Meter For Measuring High Current - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: January 20, 2023, 05:04:29 am
content: The multimeters I have go up to 10A I think, and I have an AC motor where I'd like to measure the steady state current.The issue: the startup could be 40A, so that would blow the fuse in the Fluke.I thought of bypassing the meter with a banana jack until the motor reach steady state, then removing the banana jack, but that can be dangerous (because it's AC), and I may forget to bypass the meter during a re-power thus blowing the fuse.A clamp on current meter works well, but I'm uncertain if I'm getting accurate enough readings. I'd like to measure out to two digits, so I'm not looking for high accuracy.Does anyone know a relatively cheap meter that I can connect in series that can handle over 40A?

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2023, 05:11:14 am
content: One option is you make/buy a low value resistor of known milliohms that will drop a measurable voltage under 40A of current.Place that in series with the motor. Then measure the AC voltage across that resistor.You can do it with just a piece of long wire if you can measure the ohms of it accurately. This usually involves putting a known current through the wire and measuring the voltage, then you can calculate its ohms and use it elsewhere to measure other currents.Another option is to buy a clamp meter.Something like a Uni-T UT202+ for ~$40 will do AC at 4A, 40A, 400A ranges.The AC-only ones are usually cheaper.Try to get a known brand if you can. You can get unknown brand ones for like $30 that are AC/DC but its hard to trust the readings

author: bostonman
author_rank: Super Contributor
timestamp: January 20, 2023, 05:18:17 am
content: QuoteI thought of this. I have some small value resistors, 50m ohms 100W, but, at a worst case (or future needs) of 50A, that's over 100W.It is, however, for a very short spurt, but risk them blowing up. Maybe I can parallel two or three resistors.

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2023, 05:23:58 am
content: Yeah 50mR is a bit too high, 2.5V drop is going to waste a lot of power.You want something that will drop like 100mV. So around 2mR.If you're saying that your 50mR resistors are rated 100W then they will handle more than that for a short time.But they are intended to be mounted on a heatsink. You could submerge them in a bowl of water, that's pretty common for dummy loads.You can make something out of wire lengths if you carefully measure the wire to work out its resistance accurately.Either one piece of wire, or many wires you add together to lower the resistance to what you need.

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 20, 2023, 05:24:02 am
content: Clamp meters are quite cheap (they are also sold as multimeter accessory). For your use case it can be also current transformer. If you have one for bigger current, than you can add few more loops to increase the output current. Current transformer is good for permanent installation.

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2023, 05:29:55 am
content: If this is an 110V/220V AC motor you could use a different device, like an electric heater, with DMM in series to figure out X volts across a piece of wire = X amps on meter so wire = Z ohms. Then you can swap to the motor and calculate motor current based on voltage across the same wire.This way you can keep everything setup between the calibration using heater and the measurement using motor, you only have to swap what is plugged in. (I guess you would need 2 meters to do that, but the amps is unlikely to change much from having the DMM in series vs not having it)But yeah, buying a clamp meter for <$50 is your best/safest option

author: bdunham7
author_rank: Super Contributor
timestamp: January 20, 2023, 05:54:26 am
content: Quote from: bostonman on January 20, 2023, 05:04:29 amRunning the current through the meter (or a shunt) is probably unnecessary for your stated goal and I don't recommend it. Any halfway decent clamp meter should be fine, and if your clamp meter doesn't have a low enough range, you can loop your wire through it 10X or use a 10X line splitter to give yourself another decimal place. If you don't already have a clamp meter, take a look at the Fluke 323 that you can get used on eBay for $70 or new for $120. Pretty feature-sparse as a regular DMM, but the TRMS AC current clamp function works pretty well. It only has a 400A range, so you'll have to use the 10X method I mentioned.

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2023, 10:32:05 am
content: Uni-T in general tend to make clamp meters that are cheap but not shit quality.

author: Shay
author_rank: Regular Contributor
timestamp: January 20, 2023, 12:10:35 pm
content: Do you need to measure it constantly over time? Or momentarily?For constant, over time measurement, a current transformer is great. You'll get a good insulation, safe and reliable measurement. You can also play with the output voltages by changing the resistor in parraell to the output.For momentarily measurement, something like a uni-t clamp meter or as suggested a dummy resistors in pararell would work. Attach the power resistors to a heatsink. Should be OK. But it is good to get overall a clamp meter if you have such tasks often. A clamp meter is about 50$.

author: bostonman
author_rank: Super Contributor
timestamp: January 20, 2023, 01:15:15 pm
content: QuoteI was focusing on wattage and didn't consider the voltage drop; good call.This would be a current measurement over time such as a minute or two. I already own a few clamp on meters, but sometimes question their accuracy based on just being a magnetic field. One is a Micsig and the peak steady state line on the oscilloscope was quite thick but seemed higher than a cheap clamp on current meter. Then I have a plug in wattage meter which fluctuates and gives me an overall current draw since current for the whole unit is going through it.Typically I consider clamp on current meters good for situations where breaking the circuit isn't easy, you want a rough idea of the current, and, in the case of the Micsig, I use it for measuring startup current.Thought going through a meter would be much more accurate like a Fluke, but, as mentioned, mine only goes up to 10A and steady state may be about 12A (excluding peak start up).Seems there isn't a simple meter that exists. I searched for high current meters and only found DC, but thought something existed.

author: Berni
author_rank: Super Contributor
timestamp: January 20, 2023, 01:26:52 pm
content: Current clamp meters are actually pretty good.Sure they won't generally be accurate for low currents under 1A since at low currents there is not much magnetic field to sense while at the same time having to deal with stray ambient fields like the earths magnetic field. But those stray fields are not a problem for AC current. The good ones read down to impressively low currents. Magnetic fields are often involved in measuring current or power in electric energy distribution.If you are not too sure in the accuracy you can always put a current clamp around the test lead of a regular multimeter, apply a few amps of test current and check how close they are. If the two agree in your test then they will easily agree at larger currents too.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 20, 2023, 02:31:46 pm
content: Too trueEveryone needs a clamp meter in their toolbox.And measuring through a shunt is inconvenient and an unnecessary shock risk.

author: bdunham7
author_rank: Super Contributor
timestamp: January 20, 2023, 02:41:17 pm
content: Here is the 323 measuring 1.5A @ 60Hz from a calibrator, both with the wire just going thru 1X and then wrapped around not very neatly 10X. You can wind a loop of solid 12 AWG wire 10X as sort of test fixture and then clamp around it. Decent clamp meters should work OK if you keep them away from stray magnetic fields and don't drop them on concrete.

author: David Hess
author_rank: Super Contributor
timestamp: January 20, 2023, 04:50:31 pm
content: You should be using an external current shunt to make measurements above the current limit of your meter.