Stabilizer overload indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Surge Protectors
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Not every stabilizer with an adjustable output voltage and a load short circuit protection system has an overload mode indicator. Such an indicator, if required, can also be introduced into the finished stabilizer.
It uses a bipolar transistor as a key. The indicator uses two LEDs - green and red. The green LED indicates the normal operation of the stabilizer, and the red one (while the green goes out) indicates the overload mode.
The voltage at the base of the transistor VT1 (see the diagram in Fig. 1) is stabilized by the zener diode VD1. In the normal mode of operation of the stabilizer, the voltage at the base of the transistor is approximately 1 V higher than at the emitter, so the transistor is closed and the green LED HL1 is on. Reducing the output voltage of the stabilizer below 2 V during overload causes the zener diode VD1 to exit the stabilization mode and reduce the voltage at the base of the transistor VT1. Therefore, the transistor turns on.
Fig. 1
Since the forward voltage on the on HL1 LED is greater than on the HL2 LED and the transistor, at the moment the transistor opens, the HL1 LED goes out, HL2 turns on. The forward voltage on the green AL307G LED is about 0,5 V higher than on the red AL307B LED, so the maximum saturation voltage of the collector-emitter of the transistor VT1 must be less than 0,5 V.
In the indicator, transistors of the KT3107, KT208, KT209, KT50I, KT502 series can be used. Resistor R1 limits the current through the LEDs HL1 and HL2 (Imax=20 mA), and resistor R2 determines the current through the zener diode VD1 (Imax=81 mA). If the voltage stabilizer is designed for one fixed output voltage, the VD1 zener diode can be replaced with a resistor R3, the resistance of which is determined by the formula: R3=3,3*R2/(Uout-3,3), where Uout is the output voltage of the stabilizer, V , and R2 and R3 are expressed in kiloohms.
Author: A. Suchinsky, Balashikha, Moscow region; Publication: cxem.net
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