Clip-on Current Probe - Page 1
Forum section: electronics / projects

author: metrologist
author_rank: Super Contributor
timestamp: October 25, 2016, 04:04:02 pm
content: I'm wondering if I take a steel clip-style key ring, wrap wire around it, how well would it work as a clip-on current probe for my DMM or oscilloscope? How could I make it better?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2016, 08:27:42 pm
content: The key ring will be a poor probe: to low permeability a rather small coss-section for low Frequencies (e.g.. < 1 kHz) and too much conductivity for higher frequency (e.g. > 100 Hz). The conductivity to a certain degree could be seen as a shunt, but this wild be frequency dependent and with a high temperature coefficient.To get a reasonable current transformer, one should use a high permeability core, like a ferrite core from a common mode filter one finds at the input side of SMPSs. Some oft them have a 2 part core. Remove half the windings to have space for the conductor to measure.The second part is, there needs to be a shunt at the secondary winding to limit the voltage and make the transformer work at near short circuit.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 25, 2016, 08:33:29 pm
content: This is a current probe you can wrap around a conductor:https://en.wikipedia.org/wiki/Rogowski_coilAC or DC ?Frequency ?mA or MA ?

author: tautech
author_rank: Super Contributor
timestamp: October 25, 2016, 09:11:10 pm
content: It's just not that simple OP.For my first one I used a PCB mount CT from a old PC SMPS and shunted the output as Kleinstein described.Shunt value was set for the CT to give very similar waveform values as a mates P6022.Later I got a P6021 (60 MHz 15A) but again even these are only AC current probes.You might find some guidance and understanding in this:www.davmar.org/TE/TekConcepts/TekProbeCircuits.pdf

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 25, 2016, 09:30:09 pm
content: Theory says this won't work very well, BUT, it's simple enough to build one and it might just work "good enough" so I say give it a shot.You will need to make sure there is an air gap in the ring when it is enclosed around a current carrying conductor otherwise the ring will act as a shorted turn. Eddy/hysteresis (ie "core") losses will be high (and starting at a low frequency - perhaps in the 10s of Hz) but these losses don't really affect the current transformer's accuracy, just make the core get hot. In fact, frequency response is mainly determined by the secondary wiring inductance: the higher the inductance the better the low frequency response while the lower the distributed capacitance the better the high frequency response. Those two things tend to be mutually exclusive, btw.

author: metrologist
author_rank: Super Contributor
timestamp: October 26, 2016, 12:04:53 am
content: What do you folks mean by secondary? There is just one coil around the ring. The rest is just the leads that I would probably heat-shrink back onto the ring and connect the ends to my DMM. I thought the open circuit voltage would be representative of the current through a wire that was went through the center of the ring.Initially, I just want to measure AC current through a typical 110V/220V household extension cords, which could be <1 to 30 Amps, but would be nice to clip it to DC leads on my Arduino projects. Yes.I got the idea by seeing an image of wire wrapped around a C-clamp, and wanted to improve it!I could also likely fabricate the ring via a solid cast iron if that is better?Now, I see that Rogowski coil seems like it would be easy to make with 16 or so gauge solid core insulated wire, so maybe I'll give that a whirl.If not reasonable to build, is there something cheap enough to buy?

author: Someone
author_rank: Super Contributor
timestamp: October 26, 2016, 02:47:37 am
content: Quote from: metrologist on October 26, 2016, 12:04:53 amSecondary in the sense that you are inducing a current to flow in this secondary from the wire passing through the middle (which would be called the primary)