PRECISION RMS VOLTMETER

N. P. Kalinichenko, L. A. Naumov

(Presented by the scientific seminar of the Department of Radio Engineering)

For the development and investigation of airborne power supplies and a number of other devices, an instrument for measuring rms voltage values is urgently needed, capable of operating in the frequency range 400 Hz–10 kHz and having an error of less than 0.1%. Industry does not produce voltmeters satisfying these requirements.

The most convenient in operation are direct-reading instruments; therefore, the development of an instrument of precisely this type is an urgent task.

Consideration of a number of works shows that the greatest accuracy in measuring the principal quantities of alternating current can be achieved by methods of comparing an unknown quantity with an equivalent, in terms of its action, quantity known with high accuracy.

Fig. 1.
Block diagram of a precision rms voltmeter

The use of the principle set forth above, as well as the application of inductive dividers developed at the Department of Radio Engineering and of a photoelectric comparator, made it possible to create a comparatively simple rms voltmeter with the required characteristics.

Its block diagram is shown in Fig. 1 and consists of:

— a source of calibrated alternating voltage (1);
— a voltage divider (2);
— an emitter follower (3);
— an attenuator (4);
— a power amplifier (5);
— a comparison converter (6);
— a balanced dc bridge (7);
— a null indicator (8);
— a power-supply unit (9);
— an input divider (10).

1. The source of calibrated voltage serves to obtain a highly stable alternating voltage of 10 V, square-wave type, with a frequency of 600 Hz. The source includes a multivibrator, a trigger, and an electronic switch.

A square wave from the output of the electronic switch is fed to a precision two-sided limiter using KS194G stabilitrons, which ensure high long-term stability of the reference voltage.

1. The five-decade inductive voltage divider (2) makes it possible to regulate the reference voltage (from 1 V to 10 V) in steps with the required resolution.

2. The emitter follower (3) provides matching of the input circuit with the attenuator (4). To obtain the maximum input resistance of the follower (\(> 1\) MΩ), a compensation circuit is used in the bias circuit. The dynamic range of the follower exceeds 10 V.

3. The power amplifier (5) is assembled according to the White follower circuit. The advantage of the circuit is its sufficiently high input and low output resistances. The amplifier provides limiting of the output voltage at the 3 V level, which protects the compensating converter (6) from overload.

4. The photoelectric comparator consists of an incandescent lamp (6), a photoresistor connected into one arm of the bridge, and a null indicator (8).

The division value of the instrument is 0.02% of the value of the voltage being measured.

1. The input inductive divider (10) makes it possible to extend the measurement limits of the instrument to 300 V.

2. The power-supply unit consists of DC voltage sources of 10 V and 34 V. The sources are made according to a compensation circuit.

Principle of Operation

The instrument uses the principle of alternating comparison of the measured and known voltages. Since the photoelectric comparator is a nonlinear element, the comparison is carried out at one level: 1 V. The measured voltage \(U_x\), by means of the input divider (10), is brought to the level \(1\text{ V} \leq U_x \leq 10\text{ V}\); then, by the attenuator (4), to the level of 1 V, and the bridge measuring circuit is balanced. Then a “compensating” voltage is passed through this same measuring channel; it is regulated by the inductive divider (2) until the bridge is again balanced. This state indicates that \(U_x = U_k\). From the positions of the knobs of the inductive divider (taking into account the input divider), information is obtained about the magnitude of the measured voltage.

Technical Data of the Instrument

Basic error — less than 0.1%;

division value of the indicator scale — 0.02%;

dynamic range — from 1 V to 300 V;

frequency range — 400 Hz ÷ 10 kHz;

power supply — AC mains, voltage 220 V, frequency 50 Hz;

power consumption — not more than 5 VA;

minimum input resistance at a frequency of 400 Hz — 40 kΩ.

REFERENCES

1. V. P. Balashov et al. Automation of Radio Measurements. “Sovetskoe radio,” Moscow, 1966.

2. A. P. Lozhnikov, E. K. Sonin. Cascade Circuits on Transistors. “Energiya,” Moscow, 1969.

3. T. B. Rozhdestvenskaya. Electrical Comparators for Precision Measurements. “Energiya,” Moscow, 1964.