Phase meter on the OS. Scheme, description

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Phase meter on the OS. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Many electronic phase meters have a very significant drawback, which consists in the fact that their inputs are galvanically coupled. This feature limits the scope of practical application of the device.

The proposed phase meter is free from this disadvantage. This was achieved by replacing the comparison logic element with an optocoupler. The device also provides decoupling of the measuring channels and power supply. All this expands the functionality of the phase meter, bringing it closer to similar devices of the electrodynamic system. It allows you to measure the phase angle not only between voltage and current, but also between two voltages or two currents.

Main technical characteristics:

Voltage Uin1 (Uin2) applied to the channel input, V.......0,03...100
Current Iin1 (Iin2) in the channel input circuit, А.......0,01...5
Frequency band of measured signals, kHz.......0,01...100
Measurement error, not worse, %.......±3
Power consumption, W ....... 1

The schematic diagram of the device is shown in Fig.1. The device consists of two measuring channels, similar in structure. The input signal in each of them is limited in amplitude by the diodes (VD1-VD4), and the comparators on the op-amp (DA1, DA2) convert it into rectangular pulses, antiphase to the input voltage, the amplitude of which is constant and close to the supply voltage.

Fig.1 (click to enlarge)

Let us assume that the phase shift angle of the input signals is equal to zero (Fig. 2, a). Then, in the negative half-cycle of the output voltage of the comparator DA1, the transistor VT1 opens and current flows in the LED circuit of the optocoupler U1. The negative output voltage of the DA2 comparator is applied to the VD5 diode in the opposite direction, so the current in the PA1 microammeter circuit does not flow.

OS phase meter
Ris.2

With a positive voltage at the output of the comparators, the transistor VT1 is closed, the LED is turned off, the optocoupler U1 is closed, and there is also no current in the PA1 microammeter circuit. Thus, the average value of the current flowing through the microammeter over the period of the input voltage is zero.

If the input signals are shifted relative to each other by a certain angle (the points of change in the sign of the output voltage of the comparators are shifted in time, Fig. 2, b), then during the time interval from t1 to t2, proportional to the phase shift angle between the input signals, the optocoupler will be opened . The average value of the current flowing through the microammeter during the period of the input voltage is proportional to the measured phase angle.

Measuring channels of the device are powered by separate rectifiers, galvanically not connected to each other. Two zener diodes (VD1, VD14) connected in anti-series are introduced into the primary winding circuit of the network transformer T15. Therefore, the voltage amplitude on the primary winding is stabilized. The excess mains voltage extinguishes the ballast capacitor C9, and the resistor R6 discharges it after the device is turned off.

An almost rectangular voltage is removed from the terminals of each of the secondary windings of the transformer, which rectifies the diode bridge and smoothes the capacitive filter. This circuit design of the power supply provides a very low level of ripple, and the average value (constant component) of it is much higher than that of sinusoidal voltage rectifiers. This, in turn, reduces the requirements for smoothing filters and increases the rigidity of the external characteristic of the entire rectifier.

The device uses resistors MLT and SP3-1B (R5). Instead of OU K140UD1A, standard voltage comparators, for example, K521SA3, are suitable. The KT203V transistor can be replaced by any pnp silicon structure with a permissible collector current of 10 ... 20 mA. Diodes VD1-VD4 are selected in accordance with the limit value of the measured current, but they should have the smallest forward voltage drop possible. Zener diodes VD14, VD15 can be replaced by others with a stabilization voltage of 30 ... 100 V and a stabilization current of 30 ... 10 mA, respectively, but in this case it is necessary to change the number of turns of the primary winding of the transformer T1 (with a higher voltage, the number of turns is increased).

Transformer T1 is made on the magnetic circuit Ш10Х20. The network winding contains 600 turns of PEV-1 0,21 wire, and each secondary winding contains 2X180 turns of PEV-1 0,13 wire. Microammeter RA1 - magnetoelectric system with a total deflection current of the needle 50 ... 100 μA.

The electronic phase meter has a uniform scale, and its adjustment consists in setting the maximum deflection angle of the microammeter needle with resistor R5. In this case, the input of the phase meter is connected to a source of antiphase sinusoidal voltage, the parameters of which correspond to the input voltage and frequency of the device.

When operating the phase meter, remember that the maximum voltage that can be applied between any input of one measuring channel and the corresponding input of the second should not exceed the allowable for the optocoupler (about 100 V).

See other articles Section Measuring technology.

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