The effective value-to-DC conversion circuit can obtain the real effective voltage of complex waveforms or pulses

The effective value-to-DC conversion circuit can obtain the real effective voltage of complex waveforms or pulses

Function of the circuit

When measuring AC current or voltage, if the waveform is a sine wave, an average value rectifier circuit or a central value detector circuit can be used to convert it into an effective value for measurement. However, for pulse waveforms, the ordinary rectification method has a large error and is not practical. Therefore, an operation circuit that can obtain the true effective value of the waveform is required. This circuit is usually composed of an absolute value circuit and an integration circuit. This circuit uses a single-chip IC to simplify the circuit.

Circuit working principle

The output circuit is a resistor attenuator. In order to be able to input a voltage as high as 700V, the circuit attenuates in units of 1/10.

Because the full-scale output of the RMS converted IC is 7V, a preamplifier should be added when the input voltage is less than 1V.

VR1 is used for gain calibration of the whole circuit. In order to avoid the influence of offset voltage, a DC blocking capacitor C2 is added, and the input resistance is about 16K. If FC is 10HZ, C2 is UF.

Output offset adjustment is accomplished by applying a voltage to lead 9.

The difficulty in achieving RMS conversion is how to determine the capacity of the equalizing capacitor. In order to improve the accuracy of DC output, the capacitance can be increased to increase the time constant, but the response time will also be longer. Usually, when measuring a waveform around 50HZ, if the allowable error is within 0.1%, the capacity of C3 can be more than 1UF. If the allowable error is 1%, C2 can be 0.33UF.

If you want to speed up the response and improve the accuracy, you can choose an amplifier with a buffer and add a 12DB/KCT low-pass filter to reduce the ripple. For details, please refer to AD's comprehensive product sample P6~10.

Component Selection

Since the input attenuation resistor R1 consumes about 0.5W of power when the input signal is 700V, a 1W oxide metal film resistor is selected for safety considerations. The device A1 can be 741 type. The frequency band of AD536A changes with the input signal level. If the allowable error is 1%, when V1N is greater than 1V, the frequency is 100KHZ. Therefore, if the input signal level is low, the gain of A1 must be increased.

C2 can use a 1UF non-polarized capacitor or two 2.2UF tantalum capacitors with the same polarity connected in series.

One end of the averaging capacitor C1 is connected to +VCC, so a polarized capacitor can also be used.

The input range switch can be a suitable relay or a changeover switch.

Adjustment

The effective voltage of a sine wave without distortion is 1/(2√2) of the peak-to-peak value, so 0.7V=1.98VP-P. Add it to the input terminal, and then adjust VR2 to make the output zero. The effective value of a triangle wave is 1/2 (2√3) of the peak-to-peak value, and that of a square wave is 1/2. A function generator should be used to determine the error of the output voltage. True effective value measurement is essential for noise measurement. In addition, this circuit can be used in many other areas.