Safety switch for DC power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies
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If the device is designed to be powered from an external DC voltage source, there is always a danger of applying it in the wrong polarity or exceeding the permissible value of this voltage. In the first case, a diode connected in series to the power circuit helps. But it won't save you from stress.
A simple and reliable circuit breaker for incorrectly supplied or increased supply voltage is proposed with the possibility of estimating the voltage by the brightness of the glow of an incandescent lamp. Its scheme is shown in the figure.
When a voltage of 5 V is applied to the input of this node (plus to the emitter of the transistor VT2), which is less than the stabilization voltage of the zener diode VD1, the transistor VT1 is closed, and the transistor VT2 is in saturation mode and passes the input voltage to the output with minimal losses. A base current sufficient to saturate the transistor flows through the HL1 incandescent lamp. It glows faintly, as it should, when less than 26 V is supplied to a lamp with a nominal voltage of 5 V.
Fig. 1
If the input voltage exceeds the stabilization voltage of the zener diode VD1 and becomes sufficient to open the transistor VT1, the emitter-collector section of the opened transistor will bypass the emitter junction of the transistor VT2 and the latter will close. The HL1 lamp, as a result of an increase in the voltage applied to it, will shine brighter. The resistance of its heated filament will increase. This will limit the current flowing through transistor VT1 and eliminate the need to remove heat from it. Transistor VT2 of the type indicated in the diagram at a load current of 1 A must be installed on a heat sink with an area of 20 cm2.
Such a switch will protect the powered device from voltage of the wrong polarity. Its switching element (transistor VT2) can be transferred from the positive power wire to the negative one if both transistors are replaced with similar parameters, but npn structures. The switch can also be installed at the output of any power supply to eliminate the risk of exceeding the voltage allowed for its load, for example, as a result of a breakdown of the control transistor.
It should be borne in mind that on the open transistor VT2 of the switch, several tenths of a volt of the input voltage inevitably drops. Therefore, this transistor should be selected with a minimum saturation voltage for a given maximum load current. If at this current the transistor leaves the saturation mode, which is accompanied by an increase in the voltage drop across it and dissipated power, a more powerful HL1 lamp should be installed.
The turn-off voltage is determined mainly by the zener diode VD1. In order to fully open the transistor VT1 in emergency mode. you may need to reduce the value of the resistor R1.
Author: V. Solonin
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