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Accurate Low Amp Current Probe Advice

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KungFuJosh:

--- Quote from: MarkL on November 10, 2024, 04:55:40 pm ---Double checking, the 503B I've been using for the above tests indeed only has (6) 100uF.  However, the other 503B I have has (7) like yours, and I now see the empty pads on the first unit.  The pads look factory fresh, so I guess Tek decided it was unnecessary.  The board layouts look identical and also have the same part number (A9F-1631-00, at the edge near the front panel).

For the record, the 100uF that's missing is the one that's in the center, flanked by two diodes on the top and two TO-92 transistors on the bottom.

--- End quote ---

Interesting! Now the question is, does one of them appear to have better stability? Then again, there might be something else different component wise that eliminated the need for the 7th cap.

Thanks,
Josh

MarkL:

--- Quote from: KungFuJosh on November 10, 2024, 05:07:17 pm ---
--- Quote from: MarkL on November 10, 2024, 04:55:40 pm ---Double checking, the 503B I've been using for the above tests indeed only has (6) 100uF.  However, the other 503B I have has (7) like yours, and I now see the empty pads on the first unit.  The pads look factory fresh, so I guess Tek decided it was unnecessary.  The board layouts look identical and also have the same part number (A9F-1631-00, at the edge near the front panel).

For the record, the 100uF that's missing is the one that's in the center, flanked by two diodes on the top and two TO-92 transistors on the bottom.

--- End quote ---

Interesting! Now the question is, does one of them appear to have better stability? Then again, there might be something else different component wise that eliminated the need for the 7th cap.

Thanks,
Josh

--- End quote ---
I'll run the same stability test on the second AM503B in the next day or two for curiosity's sake.  You're right that there might be other differences between the two.  One test could be adding the cap to the first, or removing it from the second.  But if they're both equally stable, I'm probably not going to mess with either.

KungFuJosh:

--- Quote from: MarkL on November 10, 2024, 05:48:12 pm ---I'll run the same stability test on the second AM503B in the next day or two for curiosity's sake.  You're right that there might be other differences between the two.  One test could be adding the cap to the first, or removing it from the second.  But if they're both equally stable, I'm probably not going to mess with either.

--- End quote ---

If one is more stable than the other, I'd start with the big 4 caps with low ESR versions like I used. Those had a bigger impact than anything else so far.

I have it all warming up now, and we'll see if the 220uFs did anything useful. The old caps were ~450mΩ ESR, and the new ones are ~45mΩ.

Is your 1uF cap bipolar?

Thanks,
Josh

Martin72:
Did you measure the old ones you had already swapped?

KungFuJosh:

--- Quote from: Martin72 on November 10, 2024, 06:30:47 pm ---Did you measure the old ones you had already swapped?

--- End quote ---

Yup:

35V 100uFOld: 1.3Ω ESRNew: 55mΩ ESR16V 220uFOld: 450mΩ ESRNew: 45mΩ ESR50V 1uFOld: 1.3Ω ESRNew: 3.4Ω ESR50V 2000uFOld: 60mΩ ESRNew: 13mΩ ESR50V 1000uFOld: 120mΩ ESRNew: 11mΩ ESR
All values measured on IM3570 after warmup and corrections.

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