This circuit is ideal to
control the cooling fan of heat generated electronic gadgets like power
amplifiers. The circuit switches on a fan if it senses a temperature
above the set level. The fan automatically turns off when the
temperature returns to normal.
The circuit uses an NTC (Negative Temperature Coefficient) Thermister to sense heat. NTC
Thermister reduces its resistance when the temperature in its vicinity
increases.IC1 is used as a voltage comparator with two potential
dividers in its inputs. Resistor R1 and VR1 forms one potential divider
connected to the non inverting input of IC1 and another potential
divider comprising R2 and the 4.7K Thermister supplying a variable
voltage to the inverting input of IC1. VR1 is adjusted so as to give
slightly lesser voltage at the non inverting input than the inverting
input at room temperature.
In this state, output of IC1 will be low and the Fan remains off.
When the temperature near the Thermister increases, its resistance
decreases and conducts. This drops the voltage at pin 2 of IC1 and its
output becomes high. T1 then triggers and fan turn on. Red LED
indicates that fan is running. Capacitor C1 gives a short lag before T1
turns on to avoid false triggering and to give proper bias to T1.DC fan
can be the one used in Computer SMPS.
Keep the Thermistor near the heat sink of the Amplifier PCB
and switch on the amplifier for 10 minutes. Then adjust VR1 till the
Fan stop running.When the temperature rises, Fan will automatically