Using DC voltmeter to measure RMS AC voltage

Shamoooot

Joined Apr 3, 2023
27
Hello

I have a DC voltmeter and I would like to design a simple circuit to that can allow measuring the RMS AC voltage on the same meter.

I was wondering if using a bridge rectifier is good practice or does it have any drawbacks?

The voltage range is 0-20Vrms, so:
Vmax = Vrms/0.707
Vmax = 28.288V

Vdc = 0.637Vmax - 2Vdiode
Vdc = 16.6

Is there a way to compensate for the drop of voltage?

Sensacell

Joined Jun 19, 2012
3,506
1) Your circuit is a peak detector, you can divide the voltage output down to the RMS equivalent, but it will only be accurate for pure SINE wave signals
2) At lower voltages, the diode drops will make the circuit very inaccurate, use an opamp precision rectifier instead of simple diodes. This trick however will fail at higher frequencies.

What kind of signals are you wanting to measure?

Shamoooot

Joined Apr 3, 2023
27
Thank you for the good info..
The signal is Sinewave and the frequency is up to 1kHz.

For precision rectifier can you suggest me an amplifier that can handle the 20V/28Vmax. I have 15V and -15V power available.

Sensacell

Joined Jun 19, 2012
3,506
Perhaps divide the incoming voltage down to +- 10 Volts first, then you can use garden variety opamps to do the rectification.

Or better yet- use an integrated solution like the AD736

Shamoooot

Joined Apr 3, 2023
27
Thank you again.

I would like to ask if the Vdc resulted from the precision rectifier would be the exact same as Vrms, and is it ok if the DC output was pulsating or does it need smoothing .

Perhaps I will go with an opamp like AD822, with summing the -15 and 15 sources to get 30 supply should be good.
datasheet

With a circuit like the one here:
design procedure

crutschow

Joined Mar 14, 2008
34,855
I would like to ask if the Vdc resulted from the precision rectifier would be the exact same as Vrms
No.
The precision rectifier generates the average value of the signal.
You increase that by a gain of 0.707/0.637 = 1.1099 to get the RMS value of a pure sinewave.

For the true RMS value of of a distorted sinewave or any other signal (square-wave, triangle-wave, etc.) you need a true RMS converter as suggested by Pyrex.

Last edited:

Sensacell

Joined Jun 19, 2012
3,506
No.
The precision rectifier generates the average value of the signal.
You increase that by a gain of 0.707/0.637 = 1.1099 to get the RMS value of a pure sinewave.

For the true RMS value of of a distorted sinewave or any other signal (square-wave, triangle-wave, etc.) you need a true RMS converter as suggested by Pyrex.
Why not peak detect and divide by 1.414?

crutschow

Joined Mar 14, 2008
34,855
Why not peak detect and divide by 1.414?
That also only works for pure sinewaves, and is suspectable to any high frequency noise or spikes riding on the sinewave.
The precision-rectifier circuit averages most of that noise.

Shamoooot

Joined Apr 3, 2023
27
I get that it's gonna be tough for digital meters, I was wondering if it is the same for analog meters or perhaps simpler.

crutschow

Joined Mar 14, 2008
34,855
I get that it's gonna be tough for digital meters, I was wondering if it is the same for analog meters or perhaps simpler.
It will say True RMS or TRMS in its description.
Examples from Amazon.

Pyrex

Joined Feb 16, 2022
313
I get that it's gonna be tough for digital meters, I was wondering if it is the same for analog meters or perhaps simpler.
it doesn't matter if it's a digital or analog display. Signal processing is the same

crutschow

Joined Mar 14, 2008
34,855
it doesn't matter if it's a digital or analog display. Signal processing is the same
Except digital does it in discrete samples and analog does it continuously.

Casperson

Joined Aug 11, 2021
4
it doesn't matter if it's a digital or analog display. Signal processing is the same
No, that's wrong. Analogue true RMS meters don't need any signal processing. The RMS value is measured directly using a moving iron instrument.

Pyrex

Joined Feb 16, 2022
313
No, that's wrong. Analogue true RMS meters don't need any signal processing. The RMS value is measured directly using a moving iron instrument.
The topic starter wanted to use DC voltmeter. So,the proposal was to apply TRMS converter

panic mode

Joined Oct 10, 2011
2,892
Is there a way to compensate for the drop of voltage?
sure...
add battery and potentiometer so that your meter gets sum of input and compensation voltage.

or ... do not use diodes in the bridge. use mosfets. then the voltage drop is next to nothing. be mindful of gate voltage limitations