This circuit was specifically created to safeguard computer monitor from overheating. It is highly recommended to connect this circuit to the monitors used by power users. CRT-type computer monitors commonly face failure due to excessive heat. After one or two hours of operation, the back of the monitor can reach temperatures as high as 45 degrees Celsius, which is 20 degrees above the surrounding ambient temperature. The main sources of heat in monitors are the VGA gun drivers, horizontal circuit, vertical circuit, and power supply. To effectively dissipate heat and extend the lifespan of the monitor (while also contributing to environmental conservation), it is advisable to incorporate a brushless fan. These fans are lighter, more energy-efficient, and superior in performance compared to regular fans.
Temperature Sensing Diodes and Power Supply Connection
In this setup, diodes D2, D3, and D4 are utilized to sense the monitor’s temperature. These diodes collectively exhibit a negative temperature coefficient of 6 mV per degree Celsius. To ensure accurate readings and eliminate noise, it’s crucial to employ shielded wire for connecting the temperature sensor to the circuit. The +12-V supply voltage is sourced either from the computer’s power supply or an external mains adapter delivering 12 VDC. Decoupling capacitors C1 and C2 play a pivotal role in smoothing out any ripples generated due to switching or oscillation.
R1 establishes bias current for D1, a 6-V zener diode functioning as a reference on the non-inverting pin of opamp IC2.B. IC1, acting as a ‘precision shunt regulator,’ elevates the sensor diodes’ voltage slightly above 6 V, contingent on the adjustment of P1. C4 functions as the decoupling capacitor within the sensor network. Integrator network R4-C5 introduces a delay of approximately 3 seconds, transforming IC2.B’s on/off output signal into an exponentially decreasing or increasing voltage. This voltage is fed into pin 3 of the second opamp, IC2.A.
Mitigating Noise with PWM Technique
The conventional hard on/off technique could potentially introduce substantial noise each time the load is switched. To address this issue, an alternative approach is implemented. IC3, a TLC555, serves as an astable multivibrator. R5 and C6 regulate the charging network, generating a sawtooth voltage with a frequency of around 170 Hz. This sawtooth waveform is coupled to pin 2 of IC2.A, which compares the voltages at its input pins and produces a pulse-width modulated (PWM) output voltage. The sawtooth wave is integral to the PWM signal sent to power output driver T1 via stopper resistor R6. The power FET, controlled by the PWM drive signal, toggles the fan on and off accordingly. High-speed diode D7 clamps the back electromotive force (emf) pulses that occur during T1’s switching on and off.
Calibration Process and Circuit Performance
During the calibration process, P1, the variable resistor, should initially be set to maximum resistance. Warm air from a hair dryer is directed onto the sensor diodes for about a minute. A temperature meter is placed near the sensor diodes, and P1 is gradually adjusted toward the minimum resistance position, with a digital meter connected to pin 7 of IC2.B. The calibration is typically set around 40 degrees Celsius. At this temperature, the meter should indicate approximately 12 V. The circuit draws about 120 mA from its 12-V supply during operation.