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If I measure a full wave voltage with a non true-RMS common variety
DMM*, will it be off by a known factor, from the true RMS value?

Or is there a rule of thumb about the error.  E.g., the reading will be

Thanks,
Bob

* - Extech Ex320 if it matters

On 2019/07/08 7:47 a.m., Bob Engelhardt wrote:

ll be

Typically these inexpensive digital meters are only fairly accurate for
60 or 50 Hz sine wave AC voltages. The further away from 50/60 Hz and/or

true sine wave you get the worse the reading accuracy...

Here is a good explanation:

http://sound.whsites.net/appnotes/an012.htm

I'd use a 'scope and do the math if the readings are critical and you
can't afford a proper AC meter

John :-#)#

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On 7/8/2019 10:56 AM, John Robertson wrote:

The readings aren't at all critical - a single-digit correction factor
would be good enough.

I wonder if the meter reading has a consistent relationship to the RMS
value.  E.g., if the meter always used the peak value, the actual value
would simply be 0.7 the read value.  If it was consistent, I could
calculate the correction factor by measuring the peak on a scope.  But I
wouldn't want to have to generate correction curves.

On Monday, 8 July 2019 18:17:32 UTC+1, Bob Engelhardt  wrote:

When I looked into this decades ago, digital meters typical read the peak &
reported 71% of that. So as you depart from sine, all bets are off. But as
mentioned, limited frequency response also means as you depart from 50/60H
z it's all going to go out of cal.

If you're measuring a consistent waveform, eg CRT filament supply in TVs, t
he waveform & f are consistent so you could apply a fixed correction factor
. If your waveform or f varies, fuggedit.

NT

** Nutcase Thornton spewed:

** Nope - non RMS multimeters do just what analogue multimeters do and scale the average value of the rectified AC wave.

I just tried two '80s 3.5 digit DDMs and found they read 125mV on a wave that was 300mV peak. The wave was 50Hz with a few harmonics, the current draw of a small amplifier.

.....  Phil

On Thursday, 11 July 2019 04:54:24 UTC+1, Phil Allison  wrote:

Ah yes, the sample of 2 proves how the rest of the universe is.

DMM work perfectly with sine wave, but not with other signals.

On 7/8/19 10:47 AM, Bob Engelhardt wrote:

Depends.  If it reads the peaks, it'll be high by a factor of sqrt(2).

If it reads the mean, it'll read 2/pi times the peak value, which is low
by  a factor  2*sqrt(2)/pi = 0.9003.  So 10% is right in that case,
except that it would read low.

Of course it could do some random third thing instead.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant
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I got a problem measuring efficienty on an SMPS unit.

The meter I used was indicating about 50% which was absurd.
I brought my personal old analog meter, it said about 85% !
That's why I don't like todays's meters.

On 7/8/19 2:04 PM, Look165 wrote:

You can get a true-RMS meter for way under \$50.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal Consultant
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NO ; with today's DMM the value is only true with sine wave.
Some use triangular signal, other Dirac comb.
They sample the signal and then computation and mathematical process.
The older one (AMM) were making true measurement with a rectifier and a
filtering cap.

This is particularly obvious in Amperemeter operations.

On 7/9/19 5:02 AM, Look165 wrote:

You're cracked.  (Plus you top-post.)

Rectifier + filter is _not_ a true-RMS meter.

<https://www.walmart.com/search/?cat_id=0&query=true+rms+multimeter

Cheers

Phil Hobbs

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Principal Consultant
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Bob Engelhardt wrote:

----------------------

** I wish folk would use correct terminology and not private shorthand.

Do you mean " full wave RECTIFIED voltage" or not ???

** Standard DDMs take the average, AC coupled rectified value and scale to coincide with the rms value for sine waves

For any other wave there is an inherent error and the DC component is missed with rectified waves etc.

Try explaining what you are actually doing instead of being too clever.

.....  Phil

On 7/8/2019 5:23 PM, Phil Allison wrote:

...

Sorry for the offense. I thought that it was obvious & didn't know that
there was any other kind.  What other kind of full wave is there, other
than full wave rectified?

Bob Engelhardt wrote:

** A full wave voltage is any continuous wave.

Rectified waves are single polarity - big difference.

....  Phil

Oh, wait ... the light dawns.  The DC that I'm trying to measure is the
output of a bridge.  It's RMS value will just be the RMS value of the AC
input, less 2 diode drops.  Unless I'm missing something ... not unheard of.

Bob Engelhardt wrote:

** Now you are thinking well into the problem.

The rms value is the equivalent heating effect of a wave expressed as a number.

So, the addition of a bridge between an AC supply and it's load has little effect on the heat in that load except for losses in the bridge.

Ergo, nearly the same rms value.

....  Phil

Definition of RMS :
The DC value that causes the same thermal effect on a resistor as the
original signal.

Joule is the quantity of energy neceassar for heatin 1g of water up to

Watt is the power related to J but per second.