I am wanting to buy a multimeter that can measure AC current at very low volts (0.01V to 20V).
I have not really used them before, the ones I am seeing keep saying things like 200V, 500V 750V for AC current.
Does that mean if I set it to 200V setting, it’ll measure 0-200V? What kind of resolution do entry level multimeters get for that? 0.1V? 0.01V?
you’ll probably be better off with an oscilloscope.
it really depends. Some are auto-ranging, others require you to set it for the expected voltage you’re going to measure. IMO anything under $1,000 is not going to be very accurate if you want to measure tenths or hundredths of a volt.
Almost any digital voltmeter bought now will measure voltages at low ranges to a thousandth of a volt precision. I have an inexpensive (don’t remember how much, but it was a pretty cheap meter from Radio Shack) that will give me AA battery voltages reading at something like 1.612 volts.
You can buy this sort of thing at any hardware or electronic store. All I’ve seen are very accurate.
that it goes out to three decimal places doesn’t mean it’s telling you the right thing. Accuracy and precision are not the same thing.
You’ve confused me a bit. Current is measured in Amps. In order to measure current, you need to break the circuit and put the meter into the break (or use an inductive pickup around the wire). Because they become part of the circuit, it’s worthwhile to know how much potenial (measured in Volts) they can take before you burn the meter out.
So when you see the voltage spec for current measurement, it doesn’t tell you what kind of current it measures.
I don’t have specifics handy, and I’m ashamed to say I haven’t used my meters in a year or more. But most multimeters measure fairly accurately in the low milliamp range. Does that work for what you need?
A common way that multimeters are quantified is their “count”, which is the number of discrete levels between 0 and the maximum given range. A mid-grade multimeter might have a count of 4000, so for a setting of 400 V that means a theoretical 0.1 V precision.
In practice, it’s not that simple and you might be 10 or more “counts” away on any given setting. Sometimes there is a percent error and count (or digit) error. For instance, I have this model (which I can highly recommend):
http://www.bkprecision.com/downloads/datasheets/2709B_datasheet.pdf
It has a 660 mV range for AC, with this error bar:
660 mV range at 50 Hz to 60 Hz ±(1.5 % rdg + 8 dgts)
So, when measuring a 100 mV signal, it might report anywhere from 97.9 mV to 102.3 mV.
I’m not sure I agree with jz that you’d be better with an oscilloscope, or that anything under $1k is worthless. A tenth of a volt isn’t that small. However, I will say that anything under about $100 is dicey for all kinds of reasons.
Also, as others have said, you’re being a bit confusing saying “AC current”. I’m assuming you are actually using this in the same way as “ATM machine”; with a redundant word at the end. When electrical types say “AC current”, they mean they’re measuring current (as opposed to voltage) for an AC signal. You say “AC voltage” if that’s what you’re trying to measure.
I believe I used both the word “precision” and the word “accurate” in my post. In the right context.
Voltage rating doesn’t apply to current measurements; the only voltage that the meter sees is the relatively small (millivolts) voltage developed across the internal resistive shunt; sometimes you will see a voltage rating for said shunts but it is nothing more than the product of its resistance and current rating. For example, the 50 millivolts shown for this shunt is the output voltage at its full rated current; you could use it on a 1,000 volt circuit if it were insulated for that voltage.
Of course, there is a maximum voltage rating for the meter as a whole, based on the insulation rating of the cables and meter housing, but that doesn’t have anything to do with measuring current.
you may have used them in the right context, but you appear to have tried to use precision to prove accuracy. Which is not right.
OP - Maybe you should explain what you plan on doing.
I think an important question is…When measuring these small voltages, do you need it to tell you what the voltage is, or just the presence of the voltage. I have two multimeters, both probably under $75 that can do this, but I don’t think I would trust them to tell me the difference between 5v and 6v, but they have no problem telling me the difference between 5v and 0v. But I usually use them more as a voltage detector or a continuity tester.
Also, one of them is this one, which drives me nuts. It flips between volts and millivolts and the decimal is always moving around. It’s annoying when you think a 120 wire is live because it started bouncing around, but then you look closer and see that it’s bouncing around between 12.0 and 33.24mv and realize that it’s just being overly touchy and there’s no power at all.
I don’t think I did this. I merely said that in my experience the meters are quite accurate, and don’t think any intimation was made that the three place precision was what led me to this conclusion. I should have also stated that the thing that led me to believe that they are also accurate was the close conformance that I had noticed between quite some of these meters, including a Fluke multimeter with a certificate of calibration. Forgive me for not going into enough detail.
I’ve had my Flukes (and others) up against O-scopes and the multimeters are as accurate as I need. If within a few hundredths of a volt is good enough, I don’t think you will have any issue with a meter that you spent over $40 on.
Most oscilloscopes have accuracy no better than 1%. Probably a lot worse. Especially since you have to contend with the finite width of the trace.
Even the cheapest DVM will have an accuracy of .5% or better.
Where an oscilloscope will usually win is measuring small voltages - most scopes will measure to 5mV/division, while the lowest range on most DVMs is 2v (or 1mV resolution for a 2,000 count DVM).
What would be a good o-scope – doesn’t have to be good, actually, or particularly accurate – for home use? I’d just be using it to see the waveform from a digital clock signal for playing around with (the signal I’d just get from a little 555 IC – I think you use a voltage regulator and a capacitor and some other stuff, from what I see online). I always thought they were pretty mad expensive, but maybe there’s something out there like a bargain.
OK, DMMs are really not that hard to understand, and in reality, even cheap ones are ridiculously accurate. There are other things that are important.
Your typical basic model is a 3.5 digit device - this means that it has a display which can only display either 0 or 1 as the first digit, and 0-9 for the next 3. So it can read from 0.001 up to 1.999. Changing the range typically just moves the decimal point. So the same display will read through to 19.99, 199.0 and maybe 1999. As Dr Strangelove points out, the accuracy is more usefully defined in counts - a 4000 count DMM will get you a base accuracy where the last digit in a 3.5 digit display gets two counts per increment. Which is just good enough to consider the last digit as useful. A 2000 count DMM and the last digit can wobble about +/- an increment. This does not mean that the absolute accuracy of the meter is that good, but delta readings are useful.
You need to be careful about maximum ratings. A 750 volt rated DMM is not safety rated above its design voltage. If you intend measuring mains voltages this can matter, as faults on the supply can yield situations where a DMM can become lethal. Another safety issue is the quality of the leads. You get what you pay for, and should avoid skimping here.
As hinted in a few postings above, measuring AC current isn’t what you meant. You mean measuring AC voltage. Most DMMs will measure current too, usually up to 10 amps.
DMMs will measure both AC and DC. However, there is a hidden assumption. Most cheaper meters assume that the AC waveform is a pure sine. For mains supply, or those where the voltage is derived via a transformer, this is a reasonable assumption. But if the waveform is not a sine wave, many meters get the wrong answer. Better meters have a “True RMS” capability which gives you the voltage that takes into account the waveform shape. By “take into account”, it means that if you apply use the result to calculate the power dissipated into a resistive load, you will get the right answer. What few meters do is provide you with the peak voltage, or other information about the waveform, for that you need something pretty sophisticated, or an oscilloscope.
Measuring AC volts at low voltages is actually a slightly odd thing as a primary use case, so it may pay to describe what you want to do.
I have a Tektronix 2213 60 MHz dual-trace oscilloscope which has served well over the 10 years or so I’ve had it; I got it off eBay for about $100. Still works perfectly (i.e. displayed voltage agrees with my multimeter and frequencies from crystal oscillators fit the time divisions correctly) as far as I can tell despite being 30 years old and having seen very heavy usage (at least as I have used it; I leave it on all the time while I am working, even if I don’t immediately need it, just connected to the same switched circuit as the lights and everything else). Also, it goes down to 2 mV per division on the low end and 10 volts per division on the high end; I find the latter more important since you can measure up to 400 volts with just a 10x probe, or more if you adjust the trace; I use probes (10x and 100x) I made and calibrated myself since they don’t normally handle more than 600 volts - I burned out a probe measuring horizontal deflection voltages before (I also made a 1000:1 DC-only HV probe to measure CRT HV, which also works with a DVM).
Not just small voltages; a DVM is pretty useless in measuring complex waveforms, either DC or AC; try designing a switchmode power supply with only a DVM to verify operation, from the oscillator to output (although I use a DVM to set the output voltage), or complex logic circuits (where I often use both inputs to ensure they are properly timed). Although you can use a DVM in troubleshooting if you have an existing circuit and a schematic with voltages marked, or even by itself if you know what to expect with a particular circuit.
Agree.
We need more details from the OP. Measuring low-level AC voltage can be particularly challenging due to noise. At work I use a Keithely 2001 to make such measurements.
What uncertainty and precision do you need? As an example, when you say you want to be able to measure down to 0.01 V, is a reading of X.XX good enough, or do you need X.XXX?
Do you want peak or RMS? (Most people want to measure RMS when measuring AC voltage.)
Are you looking at a sine wave? Square wave? Noise? Some complex wave shape? Is it a periodic signal? What frequency? Is there a DC component?
Would a “regular” RMS measurement be O.K. (where the meter assumes it’s a sine wave), or do you need a “true” RMS measurement?
If there’s a DC component to the signal, do you want the meter to display the RMS voltage of the entire signal (AC + DC), or just the AC component?
What’s the approximate output impedance of your signal? Is it low or high?
Is it an isolated or grounded signal?
Neat. Thank you.
