Homemade high-resistance voltmeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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You can make a multi-limit high-resistance voltmeter yourself if you have a microammeter of a magnetoelectric system with a measurement limit of 50 μA. These include microammeters of types M24, M263M, M265M, M494M, M901, M2003 and others, differing in accuracy class, scale size and frame resistance, which is usually indicated on the scale or in the device passport. If the loop resistance is unknown, it can be determined in the following way. A DC source with a voltage of approximately 1,5 V (for example, element 332, 316 or 343), a resistance box set to 60 kΩ, and a pointer device are connected in series, of such polarity that the arrow deviates to the right from zero.
By adjusting the resistance box, the device needle is deflected exactly by a division of 50 μA and the resulting resistance is recorded, which should be about 30 kOhm. Then, by adjusting the store, the arrow is deflected exactly by a division of 25 μA and the resulting resistance is recorded again. The resistance of the device frame will be equal to the difference between the second reading of the resistance and twice the first reading.
It is not necessary to determine the frame resistance very precisely. Therefore, in the absence of a resistance store, you can use a set of conventional resistors such as MLT or BC with a tolerance of ± 10%.
The figure shows a schematic diagram of a home-made four-limit voltmeter with an input resistance of 20 kOhm / V. In the upper position of the switch, the measurement limit is 5 V full scale, in the next position - 20 V, in the next - 100 V and in the lower position - 400 V.
Resistors must be taken of the type ULI, BLP, S2-29V or S2-14 with a tolerance of ± 1%. The accuracy of measurements will depend on the tolerance of the resistors and the class of the pointer device. To improve the accuracy of the voltmeter, the resistances of resistors R1 and R2 must be taken not as indicated in the diagram, but reduced by the value of the resistance of the device frame. So, if the resistance of the frame is 2700 ohms, the resistance of the resistor R1 is taken equal to 97,3 kOhm, and R2 - 397,3 kOhm.
The resistances of resistors R3 and R4 can be taken as indicated in the diagram, since the error introduced by the frame resistance at these measurement limits will be quite small, less than the tolerance for resistor resistances.
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