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Current probe for Oscilloscope

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2X:

--- Quote from: alm on July 16, 2023, 04:49:20 pm ---
--- Quote from: 2X on July 16, 2023, 04:25:44 pm ---"If you can live without DC and moderate bandwidth, then there are substantially cheaper options from multimeter manufacturers as industrial current clamps with BNC or banana plug output."
Do you have something to suggest me from a good company or whatever do you beleive it is worth?

--- End quote ---
Any reputable multimeter brand will do. This does not strike me as something safety critical. Try to find something that has a voltage output rather than a current output. Keep in mind that just because a particular clamp won't measure DC current does not mean DC current can't affect your measurement due to saturation of the ferrite core. If a probe is well documented, they will tell you about max permissible DC current. To get around this, you can null out the DC current by an equal current in opposite direction, or N turns of I/N current in opposite direction. See page 2-4 from This old Tektronix CT-5 manual.

--- End quote ---

I didn't understand how to do this "To get around this, you can null out the DC current by an equal current in opposite direction, or N turns of I/N current in opposite direction." As I understand this refered when an AC signal contains DC offset for instance. If I put the two cables together in the clamp probe in order to null the DC current then also the AC current will null?



alm:

--- Quote from: 2X on July 16, 2023, 07:03:04 pm ---I didn't understand how to do this "To get around this, you can null out the DC current by an equal current in opposite direction, or N turns of I/N current in opposite direction." As I understand this refered when an AC signal contains DC offset for instance. If I put the two cables together in the clamp probe in order to null the DC current then also the AC current will null?

--- End quote ---
Yes, so make sure the current in the second wire comes from a different source without the AC signal superimposed. For example a bench supply. Now finding a 100A bench supply might be tricky, but you could use a 10A current and then wind the wire carrying this 10A ten times around the clamp. Note that this is only necessary if DC current is causing you problems, otherwise you can ignore it.

johansen:
Buy an lem current sensor off ebay for 15 to 30$. They are typically 100 or higher khz bandwidth.

mkissin:
I work with very large, high frequency currents at work and (as long as you can live without true DC capability) a rogowski is the way to go. As has already been pointed out most DC current probes derate significantly with frequency due to thermal effects in the core, so you have to be careful even with your harmonics.

We use various models (usually the mini and ultra-mini for the bandwidth) of these: https://www.pemuk.com/products/cwt-current-probe/cwt.aspx

A handy thing is that they don't care if you go even way over the maximum voltage. The output will saturate, but no damage occurs. They only care if you violate the di/dt rating.

H.O:
Be VERY careful with whatever instrument you use if you're using HF-start on the welder! If I were to use an expensive current probe on a TIG-welder I would make SURE to "connect it" after the arc is established.

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