.2 = 65 Ohms (use 68 Ohms)Having worked these out, you can adapt the circuit to any voltage, as long as it is less than 30V and 12V or more. For higher voltages, see the alternative version in Figure 2 (below). Note that with the 10 Ohm resistor for R8, the actual fan voltage will be about 10V.
PR8 = (25 - 12)² / R8 = 13² / 56 = 169 / 68 = 2.5 Watts (use 5W)
R5 = (25 - 10) / 0.01 = 15 / 0.01 = 1500 Ohms (1k5)
PR5 = (25 - 10)² / 1500 = 225 / 1500 = 0.15W