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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This measuring device is universal, and therefore can take its rightful place in the laboratory of a beginner radio amateur. It allows you to measure DC and AC voltages up to 500 V, resistance up to 100 kOhm, determine the presence of phase voltage in AC circuits from 120 to 500 V, and also "ring" the installation, signaling the connection of the circuits with a light indicator.

The main working body of the device (Fig. 1) is the switch of types and limits of measurements SA1. When not in use, it may be in the "Off" position. or other than "R". The next four positions correspond to the measurement of alternating voltage (it is supplied to the sockets X1, X2) with limits of 2, 20, 100, 500 V. Additional resistors R6-R9 come into operation, the resistances of which are selected such as to ensure the deviation of the arrow of the RA1 indicator to a final division scales when measuring the maximum voltage on a given subrange. Of course, the resistance of additional resistors depends on the microammeter used, the choice of which will be discussed later.

This is also followed by four switch positions that determine the ranges for measuring direct voltage (it is supplied to the same sockets in the polarity indicated on the diagram) depending on the included additional resistors R2-R5 (one or more connected in series).

The next switch position - - corresponds to the ohmmeter mode. Now resistors R10, R11 and galvanic cell G1 are connected in series with the microammeter. When the input sockets are closed, the arrow of the microammeter should deviate to the final division of the scale (zero reading) - this is achieved by moving the slider of the resistor R10.

When "ringing" the installation, it is convenient to use the indicator light - the HL2 LED. True, its glow may turn out to be insufficient with the power source indicated on the diagram. The way out is to use an LED with a low forward voltage.

The last position of the switch - "UF" - "belongs" to the phase voltage detection mode. It is fed to socket X1 and touched with a finger to touch contact E1. The flashing neon lamp HL1 will indicate the presence of such a voltage.

In the device, you can use any small-sized microammeter with a full deflection current of the needle of 100-200 μA, respectively, selecting resistors R2-R9. It is advisable to disassemble the indicator and stick a new scale on it (if it is not possible to use the old one). An example of such a scale for the M4762 indicator (the total deflection current of the arrow is 145 μA, the frame resistance is 800 Ohms) from a tape recorder is shown in fig. 2.

Neon lamp - any other, even from an LDS starter (fluorescent lamp), it is desirable to select an LED with the lowest possible forward current and voltage, but with sufficient brightness, diodes VD1, VD2 - with a reverse voltage of at least 300 V, VD3, VD4 - any silicon and germanium, respectively. The author used the switch from the Isochron time relay, but another small-sized one with 11 positions is also suitable. Variable resistor - SPZ-46, SP2-36, the rest - MLT. Capacitor - for a voltage of at least 200 V.

The details of the device are placed in a ready-made (possibly self-made) case with dimensions of 100x55x25 mm (Fig. 3) made of insulating material. Installation - hinged, the leads of the parts are soldered to the contacts of the switch, the leads of the variable resistor, microammeter, neon lamp, LED (Fig. 4). As a sensor contact E1, a large-diameter screw was used, passed through a hole in the side wall of the housing (this can be seen in Fig. 3). Inside the case, a nut is screwed onto it, under which a contact tab is placed - a mounting wire in insulation is soldered to it, connecting the sensor contact to the corresponding circuits of the device.

Setting up the device is reduced to the selection of additional resistors R2-R9, for which you will need an exemplary device and appropriate power supplies connected to the sockets X1, X2. Start from the lowest subrange. On the upper subranges, it is not at all necessary to supply the required voltage to the input, you can get by with a much lower voltage, having achieved the deviation of the indicator needle to the corresponding scale mark. Each resistor can be made up of two connected in series or in parallel.

By selecting the resistor R12, the current through the LED is set within 10 ... 15 mA with the sockets closed and the G1 element fresh.

The phase voltage determination unit can be simplified by excluding the diodes and the capacitor and connecting the left output of the neon lamp according to the diagram directly to the resistor R1. In this case, the lamp will burn constantly (and not blink) when the probe inserted into socket X1 touches the wire with phase voltage.

Author: I.Potachin, Fokino, Bryansk region

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