This question draws upon elements of using a DVM for AC. My multi meter has a 1000V - 200mV AC range. Since there seems to be a consensus that multi meters can only reliably read 50Hz AC wave forms, why would you have this range on a meter? Very few are going to use a 50Hz signal for testing their kit. It might not even get through some small coupling capacitors. It doesn't really get to /from my sound card which I'm using as an oscilloscope. So unless you're measuring the mains, what's the point of such a huge AC range on my meter? In what circumstance would you be needing to measure for example 100 mV AC @ 50Hz?
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5\$\begingroup\$ What is this "50Hz only" consensus, and why have I never heard a thing about it? Even with the assumption that DMMs should only be used for mains AC, the frequency is typically 50Hz or even 60Hz depending on your countries standards. \$\endgroup\$– Al LongleyCommented Aug 19, 2016 at 2:38
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\$\begingroup\$ @AlLongley It comes from the referenced post and my meter's user manual. AC readings are spec'd at 45-500Hz but commentators have suggested that it's no use for anything other than 50Hz. \$\endgroup\$– Paul UszakCommented Aug 19, 2016 at 2:46
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\$\begingroup\$ Are you suggesting that it will accurately measure the amplitude of a 10kHz test signal @ 100mV? \$\endgroup\$– Paul UszakCommented Aug 19, 2016 at 2:47
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3\$\begingroup\$ So you never measure secondaries of transformers? \$\endgroup\$– PlasmaHHCommented Aug 19, 2016 at 9:37
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1\$\begingroup\$ The 200mV range would be useful for making comparative measurements well beyond the calibrated frequency range. Lots of troubleshooting centres around identifying pass/fail behaviour, not detailed characterisation. \$\endgroup\$– Sean HoulihaneCommented Aug 19, 2016 at 12:39
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4 Answers
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I have owned and used a large number of DMMs during my engineering career, and have calibrated the AC range against frequency on most of them.
The suggestion that the AC range on DMMs only reads 50/60Hz has elements of truth. A more accurate summary is that the AC range on all DMMs will read 50/60Hz, many DMMs will get to 1kHz, a few will cover the full audio range, and it's a rare beast that will read accurately above audio.
Not surprisingly, the cost of the meter has some correlation with the frequency range it can achieve. Also note that an audio range DMM will always specify this, as it's a cost-adding feature.
Given that to calibrate the frequency response, you need access to test gear that most amateurs do not have, the safest course of action is that, if your meter doesn't specify otherwise, only trust it at mains frequencies.
So, given that all meters will measure AC mains frequencies OK, why do you need 1000V to 200mV? You need 1000V to measure the mains coming in at the wall. Caution: this is not enough to be safe, you need a CAT2 or CAT3 protected meter to safely withstand (that is, fail safe) when hit with the all too common over-voltage spikes on mains supplies. People have died using a non-CAT meter on mains, even on the 1000V range.
You need 200mV to measure the voltage drop on a wire, across closed contacts, or a current shunt. Thanks to Chris for pointing out that there are plenty of intermediate AC voltages, like transformer secondaries, for which the 2v, 20v and 200v ranges are appropriate.
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\$\begingroup\$ PlasmaHH's comment about transformer secondaries indicates why the intermediate bit of the range are useful, rather than just "might as well". \$\endgroup\$– Chris HCommented Aug 19, 2016 at 10:59
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DMM's typically measure any sinusoidal AC audio frequency accurately, not just 50/60Hz.
But correction factors must be used if you know the waveform is not a sinusoid as they are often converted from average or sometimes peak to RMS.
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I suspect you're thinking this because you're in the US, and it seems like too large a range for what you'd need.
Nominal mains AC in the US is 115V RMS. In Europe it's 230V RMS. If you want to test a three-phase supply then you've got 415V RMS between any two phases. Suddenly that 1000V range doesn't seem too over-specified, does it? :)
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British Standard 7671 defines low voltage as:
below 1000 volts AC (rms) or below 1500 volts DC between conductors;
below 600 volts AC (rms) or below 900 volts to earth.
your low-voltage multimeter is designed to accurately measure across this wide range of voltages.
if you require very accurate measurements at extra-low voltages, you need a specialist meter
