Measuring instruments on neon lamps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In simple measuring devices, which are described here, neon lamps of the MN-3 type with an ignition voltage of 50-60 volts serve as indicators. The neon lamp is connected to a voltage source or to a section of the circuit where voltage or current must be measured, through a potentiometer (voltage divider), which reduces the measured voltage to the ignition voltage of the neon lamp. The role of potentiometers is performed by variable resistors. Their resistance between the middle output and any of the extreme ones changes in direct proportion to the change in the angle of rotation of the axis.

The scale of each of the devices is a cardboard disk with divisions along the circumference and a hole in the center through which the axis of the potentiometer passes. A knob with an arrow is attached to the axis of the potentiometer.

Voltmeters. The voltmeter (see diagram in Fig. 1) allows you to measure direct voltages in the range from about 50 to 250-500 V. With the help of probes connected to the "Input" terminals, it is connected to the source of the measured voltage. By moving the slider of the potentiometer R1 from the bottom up (according to the diagram), the neon lamp is ignited. The value of the measured voltage is determined by the scale of the potentiometer.

Fig. 1

Resistor R2 limits the current through the neon lamp, preventing breakdown between the electrodes when a high voltage is applied to them. Capacitor C contributes to a brighter glow of the lamp at the time of ignition.

To reduce the measurement error, the neon lamp is “trained” with a constant voltage, slightly higher than its ignition voltage, before being installed in the device. As a result of "training" that lasts 50-70 hours, the lamp operating voltage and ignition voltage change by 10-15% and become more stable. During the "training" and after it, the correct polarity of the neon lamp must be observed. The cathode of the lamp is usually taken from its outer electrode, which has the largest surface area.

To calibrate the scale of the device, a constant voltage from a rectifier or batteries is supplied to its input, in parallel to which a factory (reference) voltmeter (Fig. 2) is connected (Fig. 1), through an additional potentiometer R. By moving the slider of the potentiometer R from the bottom up, the voltage at the input terminals of the voltmeter is gradually increased, achieving the ignition of the neon lamp. Then, on the scale of the potentiometer R50, mark the positions of the arrow of its handle, corresponding to the readings of the reference voltmeter. So, for example, setting the voltage of 1 V with the potentiometer R according to the reference voltmeter and having achieved the ignition of the neon lamp with the potentiometer R50, the number "XNUMX" is put on the scale against the pointer. In the same way, other measured voltages are marked on the scale of a homemade voltmeter.

Fig. 2

Both during calibration and during the use of a voltmeter, with an increase in the voltage supplied to its input, you must first turn off the neon lamp by setting the potentiometer knob to the position corresponding to the smallest measured voltage.

The scheme in fig. 1 can also be used to measure alternating voltages within the same limits. The calibration and use of an AC voltmeter remain the same as for a DC voltmeter. But the ignition of a neon lamp at an alternating voltage voltmeter will be at an amplitude voltage value that is times greater than the effective voltage recorded by the reference voltmeter.

A device for measuring low DC voltages can be assembled according to the circuit shown in fig. 3. The device is powered by a rectifier or battery, giving a constant voltage of 250-300 V. The slider of the variable resistor R5 turned on by the rheostat is set to zero of its scale (in Fig. 3 - to the extreme right position), and the "Input" clamps are shorted. Potentiometer R3 achieve the ignition of a neon lamp.

Fig. 3

After that, the input terminals are opened and a small measurable voltage is applied to them. At the same time, the neon lamp goes out. The lamp lights up again if the resistor R6 increases the voltage on it by the amount of the measured voltage supplied to the "Input" of the device.

The reading of the measured voltages is made at the moment of ignition of the neon lamp on the scale of the resistor R5, graduated in volts.

A voltmeter for measuring alternating voltages from 2 to 220 V (Fig. 4) is a combination of an autotransformer Atr with two switches P1 and P2 and the alternating voltage voltmeter described above. Autotransformer tap contacts, marked with numbers 2-20; refer to switch P1, and contacts 0-200 to switch P2.

Fig. 4

The indication of the device is the moment when the neon lamp glows, which is achieved by switches P1 and P2. The value of the measured voltage in volts is determined by summing the numbers near the contacts of both switches. So, for example, if the slider of switch P1 is on pin 6, and the slider of switch P2 is on pin 120, the measured voltage will be 126 V.

In order to avoid shorting the measured circuit, the instrument switches before the measurement must be: P1 in position 20, and P2 in position 200.

For the autotransformer of the device, you can use the core of a transformer designed for a power of 10-12 W (core cross section 4-5 cm2). The winding is wound with PEL wire 0,2-0,23. The part of the winding, the leads from which are connected to the contacts 2-20 of the switch P1, contains only 200 turns with taps every 20 turns, and the part of the winding, the taps of which are connected to the contacts 0-200 of the switch P2 - 2000 turns with taps every 200 turns.

AC ammeter (Fig. 5) consists of a transformer Tr with a transformation ratio of 1:40-1:60 (you can use the output transformer of a tube receiver, for example, the Record type) and an AC voltmeter.

Fig. 5

Winding I (lowering) the transformer of the device is connected to the electrical circuit in series with the load and the reference ammeter (shown in dotted line in Fig. 5), and an AC voltmeter is connected to winding II (stepping up). By moving the slider of the potentiometer R1, the neon lamp is ignited and the current value of the reference ammeter is noted on the potentiometer scale. By changing the magnitude of the load in the circuit, marks of currents of other values ​​\uXNUMXb\uXNUMXbare put on the scale.

The limits of measurement with such an ammeter depend on the number of turns and the cross section of the wire of the winding I of the transformer: with a decrease in the number of turns and an increase in the cross section of the wire of this winding, the measurement limits expand. When using an output transformer of the "Record" type, the device can measure currents up to 3-4 A.

AC wattmeter. If the mains voltage is stable, then the AC ammeter (Fig. 5) can be used to measure the current power. To calibrate such a device (in the absence of a reference wattmeter), an active load is connected to the circuit - an incandescent lamp, an electric stove or an iron with a known power value in watts. Then, in series with the load, turn on a homemade ammeter and, slowly turning the knob of the potentiometer R1, increase the voltage on the neon lamp until it lights up. At the moment of ignition of the neon lamp, the value of the power consumed by the load is noted in the corresponding place on the potentiometer scale. By connecting various loads of known power, a scale calibrated in watts is obtained.

Thus, on one cardboard disc of the ammeter potentiometer, you can have another scale - the AC power scale.

RC Meter. This device (Fig. 6) is designed to measure the resistance of resistors from 10 Ohm to 10 MΩ and the capacitance of capacitors from 10 pF to 10 microfarads. It consists of an audio frequency generator and a measuring bridge. Phones are used as an indicator of the device. Measurement error 10-15%.

Fig. 6

The generator, formed by a neon lamp, winding I of the transformer Tr, capacitor C4 and resistor R5, is powered by an external DC source (rectifier) ​​with a voltage of 80-250 V. The electrical oscillations generated by it with a frequency of about 1000 Hz are induced in the winding II of the transformer and feed the measuring bridge, one of the arms of which is connected to the measured resistor (terminals Rx) or capacitor (terminals Cx). The bridge is balanced with a potentiometer, focusing on the weakest or completely disappearing sound in phones.

The meter transformer is small-sized, with the ratio of the number of turns in the windings from 1:1 to 1:10. For a homemade transformer, you can use a core with a cross-sectional area of ​​​​3-3,5 cm2. Its primary winding can have 1000 turns, and the secondary from 1000 (ratio 1:1) to 10000 (ratio 1:10) turns of PEL wire 0,12-0,13. The values ​​​​of resistors R1-R3 and capacitors C1-C3 must be selected as accurately as possible, since the measurement error depends on them.

Potentiometer R4 is supplied with a scale graduated by resistors and capacitors with the smallest possible deviations from the ratings.

Device designs can be arbitrary. It is only important that they are comfortable to use.

Author: V.Shilov

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