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Atilla 2nd December 2011 01:40 PM

Those DMMs give me the creeps.
So I've got 3 DMMs around, which complately disagree with the voltage output of a transformer I've got. They're creeping me out a bit.

The transformer's rate output is 12VAC. I measure it unloaded and:

Multimeter 1 says 10V
Multimeter 2 says 12.2V
Multimeter 3 says 14.6V

I know DMM 1 is old and its calibration is off a bit, but the other two are just a few years old and fairly good.

Scumbag DMMs.....

Forman313 2nd December 2011 03:02 PM

Do you know how they measure AC? RMS, peak ... might be intentional?

New batteries in all three?

Atilla 2nd December 2011 04:04 PM

Hmmm, I didn't think of that I need to look at their specs and figure out what they actually measure.

Otherwise, yes the batteries in all are relatively new.

Forman313 2nd December 2011 04:38 PM

My experience with DMMs are quite different. I have a $12-13 one ( .. hehe), a Gold tool ($25), a UNI-T ($50) and a Fluke ($250). There is no difference in readings. At least not AC, DC and resistance.

Kinda funny... A DMM that costs the same as a pack of siggarettes is just as accurate as the Fluke.
However, after a few months the probes came apart on the cheap one. With cheap stiff leads, it goes with out saying it wont last.

KatieandDad 2nd December 2011 04:53 PM

It's normally down to batteries.

noSmoking 2nd December 2011 04:58 PM

hi Atilla,
Any meter unless calibrated recently can be off some meters read AC as a RMS value witch is a average of the voltages,some read peak that will throw you off too.
Because that can vary with your house main load. If you have another exact trans former compare it ,most of the time if you get a reading it;s good and if you don't it's open ,and one that gets extremely hot is a bad too.
Leads can be bought anywhere ,get a set made by fluke they last years.
I hope this has helped you some ,you can measure (with good leads) your house voltage and each meter should be close to all the same reading,also a battery 1.5 v for DC all should read the same , more cost, better parts= better meter.

thaumaturge 2nd December 2011 10:17 PM

Things are getting better as True RMS converters find their way into more and more meters. But at one time an AC measurement was best viewed with more skepticism than confidence. There were three different schemes: Peak, Average and RMS. I recall one incident where a panel meter on a test fixture wouldn't calibrate correctly when used to monitor the output of a variac. I was just a young buck and was saved by a wise old sage who looked at the output using differential scope probes. The waveform was fine until the variac passed the input winding into auto transformer mode; then it visible distorted from a more perfect sine wave, which is where the average responding meter circuit lost linear tracking. Sine, Triangle and square waves all read different depending on the type of rectifier circuit used. True RMS (the equivalent DC heating value ) converters are now about $4.50 a pop[. So even now you wont find them in a $2.99 cheapo DMM. The very best I know of is the Agilent 3458A, which has an unimaginable AC accuracy of 100 parts per million from 10Hz to 10Mhz. Of course they sell for $7,000 used. But it is easy to understand why many of us old coggers have old HP 3400C's on our benches, or even older HP 400's.

Forman313 2nd December 2011 10:55 PM

I dont know how you did it... but you completely lost me and made perfect sense at the same time.

Talking about rectifier circuit .. IE. half wave and full wave ?
Wouldnt a large enough cap even out the difference?

EDIT: That is, a cap integrated in the DMM? For RMS measurements, a cap is needed, right? With out, the value is either peak or average?

thaumaturge 3rd December 2011 12:06 AM

Every value cap has its own time constant that will effect the final result. AC measurement conversion is a real evolving science unto itself. The old simpson and triplett analog meters used just simple diode and cap rectifiers. The smaller the cap the more peak responding the result. Early true RMS conversion was performed using resultant heat measurement "Bolometer" systems, as many RF power measurement systems still do (Agilent/HP RF power sensors such as 8481 & 8484) The Agilent 3458A DMM uses a unique high speed phase lock to determin A/D sample rates then superimposes a high speed jitter clock that essentially moves the sample points around for a closer approximation of the final result. The HP 3400 used a Bolometer type system. Other various schemes have used double balanced demodulators before rectification. But I fear I've just muddied the waters instead of providing clarity...

Maybe this will help: Measurements of AC magnitude : BASIC AC THEORY

Enzo 3rd December 2011 12:23 AM

Meters have impedance, but not low enough to upset the AC voltage leaving a power transformer.

A fair test would be to connect all three meters to the transformer at the same time - in parallel. That way you can watch the readings simultaneously. And that will eliminate any error from changing mains voltages. and differential loading. They still may give wide ranging readings, but at least certain errors would be eliminated.

We used to have some cheap meters that worked well enough for what we used them for, but the AC function basically just put a rectifier in front of the DC input, and put a correction factor on the display reading. Worked OK for just reading the mains voltage or something. Unfortunately it also could read DC voltage that way, of course giving the wrong result. SO one could be distracted and not set the meter correctly and get a funny voltage reading. Also couldn;t measure ripple on a DC line.

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