This circuit was designed to protect a computer monitor from overheating. It is recommended to attach this circuit to power users’ monitors! Most computer monitors of the CRT type fail owing to over-heating. After one or two hours of use, the rear of a monitor may become as hot as 45 degrees C, or 20 degrees above ambient temperature. Most heat comes from the VGA gun drivers, the horizontal circuit, vertical circuit and power supply. The best possible way to extract heat and so prolong monitor life (and save the environment) is to add a brushless fan, which is lighter, energy-wiser and more efficient than a normal fan.
In the diagram, diodes D2, D3 and D4 sense the monitor’s temperature. These diodes have a total negative temperature coefficient of 6 mV per degree Celsius. To eliminate noise, shielded wire should be used for the connection of the temperature sensor to the circuit sensor. The +12-V supply voltage is borrowed from the computer’s power supply. Alternatively, a mains adapter with an output of 12 VDC may be used. C1 and C2 are decoupling capacitors to eliminate the ripple developed by switching or oscillation. R1 provides bias current to D1, a 6-V zener diode acting as a reference on the non-inverting pin of opamp IC2.B. IC1, a ‘precision shunt regulator’ raises the sensor diodes’ voltage to just over 6 V depending on the adjustment of P1. C4 is the decoupling capacitor with the sensor network. Integrator network R4-C5 provides a delay of about 3 seconds, transforming the on/off output signal of IC2.B into an exponentially decreasing or increasing voltage. This voltage is fed to pin 3 of the second opamp, IC2.A.
The hard on/off technique would produce a good amount of noise whenever the load is switched, hence an alternative had to be found. IC3, a TLC555, is used as an astable multivibrator with R5 and C6 controlling the charging network that creates a sawtooth voltage with a frequency of about 170 Hz. This sawtooth is coupled to pin 2 of IC2.A, which compares the two voltages at its input pins and produces a PWM (pulsewidth modulated) output voltage. The sawtooth wave is essential to the PWM signal fed to power output driver T1 by way of stopper resistor R6. The power FET will switch the fan on and off fan according to the PWM drive signal. The back emf pulses that occur when T1 switches on and off are clamped by a high-speed diode, D7. Initially, turn P1 to maximum resistance. Blow hot air from a hair dryer onto the sensor-diodes for a minute or so, then get the temperature meter near the sensor diodes and adjust P1 slowly towards the minimum resistance position with a digital meter hooked up on pin 7 of IC2.B. Roughly calibrate the temperature to 40 degrees C. At this temperature, the meter will show approximately 12 V. The circuit will draw about 120 mA from its 12-V supply.