AC voltage sensor for 250 V. Scheme, description

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AC voltage sensor for 250 V. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Indicators, sensors, detectors

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Current and voltage sensors are most often used to measure alternating currents and voltages of industrial frequency 50 Hz. The use for these purposes of current and voltage sensors with galvanic isolation based on the Hall effect of the company LEM, which is a monopolist in this area, is impractical because of the large input currents for voltage sensors (10 ... 20 mA), a large error of the order of 1% of the upper scale, complexity and high cost [1]. The advantage of sensors using the Hall effect is the ability to measure direct currents and voltages.

The most appropriate for measuring alternating currents and voltages is the use of current and voltage transformers with an electronic signal processing circuit that is convenient for the user. The transformer converts voltage over the entire range with the same error, whether it be hundreds of volts or units of microvolts. The signal power from a transformer of the smallest feasible size is several orders of magnitude greater than the signal power of the Hall sensor. This makes it possible to implement voltage sensors with an input current of 1 mA and current sensors with a scale of 0,5 A and an error of 0,1 ... 0,5%.

When measuring alternating currents and voltages with sensors based on the Hall effect, the error can be significantly reduced by cutting off the constant component with a capacitor, but this further complicates the sensor and is impractical.

In the proposed voltage sensor, in order to simplify the design and reduce the dimensions, instead of a voltage transformer, a current transformer with additional resistance is used (Fig. 1).

250 V AC voltage sensor

The sensor has the following characteristics:

Input voltage ....... 0...250 V
Input current (at a voltage of 250 V) ....... 1 mA
Output current (at a voltage of 250 V) ....... 5 mA
Load resistance.......0...1 kOhm
Error from the measured value in the measurement range from 25 V to 250 V ....... no more than 0,1%
Supply voltage......±15 V

The additional resistance consists of three resistors R1R3 and determines the input current of the sensor. The resistor R1 is precision with an error of 0,5%, the resistors R2, R3 are used to select the value of the additional resistance, the selection is carried out by shorting the resistors R2, R3.

The maximum power dissipated by resistors R1R3 is not more than 0,25 W. The main error is determined by the accuracy of the selection of resistors R1-R3. Additional error is determined by the TCS resistors R1-R3.

The signal from the secondary winding 3 of the transformer T1 is fed to the input of the operational amplifier A1. To eliminate the residual magnetization of the core and improve the frequency response, the amplifier is covered by a negative feedback on the magnetic flux. The output signal A1 creates an output current in winding 2, which creates a magnetic flux in the core equal in magnitude and opposite in direction to the magnetic flux generated by the input current.

The sensor has a current output (the output current does not depend on the load resistance):

Iout=IinW1/W3,

where W1 is the number of turns of winding 1, W3 is the number of turns of winding 3.

The transformer core is assembled from L-shaped plates and has dimensions of 30x22x5 mm (5 mm - the thickness of the set). Windings 1, 2 contain 2000 turns of wire PEV-0,1, winding 3 contains 400 turns of wire PEV-0,2.

Figure 2 shows a version of the sensor with a rectified current output.

250 V AC voltage sensor

The sensor has the following characteristics:

Input voltage ....... 0...250 V
Input current (at a voltage of 250 V) ....... 1 mA
Output current (rectified, peak value) ....... 10 mA
Load resistance.......0...620 Ohm
Error from the measured value in the measurement range from 25 V to 250 V ....... no more than 0,5%
Supply voltage......±15 V

The variable signal from the resistor R4 is fed to the input of a precision rectifier assembled on the operational amplifier A2 and diodes V1, V2 according to the standard scheme. The signals from the diodes V1, V2 are fed to the inputs of the voltage-to-current converter, assembled on the OU A3 and the transistor M3. In order for the output current not to depend on the load resistance, it is necessary to fulfill the equality R11/R9=R12/R10. Adjustment of the sensor consists in setting the value of the output current at an input current of 1 mA by selecting the value of the resistor R4.

References:

Prospectus of the firm "LEM".

See other articles Section Indicators, sensors, detectors.

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