Using DC voltmeter to measure RMS AC voltage - Page 1
Forum section: electronics / projects

author: shamooooot
author_rank: Regular Contributor
timestamp: May 15, 2024, 04:49:42 am
content: HelloI have a DC voltmeter and I would like to design a simple circuit to that can allow measuring the RMS AC voltage on the very 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/0.707Vmax = 28.288VVdc = 0.637Vmax - 2VdiodeVdc = 16.6Is there a way to compensate for the drop of voltage?

author: jonpaul
author_rank: Super Contributor
timestamp: May 15, 2024, 05:25:40 am
content: https://www.analog.com/media/en/training-seminars/design-handbooks/rms-dc-app-guide-appendD.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2024, 09:43:07 am
content: With a passive circuit there is no easy way to compensate for the voltage drop due to the diodes. It is possible with an active rectifier circuit, but this is more complicated and needs extra power.What can be done is using diodes with a low drop (Ge or schottky types) and possibly use a 1 way rectification. There is also a way to replace 2 of the diodes in the bridge with resistors and this way get less diode drop at the cost of additional input current. It can be a bit tricky with the ground, but was used in some analog meters.The curve at the low end is a bit nonlinear. For this reason the old analog meters had a nonlinear scale for the AC ranges to compensate for much of this.For higher voltages, like 5 V and up the diode drop is not that bad.

author: shamooooot
author_rank: Regular Contributor
timestamp: May 15, 2024, 09:56:30 am
content: Thanks guys..How about using precision rectifier?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.I have -15V and 15V power sources on the panel, perhaps I can use opamp like AD822, by summing these supplies to get 30V supply.https://www.analog.com/media/en/technical-documentation/data-sheets/AD822.pdfCircuit like this:https://circuitdigest.com/electronic-circuits/half-wave-and-full-wave-precision-rectifier-circuit-using-op-amp

author: coromonadalix
author_rank: Super Contributor
timestamp: May 15, 2024, 10:38:05 am
content: change your meter simple no ? redesigning and adapting into existent stuff could be more challenging,you have low priced good rms voltmeters aroundsearch any ad636 637 circuits ...

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2024, 10:43:49 am
content: If one has a supply available, one could use an active (precision) rectifier circuit. An analog meter movement or integrating ADC (with a DC panel meter) can do the averaging. So one may get away without extra filtering.For the supplies it may take care on how the ground is connected (e.g. supply from a generator or amplifier circuit)The result is not the same as RMS, but the rectifier value. The ratio depends on the wavefrom (can be 1 for square wave).

author: xvr
author_rank: Frequent Contributor
timestamp: May 15, 2024, 10:47:49 am
content: Quote from: shamooooot on May 15, 2024, 09:56:30 amNo, rectifier will provide rectification, not RMS calculation. You will get pulsating output. If you add "some smoothing" (simplest ways is to add RC filter) you will getaveragevalue, not RMS.You can use it if average is suite you. If you need True RMS you have to use special IC (as already mentioned chips from Analog Devices)

author: Sacodepatatas
author_rank: Regular Contributor
timestamp: May 15, 2024, 01:55:17 pm
content: Quote from: xvr on May 15, 2024, 10:47:49 amIf you square, filter, and then square root, then you get the more o less accurate RMS value.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2024, 05:04:03 pm
content: Quote from: Kleinstein on May 15, 2024, 09:43:07 amA second diode can be used to compensate for the forward voltage drop of the first diode. This rectifier circuit is common at higher frequencies where a precision rectifier is too slow, but it works down to DC. It is not quite as accurate as a precision rectifier.Quote from: shamooooot on May 15, 2024, 09:56:30 amA precision rectifier is commonly used, but produces an averaging response rather than an RMS response. Typically the precision rectifier is calibrated to produce an RMS response for sine wave inputs, but other wave-shapes will then have an error. Square waves for instance will then read 11.1% high.

author: bdunham7
author_rank: Super Contributor
timestamp: May 15, 2024, 05:17:50 pm
content: Quote from: shamooooot on May 15, 2024, 04:49:42 amBy "DC voltmeter" do you mean a panel meter, a DMM or what? Is it analog or digital, is it passive or powered and what is its input impedance? Does your circuit have to be entirely passive or powered by the AC input voltage or can it have a separate supply? Does it have to read accurately down to zero?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 16, 2024, 09:22:18 pm
content: Quote from: shamooooot on May 15, 2024, 04:49:42 amWhat is your desired AC range ?The simplest passive circuit is a bridge, or even a single diode.You can correct for the diode using a offset table, or custom scale, or just do a best-fit for your range.You can buy a better diode, to help a little. eg Schottky diodes have lower drops.You need to consider your DC voltmeter input impedance, and your AC source impedance.Schottky diodes have lower drops, but somewhat trade off VR,VF, IR and vary with temperature.A bridge design means there is a path for leakage currents, so you could experiment with varying diodes and graph VDC vs your AC range.Higher current diodes have lower VF, but also higher leakage currents.Quote from: shamooooot on May 15, 2024, 09:56:30 amNo.VRMS means what the letters say - it is theroot of the mean of the squares.That matters if the waveform is not sine.Who has specified RMS and why ?Voltage waveforms are usually close to sine than current waveforms, so you may get away with a 'best guess' ratio.Quote from: shamooooot on May 15, 2024, 09:56:30 amYes, if you have addition power and parts, then you can add smarts to reduce errors.You can even use better RMS to DC ICs, if that RMS matters.

author: Benta
author_rank: Super Contributor
timestamp: May 16, 2024, 09:37:34 pm
content: If you really want true RMS measurement, the basic way is a power resistor coupled with a temperature sensor.This will take care of all phase-shift, crest-factor, sine distortion etc. factors. True RMS as physically defined.Not very practicable except for limited-range measurements, though.