Using a coil to increase clamp meter precision from 0,1A to 0,01A

Thread Starter

injtsvetkov

Joined Dec 12, 2015
5
Hello guys, I have a clamp meter with precision of 0,1A. I tried to measure the AC current which a battery charger draws from the mains (220V) but it is too low (0,2A-0,3A) so I made a coil to boost the measurement precision to 0,01A. I thought that the coil must have 10 turns but in fact it needed 8 turns. I made a test with two identical clamp meters (one on the coil and the other on the wire next to the coil) and the result is great. However I need to know is there a problem if the current passes trough the coil all the time untill the battery is fully charged (it may take several hours) or I have to make a switch to pass the current trough the coil only when I want to measure it and then switch it back trough straight wire?
 

rsjsouza

Joined Apr 21, 2014
335
According to the working principle of AC-only clamp meters (basically a transformer), the turn ratio is linearly correlated with the measurement, therefore you should use 10 turns to get a 10x higher gain. If you are getting a different result as you said, either the accuracy of the clamp may be skewing the results or it is a Hall effect clamp (one that can measure DC currents as well) that is pre-magnetized.

The DC clamps can have some residual magnetization that influences the readings, especially after a high intensity measurement. The clamps usually have a way to offset this magnetization (check this document). Also, due to the higher sensitivity, these clamps are usually recommended to have the conductor intersect the jaw opening as close as possible to the middle.

If the coil wire is thick enough, I don't see a problem in leaving it permanently attached. The AC-only clamp is also more robust in this sense, as it is a pure transformer and magnetization effects are much reduced. Hall effect clamps reading AC currents are less susceptible to the aforementioned offset (as the magnetic field switches directions constantly), but might need to be zeroed from time to time (I haven't used Hall effect clamps for long periods attached, so others may comment further).
 

crutschow

Joined Mar 14, 2008
28,161
As injtsvetkov noted, if the coil wire size is sufficient to continuously carry the current, there should be no problem in leaving it connected during the duration of the charge.
 

Thread Starter

injtsvetkov

Joined Dec 12, 2015
5
Thank you all for your replies!

First I have to make correction! Both clamp meters are “MT87” with precision of 0,01A (and not 0,1A as I wrote in the OP). So I had a reading of 0,02A-0,03A (drawn by the battery charger) which I tried to amplify 10 times. I took a lamp with a dimmer and I made a coil with 10 rounds. I measured about 0,08A on the straight wire and about 1,15A on the coil when I expected to be around 0,8A. Then I removed 2 rounds and it got around 0,8A. Then I tried with the dimmer and both measurements were changing accordingly so I left it that way. Here is a picture:

coil.jpg

I presume that the clamp meter reading of 0,08A (on the 20A range) could be anywhere from 0,05A to 0,11A so maybe I should have trusted the 1,15A measured on the ”10 round coil” meaning that there were actually 0,11A on the straight wire and not 0,08A.

However my concern is about the AC device when I put the coil between it and the mains. In the first case it’s a battery charger but I need to measure my PC’s current too. So I thought that there could be some risk for example if the mains voltage (220V) gets slightly lower or higher and the coil amplifies that… or some other unwanted effect that I’m not aware of which could damage the AC device after continuous use with the coil.
 

#12

Joined Nov 30, 2010
18,223
I have used this method for decades because a clamp meter isn't sensitive enough to set the heating anticipator in a wall thermostat, but I never measured to verify the accuracy of the result. Thank you for bringing this to my attention. My much more modern AC VOM will be sufficient to test my very old hack.
 
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