Circuit Description:
The circuit is based around the LM324 quad Opamp configured as differential amplifiers.
QA1a generates a very stable voltage, set to 10v, while QA1b will compare this stable voltage against the voltage difference generated by the thermistor. The thermistor is an NTC 47K. NTC stands for Negative Temperature Coefficient i.e. the resistance decreases when the temperature rises.
Opamp QA1b inverting input (-) is preset to 8v while the non-inverting input (+) is fed from the voltage difference between the 47K preset and the 47K thermistor. When the temperature rises, the thermistor resistance will decrease and therefore the the non-inverting input (+) voltage will rise. Once the non-inverting (+) input is higher than the inverting input (-), the output of the opamp will go high, turning on the transistor Q1 which in turn will turn on the relay RLY1. The relay will in turn switch on the cooling fans.
Capacitors C2, C3, C4 & C6 offer some hysteresis so that the fans will not switch on/off continuously.
U1 is a 7812 voltage regulator. Since the available voltage inside the inverter is 24v and the circuit needs 12v, the regulator is used to step down the voltage, smoothed further by C1.
Parts List:
D1 - 5.6v Zener Diode
D2, D3 - 1N4001
D4 - Red LED
U1 - 7812 voltage regulator
OA1a, OA1b - LM324
R1 - 4.3K
R2 - 5.6K
R3 - 1K
R4 - 5.6K
R5 - 47K Preset
R6 - 1K
R7 - 47K Thermistor (NTC)
R8 - 47K Preset
R9 - 1.2K
C1, C5 - 2200uF
C2, C3, C4, C6 - 1000uF
Q1 - TIP122
RLY1 - Relay
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