Voltage difference indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Sometimes in amateur radio practice it may be necessary to compare two voltages with each other and monitor the change in their difference. Of course, they can say that for this purpose you can use two voltmeters, according to the dial indicators of which it is not difficult to make the necessary assessment. However, this method is not always acceptable, both because of its imperfection, and because of the low accuracy in estimating a small voltage difference.

This is where electronics will come to the rescue, in particular, the proposed indicator, the scheme of which is generally shown in Fig. 1. This is a differential amplifier made on transistors VT1 and VT2, the loads of which are LEDs HL1 and HL2. A distinctive feature of the indicator is the combination of the functions of comparison and indication devices in one cascade.

Ris.1

If the input constant voltages applied to connectors XS1 and XS2 are equal, the brightness of the LEDs is the same. But as soon as one of the voltages changes by about 3%, the difference in the brightness of the glow becomes noticeable, and if the difference in input signals is more than 20%, only one LED will light, which determines the sign of the voltage difference.

What are the practical possibilities of such an indicator? Here are two examples that radio amateurs can use when developing various designs.

Amplitude calibrator - this is how you can call the indicator, the diagram of which is shown in Fig. 2. An alternating voltage is supplied to the XS1 connector, the amplitude of which must be monitored and maintained at a certain level - it is set by the exemplary voltage (from 0,5 to 5 V) set on the basis of the transistor VT2 with a trimming resistor R3. As long as the amplitude of the input signal is comparable to the set value, the brightness of the LEDs is the same. When the amplitude deviates in one direction or another, the corresponding LED stops glowing.

Ris.2

The indicator is calibrated to a given signal level as follows. A signal is applied to the input of the indicator, for example, with an amplitude of 1V, and by moving the trimming resistor slider, the same brightness of the LEDs is achieved. In this case, the error in setting the reference voltage will not exceed 3%. If the tuning resistor is replaced by a variable one and provided with an appropriate scale and calibrated, in the future you can quickly set the desired value of the reference voltage, and hence the controlled level of the input signal.

The frequency range of the calibrator is 20 Hz...100 kHz. It should be powered from a stabilized DC source with a voltage of 10 ... 25 V. However, at a voltage of more than 20 V, the resistor R2 should be 2,2 kOhm.

One of the practical applications of such a calibrator is the recording level indicator of a monophonic tape recorder. Of course, the calibrator can also work in a stereo tape recorder, allowing you to more accurately set the same gain across channels. In this case, the same circuit is connected to the base of the transistor VT2 instead of the resistors R3, R4 as to the base VT1. There will be another entrance. Now each input is connected to its own amplifier channel. Playing any record in the "Mono" mode, the tape recorder controls set the same brightness of the LEDs. In other words, the calibrator becomes in this case an indicator of stereo balance.

The inputs of the calibrator are connected to the same circuits of channel amplifiers, in which the signal amplitude lies within the above limits (0,5 ... 5 V).

If, when the calibrator is connected, sound distortion in the tape recorder is observed, it will be necessary to install emitter followers in front of the calibrator inputs, made according to the generally accepted scheme.

Having slightly transformed the circuit of the previous device, you will get an indicator of battery discharge, for example, galvanic cells (Fig. 3). The reference voltage in it is formed by a parametric stabilizer, composed of a ballast resistor and a zener diode (details R4 and VD1).

Ris.3

The operation of the indicator is illustrated in Fig.4. In the battery voltage range of 12,6 ... 7 V, the HL1 LED lights up, and its brightness almost does not change. If the voltage drops below 7 V, the HL1 LED starts to burn and at the same time the brightness of HL2 decreases. The same brightness of both LEDs may indicate the need to recharge the battery (if it is made up of batteries) or replace it. In the voltage range of 6 ... 2,5 V, the HL2 LED will light up, informing about the decrease in battery voltage below normal.

Ris.4

The trimming resistor R2 can shift the boundary region (dU in Fig. 4) from 3,8 ... 4,3 V at the lower, according to the scheme, position of the engine to 11 ... 12,3 V at the upper position.

It is convenient to use such an indicator, say, in a car to control the voltage of the on-board network. At the maximum supply voltage, the indicator consumes a current of about 2 mA, and at a voltage of 6V - about 1,2 mA. The LEDs may be different, but then you will have to select the R3 resistor to obtain the desired brightness.

When replacing the silicon transistors indicated in the diagram with germanium structures npn (MP37B), some expansion of the dU zone was observed, within which both LEDs are lit, up to 1,5 V. connection of LEDs, zener diode and power supply.

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