Full wave voltage reading

ll be

Typically these inexpensive digital meters are only fairly accurate for

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

Here is a good explanation:

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

John :-#)#

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.

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

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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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

...

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?

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.

Rectified waves are single polarity - big difference.

.... Phil

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

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

Cheers

Phil Hobbs

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.

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.

You're cracked. (Plus you top-post.)

Rectifier + filter is _not_ a true-RMS meter.

Cheers

Phil Hobbs

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

Not considering losses, The mean rectified signal is 2^^(1.5)/pi the RMS input value

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

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.