LEM HEME PR-1001 current probe teardown, schematics & questions

[fenugrec]

I just got a LEM HEME PR 1001 clamp current probe.
Basic specs : 200A / 1000A ranges , scaling of 1mV / A on both.

Other meters in the same series (PR2000) have 10mV/A scaling for the lower range, so I was hoping to change the gain of mine to match that - I'm after the increased resolution of course, this could only at best keep the same accuracy but possibly deteriorate it.

"why didn't I get the PR200 instead" : this one was affordable.


Notes on the overlay and schematics :
[see proper schem in next post]

- part numbering is entirely arbitrary of course
- P1 and P2 blue 0603 are 0-ohm jumpers (R101, R102 in schem)
- currently (!), it draws 14.5mA from the 9V battery. Looks like 5mA for the 78L05's quiescent current, 3.5mA for the LED, 2mA for the voltage divider driving Q1. I'm tempted to swap in an ultrabright LED, and an LDO with the proper caps although this may require calibration. [EDIT : Done. Replaced regulator with LP2950A , new total current draw is ~ 8.2mA. Calibration was indeed required since Vcc changed from 5.11V to 4.99V]

- That thing around IC3, TL061. It seems to say "relaxation oscillator", but: [ ignore the following; see next post]

A) it's not feeding anything except the LED ? For a moment I thought "chopper amp" but no.
B) if it's just driving an LED, why do this ?
C) it ain't oscillating. I'm willing to suspect a bad opamp or bad cap C9 though :

IN+ 4.38V
IN- 3.78V
OUT 4.13V
Why isn't IN- almost identical to OUT ? They're JFET inputs, I_bias should only be around 200pA. And the datasheet says "Common-Mode Input Voltage Range Includes VCC+" , as well as "Latch-Up-Free Operation" so I can't explain this behaviour.


Adjustments:
-VR1: zero adjust. Accessible from front panel
-VR2: no idea. hall effect balance ?
-VR3: low range gain
-VR4: high range gain
-VR5: goes with that TL061-based mystery ?

As far as gain is concerned, it would seem simply a matter of swapping out VR3. I'm not seeing exactly how the math works out for this kind of balanced output stage - any clarifications on this are welcome.

[EDIT : 2020/12/12, updated current draw post-mod]

[fenugrec]

Ok, finally cleaned up the schem.
I've double- and triple-checked the circuit around the TL061 and I'm positive that it doesn't go anywhere except the LED. And still no sign of oscillation there....

[EDIT] Of course, the answer was in the manual : the LED blinks when battery is getting low ! So obvious.

[Fraser]

I know this is an old thread but I just bought a PR200 and wondered whether you had any success improving the output mV/A ratio for lower current use ?

Fraser

[fenugrec]

I know this is an old thread but I just bought a PR200 and wondered whether you had any success improving the output mV/A ratio for lower current use ?

Yes; I installed a 1k in parallel and adjusted VR3 to give 2x the original mV/A ratio.
But doesn't the PR200 already have a more versatile gain already ?

I also replaced the 78L05 with an LP2950 , and replaced the LED with a super bright part (and changed its resistor).  This reduced total current draw to ~  8mA instead of 14.5 !
Only problem is that the regulator change requires calibration. But I was going to do that anyway after changing the gain.

[Fraser]

Many thanks 

The PR200 is providing 10mV per Ampere on its 20A range but I would like to see if I can get it up to 100mV per Ampere  The quiescent noise levels may become an issue for lower current work below 1A but I am just interested in the possibility of improving the low current capability for use with an oscilloscope. If it cannot be achieved, I will still be content with 10mV/A 

Thanks again

Fraser

[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]

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).

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 !