Small voltmeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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You don't always need a multi-range measuring instrument, such as an autometer or multimeter. Sometimes it is enough to measure the voltage or resistance in some circuit of the device. For this purpose, the proposed volt-ohmmeter is quite suitable.

Balancing on a shaky stepladder, holding with difficulty the uncomfortable Ts57 or Ts20, you try to find a malfunction in the wiring of the television antenna in the stair shield and ... you drop the measuring device. A familiar picture, isn't it? The device will also become unusable if, having forgotten to switch the resistance measurement mode, you suddenly touch the probes with the relatively high-voltage circuit of the radio device under test.

All this can be avoided if you have a small-sized volt-ohmmeter (Fig. 1) with a convenient system for switching measurement modes in your hands.

Such a device can be held in the hand (Fig. 2) and the necessary buttons of the mode switches and measurement limits can be pressed with your fingers.

The logarithmic characteristic of the device made it possible to cover a larger measurement range compared to the linear characteristic. Hence, the reduction in the number of ranges to two: direct voltage 0 ... 30 V, 0 ... 300 V, and alternating voltage 0 ... 60 V, 0 ... 600 V. Read readings on these ranges from different scales without additional switching [1]. To measure resistance, two ranges are also used: 0...2 kOhm...infinity (on a scale with direct reading) and 0...100 kOhm...infinity (on a scale with reverse reading) [2]. Current measurement is not provided, but its strength is easy to calculate from the voltage drop across a resistor of known resistance.

The main distinguishing feature of the volt/ohmmeter is that the desired range and mode are selected by pressing the buttons of the switches SB1-SB3. If the buttons are released, the device automatically returns to the maximum voltage measurement mode. Another feature of the device is that even in uncomfortable positions (for example, in the hold of a boat), it is possible to unmistakably switch on the desired mode with one hand, moreover, by touch, while holding the device overhead. In this case, the scale, the measuring probe and the circuit of the measurement object are simultaneously in the field of view. So that this convenience does not depend on the polarity of the measured voltage, a switch SA1 is provided.

If the contacts of this switch are in the position shown in the diagram, the X1 probe should be connected to the positive voltage of the circuit under test, and the X2 clamp to the negative one. The VD2 diode will turn out to be open. In order not to interchange the probe and clamp with the reverse polarity of the measured voltage, set the movable contact of the switch to a different position.

When measuring alternating voltage, the diode VD1 is equalizing, VD2 is a rectifier. When the movable contact of the SA1 switch is in the right position according to the diagram, the functions of the diodes change. In this case, it becomes possible to quickly detect some types of distortions in the form of an alternating voltage - by the difference in the amplitudes of the half-cycles. Naturally, the diodes must be the same.

Switches SA1 and SB2 are located on the upper part of the case, easily accessible for switching with the index finger, and SB1 and SB3 - on its side wall. This secures the priority of the top two switches over the two side switches. It becomes possible before each inclusion of the resistance measurement mode to make sure that there is no voltage on the circuit under test. To measure low resistances, the buttons SB1 and SB3 are pressed simultaneously, and the movable contact of the switch SA1 is moved to the right position according to the diagram. If you need to measure low resistances, press the SB1 button, and set the movable contact of the switch to the left position.

As SB1-SB3, microswitches MP5 (permissibly MP1) are used, which have a relatively easy stroke of the drive mechanism. Their work is clearly felt tactilely and by ear. You need to solder them carefully so that the flux does not flow into the case. Power source G1 - battery D-0,26. It can be periodically recharged without removing it from the instrument case. To do this, you need to fix the SB1 button in the pressed position, put the movable contact of the SA1 switch in the right position according to the diagram, and apply a constant voltage of 1 ... 2 V to the X2 probe and X3 clamp (plus to the clamp). Resistor R3 (SP5-3) sets the charging current to 26 mA.

Arrow indicator RA1 - recording level indicator M4761-M1. The case for the device can serve as an opaque part of the plastic packaging from the popular "Electronics" watch. The landing dimensions of the indicator and the packaging, by chance, exactly match. If possible, one should be selected from several indicators that has a well-balanced moving system - when changing the position of the device, the arrow should not deviate from the zero mark.

The indicator must be provided with a new scale. To do this, open the cover of the indicator with a scalpel and push the stops (if they are in this instance of the indicator) so that the span of the arrow increases to 90 °. After loosening the thrust bearing fixing nut, set the arrow 3 mm to the left of the black sector of the factory scale, then tighten the nut and fix it with a drop of glue. The magnetic system is adjusted and fixed so that the needle maintains the same (about 1 mm) distance to the scale along its entire span.

A new scale (Fig. 3) with dimensions of 53x30 mm is made from coated paper and glued, and then the cover is glued with dichloroethane.

Plastic plates (from the top cover of the watch package or from the package of the MK-60 cassette) must be glued flush with the cutouts on the bottom of the case. Having simulated the location of the parts in the housing (Fig. 4), drill holes with a diameter of 1 mm against the buttons of the switches SB3 -SB5 and countersink them with a drill with a diameter of 10-15 mm.

Under the SA1 switch engine, a rectangular groove is cut (or drilled) in the housing wall. These parts, as well as the resistor R3 and the battery, are glued to the case with Moment glue. But beforehand, trying not to overheat it, quickly solder pieces of MGTF wire to the battery. Fixed resistors and diodes are soldered to the terminals of the switches (the lower terminal of the resistor R2 according to the diagram and the upper terminals of the diodes are soldered together on weight).

Probe X1 - a piece of brass wire with a diameter of 1,5 mm and a length of 76 mm. On the one hand, it is sharpened, and on the other, it is bent 13 mm from the edge at approximately a right angle. The probe is inserted from the outside into a round hole in the SB2 switch housing and the backlash is eliminated by winding a strip of foil 7 mm wide onto the probe. A spring, a pre-tinned washer, a bare end of the connecting wire (it fits SB1 and SB3) are put on the end of the probe protruding from the hole, and the connection is soldered. On the outer side of the body, three grooves about 1 mm deep are melted for a clear fixation of the probe in three working positions, and one groove along the diagonal of the body for laying the probe in the non-working position. A bright thin-walled cambric is put on the probe.

Clamp X2 is made from a sewing needle No. 130 from a sewing machine. A piece of brass wire with a diameter of 0,5 mm is fixed in the ditch of the needle. The wire from the clamp to the device is best used in fluoroplastic insulation.

When carrying the device, the clip is inserted into the "sheath" from a flattened tube from the "chupa-chups" 20 mm long, glued to the body with melted nylon.

When adjusting the device, by selecting the resistor R2, the arrow is deflected to the final division of the scale when a constant voltage of 30 V is applied to the input (probe and clamp) and the SB2 button is pressed. They check the calibration of the DC voltage scale, and then the AC voltage, applying 60 V to the probe and clamp. Next, they select a resistor R1 of such resistance that when the SB2 button is released, the arrow deviates for the first supply voltage of 3 V, and for the second - by 6 V on the corresponding scales.

The calibration of the resistance scales is checked by connecting resistors of known resistances to the instrument input.

If the deviations of the indicator needle do not coincide with the calibration of the scales, you must either redraw them again or compile a correction table.

After that, you need to tightly fix the dial indicator in the housing, applying a layer of plasticine around its perimeter.

To avoid the "tragedy", which was discussed at the beginning of the article, it is enough to place a piece of thin fishing line inside the device, the end of which in the form of a loop can be thrown over a button when working at height.

Literature

1. Ladyka A. Miniature tester with a probe. - M .: DOSAAF, VRL, vol. 81, p. 1-6.
2. Kuzin V. M. Portable combined devices. - M.: Radio and communication, 1991, p. 19.

Author: A. Ladyka, St. Petersburg

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