Multirange voltmeter with a linear scale. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Multirange voltmeter with a linear 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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Working with a conventional tester is often annoying with the need to change probes when changing the polarity of the measured voltage. The uneven scale of variable voltages is also inconvenient. The proposed device is devoid of these disadvantages.

Schematic diagram of the voltmeter is shown in the figure.

(click to enlarge)

The main difference between the construction of the device circuit and the generally accepted ones is the use of a bridge formed by germanium transistors of different structures VT1 - VT4, which is included in the measuring circuit, regardless of whether the voltage is measured AC or DC. This eliminates the need to change the probes when changing the polarity of the measured voltage. Switching the voltmeter from measuring DC to AC voltage is achieved by closing the additional resistor R16 using the SA2 button to eliminate the difference in readings.

For measurements of alternating voltages in traditional circuits, rectifier diodes are used, their non-linear current-voltage characteristic leads to a non-linearity of the scale. The transistor bridge has a linear characteristic. The voltmeter circuit provides protection for transistors and pointer device from overloads. For this, the zener diodes VD1 and VD2 are used, as well as the resistor R10 and the fuse FU1. When assembling the device, it is necessary to select bridge transistors with the same values of the current transfer coefficient, which must be at least 40.

Structurally, resistors of additional resistances are placed directly on the slats of the switch of measurement limits. The details of the transistor bridge are assembled on the printed circuit board shown in the figure.

In the author's version of the voltmeter, a pointer device with a total deflection current of 300 μA was used. In this case, the input resistance of the voltmeter turned out to be 2 kOhm/V. If you install a microammeter with a total deflection current of 50 µA, the input resistance will increase to 10 kΩ/V. In this case, it is necessary to increase the resistance of additional resistors.

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