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LEM HEME PR-1001 current probe teardown, schematics & questions
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Fraser:
My PR200 is the earlier model but possibly contains a similar circuit.
Picture attached.
richardash1981:
I have another "old" PR200 probe and IM200 separate amplifier. Marked up as RS stock code 654-607 with a white sticker on the probe body.

The 200A range works fine, but the 20A range will not zero - the offset goes down as I turn the adjustment, but I run out of potentiometer before I get to a zero reading. I haven't been able to find any documentation (not even the user guide) online for this sort of PR200, as opposed to the all-in-one version which became LEM and then Fluke.
jimmc:
I don't know how I missed this thread at the time but this might still be of interest.

I rescued a later version of the PR200 after it had been binned at work for excessive drift, it wasn't until I used it with a 'scope that I found it was oscillating badly. (The output cable had been replaced at some time, possibly increasing the capacitive load on the output.)
After reverse engineering it, I found that there appeared to be an error in the design, the feedback capacitors (C1,2) were connected to the load side of the isolating resistors (R14, 15).
After moving the caps to the Op-Amp outputs, the oscillation stopped and the drift was tamed.

It would appear that the PR1001 has a similar circuit and may have suffer in the same way. (I'm not sure about the differences around TR1, I looked at this about 9 years ago and can't remember the details.)

Jim
Fraser:
Jimmc,

Thank you for this useful information  :-+

Fraser
fenugrec:

--- Quote from: jimmc on September 01, 2022, 08:44:02 pm ---After reverse engineering it, I found that there appeared to be an error in the design, the feedback capacitors (C1,2) were connected to the load side of the isolating resistors (R14, 15).

--- End quote ---

Interesting... I can't see why they wired it that way, but also surprised they would have done that by mistake over a few models spanning many years...

Thanks for sharing your schematics - we even have almost the same layout !
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