Multimeter AC current variance - Page 1
Forum section: electronics / general

author: BrianNStPaul
author_rank: Newbie
timestamp: June 20, 2025, 11:28:35 pm
content: Hello, I am an electrician and only dabble in electronics projects. I was recently adding a cooling fan to 120VAC 60Hz powered device and on a lark wanted to measure the current of the small cooling fan. The muffin fan was listed as 80-240VAC and 1.5W. When measuring, I saw a lot of variance in current depending on which meter I used. It looks like meters with higher AC current bandwidth were rather inaccurate in this application ( measuring current in series with cheap cooling fan ) and low bandwidth meters worked well. Here are some of the current testing results:Fluke 87V: 27.8mA, 60-75HzGreenlee DM-830A ( Brymen BM829s ) : 34.4mA, 210kHzUni-T 139s : 21.3mA, 60Hz this meter seemed accurateFlir DM92 normal/auto mode : 33.0mA, 520HzFlir DM92 with low pass filter enabled in current mode : 21.1mA, 60Hz this seems accurateFluke 17B max : 15.3mA, 25.8kHz"Kill-A-Watt" energy meter : 21-23mA, 1.2W, 2.6VA, 0.46 power factorI normally measure much larger currents with clamp meters and have not seen this variance between meters before, is this an application for which otherwise reliable meters do not perform well?Thanks in advance for any insight you may have on this measurement variance.

author: Whales
author_rank: Super Contributor
timestamp: June 21, 2025, 08:07:07 am
content: Woah that's a lot of variation, more than 50% in some cases.Higher current readings when using higher bandwidth: thatcouldbe explained by noisy mains power (ie lots of different frequencies in the mix, not just 50/60Hz). It could be causing heating in the fan (real power) or not (reactive power) or even re-radiating in another form (sound or radiowaves). I would have thought an inductive load wouldn't do this, but I guess the average fan is more complex than that.The lower current reading from the Fluke 17B: my theory falls apart there.If you only want to compare the current draw with the fan's expected spec then I'd use the low-pass filtered (narrower bandwidth) results.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2025, 09:15:21 am
content: The wide supply range suggest that this is using some SMPS. At the low power there is usually no PFC and the current waveform can be quite ugly. This could really challange RMS solutions and the BW can also make a difference. The low power factor is likely not just a phase shift, but more like harminic content. If really bad, that could even be a DC component, that different meters may handle differently.The kill a Watt meter is very much at it's lower end and may have problems for this reason.

author: Whales
author_rank: Super Contributor
timestamp: June 21, 2025, 11:49:11 am
content: Good point. How is this only 1.5W? The smallest 240V fan I have ever seen still pulls 10's of watts. Can you share the part number or a photo so we can work out what is inside it?

author: Whales
author_rank: Super Contributor
timestamp: June 21, 2025, 12:00:13 pm
content: Woah!https://www.alibaba.com/product-detail/Gdstime-GEA8025-80x80x25mm-EC-Fan-110V_1600267524606.htmlHub is the size of a DC fan, wiring is of DC fan type (including LVDC level insulation), but only 1.5W power draw for the lowest speed 80mm model. I'm not sure if a capacitor dropper could even fit in that hub, so it must be some kind of SMPS? I want a teardown!This looks so wrong!

author: BrianNStPaul
author_rank: Newbie
timestamp: June 21, 2025, 06:36:38 pm
content: It is indeed one of the "GDSTIME" fans, specs attached. Seems to work fine, but haven't ran it for very long.

author: CosteC
author_rank: Regular Contributor
timestamp: June 22, 2025, 06:38:36 pm
content: Quote from: BrianNStPaul on June 20, 2025, 11:28:35 pmThe differences are indeed massive (15.3 mA-34.4 mA RMS) and those are from meters well suited for this range.You are providing results with 0.1 mA resolution, suggesting your chosen range correctly.Differences in filter ON/filter off results suggest this fan has horrible power factor and generates so much harmonics it confuses TRUE-RMS, or simply exceeds bandwidth significantly. Reading from power meter is inline with this hypothesis.This is however hypothesis. Current waveform shall be observed to make sensible diagnosis.

author: electr_peter
author_rank: Unknown
timestamp: June 22, 2025, 06:49:11 pm
content: Fan draws highly irregular current waveform that is not easy to measure with low bandwidth DMM. Higher BW DMM should show more accurate reading. Ideally use scopemeter with voltage/current probes or power meter.

author: bdunham7
author_rank: Super Contributor
timestamp: June 22, 2025, 07:03:30 pm
content: Quote from: BrianNStPaul on June 20, 2025, 11:28:35 pmHow do you know which meters are accurate? A higher BW meter will see higher frequencies and harmonics, the lower BW meter will not and thus will typically read lower. You can clearly see this with the DM92 as you change the BW. The Fluke 17B is not TRMS so it will be way off for a signal with a high crest factor. You'd need to use a scope to look at this to see why all of your meters read differently.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 22, 2025, 10:15:33 pm
content: Quote from: BrianNStPaul on June 20, 2025, 11:28:35 pmA simple sanity check here would be to use a 10k 5-10W resistor load to confirm their calibrations with a simple resistive loadof the same currentas you are testing.Any variations from those check values, are then down to current distortion and how it is handled.

author: J-R
author_rank: Super Contributor
timestamp: June 23, 2025, 05:03:22 am
content: At a basic level, each DMM likely has a different mA burden voltage (shunt) which could cause quite a bit of variance in the readings unless you have all of the DMMs connected in series at the same time.However, it's doubtful the DMMs are able to properly account for complex loads like this motor. There are some points in time when the load is actually sending power back to the supply. You need a device that can observe the phase angle between the voltage and current.So the Kill-A-Watt is the only device in the list that is appropriate for making this measurement, and further strengthened by the very low power factor value.

author: Whales
author_rank: Super Contributor
timestamp: June 23, 2025, 07:13:23 am
content: Quote from: J-R on June 23, 2025, 05:03:22 amStrongly disagree, this is going to cause negligible error in this scenario. Eg the Fluke 87V has a burden resistance of about 2ohms in mA range, or about 50mV dropped out of the total 110V or 240V. This is several magnitudes smaller than what OP is seeing.QuotePower factor is nice to have, but it (EDIT willnot) help with the OP's issue of why every DMM is reading differently.Sidenote: TI would not trust a kill-a-watt to be accurate at these low power levels. They tend only have a single shunt & measuring circuit for everything up to max load (about 1000x bigger than these measurements). It's worth double-checking their readings against something else at these low power levels.

author: bdunham7
author_rank: Super Contributor
timestamp: June 23, 2025, 02:26:49 pm
content: Quote from: J-R on June 23, 2025, 05:03:22 amThey don't need to--phase angle has nothing to do with measuring the actual current.As for how the DMM resistance affects reactive current, although I'd guess that a few ohms wouldn't matter you don't know without a scope. Sometimes weird stuff happens and I could see how DMM impedance could change a highly resonant circuit. However, my first guess would be that this is all due to frequency response differences.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2025, 04:09:28 pm
content: The difference is likely from the frequency response, but also from a DC component and possibly clipping with a very high crest factor waveform. Another issue could be the mains waveform effecting the current - so the current could change between the measurements.

author: BrianNStPaul
author_rank: Newbie
timestamp: June 25, 2025, 12:58:57 am
content: Hello everyone, thanks much for your thoughts on this particular measurement case. As to why I thought some measurements "seemed correct" - I just meant that the measurements reported correct mains frequency ( 60Hz ) and current draw in line with a poor power factor appliance ( cheap muffin fan ). Line mains at my house can easily and readily be measured at 60Hz, so if a meter reports other than 60Hz I am unlikely to regard its current measurement as accurate. Several instruments reported very close measurements, while several others reported widely varying measurements. Not sure how one is expected to develop measurement confidence with widely varying results. I was thinking that the several closely spaced measurements at low ac current bandwidth were likely to be the more accurate measurements. I did measure current with all of the previously used meters in series with 3 22kOhm resistors in parallel to simulate an equivalent load and they all measured about 16.5mA, and spot on at 60Hz. Seems to me that this is a measurement "edge case" where otherwise accurate meters do not perform well. I don't have a high bandwidth low current probe, but will try to get an oscilloscope trace of voltage with and without fan on, might be able to see fan's effect on line.

author: bdunham7
author_rank: Super Contributor
timestamp: June 25, 2025, 04:24:01 am
content: Quote from: BrianNStPaul on June 25, 2025, 12:58:57 amThere's your mistake--the applied voltage is surely 60Hz, but you're measuring current and that could be anything. A scope will answer your questions.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 25, 2025, 04:33:35 am
content: Quote from: BrianNStPaul on June 25, 2025, 12:58:57 amMeters base lines will all be 50/60Hzsine waves, and some will infer the RMS from a simple average calc, rather than properly RMS measure it.As the conduction angle drops, the Average (Simplest) and RMS (more power useful) numbers diverge.See alsohttps://electronics.stackexchange.com/questions/18368/what-value-does-ammeter-or-voltmeter-measures-rms-average-or-peakQuote from: BrianNStPaul on June 25, 2025, 12:58:57 amIt does not need to be high bandwidth.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2025, 07:31:56 am
content: The meter accuracy is specified for a clean sine, though the RMS solution should also work with different waveforms, if within reasonable limits for the crest factor. To high a crest factor can cause false readings also for true RMS meters. The poor power factor reported by the Kill a Watt meter suggest a high crest factor, as I would not expect much phase shift, more a non sine waveform.A lot of meters are not good in handling too high a peak value. It is rare to get a warning for internal clipping.Many analog RMS solution also don't handle very low frequencies (e.g. < 40 Hz from sub-harmonics) well. Probably not the case here, but for heaters there are modulations like 2 cylces on and 8 cycles off, that can through off a true RMS meter. A poorly working speed control at a fan can also contain low frequencies below mains.

author: bdunham7
author_rank: Super Contributor
timestamp: June 25, 2025, 04:08:15 pm
content: Quote from: Kleinstein on June 25, 2025, 07:31:56 amI think most of the meters listed in the OP are not at all close to the top of the range in use, so the crest factor becomes a BW issue as much as anything else. The readings for the 87V are listed to 0.1mA which implies the 400mA range, unless the OP truncated and is using the 60mA range. Given what some of the other meters read I'm guessing there's some pretty HF switching going on.QuoteThat's an interesting thought, sub-harmonics hadn't occurred to me. But the analog RMS section isn't what is typically lacking in low frequency response, those typically go to DC. It's the input filtering and for the 87V the stated frequency response is 45-2000Hz. The OP needs a TRMS AC+DC meter and an AC/DC current clamp or an equivalent system using a shunt, all with higher BW. One meter was reading the frequency of the current as 210kHz, that's interesting.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2025, 06:35:31 pm
content: The analog RMS circuitry works at DC, but it still has problems with low frequencies like 20 Hz. It uses integral over U² divided by the average output. The problem is that the average of the output is from some low pass filter and if the frequency is too low there is some residual ripple. So the calculation is no longer accurate RMS. With short current pulses it gives too much weight (rel. low output voltage) to the start of the pulse and less than correct to the later half of the pulse. Depending on the pulse shape this can give an error.The filter is responsible for the time it takes for the reading to settle. So the time constant here is a compromise between accuracy below some 40 Hz and the response time for the meter. As frequencies below mains frequency are relatively rare the time constant is often choosen to get fast response without sacrificing accuracy at mains frequency very much. This however means accuracy gets worse not too much below mains frequency. This may not be reflected that much with a sine waveform, but can be an issue especially with a more pulsed waveform, possibly aready with the actual frequency at mains frequency.

author: bdunham7
author_rank: Super Contributor
timestamp: June 25, 2025, 09:46:45 pm
content: Quote from: Kleinstein on June 25, 2025, 06:35:31 pmYes, I hadn't thought of it, but if your frequency is very low, less than a multiple of either the reading rate or filter time constant, then even TRMS AC+DC will have issues. However, unless you are very lucky (or perhaps unlucky?) this will manifest itself as an unsteady reading more than an error. A quick experiment with a Fluke 189 shows that using TRMS it reads accurately and steadily down to ~8Hz for both sine, square and pulsed, then it becomes unsteady but without gross errors until you hit exactly 2Hz and then it shows a somewhat steady reading that is off--but not the same each time. The difference between 30Hz and 10Hz for both sine and square was less than 1%. I'd note that the standard AC response for this meter rolls off 3% @ 10Hz, 5% @7Hz and 10% at 5Hz. I suspect this is about as good as it gets for standard handheld meters.I doubt any of these issues are affecting the OP.

author: BrianNStPaul
author_rank: Newbie
timestamp: June 26, 2025, 01:48:17 am
content: Thanks everyone who took the time to help me understand the results I measured. I placed a 2 ohm resistor in series with the fan and measured voltage drop across the resistor with the following results:Fluke 87V : 75.0mV, 60HzFluke 87V with low pass filter on : 40.0mV, 60HzFlir DM92 : 96.0mV, 16.6kHzFlir DM92 with low pass filter on ( "VFD" mode ) : 40.0mV, 60HzUni-T UT139s : 40.0mV, 178kHzThe other meters I checked varied between 50-110mV and 60Hz to 340kHz. As an electrician, I've used the low pass filter on meters when measuring voltage output of inverter drives - where the voltage output is rapidly switching at what I've heard described as the "carrier frequency". I have a cheap portable oscilloscope I've used for automotive ( PWM and Canbus ) troubleshooting and a 30A current probe/clamp. If I loop the wire through the clamp several times should be able to get the probe into a good working range and maybe have a better idea of what's going on.

author: BrianNStPaul
author_rank: Newbie
timestamp: June 26, 2025, 10:02:25 pm
content: Hello, attached are pictures of the current waveform using a 100mV/A current probe with 10 wraps of wire through probe jaws. Should put signal at 1mV/mA, saved an image across several time bases. Not very experienced with oscilloscopes so not sure how to interpret what I'm seeing, but it looks a load rapidly switching itself on and off. In terms of calculating power which of the measured values for current that I've seen would be correct to use? The low bandwidth meters ( or meters set to low pass filter ) measured current ( or voltage drop ) in line with fan specs and matched fairly closely with readings from the "Kill-A-Watt" device so I was considering them to be "accurate" power purposes.Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: June 26, 2025, 10:20:27 pm
content: Quote from: BrianNStPaul on June 26, 2025, 10:02:25 pmNone of them will be very good because for calculating power you need to measure instantaneous current and voltage at the same time and then average that value over some period. A mess like that could be interpreted by a fairly good scope with a power analysis feature or even just plain math if it will take formulas. But there's no one number for current that you can get and then multiply by voltage to get power. A simple small filter capacitor might change things enough that you wouldn't see so much HF current noise and that might make for easier measurement--but I'm sure it will still look interesting.Low pass filtering isn't effective for current like it is for voltage. The "VFD" feature--the 400Hz low pass filter on the 87V--sort of works because of the assumption that the inductance of the motor circuit will not allow it to respond to the switching (you called it "carrier") frequency so the current will be something like a sine wave corresponding to the primary (low) frequency. This works when the switching frequency is 50-100kHz but the motor is being driven at 30Hz or something like that. In that case, you want the meter to respond to what the motor is responding to--the 30Hz. In your case, all that goes out the window.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 27, 2025, 03:35:32 am
content: The current waveform looks really nasty. It has a lot of higher frequency content and is likely way beyond the EMI limits. The crest factor (ratio from peak to RMS) also looks quite large (maybe 5 - 10). It looks like a SMPS without any fitler cap at the input.For this I would not trust any of the handheld meters. It is way beyond what the meters are made for.

author: 1audio
author_rank: Frequent Contributor
timestamp: June 27, 2025, 10:20:34 pm
content: Its a little bit of the shotgun to kill a flea but the right instrument would be a Clark Hess 255https://testequipment.center/Product_Documents/Clarke-Hess-255-Specifications-1A562.pdf