This is a true r.m.s. Converter circuit. it use parts that easy to find and the normal quality. But we must to use normally parts that Low tolerances and more precise tuning.
This use principle of the automatic gain control circuit; AGC). Which include two ICs, number – XR 13600 (A1 and A2) and XR 1458 (A3, A4).
The circuit will adjust automatically its gain, cause the output of A1 is always constant. This output level will be monitored by the amplifier A2.
Figure: schematic diagram of true R.M.S Converter circuit
Then if compare the average level and the reference voltage by A3. Next the output will be sent to the diode input of A1 that is set the bias current by resistor-2 K and the transistor T1, and is also attenuated the input signal.
As above, the output power of A1 is steady. Thus the RMS level also steady. The attenuation is ratio directly between the RMS of output voltage and current bias of diode.
A4 is booster to adjust the gain of current that flow through pair diode to same. Thus the output voltage of A4 will be ratio with the input RMS voltage.
Adjustment and usage
We should adjust the output to correctly reading at sine wave input. Which may adjust compare with the normal meter.
Parts you will need.
C1___100nF 63V Polyester Capacitor
C2___1nF 63V Polyester Capacitor
C3___4.7uF 50V___Electrolytic Capacitors
C4___22uF 25V___Electrolytic Capacitors
C5___47uF 16V___Electrolytic Capacitors
All Resistors look at the Figure