Voltage indicator for power strip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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I would like to share my experience in manufacturing an original indicator of the presence of voltage in the network ~ 230 V. Many radio amateurs remember the time when computer system units had processor clock indicators, consisting of a three-digit seven-element LED indicator and many jumpers, by rearranging which, it was possible to display any number on the indicator from 000 to 999.

This indicator has been lying on my shelf for a very long time. I did not want to unsolder it, because I hoped to make something original out of it, and not just get a three-digit indicator and two dozen unnecessary jumpers ...

And recently, when I was making a network extension cord with a voltage indicator on a blue LED, this indicator caught my eye. The blue LED was removed without regret, and an indicator was inserted into the extension cord, on which the green number 230 is lit, indicating the rated voltage in the network. I powered the indicator from a simple power supply with a quenching capacitor according to the circuit shown in fig. 1.

Rice. 1. Indicator power circuit

In order not to accidentally touch the parts of the indicator board that are under mains voltage, the gaps between the hole in the extension housing and the indicator must be covered with an insulating material. To limit the inrush current when turned on, a resistor with a resistance of 20 ... 30 Ohm and a power of 0,25 ... 0,5 W should be installed in series with the fusible link.

But first, the indicator had to be connected to a 5 V DC voltage source, after setting the jumpers so that the number 230 lit up, and measure the current consumption with a multimeter. You need to know it in order to correctly select the capacitance of the quenching capacitor C1. Formulas for its calculation can be found, for example, in the article by S. Biryukov "Calculation of a network power supply with a quenching capacitor" ("Radio", 1997, No. 5, pp. 48-50). With sufficient accuracy, we can assume that if the capacitance of this capacitor is 1 μF, and the rectifier is full-wave (as in the case under consideration), then this current will be about 60 mA. Of these, 50 mA will flow through the HG1 indicator, and the remainder will be taken over by the Zener diode VD2. If the indicator is accidentally turned off, the zener diode will protect the smoothing capacitor C2 from breakdown, the voltage on which will not exceed 6 V. If you use an indicator with a different current, then the capacitance of the capacitor C1 must be changed in proportion to the current.

Fusible insert FU1 is needed in case of breakdown of capacitor C1. If it burns out, it will prevent damage to the mains supply wires and elements of the protected device that threatens with great trouble. It was decided to test the fuse-links at 0,16 A and 0,25 A. In order to accurately determine whether the 0,16 A fuse would burn out from the surge of the charging current of the capacitor C1 during the initial turn-on, about a dozen slow turns on the mains plug were made in socket and turn it off. Many of them were accompanied by sparks. But the 0,16 A insert passed the test. It is clear that a 0,25 A insert will withstand it all the more.

Resistor R1 is designed to quickly discharge the capacitor C1 after the device is disconnected from the mains. Otherwise, you may receive an electric shock if you accidentally touch the pins of the mains plug that has been disconnected from the socket.

Since the indicator must work around the clock to ensure the required reliability, an imported analogue of the K1-73 film capacitor with a permissible direct voltage of at least 17 V (or alternating at least ~ 630 V) should be used as C275. Unfortunately, the domestic industry does not produce K73-17 capacitors for 630 V with a capacity of more than 0,47 microfarads, therefore, if there is no suitable imported capacitor, you will have to connect two such capacitors in parallel.

You can go the other way - use a network charger for a cell phone. The main thing is that its board fits into the extension housing. This will significantly increase the safety of the extension cord. But you should make sure that the output voltage of the charger is 5 V (all modern chargers with a micro USB connector satisfy this requirement).

If the charger was intended for an old model phone and its output voltage is more than 5 V, it is necessary to connect a limiting resistor in series with the indicator, choosing it so that the indicator current does not exceed the previously measured value.

Rice. 2. Scheme for switching on the indicator with common anodes

Rice. 3. Scheme of turning on the indicator with common cathodes

Instead of a board with a clock indicator from an old computer, if it could not be found, you can use any LED three-digit seven-element indicator, the digits of which have separate element outputs (the total number of outputs of such indicators is 28). An indicator with common discharge anodes is switched on according to the circuit shown in fig. 2, and with common cathodes - in fig. 3. Of course, you can use three single-digit indicators or a four-digit one without using one digit in it. Selecting resistors R2-R4, set the desired brightness of the glow of the numbers.

Author: A. Karpachev

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