Voltmeter with linear scale. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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A voltmeter with a linear scale allows you to measure DC and AC voltages and resistances.

The main parameters

The voltmeter amplifier (Fig. VII.4) is a three-stage differential with a single-ended output. The first stage (on field-effect transistors V2, V3) is designed to increase the input resistance of the device. The second and third stages are assembled on transistors V6, V7, V9 and V10. The amplifier in the second stage is balanced by resistor R39. The load of the output stage is the microammeter P1, included in the collector circuit of the transistor V10.

(click to enlarge)

Resistors R29*, R30* are connected in series with the microammeter, on which a negative feedback voltage is allocated, proportional to the current through the microammeter. The device circuit is designed in such a way that only the positive voltage applied to the gate of the transistor V2 (inverting input of the differential amplifier) ​​and the negative voltage applied to the gate V3 (non-inverting input) are amplified. At voltages of reverse polarity, the transistor V10 closes and the reverse deflection current of the microammeter can only be obtained by auxiliary power. Therefore, a diode V9 is included in the collector circuit of the transistor V11, and voltage is removed from it to compensate for the voltage drop across the resistor R42 created by the initial current of the transistor. This current is chosen in the range of 0,04 ... 0,05 mA to increase the linearity of the initial section of the amplifier characteristic. Buttons S1-S3 serving * for selecting the type of measurement. Switch S4 change the measurement limits. Protection of the device against overloads when measuring voltages is carried out by resistor R28 and diode V4, when measuring resistance - by resistor R35 and diode V16. Diodes V1 and V8 also serve to protect the amplifier transistors from overloads.

The supply voltage stabilizer is made on transistors V12, V13, diode V15 and zener diode V14.

When measuring at direct current, capacitor C1 protects the input of the device from the variable component. If voltage is applied to the input of the device in the wrong polarity, then the arrow of the device will not deviate.

When measuring on alternating current, the amplifier acts as a half-wave rectifier and, therefore, amplifies only the positive half-wave of the measured voltage. Limit switch S4 - type PPZNPM, S1-S3, S5, S7-P2K, S6 - KM button.

The device uses an M4208 microammeter with a total deflection current of 300 μA. You can use other microammeters. If the total deflection current of the arrow is less than the specified one, you should select a shunt to the milliammeter, if more, change the feedback depth. Transistors KP102E can be replaced by any of the KP102 or KP103 series. Instead of KT365CA microassemblies, you can use K2HTI71-K2NT173 or KT315 transistors. Instead of the V4 diode, you can use one junction of the KT315 transistor.

The adjustment of the device is reduced to the selection of the operating mode of the first stage of the amplifier. With an open feedback circuit (soldered resistors R29 * and R30 *) and the middle position of the resistor R39 slider, resistor R3 * is selected so that the voltage drop across it is 0,7-0,9 V; resistors R31 *, R32 * are selected until a voltage drop across them is 5,0 ... 5,5 V. The modes of the second and third stages are set automatically.

Further selection of the resistor R31 * or R32 * ensure that the "zero" of the device is set at the middle position of the variable resistor R39.

Self-excitation is eliminated by including a 15 ... 50 pF capacitor between the bases of transistors V6, V7, and by shunting the amplifier input with a 1 ... 2 MΩ resistor, which is soldered in parallel with the V4 diode. At the first measurement limit (0,3 V), the boundaries are set by selecting a feedback resistor. First, by applying an alternating voltage of 0,3 V to the input, by selecting the resistor R30 *, the arrow is deflected to the full scale. Then, by selecting the resistor R29 *, the measurement limit is set to 0,3 V at direct current. On the remaining limits, the measurement accuracy depends on the accuracy of the choice of R1*-R11*.

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