AC ammeter with linear scale. Scheme, description

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AC ammeter with linear scale. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Using synchronous AC rectification, the author linearized the scale of a magnetoelectric-type shunt ammeter without any amplifier. The article proposes variants of circuits with a half-wave and ring synchronous rectifier, usually used in ring modulators.

The scale of an AC ammeter built using a magnetoelectric pointer device with a shunt and a simple rectifier is usually non-linear. It's related to that. that when the voltage decreases below a certain threshold (0,2 ... 0,6 V), the rectifying properties of germanium and silicon diodes deteriorate sharply. As a result, it is required to increase the voltage drop across the shunt or to use linear rectifiers based on AC voltage amplifiers. However, an increase in the voltage drop across the shunt inevitably leads to power losses and an increase in the output impedance of the power supply. In addition, this method only reduces the non-linearity, but does not eliminate it completely. True, the use of amplifiers makes it possible to almost completely eliminate the nonlinearity, but greatly complicates the meter.

Meanwhile, the linearity of simple semiconductor diode measuring rectifiers can be significantly improved without much complication if synchronous rectification is used.

On fig. 1 shows a diagram of a half-wave synchronous rectifier for an ammeter with a linearized scale.

AC ammeter with linear scale
Fig. 1

In the positive half-cycle of the alternating voltage (plus at the upper ends of the windings II and III), the diodes VD1 and VD2 open, connecting the microammeter to the Rsh shunt. In the negative half-cycle, the diodes are closed. In the open state, the diodes have a small differential resistance, and the nonlinearity of this resistance is small, so the scale is almost linear.

When using microammeters with a scale of 50 ... 200 μA with a maximum voltage drop on the frame of not more than 150 mV, the minimum voltage on winding III can be 1,5 ... 2 V for germanium and 2 ... 2,5 V for silicon diodes (at a lower voltage, its instability noticeably affects the readings of the ammeter). The maximum voltage is limited by the maximum allowable reverse voltage of the diodes used. The minimum current of the diodes should be 10...20 times higher than the maximum current of the microammeter. An additional winding can be made independently by winding several turns of a thin insulated wire around the transformer coil, if its design allows this.

Resistors R3 and R4 are used to adjust the zero of the ammeter, the shift of which occurs due to the current of the diode VD2. flowing through the shunt, and the spread of diode parameters.

The in-phase connection of the windings II and III is important at a relatively low voltage of the winding III (less than 2 V), since when these windings are turned on in antiphase (in this case, the polarity of the microammeter connection must be changed), the scale appears non-linear (the division value at the end of the scale gradually increases) which, by the way, can sometimes be useful. However, when the voltage on winding III is higher than 4 ... 5 V, this nonlinearity is practically not noticeable, and you can ignore the turn-on phase of the windings.

To protect the microammeter from accidental overloads in parallel with its outputs, it is useful to turn on the silicon diode D220, KD522 or KD521 in the forward direction, after making sure that it does not affect the readings of the microammeter at the end of the scale.

By adding two more diodes and one resistor, the synchronous rectifier can be converted to a full-wave rectifier (Fig. 2). As a source that opens the diodes, the working winding of the transformer is used here.

AC ammeter with linear scale
Fig. 2

The advantage of a full-wave rectification circuit over a single-half-wave rectifier is that. that the required voltage drop across Rsh is approximately two times less for the same current of the full deflection of the microammeter. So, if in a half-wave rectifier with D220 diodes, for a full deflection of the microammeter needle by 200 μA (with a frame resistance of about 670 Ohms), a voltage drop across Rsh of about 0,4 was required .0,2 V, then in the full-wave this voltage did not exceed XNUMX V.

The above circuit is a modification of a conventional ring modulator. When the voltage at Rsh increases to 0,4 V (peak value) for germanium and 1,2 V for silicon diodes, a through load current begins to flow through the diodes VD1, VD3 and VD2, VD4. Therefore, resistors R3-R5 serve not only to balance the bridge. They limit the current through the diodes when overloaded.

Based on these considerations, it is better to use silicon diodes in a full-wave rectifier and calculate the ammeter for a maximum voltage drop across Rsh of not more than 0,5 ... 0,6 V.

In case of overload or K3, additional measures can be taken to limit the current through the diodes. This may be an increase in the resistance of resistors R3-R5, a quenching resistor and shunt diodes or zener diodes.

AC ammeter with linear scale
Fig. 3

To open the diodes of the measuring bridge of the ammeter with a linear scale, it is not necessary to use a transformer. On fig. 3 shows a method for obtaining the opening voltage directly from a 220 V network. The Zener diode VD1 limits and stabilizes this voltage. Diode VD2 reduces the heating of the quenching resistor R5. It is also advisable to use such a power supply in the case of power supply from a transformer if its output voltage exceeds several tens of volts. When using a full-wave rectifier in such a case, the VD2 diode must be excluded, and one more counter (of the same type) should be connected in series with the VD1 zener diode or a two-anode zener diode should be used.

When calculating the elements of a half-wave rectifier and taking measurements, you need to remember about the features of measuring non-sinusoidal current or voltage, taking into account the shape factor.

When manufacturing a multi-limit ammeter with measured current limits of less than 0,2 ... 0,4 A, it is necessary to take into account the following feature of these bridge circuits. The current that opens the VD1 diode in fig. 1 (or VD1, VD2 in Fig. 2), closes directly to the power source, and the diode current VD2 (or VD3, VD4 in Fig. 2) passes through the resistor Rsh and creates a voltage drop on it, which, as mentioned above, compensated by adjusting the resistor R4.

When the resistance of the resistor Rsh is not more than 0,1 ... 0,2 Ohm, the voltage drop across it from the current of the diode VD2 (1 ... 2 mA) does not exceed 0,1 ... 0,4 mV At the maximum voltage drop across shunt 100...200 mV it can be ignored. If, at the minimum measurement limit, the resistance Rsh has a greater value, then it is necessary to take measures to maintain zero when switching the measurement limits.

If the bridge is powered from an additional winding, then at the minimum limit it is possible to make a shunt of two halves and connect the output of the bridge power winding to the midpoint of the shunt. It is also possible to use an additional section of the transfer switch, so that when switching the limits, the current in the power circuit of the individual legs of the measuring bridge is not interrupted.

In the manufacture of ammeters according to the above schemes, it is necessary to take measures to increase the temperature stability of the instrument reading, which is mainly determined by the equality of temperatures of the measuring bridge diodes. To do this, it is advisable to use diode assemblies in one package or place the diodes next to each other and ensure good thermal contact by filling them with a compound.

Author: V.Andreev, Togliatti, Samara region

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