Electronics > Projects, Designs, and Technical Stuff
DIY DC Current Probe?
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Jeroen3:
During assembly of another one I discovered the choice of capacitor is essential to output quality of the supply. Is it strange to put a series resistor of a few ohm before the output caps to adjust ESR? I a currently down to the background noise of my scope, or I am probing wrong. I think it's nice!

Meanwhile I also added idiot protection on the input. Due to using a non-standard USB to DC barrel cable, someone will eventually plug in a 12V supply, or a reverse polarized one.

Now, the TPS65131 has absolute maximum Vin of -0.3 to 6V, this means a typical TVS and reverse polarity diode won't do it. Instead, I found this NCP361 which is an USB protector P-channel fet that switched off above 5.6V and has overcurrent protection. Although I don't need overcurrent, it's a nice bonus (especially the soft start).
I've also added an N-channel reverse polarity protection fet suitable for up to -20V based on a logic level fet.



Any thoughts?
IDEngineer:

--- Quote from: Jeroen3 on November 11, 2017, 05:14:21 pm ---Is it strange to put a series resistor of a few ohm before the output caps to adjust ESR?
--- End quote ---
Not necessarily, depending upon the regulator and capacitor. Some voltage regulators depend upon a minimum ESR in the output cap. You can find datasheets that openly discuss this, and the reasons for it. This is why many regulator spec sheets actually note they may be used with ceramic capacitors - they have been designed NOT to rely on minimum ESR.
Jeroen3:
I've put my efforts so far on github!

https://github.com/Jeroen6/IdcProbeV1
Jeroen3:
Well, I was assembling and testing a better smps board layout. And I've actually needed the previous one to debug the issue of the missing -12 rail.

On V1.1 I changed the input circuitry with fuse, reverse polarity fet and NCP361 USB voltage "voltage window power switch" and overcurrent protection.
However, when assembled the -12V rail went down quickly when the output opamp was put on.

During debugging I used my V1 prototype, and behold, obvious reason. And why this thing is valuable.
1: Input voltage (from DP832)
3: Current (mV=mA)
I have some inrush problems...

Upon further investigation, the inrush problems arise from the by Sensitec recommended 47uF electrolytics on the + and -12V rail. This kept the current high for enough time that the NCP361 would trip and would not properly restore due to being stuck in perpetual inrush condition. It's a thermal trip after all.
Jeroen3:
Sensitec discontued the CMS current sensors. Back to square one.
Or if I can aqcuire some to build a few units, since the prototype has been extremely useful.
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