Clamp meter mystery. - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: September 23, 2012, 09:02:54 am
content: So I have the classic cheap uni-t clamp meter. Now i always thought that the two halves of the clamp should touch, the metal that is. But on mine one half is covered in plastic. So I decided to file it down so that the metal can touch, I usually get a reading offset of 200mA so thought the gap was a problem. but on filling off the plastic from the covered side I found it has some sort of sensor (Hall ?) on the end. What is gong on ? is this normal or the cheap and unchearful way of doing it ?At the moment there is a fair bit of gap, about 5mm, I could file some of the other side down until they almost meet.

author: BravoV
author_rank: Super Contributor
timestamp: September 23, 2012, 09:30:37 am
content: Its normal Simon, like my Agilent u1213a clamp here :The other endAbove pics are from my U1213A teardown thread ->Agilent U1213A clamp meter teardown.

author: Psi
author_rank: Super Contributor
timestamp: September 23, 2012, 10:51:10 am
content: I think this is because the magnetic field gets concentrated at the gap which is why the Hall is there.Same as a VCR or TAPE read/write head which has a slit cut to provide a stronger point source to read/write the tape.

author: Simon
author_rank: Unknown
timestamp: September 23, 2012, 10:53:26 am
content: well the field is going round the loop so putting the sensor in it's path makes sense but surely you want to loop as closed as possible to help the flux? circulate.

author: Psi
author_rank: Super Contributor
timestamp: September 23, 2012, 10:59:01 am
content: Have a look at a short bit of this video starting at 10m 40s

author: Neilm
author_rank: Super Contributor
timestamp: September 23, 2012, 11:00:52 am
content: Clamp meters are magnetic devices. They work on the signals picked up by the head. How this is detected can be done in many different ways. One is a hall effect sensor to detect the flux in the head but coils on the head are also used.The offset is more likely due to poor adjustment of the circuit not the gap in the head. The chances are that this gap has been designed to be in there and that gap is actually fairly tightly controlled. Removing it will cause issues - either due to the safety rating but almost certainly due to the core material. Removing the gap will change the parameters of the magnetic circuit and cause additional inaccuracies. Also, you might find that if you use with the side touching you will saturate the core which is not good.YoursNeil

author: T4P
author_rank: Super Contributor
timestamp: September 23, 2012, 11:18:15 am
content: How about picking up DC? That's the part that seems like witchcraft to me

author: Psi
author_rank: Super Contributor
timestamp: September 23, 2012, 12:01:19 pm
content: Quote from: T4P on September 23, 2012, 11:18:15 amAny current through a cable produces a magnetic field around the cable.Did you ever see the high school experiment where you setup two long cables next to each other then a current through them? You can see them bend inwards as the magnetic fields attracts them.This video is an extreme example with lots of current so its really obvious.

author: SeanB
author_rank: Super Contributor
timestamp: September 23, 2012, 12:36:34 pm
content: These sensors are compound units, with the hall sensor doing the Dc and low frequency ( up to around 20Hz and then rolling off) while the coil wound around the lower section of the core does the AC side. You then mix the 2 signals to hopefully ( and this is a reason the more expensive ones are so, they control the manufacture and assembly more tightly and test each one and calibrate them to be flat) provide an equal response from DC to the top end, using a true RMS converter to drive the display.

author: Unknown
author_rank: Unknown
timestamp: September 23, 2012, 02:18:52 pm
content: Why would they do that? Hall effect sensors alone can easily have a bandwidth up to the tens of kHz, much higher than most clamp meters. I know the technique of combining a Hall effect sensor and current transformer is used in some oscilloscope current probes, but these go up to the tens of MHz, not kHz, and cost about an order of magnitude more.

author: Psi
author_rank: Super Contributor
timestamp: September 23, 2012, 02:50:51 pm
content: All the hall current sensors i've used haven't been very accurate.eg, a 10A hall sensor might only be accurate/repeatable to +/- 200mAA CT however, can be quite accurate.

author: Unknown
author_rank: Unknown
timestamp: September 23, 2012, 08:58:46 pm
content: Clamp meters are not usually precision instruments. AC only (CT) clamp meters can be somewhat better than AC+DC models, but AC+DC meters usually have better accuracy for DC than for AC. The difference is probably not enough for Agilent to bother with the intricate and complex dual CT/Hall arrangement. Last time I checked there were three companies (Tektronix, LEM, Hioki) manufacturing these beasts, and Agilent was not one of them.