Author Topic: Accurate Low Amp Current Probe Advice  (Read 17953 times)

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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #200 on: November 08, 2024, 09:53:48 pm »
A good result that I am tempted to reproduce.

I'd love to see how your AM503A compares!

For anybody with an AM503B that wants to recap or convert the NVRAM, this is my Mouser BOM of what I ordered.

So far I've upgraded the NVRAM and changed only the 4 large caps. I will probably do the rest of the caps on that list too.

Thanks,
Josh
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #201 on: November 08, 2024, 09:55:18 pm »
So... If you're still using 10mA/div to be comparable with previous tests, that's a drift of 0.235mA.  Half of what I was getting.  Not bad!

Thanks! I'm happy with the result.

I'm curious now if replacing the rest of the electrolytic caps with lower ESR caps will change anything, and if so, for better or worse. 🤔

ETA: Mark, can you please confirm the values of your electrolytic caps in your AM503B? 4 caps were already replaced before I got mine.

Thanks,
Josh
« Last Edit: November 08, 2024, 10:10:27 pm by KungFuJosh »
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #202 on: November 08, 2024, 10:01:17 pm »
Then, to be consistent, you would have to monitor the voltages that the electrolytic capacitors filter.
If these are/were unstable due to aging of the electrolytic capacitors, it will affect everything that is supplied by them.

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #203 on: November 08, 2024, 10:13:48 pm »
Then, to be consistent, you would have to monitor the voltages that the electrolytic capacitors filter.
If these are/were unstable due to aging of the electrolytic capacitors, it will affect everything that is supplied by them.

What will monitoring them help? I don't have a schematic or reference of any kind for the 503B. If I swap out a set of caps and test the performance of the probe, won't that tell me what I need to know?

I'll also be connecting all caps to the IM3570 to test ESR at 100kHz and µF at the proper range for the value. I tested some of them in circuit with the ST42, and it did pretty well too.
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #204 on: November 08, 2024, 11:00:53 pm »
Quote
If I swap out a set of caps and test the performance of the probe, won't that tell me what I need to know?

Of course you can do it that way...
I'm always interested in the background of such things, because even years ago I always had a problem in the test field.
Don't ask, just replace...
That was always not enough for me.
Forget what I said. ;)
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #205 on: November 08, 2024, 11:24:26 pm »
I'm always interested in the background of such things

Sure, but you're also more likely to try and trace out the circuit and make your own schematic for it. I won't. 😉
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #206 on: November 09, 2024, 09:25:31 pm »
Please summarize your setup again.
Power supply, resistor, SDM3065X with the Tester software, then the Tektronix.
You connected the output of the AM503 to the SDM via 50 ohms, right?

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #207 on: November 09, 2024, 09:50:23 pm »
Please summarize your setup again.
Power supply, resistor, SDM3065X with the Tester software, then the Tektronix.
You connected the output of the AM503 to the SDM via 50 ohms, right?

There's a mediocre photo in reply 196, but here's my setup:

PSU red terminal to DMM-1 current terminal, DMM-1 black wire to 100Ω resistor to PSU black terminal.

A6302 connected on negative wire near PSU (obviously pointed at PSU).

AM503B connected to DMM-2 via 50Ω terminator and BNC to banana adapter.

TestController set to monitor reference current on DMM-1 and AM503 output on DMM-2.

My DMMs had 4 hours of warmup. AM503B and PSU had over an hour warmup. Maybe 2, I don't remember. I wouldn't do this test with less than an hour warmup on anything.

Thanks,
Josh
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #208 on: November 09, 2024, 10:46:10 pm »
Ah, that's right, you had used two DMMs.
I'll have to solve that differently then, I only have one that can connect to the test controller software.
I would warm up the DMM and the resistor, and I would record the AM503A including the cold start.
I need to see what the trend function on the SDS3104X HD can do before I have to buy a second DMM.

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #209 on: November 10, 2024, 12:30:26 am »
Ah, that's right, you had used two DMMs.
I'll have to solve that differently then, I only have one that can connect to the test controller software.
I would warm up the DMM and the resistor, and I would record the AM503A including the cold start.
I need to see what the trend function on the SDS3104X HD can do before I have to buy a second DMM.

In reply 181 I used my SDS2504X HD instead of the second DMM with TC. You can try that too. Replace DMM-2 with your scope to measure the current from the AM503.

I added the SDS3000X HD series to the attached device file for TestController. Let me know if it works for you. It should.

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #210 on: November 10, 2024, 02:19:23 am »
Ah, that's right, you had used two DMMs.
I'll have to solve that differently then, I only have one that can connect to the test controller software.
I would warm up the DMM and the resistor, and I would record the AM503A including the cold start.
I need to see what the trend function on the SDS3104X HD can do before I have to buy a second DMM.
What DMM do you have?  If you have one that has a jack in common with a volts input and a current input, you can wire it up and use your computer to read current through the 100R resistor, then switch DMM functions to read the voltage output from the AM503B.  Keep flipping back and forth reading current and volts.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #211 on: November 10, 2024, 02:34:23 am »
...

Mark, could you please check your AM503B and count the electrolytics? Mine has three 220uF caps, but I want to confirm that's the correct value before I replace them (they were the only new caps present when I received the 503B).

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #212 on: November 10, 2024, 03:22:35 pm »
...

Mark, could you please check your AM503B and count the electrolytics? Mine has three 220uF caps, but I want to confirm that's the correct value before I replace them (they were the only new caps present when I received the 503B).

Thanks,
Josh
Sure!  I have:

  (3) 220uF @ 16V, Nichicon VX(M) series
  (6) 100uF @ 25V, Nichicon VX(M) series
  (2) 1000uF @ 50V, Nippon Chemi-Con SME series
  (2) 2200uF @ 50V, Nippon Chemi-Con SME series
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #213 on: November 10, 2024, 03:29:43 pm »
Sure!  I have:

  (3) 220uF @ 16V, Nichicon VX(M) series
  (6) 100uF @ 25V, Nichicon VX(M) series
  (2) 1000uF @ 50V, Nippon Chemi-Con SME series
  (2) 2200uF @ 50V, Nippon Chemi-Con SME series

Thank you! There's (7) 100uFs in mine, I think one is hiding from you. 😉

There's also a cute little 1uF near the 220uFs.

Thanks for checking that, now I can replace those last 4 caps.

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #214 on: November 10, 2024, 04:55:40 pm »
Sure!  I have:

  (3) 220uF @ 16V, Nichicon VX(M) series
  (6) 100uF @ 25V, Nichicon VX(M) series
  (2) 1000uF @ 50V, Nippon Chemi-Con SME series
  (2) 2200uF @ 50V, Nippon Chemi-Con SME series

Thank you! There's (7) 100uFs in mine, I think one is hiding from you. 😉

There's also a cute little 1uF near the 220uFs.

Thanks for checking that, now I can replace those last 4 caps.

Thanks,
Josh
You're right I missed the 1uF @ 50V near the output amplifier.

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.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #215 on: November 10, 2024, 05:07:17 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.

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
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #216 on: November 10, 2024, 05:48:12 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.

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
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.
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #217 on: November 10, 2024, 06:19:08 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.

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

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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #218 on: November 10, 2024, 06:30:47 pm »
Did you measure the old ones you had already swapped?

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #219 on: November 10, 2024, 06:46:53 pm »
Did you measure the old ones you had already swapped?

Yup:

35V 100uFOld: 1.3Ω ESRNew: 55mΩ ESR
16V 220uFOld: 450mΩ ESRNew: 45mΩ ESR
50V 1uFOld: 1.3Ω ESRNew: 3.4Ω ESR
50V 2000uFOld: 60mΩ ESRNew: 13mΩ ESR
50V 1000uFOld: 120mΩ ESRNew: 11mΩ ESR

All values measured on IM3570 after warmup and corrections.
« Last Edit: November 10, 2024, 06:53:22 pm by KungFuJosh »
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #220 on: November 10, 2024, 07:29:15 pm »
Well, this is also interesting. I replaced the final 4 caps (220uF x4 and 1uF).

Before replacing those, after degauss/zero button would show around 0.25mV on the DMM.

Now it's slightly better.

So far:

Big caps (1000uF and 2200uF) affected the stability/drift drastically. Went from ~5mV PP to 0.2mV PP.
100uF caps...nothing noticeable.
220uF and 1uF: much closer to zero after degaussing.

Now to check the stability again.

Thanks,
Josh
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #221 on: November 10, 2024, 08:24:42 pm »
The TM501 unit houses the power supply for the plug-in units.
Ultimately, the capacitors would also have to be replaced there.
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #222 on: November 10, 2024, 08:49:05 pm »
The TM501 unit houses the power supply for the plug-in units.
Ultimately, the capacitors would also have to be replaced there.

I agree, those should be replaced too. I'll get there eventually. 😉
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Offline nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #223 on: November 10, 2024, 09:47:23 pm »
The TM501 unit houses the power supply for the plug-in units.
Ultimately, the capacitors would also have to be replaced there.
I'd do a frequency analysis (FFT) of the noise (it it isn't mains related) and try to correlate that with noise in the same frequency band(s) from power supply lines or other places in a circuit before attempting to make seemingly random changes to equipment. It is very possible there are external sources which cause the noise. For example: in my own lab I can see interference around 30MHz coming and going at random times.

I'd also be wary about changing capacitors for ones with a much different ESR without checking the specification for the original capacitor. Tektronix may have designed the circuit for a specific ESR range. For example: in the TDS500/600/700 series scopes there are a few electrolytic capacitors which can't be changed by any other type like ceramic or tantalum because the ESR is too different.
« Last Edit: November 10, 2024, 09:56:44 pm by nctnico »
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #224 on: November 10, 2024, 10:01:23 pm »
When the capacitor was replaced, it was more a matter of drift than noise, at least that's how I understood the posts, and that's how an improvement was achieved.
So if he's already replacing electrolytic capacitors, then he might as well replace the electrolytic capacitors of the actual supply.
If I remember correctly, all supplies in the system are linear and not switched, so the ESR question hardly arises in this respect.
However, I agree with you about other capacitors in other circuits of the system.


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