Voltage divider probe for digital multimeter. Scheme, description

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Voltage divider probe for digital multimeter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Most digital multimeters are designed to measure DC and AC voltages up to no more than 1000 V. For the M890C + multimeter with an input resistance of 10 MΩ (on all ranges of measuring DC and AC voltages), the author developed and manufactured a high-voltage probe-divider 1:10, which increases the input resistance of the device to 100 MΩ and increases the limit of the measured direct voltage to 10 kV. A sinusoidal alternating voltage device with a divider measures up to 7,5 kV with an accuracy of ± 5%. The input current when changing the DC voltage of 1000 V is about 10 μA, and for 15V it is only 0,15 μA. The dissipated power on all probe resistors when measuring a voltage of 10 kV does not exceed 1 W.

The dividing probe diagram is shown in fig. one.

Voltage divider probe for digital multimeter

As resistors R1 - R3, high-voltage resistors KEV-1 with a nominal value of 33 MΩ were used from scanners from obsolete color tube TVs ULPCT-59/61.

To get a voltage division factor of 10, the probe must have a resistance of about 90 MΩ. The resistance of most of the KEV-1 resistors tested by the author with a nominal value of 33 MΩ ± 20% turned out to be less than 30 MΩ, so the selection of resistors to obtain the desired probe resistance did not cause difficulties. Resistor R4 (for example, MLT-1) is the final finishing of the probe.

To avoid damage to the DMM due to high input voltage, it must be equipped with an air gap, as shown in Fig. 2.

Voltage divider probe for digital multimeter

Two small pieces of thick copper wire with sharpened and directed ends are soldered to the "COM" and "V / -" sockets. The distance between the tips of the wires is 1,2 ... 1,3 mm.

An exemplary design of a high-voltage probe is shown in fig. 3.

Voltage divider probe for digital multimeter

The needle 1 of the probe is fixed with a nut 2 in the housing 3. The flexible cable 4 connecting the resistor R4 with the multimeter, as well as the common wire with the crocodile clip, is made with a wire in strong insulation. As a body, the author used two hot-glued markers, one of which was shortened by 25 mm. Any other suitable tube made of polystyrene or polyethylene with a wall thickness of 1,5...2 mm can be used.

The flexible terminals of the resistors are bitten, the remaining metal tips are cleaned of paint on an emery wheel or paper. Resistors are connected end-to-end with a large amount of solder. Soldering should be neat and smooth.

Before installing the resistors in the probe body, it is advisable to wrap them with several layers of thin fluoroplastic film. You can use a film of capacitors from failed voltage multipliers UN8,5 / 25-1,2.

For the M830V multimeter, which has an input resistance of 1 MΩ when measuring DC voltage, a similar probe with a resistance of 9 MΩ was made, consisting of nine matched MLT-2 resistors of 0,91-1 MΩ each. However, if a voltage of 10 kV is measured with this divider, about 10 W of power will be dissipated on the probe resistors, which in most cases is unacceptable for the circuit being measured. Therefore, the dividing probe by 10 for multimeters with an input resistance of 1 MΩ is most advisable to use only to increase its input resistance. If the total resistance is increased to 99 MΩ in the proposed probe, then a 830:1 divider is formed with the M100V multimeter and the limit of the measured voltage increases to 10 kV. In this case, the instrument readings should be multiplied by 100.

Attention! When measuring high voltage, you must comply with the relevant electrical safety requirements. During the measurement, do not touch the chassis and the common wire of the measured device!

Author: A. Butov, village of Kurba, Yaroslavl region

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