Author Topic: Accurate Low Amp Current Probe Advice  (Read 17946 times)

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Online nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #225 on: November 10, 2024, 10:05:01 pm »
Personally I rather leave components which work OK alone. Whatever you take apart is not getting better; only thing you can do is try to minimise damage.  ;) In Dutch we have a saying which kind of translates to 'repair it until broken' (which is not a good thing).
« Last Edit: November 10, 2024, 10:08:59 pm by nctnico »
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Online Martin72

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Re: Accurate Low Amp Current Probe Advice
« Reply #226 on: November 10, 2024, 10:40:33 pm »
Quote
Personally I rather leave components which work OK alone.
I am with you, never touch a running system - normally.
We (the company) consistently replace electrolytic capacitors after 10 years in the device to ensure continued operational reliability.
However, these are also stressed accordingly.
Electrolytic capacitors dry out over time, faster or slower depending on the load/ambient temperature.
That's why I suggested taking the appropriate measurements.


Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #227 on: November 10, 2024, 10:42:58 pm »
Personally I rather leave components which work OK alone. Whatever you take apart is not getting better; only thing you can do is try to minimise damage.  ;) In Dutch we have a saying which kind of translates to 'repair it until broken' (which is not a good thing).

If it worked ok, I would have left it alone. 😉

It went from 4.5mV PP to 0.2mV PP. I'd call that an improvement. I like stability.


Electrolytic capacitors dry out over time, faster or slower depending on the load/ambient temperature.
That's why I suggested taking the appropriate measurements.

This is why I went after those caps in ~37 year old device.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #228 on: November 10, 2024, 10:44:41 pm »
PSU:
Nippon Chemi-Con
81D
18mF 16V  (1)   Maybe 81D183M016KD5D ?
4.7mF 50V (2)   
   
LS: 10mm
D: 30mm
H: 40mm

A whopping 3 caps.
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Online nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #229 on: November 10, 2024, 10:47:08 pm »
The date codes on various seem to suggest your unit was manufactered after 1995. What date does it say on the 'tested' sticker? Interestingly, I'm using the same stickers BTW.
« Last Edit: November 10, 2024, 10:49:05 pm by nctnico »
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #230 on: November 10, 2024, 10:57:51 pm »
The date codes on various seem to suggest your unit was manufactered after 1995. What date does it say on the 'tested' sticker? Interestingly, I'm using the same stickers BTW.

That's almost 30 years. I said 37 based on the copyright on the PSU board. The PSU is older than my AM503B. There's a Copyright 1993 marked on the AM503B. Neither are young. 😉
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #231 on: November 10, 2024, 11:02:58 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.
Was your measurement technique consistent in your comparison?  You had swapped in the DMM in place of the scope at some point.

I would expect the large filter caps to affect noise and not so much the slowly changing DC drift.

Quote
Is your 1uF cap bipolar?
Yes, on both units.
 
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Online nctnico

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Re: Accurate Low Amp Current Probe Advice
« Reply #232 on: November 10, 2024, 11:09:15 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.
Was your measurement technique consistent in your comparison?  You had swapped in the DMM in place of the scope at some point.

I would expect the large filter caps to affect noise and not so much the slowly changing DC drift.
I probably missed this but are there any schematics available? The only cause for slow drift I can think of where it comes to electrolytics would be leakage current. But this would mean the large capacitors would be part of some kind of (slow) servo mechanism. Maybe to cancel DC magnetic field in the probe head? The schematics should tell what is what.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #233 on: November 10, 2024, 11:09:41 pm »
Was your measurement technique consistent in your comparison?  You had swapped in the DMM in place of the scope at some point.

I would expect the large filter caps to affect noise and not so much the slowly changing DC drift.
Yeah, I left everything setup exactly the same. Nothing was moved.

The initial change from 4.5mV to 0.2mV was done using the scope. Test 3 with the DMMs was 0.236mV over a longer period (see attached, and note the names displayed for each device). The only change was the 4 larger caps in the 503B at the time.

Quote
Quote
Is your 1uF cap bipolar?
Yes, on both units.

Thanks!
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #234 on: November 10, 2024, 11:15:47 pm »
I probably missed this but are there any schematics available? The only cause for slow drift I can think of where it comes to electrolytics would be leakage current. But this would mean the large capacitors would be part of some kind of (slow) servo mechanism. Maybe to cancel DC magnetic field in the probe head? The schematics should tell what is what.

There is no known publicly released schematic for the AM503B. The AM503A might be similar enough for those parts though.
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #235 on: November 10, 2024, 11:26:35 pm »
Here's the manual for the TM502A & its PSU: https://w140.com/tekwiki/images/e/ee/070-6502-00.pdf
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #236 on: November 10, 2024, 11:32:00 pm »
For the PSU caps, I'm leaning towards these:

MAL205658472E3

LGU1E183MELB


Thanks,
Josh
« Last Edit: November 11, 2024, 04:17:02 am by KungFuJosh »
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #237 on: November 11, 2024, 12:12:21 am »
Was your measurement technique consistent in your comparison?  You had swapped in the DMM in place of the scope at some point.

I would expect the large filter caps to affect noise and not so much the slowly changing DC drift.
Yeah, I left everything setup exactly the same. Nothing was moved.

The initial change from 4.5mV to 0.2mV was done using the scope. Test 3 with the DMMs was 0.236mV over a longer period (see attached, and note the names displayed for each device). The only change was the 4 larger caps in the 503B at the time.
As I mentioned previously, the 4.5mV Pk-Pk measurement looks more like random noise to me, which is completely within reason as being corrected by the new caps.  I think it's difficult to conclude anything about slower moving, and possibly much smaller, DC drift because of the randomness of the measurement.

Perhaps the measurement selected was capturing higher frequency noise, or line-related noise not on a cycle boundary (like measuring RMS on the whole screen with only a cycle or two visible).  And all of these things depend on the sample rate and possibly capture size (don't know - I don't have a Siglent scope).

I would pick a slow sweep rate (like 100ms/div), hi-res (aka box car averaging), and no trigger (free-run).  Then ask the scope to read the RMS of the entire screen or capture buffer (whatever is possible on the Siglents).  You could also ask the scope for the statistical average of a number of RMS measurements as a different way to do averaging.

But still a DMM is a better way to measure this drift.  The DMM "before" cap replacement measurement is not available for a direct comparison.
 

Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #238 on: November 11, 2024, 12:21:50 am »
...
I probably missed this but are there any schematics available? The only cause for slow drift I can think of where it comes to electrolytics would be leakage current. But this would mean the large capacitors would be part of some kind of (slow) servo mechanism. Maybe to cancel DC magnetic field in the probe head? The schematics should tell what is what.
I'm not aware of any schematics either.  The board layout is definitely different between the A and B, but a quick glance says that some of the components are the same (processor, attenuator, output amplifier, etc.).

The four big filter caps in the A schematic are involved in supply filtering, and are located in the rear near the backplane connector as would be expected.  In the B, they are also in the same area, although the layout differs a little.  And the B uses 2200uF instead of 1000uF.  They are almost certainly performing power filtering.

My guess is that there was a lot of power line hum showing up in the output which was resolved by the new caps.
 

Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #239 on: November 11, 2024, 01:05:09 am »
As I mentioned previously, the 4.5mV Pk-Pk measurement looks more like random noise to me, which is completely within reason as being corrected by the new caps.  I think it's difficult to conclude anything about slower moving, and possibly much smaller, DC drift because of the randomness of the measurement.

Perhaps the measurement selected was capturing higher frequency noise, or line-related noise not on a cycle boundary (like measuring RMS on the whole screen with only a cycle or two visible).  And all of these things depend on the sample rate and possibly capture size (don't know - I don't have a Siglent scope).

I would pick a slow sweep rate (like 100ms/div), hi-res (aka box car averaging), and no trigger (free-run).  Then ask the scope to read the RMS of the entire screen or capture buffer (whatever is possible on the Siglents).  You could also ask the scope for the statistical average of a number of RMS measurements as a different way to do averaging.

But still a DMM is a better way to measure this drift.  The DMM "before" cap replacement measurement is not available for a direct comparison.

Regarding the scope version of the tests, nothing was changed except the caps. It could very well have been AC garbage causing the higher PP, which is why I tried that. Whether or not that's affecting the drift directly, I dunno. 4.5mV PP noise certainly could mask a much smaller drift that was hiding in there.

The only thing that makes me wonder about that though is that before changing the caps, if I waited long enough, the signal would drift right off the scope screen. 🤷
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #240 on: November 11, 2024, 04:21:52 am »
I removed and tested the 3 caps from the PSU, and...it seems pointless to replace them. They look new, I don't know if they're original or not. I assume not original, because there was some flux residue on the PCB around them.

Also, the ESR is super low. I can't get anything lower, so what's the point if they're not bad.

Does anybody else have a TM502A they can check the PSU caps in? I'm curious what's normally in there.

Thanks,
Josh
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Offline jonpaul

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Re: Accurate Low Amp Current Probe Advice
« Reply #241 on: November 11, 2024, 04:32:21 am »
KunfFu: In the lab since 1990s, we have many TM50x MF , AM503A, AM503B, AM503 and  probes, manuals.

Can you say what AC/DC measurements you make and what accuracy/resolution you expect?

Happy to have contact   via PM.

Have an absolutely fantastic day,

Jon
An Internet Dinosaur...
 
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #242 on: November 11, 2024, 05:05:07 am »
KunfFu: In the lab since 1990s, we have many TM50x MF , AM503A, AM503B, AM503 and  probes, manuals.

Can you say what AC/DC measurements you make and what accuracy/resolution you expect?

Hey Jon!

Accuracy of around 1% is fine. Resolution isn't that important; 3 or 4 digits is plenty.

Messing with this right now is mostly education/entertainment. I was going after different caps to see about improving noise and stability. Stability is pretty decent now.

I took a look at the PSU electrolytics to see if any further improvements could be made with those, but as I posted above, it doesn't look like those caps are a problem. They actually seem quite nice.

Thanks,
Josh
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Offline MarkL

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Re: Accurate Low Amp Current Probe Advice
« Reply #243 on: November 12, 2024, 02:13:41 am »
I noticed on the AM503A schematic there was a manufacturing test serial port.  On the AM503B, this serial port is connected to the GPIB option connector.  Using the infinite monkeys method, I did some poking at this port and was able to figure out most of the internal commands to control the AM503B, as would the GPIB controller card.

My goal was to find a way to read and write the cal constants without having to modify the board or start pulling chips.  If GPIB could do it, then it must be happening through this serial port.  (Hint: It's the "CC" command).  Along the way I also found most of the internal commands used by the GPIB option card to control the AM503B and it's twin, the AM5030.

There's probably other things I haven't discovered, but I found what I needed so I'm not going to invest much more time in this.  At most I will probably write a Linux bash script to make it easier to extract the cal constants for backup, and also allow a user to update the cal constants based on the procedure found in the service manual.

Below are my notes.  Comments/corrections are welcome from anyone who wants to give this a try.  I'll also post this to the TekScopes group as it may be of interest there too.

AM503B Internal Serial Port Notes
---------------------------------
11/11/2024 - MarkL @ eevblog

The serial port on the AM503B's internal processor (a P87C528),
communicates with the optional GPIB controller card, and this is where
we can connect and issue text commands and receive text responses.
It's not GPIB commands at this interface, but is closely related in
functionality, parameters, and naming.

The interface could be the same as "MFG_DATA_IN" and "MFG_DATA_OUT" on
the AM503A schematic, but I don't have a AM503A to test if the same
command set is in use there.

Some of the remaining mysteries below could probably be figured out by
eavesdropping on the internal GPIB controller header on an AM5030.  An
AM503B with the GPIB option card is the same as an AM5030.

References (all are on https://w140.com/tekwiki):

- AM 503B & AM 5030 AC/DC Current Probe Amplifiers Instruction Manual,
  publication #070-8766-05

- AM 503B, AM 5030 & A6300 Series, 067-0271-00 Verification and
  Adjustment Kit, publication #070-9352-01

- AM 503A Current Probe Amplifier Service Manual, publication
  #070-8174-01

- AM503A_Schematics.pdf, unknown


Internal GPIB Controller Connector
----------------------------------

- 2x10 header directly to the right of the processor
- Serial port is 5V TTL level (not 3.3V!)
- Header pins to serial adapter:
    16: Rx (yellow, data coming from AM503B)
    14: Tx (orange, data going to AM503B)
    12: GND (black, there are other GND pins: 2, 3, 11, 20)
    Notes: Header square pad is pin 1 (lower right)
           Colors refer to FDTI TTL-232R adapter cable
- 4800 baud, 8 bits, no parity, idle high (the usual)


Commands and Queries
--------------------

- A command is two characters followed by optional arguments.
- A query is two characters followed by a "?" and then optional arguments.
- Not all commands have a query format and visa-versa, but if they do
  the query form generally shows what was last set via the command
  form.
- All numbers are ALWAYS HEXADECIMAL, including error responses.
- Everything is ALWAYS UPPERCASE.
- Commands/queries are terminated with a single carriage return (0x0D).
- Responses are terminated with a single carriage return (0x0D).
- Query forms (xx?) below show an example response, current settings
  will vary the response.

AD?: AD (unknown meaning, appears to be constant)

AMx: Set amps per division x=[A-L] (range steps 1mA/div to 5A/div)
AM?: AMB

BWx: Set BW limiter x=[0,1], toggle if x omitted
BW?: BW1

CCrr+xxxx: Set gain cal constant rr=[00,01,02] to xxxx.
CC02?: CC02+61A8   (0x61A8 == 25000, which is the default)

COx: Set coupling x=[A,D,R] (AC, DC, Ref), toggle if x omitted
CO?: COR

DAxxx: OK (unknown effect, xxx is optional and any length)

DC+xxxxx: Set DC level to xxxxx
DC?: DC-00172  (was set to -2.3 from front panel)
DC?: DC-00162  (was set to -2.2 from front panel)
DC?: DC-00012  (was set to -0.1 from front panel)
DC?: DC+00002  (was set to  0.0 from front panel)
DC?: DC+00012  (was set to  0.1 from front panel)
DC?: DC+00142  (was set to  2.0 from front panel)
DC?: DC+00000  (after initialize)

DEx: Perform degauss operation x=(0, 1, 2):
     0 or omitted: Just like pressing front panel button
     1: Forces gain calibration (see manual)
     2: Gain calibration without balancing Hall device (see manual)

EX: Exit error mode (e.g., 266 being displayed on front panel)

FPx: Front panel lock x=[0,1], lock if x omitted

GP?: GP050  (unknown, appears to be constant)

ID?: ID3.0  (returns firmware version)

IN: Initialize

KYxx: Front panel button/knob simulation(?) xx=key to press, 00=release
      (not too sure how this is used)
KY?: KY00

OV?: OV0  (status of front panel OVERLOAD light)

PO?: PO0  (status of front panel PROBE OPEN light, returns "p" when
          open, huh?)

PT?: PTA6302  (returns probe type)

SNxxxxxxx: Set serial number to xxxxxxx, set to null if xxxxxxx omitted
SN?: SNB012345

TE?: TE000  (first error from self test, 000=no error)
TE?: TE165  (0x165 == 355 dead battery, NVRAM reset)

TMxx: Run test xx, see AM503A service manual, examples:
        00 = blink all LEDs
        02 = display firmware version on front panel

TRxxx: Set probe trim to xxx
TR?: TR42D  (was set to 069 from front panel, 0x42D == 1069)

UN?: UNA  (returns output units, A=amps, V=volts, see manual)


Some Responses Seen
-------------------

- Many of the error message appear to be analogous to the ones listed
  in the GPIB section in the manual (not surprising).

OK: Command executed successfully
KP: A front panel button was pushed or knob turned
    "KP" and other key press events appear to have the 8th bit set
PW: Power up (8th bit set)

ER065: 101: Unknown command
ER0FC: 252: AM argument out of range
ER105: 261: DE argument out of range
ER10A: 266: Auto-balance not 50 ohm terminated

 
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Offline jonpaul

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Re: Accurate Low Amp Current Probe Advice
« Reply #244 on: November 12, 2024, 04:56:49 am »
Best amp is TEK AM503B.

Look at epay, ham fleas, auctions.

TM50x MF not critical, of the dozens we have just a few were bad, mostly blown pass trans, the lytics hold up.

The AM503 are not stable and low res.

AM503A is not grat but better than AM503.

Lots of bad probes ....dropping a TEK P6302 a few cm on concrete/metal may hariline crack the ferrite jaw.

Symptom is poor LF/DC or unstable...Beware.

j
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Offline KungFuJoshTopic starter

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Re: Accurate Low Amp Current Probe Advice
« Reply #245 on: November 12, 2024, 05:30:45 am »
Best amp is TEK AM503B.

Look at epay, ham fleas, auctions.

Thanks, I already got it. I got the A6302, TM502A, and AM503B. I also have a Toolbox 016-0362-00 on the way for the other slot to store the A6302 in.
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Offline jonpaul

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Re: Accurate Low Amp Current Probe Advice
« Reply #246 on: November 12, 2024, 05:53:08 am »
Kung, perfect, cant do better.

We degauss after warmup 30 min and before each use,

just be sure to term AM503B in 50 ohm on scope input.

To CAL we use TEK  PG506, (or similar,) and wind a  10, 100T magnet wire square loop  with a 50 Ohm term R on the loop in series with 50 Ohm on the gen.

Enjoy,


Jon
An Internet Dinosaur...
 
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Online tautech

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Re: Accurate Low Amp Current Probe Advice
« Reply #247 on: November 12, 2024, 07:07:16 am »
Quick first look at the factory fresh SCP5030A 100 MHz 30A active current probe measuring the current loop output of the Power Analysis deskew fixture.
So easy to use as it's scope self sensing and results are quite satisfactory from a cold start.

Used HiRes mode to reduce trace noise......

Avid Rabid Hobbyist.
Some stuff seen @ Siglent HQ cannot be shared.
 

Offline _Wim_

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Re: Accurate Low Amp Current Probe Advice
« Reply #248 on: November 12, 2024, 03:03:53 pm »
Another possibility. Not as good as the Tektronix system, but way more compact and good enough performance for what I need: a Lecroy CP015 (alternatives are CP030, AP015 and AP030)

I run this probe without a Probus interface, just with a +-12V power supply connected directly to the probe (and a BNC 90°-adaptor to connect it to the scope). There probes can be had for 300-500€ on Ebay, less if you are lucky. When power is turned on, the probe runs a degaussing cycle. So for I have not looked into trying to send I2C commands as these protocol is not well documented (and it works good enough without).

Test was run with the Yokogawa GS200 supplying a 30mA current (to mimic test conditions above). I did not log the current of the Yokogawa, but I am sure this is completely flat (max a couple of µA variation). Probe was connected to a BM869s and logged via testcontroller software. As the output of the probe is only 100mV/A, the signal was only 2.6mV. Temperature in the room was not tightly controlled, so some drift could be temperature related of the >4 hours this test has run.
 
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Offline _Wim_

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Re: Accurate Low Amp Current Probe Advice
« Reply #249 on: November 12, 2024, 03:20:41 pm »
Here some more detail how this is connected to the scope. The damage to the sticker was how a bought it, and it also had already the cut-out where the 90°-BNC adaptor fits in. I did solder the power supply cable (and UTP cable, that fits directly to the Fluke I50 power supply I already had).
 


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