Fan temperature switch
In practice, we often encounter the requirement to cool certain components. For this purpose, it is common to get coolers of various sizes and profiles. But their prices are not negligible.
In some cases, even the factory construction does not inspire confidence – e.g.. PA or resources. Source, the one I bought had a more massive cooler, but you certainly wouldn't be able to keep your hands on it after an hour of operation. For peace of mind (what if the stabilizing transistors break) I supplemented it with a circuit fan, which switches it on at elevated temperature.
Air flow effect for cooling
Flowing air can increase the cooling effect very significantly (up to 7 times!) and so it is not necessary, so that in this case the fan does not run continuously, not even at full speed. At increased temperature, the balance of the bridge is disturbed, the comparator flips over and the transistor starts the fan via the pull-down resistor.
Connection of a fan with a temperature switch
Connection is extremely simple, but at the same time very high quality. The supply voltage is 13.8V. Resistor R3 reduces the speed of the fan, which significantly reduces noise. The trimmer Rp serves to set the temperature, at which the fan starts up. The value of the components connected in the wheatstone bridge is determined by the resistance value of the thermistor Rt. It pays, that Rt=Rp=R1=R2. Capacitors C1 and C2 are blocking, I 7805 ensures, so that the fan is not accidentally started with fluctuations in the supply voltage. The operational amplifier can be arbitrary – MAA…., TL…., BM…. and so on. Transistor T must be selected according to the following parameters: Uce>15V, Ic>150mA ( it depends on the fan ). I do not give the exact values of the parts, everyone chooses them according to their component base themselves.
Hysteresis is ensured only by the thermistor (in my case, an unknown type from an old TV). The fan runs for at least a minute after starting. The connection works to my absolute satisfaction on my RM LPS120/S source.
