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Automatic DC FAN controller circuit

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Automatic Temperature controlled DC FAN Controller Circuit 

We use many electronic gadgets like power Amplifiers, SMPS, Inverters, and AC-DC converters, etc. These gadgets use ICs that generate more heat during operation and required any temperature controller for Cooling to control the heat for protecting the devices. This circuit is ideal to control the cooling fan of heat generated electronic gadgets like power amplifiers. 

The fan should be small. You can use DC fan of Computer SMPS or CPU FAN .

This circuit switches on a fan automatically when the temperature rises to a fix or a set level.  It works by senses a temperature set level. The fan automatically turns off when the temperature returns to normal.

Working

The circuit uses an NTC (Negative Temperature Coefficient) Thermister to sense heat. The resistance of NTC Thermister reduces when the temperature in its vicinity increases.

A voltage comparator ic is used to compare its input voltage at pin 2 and 3 which is varied by thermister (as per temperature rises) with two potential dividers. Resistor R1 ( VR) And R2 form one potential divider connected to the non-inverting input of IC (U1) and another potential divider comprising R3 and the 4.7K  Thermister supplying a variable voltage to the inverting input of IC1. VR (R1)is adjusted to give a slightly lesser voltage at the non-inverting input than the inverting input at normal room temperature. Output of IC (U1) will be low and the Fan remains off in this case.

And when temperature increases near the Thermister, The resistance of its decreases. This drops the voltage at pin 2 of IC and its output becomes high and then Transistor Q1 gets on and fan turns on. Capacitor C1 gives a short lag before Q1 turns on to avoid false triggering of the transistor and to give proper bias to Q1.

Attach the Thermistor as possible as near the Heat sink of the Amplifier PCB.

Switch on the amplifier for 5-10 minutes. Then adjust VR till the Fan stop running. When the temperature rises, Fan will automatically switch on.

Components

IC

CA3140-1

Transistor

BC547 or 2N2222-1

Thermistor

NTC 4.7K-1

Resistance

1K-2

1.5K-1

Capacitor

10uf 25v-1

Diode

1N4007-1

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