AC voltmeter with extended scale. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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AC 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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In the practice of a radio amateur, it is sometimes necessary to control the mains voltage and monitor its changes. The easiest way for this purpose is to make a voltmeter based on a pointer indicator and "stretch" its scale so that only a few tens of volts "fit" on it. A diagram of such a voltmeter is shown in the figure.

Stretched AC Voltmeter

The controlled voltage is fed through current-limiting resistors R1, R2 to a full-wave rectifier, made according to a bridge circuit on diodes VD1-VD4. The rectified voltage is filtered by capacitor C1 and fed to the microammeter RA1 through the trimmer resistor R4 and the emitter junction of the transistor VT1, which in this case operates as a low-voltage "zener diode".

While the voltage at the rectifier output does not exceed 6 ... 8 V, the indicator needle is at the zero mark of the scale. When it exceeds the breakdown voltage of the emitter junction of the transistor, the indicator needle will begin to deviate the more, the greater the voltage.

Resistor R3 regulate the sensitivity of the device, and R4 - the range of measured voltages. So, with zero resistance of the resistor R4, when its slider is in the left position according to the diagram, 20 V "fits" on the indicator scale, and it can be designed to measure voltage from 210 to 230 V. If the resistor slider is moved to the right according to the scheme position, the device will be able to "show" voltage from 100 to 250 V.

Microammeter - from the recording level indicator of a tape recorder, for example, M4476 / 1, M4587, M68501, or any other with a full deflection current of the arrow 50-300 μA. Transistor - any of the KT315 series, diodes, except those indicated on the diagram - KD102B, KD521A, any of the KD105 series or KTs407A diode bridge. Adjusted resistors - any type. After setting up the device, they can be replaced by constants of the desired denomination.

For adjustment, an autotransformer is used, from which voltage is supplied to the X1 plug, and a digital voltmeter that controls this voltage. Adjusted resistors R3 and R4 achieve the desired range of measured voltages, after which they calibrate the indicator scale or make a calibration graph to the scale.

Author: A.Butov

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