Croccodile clip to inject primary current into a CT? - Page 1
Forum section: electronics / projects

author: Jeroen3
author_rank: Super Contributor
timestamp: June 24, 2024, 05:59:12 am
content: I want to produce a board with a current transformer that I need to test. This is a current transformer with a hole, not a split core.To test and calibratie I need to inect primary current for testing, of course I can feed a wire through, but I suspect that setup probably won't last that long.Does someone know of a kind of clamp or clip to inject primary current?Or how else is this done?

author: soldar
author_rank: Super Contributor
timestamp: June 24, 2024, 09:30:44 am
content: It should not be difficult to make something rustic if you just need one or to design something and have it made if you need quantities.It also depends on accessibility. If you have free access all around or if it is cramped. I would say the design is more dependent on this aspect than anything else.What kind of current are we talking about here?

author: CosteC
author_rank: Regular Contributor
timestamp: June 24, 2024, 10:56:38 am
content: Consider solid round bar to go through CT and something to catch it on opposite end, crocodile may have too little surface are to provide good, stable contact. Solid clamp may be better.

author: coppice
author_rank: Super Contributor
timestamp: June 24, 2024, 11:27:00 am
content: What current do you need to pass through that hole? For 10A-20A you can put a slim 4mm plug on the end of a thick stiff wire, and leave off the plug's insulation. That can be small enough to get through the hole in many CTs, and a decent quality 4mm plug makes pretty reliable high current contact.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 24, 2024, 02:16:52 pm
content: Yes around 20A. It's short term testing/calibration.DIY is possible, convert a glueing clamp or something. But I can't be the first one with this problem?

author: CosteC
author_rank: Regular Contributor
timestamp: June 24, 2024, 06:27:56 pm
content: To what accuracy you want to calibrate this CT?And what CT it is actually?

author: ajb
author_rank: Super Contributor
timestamp: June 24, 2024, 07:10:36 pm
content: What about something like a flexible current probe? Example photo attached. Might be tricky to find a suitable connector that will fit through the hole in the CT that will handle 20A, possibly one of those miniature banana-plug-style connectors like are used in RC batteries? Connector cycle rating could be an issue depending on how many of these things you need to test.Or what about a piece of solid wire covered with heatshrink and bent into an 'L' shape that gets stuck through the CT and then an alligator clip that grabs the end of the wire on the other side?In either case, you can bundle the 'in' and 'out' wires together back to your test system to make it easier to use. I'm sure you could come up with an even easier-to-use solution with some sort of custom clamp thing, but that may not be worth while unless your volume is quite high.

author: CosteC
author_rank: Regular Contributor
timestamp: June 25, 2024, 04:16:02 pm
content: Quote from: ajb on June 24, 2024, 07:10:36 pmTwo issues with flexible current probes:1) This is Rogowski coil type probe and does not measure current - measures di/dt. To measure current an integrator is mandatory2) Accuracy - getting 0.5% in centre is hard but possible, but position error may be far higher. Personally I found coils which were also very susceptible to external currents.

author: wraper
author_rank: Unknown
timestamp: June 25, 2024, 04:25:00 pm
content: Just crimp a ferrule on the wire and use something like wago connector to quickly connect/disconnect it. If wire wears out, throw it away and use another. To center it for better accuracy, you could 3D print some centering tool that is placed on the wire. Won't get much faster than that unless you want to build hundreds of thousands and make some advanced test fixture.

author: wraper
author_rank: Unknown
timestamp: June 25, 2024, 04:28:11 pm
content: This style probably will be easier/faster to use

author: ajb
author_rank: Super Contributor
timestamp: June 25, 2024, 04:37:35 pm
content: Quote from: CosteC on June 25, 2024, 04:16:02 pmI could have been clearer in my wording, but I didn't mean literally using one of those probes. I meant something with the same mechanical arrangement with an openable loop of wire that can be fed through the CT.

author: Benta
author_rank: Super Contributor
timestamp: June 25, 2024, 04:49:39 pm
content: What's the size of the hole in the CT?

author: Jeroen3
author_rank: Super Contributor
timestamp: June 26, 2024, 05:47:37 am
content: The CT is to measure 50Hz current. The hole is 9.5mm on this 20A model.The wago idea is simple, I like it. However, wago's only have like 10 mating cycles before the lever fails.I can try to find some non-stack 4mm or 2mm banana crimps perhaps.There are some wolfcraft clamps on the way, I have some brass fasteners in stock, and I have a 3D printer. I'll let you know what I come up with.

author: wraper
author_rank: Unknown
timestamp: June 26, 2024, 06:30:24 am
content: Quote from: Jeroen3 on June 26, 2024, 05:47:37 amWhere did you get this strange idea?

author: Someone
author_rank: Super Contributor
timestamp: June 26, 2024, 06:55:42 am
content: Quote from: Jeroen3 on June 26, 2024, 05:47:37 amPlenty of options in 4mm banana, both genders and even insulated versions are available that will pass through a 9.5mm diameter aperture.

author: Overspeed
author_rank: Frequent Contributor
timestamp: June 26, 2024, 07:07:19 am
content: HelloI use 10 mm copper rod with 4 mm axial hole to handle banana male in 4 mm , rod is installed with two Omega clipsRegardsOS

author: Phil1977
author_rank: Frequent Contributor
timestamp: June 26, 2024, 07:28:04 am
content: Can you solder 5-10 pogo pins around a copper rod so that they are still fitting through the CT? Then drive the rod through the CT and press it against a hard plated contact plane.Inside an automation you could track the supply voltage or temperature of the pogo pins to detect raising resistance.