The post explains a simple temperature indicator circuit using a single BC547 transistor and complementary 1N4148 diode.
The temperature indication of a heat sink on a power transistor in high power circuits can be extremely useful. A simple, inexpensive temperature indicator would be ideal for this purpose since accuracy is not an important factor. In the design for the temperature indicator here, the voltage drop across a diode that is held at ambient temperature is used as a reference level. The temperature detection is carried out by a transistor mounted on the heat sink and/or close to the power transistor in question. In the circuit diagram the temperature detector is transistor T1 and its base emitter voltage is compared to the reference level at the junction of D1 and R1 via the preset P1. The tran- sistor will remain switched off as long as its temperature remains below a certain level, a level that · is effectively set by P1. The base emitter voltage of the transistor will drop by about 2 mV for a rise in temperature of about 1 degree Centigrade. When the base emitter voltage of the transistor drops below the voltage level at the wiper of P1 the transistor will conduct and light the LED D2. This will happen gradually and thus provide an indication over a fairly wide range. The values of R1 and R2 are of course dependent upon the supply voltage, Ub, and can be calculated as follows: R1 = (Ub - 0.6)/5 kohm R2 = (Ub - 1.5)/15 kOhm For optimum performance of the circuit it is important that the reference diode is situated in the free air at room temperature definitely not above the heatsink! The transistor should be mounted on (or even in, if drilling the heatsink is acceptable) the heatsink as near the heat dissipating element as is practical. It must be remembered however that the maximum expected temperature should not exceed 125°C if you value your transistor.
The current consumption of the simple temperature indicator circuit will be little more than the LED current, about 20 mA, and then only when things are starting to cook