Power transistors of the majority of structures in, say, laboratory block power, voltage converters, amplifiers, 3H, mounted on the heat sinks. Under severe conditions the corps of transistors heated to the temperature close to critical, - 125...150°C. In this increases the likelihood of their thermal and electrical breakdown. To warn him it is possible, by controlling, for example, the temperature of the heat sink (it is heated to 80...100°C) using the installed sensor on it, because, to place this the sensor directly on the transistors is not always possible.
One of the variants of the indicator over temperature of heat sink is shown in Fig. 1.
As the temperature sensor used germanium transistors - they rigidly attached to the heat sink through a thin insulating strip of mica or fluoropolymer. The transistors included in the power circuit of the indicator light - LEDs HL1. The current through the LEDs and transistors limiting resistor R2. It feeds the indicator from the source of the controlled node using parametric stabilizer R1VD1. The resistance of the resistor in kiloomah calculated from the formula R1 = (Un - 10)/8. If the supply voltage reaches 50 V, the resistor should be power not less than 0.5 W At a voltage of less than 16 In the part of the stabilizer it is permissible to exclude.
The led starts to glow visibly when the temperature of the transistor package exceeds 70°C.
In another embodiment, the indicator (Fig. 2) as a temperature sensor is used transistor VT2.
Transistor VT1 is made current amplifier LEDs HL1. Trimmer resistor R4 regulate the sensitivity of the device. The resistor R1 is calculated according to the above formula.
If this indicator to include in series with a corresponding led the optocoupler or opto-electronic relay will automatically disconnect controlled node from the power supply when the emergency situation.
Sensors indicators can be any germanium transistors series MP, MP, MP, GT, GT. Instead of the transistor CTB will fit any of the series KT361, CT, CT. Except as described in the diagram of the led, you can apply KIPP-K. Zener DV quite replace on DB, DG, DD, KSA. Trimmer resistor - any type of resistance 10...15 kW, constant - MLT, S2-23.
Author: A. Butov, S. CORBA Yaroslavl region