Author Topic: Accurate Low Amp Current Probe Advice  (Read 17960 times)

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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #150 on: November 03, 2024, 05:46:46 pm »


Short clip about initial conditions and degaussing the probe.

 
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #151 on: November 03, 2024, 05:53:53 pm »


Various loading situations with the electrical DC load.
You can clearly see the influence of the load's ripple, which naturally decreases as the current increases.
 
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #152 on: November 03, 2024, 06:57:23 pm »
I had already done current measurements with normal resistances:
https://www.eevblog.com/forum/testgear/accurate-low-amp-current-probe-advice/msg5692771/#msg5692771

I'll have to revive my SDL1000X thread soon... ;)

Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #153 on: November 03, 2024, 08:53:15 pm »
These are the transmission ratios that you have to set in the channel setup under user defined probe to get correct measurement results.
The only disadvantage is that you have to set them for each current range, but you don't change that often and you have two user probes that can be set.

 
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Online tautech

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Re: Accurate Low Amp Current Probe Advice
« Reply #154 on: November 03, 2024, 09:21:39 pm »
These are the transmission ratios that you have to set in the channel setup under user defined probe to get correct measurement results.
The only disadvantage is that you have to set them for each current range, but you don't change that often and you have two user probes that can be set.


Yep, active current probe terminations are a PITA whereas a passive termination is set and forget.

Once you go down the DC capable current probe route things become somewhat more complex and harder to use compared to an current probe with plug and play scope support.
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #155 on: November 03, 2024, 09:31:39 pm »
I would have preferred an SCP5030, but it's not going to fit into my budget this year.
Probably not next year either. ;)

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #156 on: November 03, 2024, 11:53:01 pm »
I took apart the amp again and took better photos. I like the chassis design; it makes repairs easier.

Here's the electrolytic caps in the 503B:

35V 100uF    LS:5mm               x7
16V 220uF    LS:5mm               x3
50V 1uF      LS:2mm?               x1
50V 2000uF    LS:7.5mm D:16mm H:35mm  x2
50V 1000uF    LS:7.5mm D:14mm H:25mm  x2

Marked 85C

It looks like 4 of the caps were already replaced at some point.

Thanks,
Josh
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #157 on: November 04, 2024, 01:17:01 am »
Here are my test results using the ET load.

I also did screenshots with the probe not connected to anything.

Thanks,
Josh
"Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before." - Steven Wright
 
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #158 on: November 04, 2024, 08:17:24 am »
Hi,
Now do the same tests with "real" resistors..
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #159 on: November 04, 2024, 01:49:36 pm »
Hi,
Now do the same tests with "real" resistors..

What resistor values? What power source? I assume batteries, but specify which type please.

Thanks,
Josh
"Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before." - Steven Wright
 

Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #160 on: November 04, 2024, 01:54:27 pm »
Battery does not have to be.
For small currents I used a 1k resistor, for larger currents 100 Ohm, and then adjusted the voltage and current with the power supply unit to achieve the desired values.
See link in reply #152
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #161 on: November 04, 2024, 02:57:53 pm »
Speaking of batteries, the battery in my AM503B is weak (.6V, should be ~1.5V), so I removed it to see what it is (390 or SR1130SW). Removing it cleared the NVRAM and reset the amp to factory default (355 error after power on).

You had mentioned before that the AM503 normally starts up with ref coupling, but mine had always defaulted to DC, never ref on start. After reinserting the dying battery, now it starts up as you said in ref coupling. I wonder what other weird things were set. 🤷 Anyway, I ordered a new battery.

Thanks,
Josh
"Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before." - Steven Wright
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #162 on: November 04, 2024, 03:22:00 pm »
This probe has some serious drift. Note the timestamps and scale offset. This was after over 30 minutes warmup time. I'll let it warm up longer and see if it ever stabilizes at all.

Thanks,
Josh
« Last Edit: November 04, 2024, 03:25:39 pm by KungFuJosh »
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #163 on: November 04, 2024, 04:39:10 pm »
Here are those tests. They're not terrible, but none are dead on. I degaussed multiple times throughout the process to try and get it closer.

I also had one of my SDMs in circuit to verify current level was accurate within at least 1 or 2 decimal places.

Thanks for the tests, now I think I understand how the current amplifier works with regards to the scope scale. 10mA/div is 1:1 with 10mV/div on scope, and each variation changes the ratio.

This needed over an hour to get to the mediocre stability it had. I'm assuming that's more the probe's fault than the amplifiers. Am I also correct to assume the amplifier seems good, but the probe is not great?

Thanks,
Josh
"Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before." - Steven Wright
 
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #164 on: November 04, 2024, 05:13:04 pm »
If you lost your NVRAM, you also lost your gain calibration constants.

Take a look here:

  https://w140.com/tekwiki/wiki/AM503B

And specifically the documents:

  AM503B/5030, A6300 067-0271-00 Verification and Adjustment Kit Manual 070-9352-01
  AM503B/AM5030 Calibration Adapters

You might want to go through the verification procedure to at least determine if the inaccuracy is due to the amp or the probe.

For the AM503B, calibration requires temporary installation of the GPIB option card (as found in the AM5030), a TM5000 mainframe, and a computer with GPIB.  Fortunately, someone has reverse-engineered the contents of the NVRAM, and it's possible to just write the new cal values directly to the NVRAM:

  https://groups.io/g/TekScopes/files/AM503B%20Calibration%20without%20GPIB/AM503B_AM5030%20NVRAM%20Replacement_V01.pdf
 
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Online nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #165 on: November 04, 2024, 05:42:02 pm »
Now I recall why I never bought that Tektronix current clamp...  8)
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #166 on: November 04, 2024, 05:55:04 pm »
Are you kidding? The custom FRAM chip update project looks fun! I'm getting my Mouser order ready now. 😉

It's already pretty close, but if a fun project and some tweaking gets it dead on, that's pretty cool. I'm not going to spend $5000 on a probe any time soon, so this is the best option.

I think the best reason to avoid this probe is the physical size of the current amp it connects to.

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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #167 on: November 04, 2024, 06:06:46 pm »
Fortunately, someone has reverse-engineered the contents of the NVRAM, and it's possible to just write the new cal values directly to the NVRAM:

Thanks! Do you know if they shared the spreadsheet shown in the PDF? I can't find it anywhere.

Thanks,
Josh
"Right now I’m having amnesia and déjà vu at the same time. I think I’ve forgotten this before." - Steven Wright
 

Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #168 on: November 04, 2024, 06:20:16 pm »
Now I recall why I never bought that Tektronix current clamp...  8)

Because you have a functioning crystal ball. ;)
The battery of my 503A was dead too, but changing it was no problem because I had followed these instructions.

Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #169 on: November 04, 2024, 06:28:00 pm »
Fortunately, someone has reverse-engineered the contents of the NVRAM, and it's possible to just write the new cal values directly to the NVRAM:

Thanks! Do you know if they shared the spreadsheet shown in the PDF? I can't find it anywhere.

Thanks,
Josh
I haven't seen it anywhere.  However, the data in NVRAM is sparse and it looks to be only about 25 bytes.  It wouldn't take long to type it in if you're looking to copy their NVRAM.  Some of the values may already have been initialized to defaults if the battery was detected as dead.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #170 on: November 04, 2024, 06:34:16 pm »
Now I recall why I never bought that Tektronix current clamp...  8)

Because you have a functioning crystal ball. ;)
The battery of my 503A was dead too, but changing it was no problem because I had followed these instructions.

It's not the same for the 503B. My test mode maxes out at 53. 52 is to degauss the amp itself. Nothing else is really listed in the manual. The battery replacement section in the manual also doesn't say anything about tests after replacing the battery.

My tests were pretty close anyway, I don't know if it really lost any data. It's been a decade since it was calibrated according to the stickers, so I would guess nothing was lost based on that alone considering how close it was.

However, if I want to get it to be more accurate, I'm guessing the easiest way is with the NVRAM replacement game.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #171 on: November 04, 2024, 06:36:39 pm »
I haven't seen it anywhere.  However, the data in NVRAM is sparse and it looks to be only about 25 bytes.  It wouldn't take long to type it in if you're looking to copy their NVRAM.  Some of the values may already have been initialized to defaults if the battery was detected as dead.

Well, I was hoping it autogenerated the cal constant hexadecimal codes so I could be lazy about that part. 😉
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #172 on: November 04, 2024, 06:45:15 pm »
Now I recall why I never bought that Tektronix current clamp...  8)

Because you have a functioning crystal ball. ;)
The battery of my 503A was dead too, but changing it was no problem because I had followed these instructions.
The AM503A has a PCF8582A EEPROM, which is probably used to store the cal data.  The AM503B only has a PCF8570 low-power CMOS memory.

The AM503A also has a PCF8570, and I guess the procedure is copying the data between the two memory chips, but I'm not clear which one it means when it says "NVRAM".

I guess my crystal ball is not so good.  I have 2x AM503, 2x AM503B, 2x A6302, 1x A6303, and 2x TCP202.  They all have various drift, noise, and fidelity issues.  I use the AM503B the most because I like the auto balance, but it has processor clock noise in the output which can be annoying.  In those cases, I put up with the fiddly drift in the AM503.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #173 on: November 04, 2024, 09:49:08 pm »
They all have various drift, noise, and fidelity issues.  I use the AM503B the most because I like the auto balance, but it has processor clock noise in the output which can be annoying. 

Is there anything we can do to mitigate any of those issues? Currently (pun intended), the drift is the most annoying issue.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #174 on: November 04, 2024, 10:03:09 pm »
This is kinda funny. I think I scared the 503B. I removed the PCF8570P and installed a socket there, then inserted the chip. Now it's behaving a little better. 🤣

The drift is still annoying, but it seems to improve depending on warmup time and room temp. I degaussed between every value change, but not between the two 100mA tests.
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