This temperature monitor is practical to monitor the level of heat from its power transistors and heatsinks and if needed, to automatically turn off the amplifier to avoid damage to the vital components. Since it is not required to monitor the temperature by strict degree resolution, this simple monitor will work very well.
The temperature monitor circuit works by comparing the voltage drop of a “cold” diode (T1) with the base emitter voltage of a “warm” transistor (T2). The transistor must be attached physically closest to the heat source, ideally it should be attached to the heatsink of the power transistors.
The diode T1 must be positioned away from the heat source to ensure that it is always at room temperature. The circuit “measures” the heat difference between the transistor and the diode. T2 should not conduct as long as the temperature level being monitored is below the set threshold. The base-emitter voltage of T2 decrease by 2 mV per 0C. Once the voltage is below the level set by P1, the T2 conducts and the LED begins to light up.
The values of R1 and R2 are dependent on the power supply voltage. The values can be computed using the following formulas:
R1 = (Ub/V – 0.6)/5 kΩ
R2 = (Ub/V – 1.5)/15 kΩ
For example: by 12 volts power supply, the R1 is 2.2 kΩ while R2 is 680 Ω. The maximum current consumption of the temperature monitor circuit when the LED lights up is 20 mA. Take nothe that the T2 must not become hotter than 1250C.