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Re: Full wave voltage reading

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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Re: Full wave voltage reading

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.

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

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:

******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 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

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

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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.

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.

Re: Full wave voltage reading

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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Dr Philip C D Hobbs

Principal Consultant

Dr Philip C D Hobbs

Principal Consultant

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Re: Full wave voltage reading

Bob Engelhardt wrote:

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

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

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

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

******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 wavesFor 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

Bob Engelhardt wrote:

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

******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

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