Electronics > Projects, Designs, and Technical Stuff
DIY DC Current Probe?
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0xdeadbeef:
BTW: found the time to quickly test my two boxes at work with a 10kHz PWM at ~12V signal driving a current through a ~3.3Ohm load. Somewhat simplified, but I didn't have the time to set up the test I wanted to do (more complex current profile, inductive load and much higher peak current).

Anyway, the signal shape looked almost identical to that of a Fluke i30s letting aside the very low current range. At the end of the active phase, the signal of the i30s dropped to a flat line while the one of my boxes showed a certain (slow) undershoot/overshoot. I'm unsure though if this was an issue of my boxes or one of the Fluke i30s. Didn't look like a bandwidth thing, probably more one of offset or sensitivity.
Besides, the signal of the i30s looked much noisier (which also made the low current range difficult to compare) but in hindsight I guess this might have been due to  different filtering settings on the scope.
Anything above the 10mA range (or so) overlapped more or less completely, so the amplitude is OK and there is no general flaw in my circuit.
Also the 1st of the current LEDs was lit or blinking depending on the PWM frequency and this worked in both directions. So I'm confident that the LED gimmick also works as planned.

I still want to perform a test with a more complex load and signal shape (+higher currents) and compare the boxes to a Fluke i30s (100kHz) and Fluke i50s (50MHz) with screenshots and all.
Still, it's already clear that the boxes work and fulfill my expectations. Even if further tests should show that there is some kind of undershoot/offset issue in the low current range, this wouldn't affect the usability for the planned purpose (to check the amplitude and shape/timing of a current profile).
0xdeadbeef:
Started testing a bit with an Electronic Load and was confused at first about getting ~0V at 100mA. Further investigation shows there is a 12mV offset on the output without any current flowing which means 120mA or ~0.5% of the 25A current range. This is a bit higher than the "typical" value of +/-0.3% given in the datasheet but still within the maximum of +/-0.8% (not taking into account an additional temperature offset).
This is a bit higher than I expected given that the datasheet defines these values for +/-15V while I'm using +/-12V. Then again, in the 100mV/div (or 1A/div) range this is not really visible and given that the currents to me measured are in the 10-20A range, this is not really an issue for me.
leonerd:

--- Quote from: Jeroen3 on September 23, 2017, 04:02:43 pm ---I found one device on Tindie that looked like what I would want. Except not quite suitable for high side.


--- End quote ---

That's mine ;)

You say "not quite suitable for high side use" - what do you mean there? The design is entirely built around having the sensing shunt resistor on high-side. This can be anywhere from 0V to 60V above the reference ground. Is that not high enough? What limits are you wanting to go to?


--- Quote from: Jeroen3 on September 23, 2017, 04:02:43 pm ---Simply put, I want to make a uCurrent with a BNC plug on it that can have some common mode voltage.

--- End quote ---

That was the exact intention of my design.
Jeroen3:
It looks to be intended to measure a devices power, and it goes in series with the power supply. You don't have high side problems there.
I would like to measure current anywhere, also in-circuit.
leonerd:
OK, so just pass the measured current between the V[in] and V[out] terminals. It will cope just fine with any input voltage from 0V up to about 60V - so it can do high-side, low-side, anything inbetween. It's really just a current shunt sensor, but arranged on a board convenient that it could measure a device's power supply as that was what I wanted it for. But it can measure whatever current really.
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