Full wave voltage reading

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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  
high by about 10%.

Thanks,
Bob

* - Extech Ex320 if it matters

Re: Full wave voltage reading
On 2019/07/08 7:47 a.m., Bob Engelhardt wrote:
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ll be  
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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 :-#)#

--  
(Please post followups or tech inquiries to the USENET newsgroup)
                      John's Jukes Ltd.
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Re: Full wave voltage reading
On 7/8/2019 10:56 AM, John Robertson wrote:
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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.

Re: Full wave voltage reading
On Monday, 8 July 2019 18:17:32 UTC+1, Bob Engelhardt  wrote:
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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

Re: Full wave voltage reading
** Nutcase Thornton spewed:  

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** 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  

Re: Full wave voltage reading
On Thursday, 11 July 2019 04:54:24 UTC+1, Phil Allison  wrote:
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Ah yes, the sample of 2 proves how the rest of the universe is.

Re: Full wave voltage reading
DMM work perfectly with sine wave, but not with other signals.


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Re: Full wave voltage reading
On 7/8/19 10:47 AM, Bob Engelhardt wrote:
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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

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Full wave voltage reading
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.



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Re: Full wave voltage reading
On 7/8/19 2:04 PM, Look165 wrote:
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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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Re: Full wave voltage reading
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.


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Re: Full wave voltage reading
On 7/9/19 5:02 AM, Look165 wrote:


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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

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Full wave voltage reading
Bob Engelhardt wrote:

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

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


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** 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  


Re: Full wave voltage reading
On 7/8/2019 5:23 PM, Phil Allison wrote:
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...

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?

Re: Full wave voltage reading
Bob Engelhardt wrote:

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** A full wave voltage is any continuous wave.  

 Rectified waves are single polarity - big difference.  



....  Phil  

Re: Full wave voltage reading
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.

Re: Full wave voltage reading
Bob Engelhardt wrote:
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** 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  




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


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Re: Full wave voltage reading
Joule is the quantity of energy neceassar for heatin 1g of water up to  

Watt is the power related to J but per second.


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Re: Full wave voltage reading
Not considering losses, The mean rectified signal is 2^^(1.5)/pi the RMS  
input value
2^^(1.5)/pi is about 0.9.


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