AC presicion metter, with current transformer - Page 1
Forum section: electronics / general

author: LuisHDR
author_rank: Newbie
timestamp: May 15, 2025, 10:38:37 pm
content: Hello, i am new en this forum.I am trying to measure AC current with decent accuracy. I’m using a current transformer model DL-CT08CL2, class 0.1,and comparing it with an Aneng AN870 multimeter. I have a 98.5-ohm burden resistor (adjusted) connected in parallelto the CT, and these are the readings I’m getting.Considering the RMS voltage value from the oscilloscope (143.69 mV rms) and that the transformer has a 2000:1 ratio,I theoretically have 2.916 A according to the oscilloscope, while the multimeter reads 2.813 A, giving avariation of 3.67%. The question is: which one should I take as the reference measurement? Which one is more accurate?(oscopte is in AC coupling)

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: May 15, 2025, 10:59:08 pm
content: Remember that, in addition to the CT’s tolerance, both the oscilloscope and the DMM have its own tolerances.Read the manuals for each. What are they?

author: jwet
author_rank: Frequent Contributor
timestamp: May 17, 2025, 02:48:14 am
content: As Unijunction said, look at the specs. I think you'll find when you stack up tolerances, the DMM will be much more accurate. Your scope has an AC accuracy of 3% off the bat and there are some other error sources. I'm not a fan of Aneng stuff for anything precise but its the best you have here. You have to spend a bit of money to get a meter that has good true RMS AC accuracy. Even a Fluke 115 ($200) only gives 1.5% accuracy for this measurement. "Decent" accuracy is pretty nebulous, is there a reason that you need more than a few percent- I would consider that decent but I don't know what you're trying to do.Have Fun.

author: radiolistener
author_rank: Super Contributor
timestamp: May 17, 2025, 03:03:59 am
content: Quote from: LuisHDR on May 15, 2025, 10:38:37 pmI wouldn't fully trust either of these devices. But you are also missing a very important detail - measurement bandwidth. It is obvious that the measurement bandwidth used in an oscilloscope is (usually) higher than in a DMM, so it is incorrect to directly compare their results. The oscilloscope captures more high-frequency details than the DMM, and in theory, the oscilloscope result should be closer to the real value. But on the other hand oscilloscope has (usually) higher measurement error specification than DMM. So, its hard to say what result is better.And since this is a Hantek... I'm not sure it provides accurate measurements, so it needs to be tested. But the same is applied to Aneng either...PS: It is good practice to enable the 20 MHz bandwidth filter when measuring RMS levels on an oscilloscope, as bandwidths vary between different oscilloscopes and can give you different results. However, most oscilloscope models include a standard option to limit the bandwidth to 20 MHz, which helps achieve more reproducible results that are generally closer to those obtained from a DMM.PPS: another finding on your photo is too low sample rate 125 kS/s. This may not be enough for reliable RMS measurements, since many SMPS have noise ripple up to 60-200 kHz and even more and this noise can significantly affect measurement results due to aliasing at 125 kS/s.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 19, 2025, 06:49:20 am
content: Y axis accuracy and resolution is crap on most usual oscilloscopes. Like mere 8 bits of ADC resolution and more than 1 % of error. It's an instrument designed for relatively high X axis (time) resolution and also accuracy, compromising Y accuracy on purpose. There are oscilloscopes with better Y resolution and accuracy but if you have one, you probably know it.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2025, 02:59:50 pm
content: With the scope one should check if the RMS value is calculated for an interval that spans hole periods or a very long. One can get wrong result if something 2.4 periods are used. As a quick check the result should not change with the trigger level or if possible trigger delay.Check the specs how accurate the scope and DMM are. In the comparison the specs for ANENG dmm may be more optimistic than for a scope. So even the numbers on paper may only tell you half the story.Especaially cheap DMM are sometimes quite poor in the higher current ranges due to self heating of the shunt.For a check of the current transformer and burden resistor part I would consider a few more turns and than a lower current. A 2nd point would at least give an extra check.