Mains voltage voltmeter with extended scale. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Mains voltage voltmeter with extended scale. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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During debugging of various devices operating from the mains voltage of 220 V, it often becomes necessary to control the magnitude of the supply voltage. In this case, the tester, which is currently on the timeline, is usually used. But from frequent changes in the type of work and measurement range, the tester switch contacts wear out quickly. And if you also forget to put the switch in the right position, then the consequences for the measuring device can be the most deplorable.

So, if, after measuring the resistance, you try to measure the mains voltage without switching the range, then a cloud of smoke from the device can ruin your mood for a long time. In order to avoid this, it is proposed to make a simple voltmeter based on a pointer indicator, which would continuously monitor the mains voltage.

The schematic diagram of the device is shown in the figure.

Mains voltage voltmeter with extended scale

The voltage of 220 V through the rectifier diode VD1 and the resistor R1 is supplied to the transistor VT1, which in this mode operates in the avalanche breakdown mode. The sensitivity of the device depends on the value of the resistance of the resistor R2. Until the voltage at the emitter-base junction of the transistor VT1 does not exceed 6 ... 9 V (depending on the specific instance of the transistor), the current through this transition is too small and the voltage between the terminals of the base-emitter VT2 is less than 0,55 V, which is not enough to open it . As soon as the voltage at these terminals becomes more than 0,65 V, the transistor will begin to open and the microammeter needle will leave zero. This transistor works as an emitter follower, which makes it possible to reduce the temperature instability of the instrument readings and dispense with special thermal compensation elements.

The current amplifier on the transistor VT2 is powered by a simple parametric stabilizer on diodes VD2, VD3, resistor R3 and LEDs HL1-HL3, acting as a zener diode with a stabilization voltage of 4,6 ... 7 V, at the same time they are designed to illuminate the scale of the device. Capacitors C1, C3 smooth out the ripple of the rectified voltage, capacitor C2 dampens excess mains supply voltage. The resistance of the resistor R5 depends on how much you can stretch the scale.

Details. Fixed resistors of types MLT, C1-4, BC. Trimmer resistor R5 type SP3-29A (used in ULPTSTI-61 TVs) or any other resistance of 4,7 ... 10 MΩ. Capacitors C1, C3 series K50, K52, K53 or their imported analogues. Non-polar capacitor C2 of types K73-17, K73-16 for a voltage of at least 400 V. VD1VD3 diodes can be replaced with any of the KD209, KD410, KD243D, KD 247G, 1N4004, 1N4007 series. Inexpensive KIPD21G-K LEDs can be replaced with brighter KIPD21P-K, L-1513src-E, L-1513src-.. Transistor VT1 - any of the KT315, KT312 series; VT2 - from the KT3102, KT342, SS9014, BC549, 2SC184 series. It should be selected with a base current transfer coefficient of at least 500.

A microammeter can be used any small-sized from the recording current indicator of a household tape recorder, for example, M4387, M4762.1, M4761. It should be borne in mind that the type of pointer indicator determines how much the scale of the measured voltage can be stretched. So, with a resistor R5 resistance of 0 ohms with an M68501 frame (300 μA), it will fit up to 15 V on the scale, and with sensitive frames M4260, M4204 with a total deviation current of 50 μA, the scale of the device is compressed to 8 V. Of course, such a sensitive device (216. ..224 V) may find limited application, but it is clear and interesting for various experiments and tests.

Adjustment. Resistor R2 regulates the sensitivity of the device, resistor R5 - the width of the range of measured voltages, with its increase, the range expands. Due to the presence of a current amplifier stage on the VT2 transistor, the device can work with almost any measuring frame with a full deflection current of the arrow from 50 to 1000 μA.

To calibrate the device, it is desirable to use an accurate digital voltmeter (no worse than ± 1%) and an autotransformer. For constant monitoring of mains voltage, it is recommended to select the range 198-242 V (deviation ±10%) or 176-242 V (-20...+10%). If more precise control is needed, then a scale of 215 ... 225 V can be made. After the final adjustment of the device, it is recommended to replace the tuning resistors with constant ones, of the same resistance as part of the tuning resistors (the stability of the tuning resistors is much worse).

By proportionally changing the resistance of the resistor R1 and the capacitance of the capacitor C2, it is possible to control a voltage that is significantly different from 220 V. LEDs are used to illuminate the scale, which is often very convenient. If there is no need for backlighting, then they can be replaced with a 4 ... 7 V zener diode.

A new scale for the device can be carefully drawn with a ballpoint pen on coated paper for office equipment. The old scale is removed from the measuring frame. According to its shape, a new one is cut out of a sheet of paper, which should be glued with Moment glue in place of the removed one. Further, during calibration, the necessary marks are made on it.

A very beautiful scale can be made on a printer with a color cartridge, but this is already a matter of possibilities.

Structurally, the device can be made as a stand-alone device in its own case or built into the case of any apparatus, which is more convenient so as not to clutter up the desktop and adjacent surfaces with a variety of measuring equipment. When setting up and operating the device, observe the precautions taken when working with a mains voltage of 220 V.

Author: A.L. Butov

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