Measure DC current with near field probe


I have a failed board, with high current running into a 100 pin micro. I just don’t know which pin

I do not have access to the source code, otherwise I could single step and find the problem

For ac one can use a near field probe to find B field from current in a trace

I would like to have a similar product for DC. Does that excist, except for botched together hall effect sensor?



I don’t recall anything. A standard clamp-on current probe (I have a MicSig CP2100A) is likely not going to work for you. Nothing to clamp around.

What about magnetic field viewing film? See this blog. Not sure about how sensitive they are…

If it’s high enough current, you can also consider a thermal camera, but heat spread also makes it challenging…


Would it damage the micro to cycle the power relatively frequently so you can sense the magnetic field change with the step in current? From my vague awareness of power glitching attacks in reverse engineering, I get the impression they are fairly resilient to this kind of repeated abuse.

A Hall sensor might be too insensitive. Maybe a magnetoresistive sensor? has a 2x2mm package that might approximate to a probe. I made a temperature sensor like this and a long thin PCB works fine as a handle/probe with a small package like this at one end.

I had picked up this I-Prober 520 - I-prober 520 Positional current probe, PCB track Touch and Measure | Aim-TTi

It sounds like it might be what you need for this, to be honest I have only done some quick measurements with it. I see there is an EEVBlog review maybe has more details (another untested recommendation, just saw it came up in search results).

1 Like

Looking at that totally reminded me. I have the Little Bee B1, but I have never really put it to use. The CrowdSupply page compares it to the i-Prober 520. Unfortunately, it’s showing no longer available on CrowdSupply and Mouser. Not sure if you can still find it anywhere…

I did test thermal, and there is no Hotspot of the micro

1 Like

I thought of that too, but thought the decoupling cap would be exercised instead. But it could work now that you mention it also :smirk:

Great, exactly what I needed. Thanks :blush:

I found the I prober at a Danish website on stock, will be interesting to try it out

1 Like

This is my first suggestion as well, but unfortunately Weston could only get 3k of the core AMR part he used. It’s still available, but only at an MOQ of like 25k parts or something absurd, so he’s expecting never to produce more. I guess the i-prober is probably the right call.

That said, my usual go-to is thermal. Of course, you say you already tried it and couldn’t find any hotter-than-average traces, so maybe not.

1 Like

How much current are you speaking of? Is the pin acting as on input or output? Also, are you sure that only one pin is affected?

I was a failure analysis engineer for the majority of my career; I’m happy to help you troubleshoot the issue if you’d like a hand.

Thanks for helping out :wink:
I am not sure at all about the single pin. In fact, I took a picture with the DYTSpectrumOwl (great and cheap thermography, DytSpectrumOwl PCB Inspection Thermal Camera | Voltlog 384 – VoltLog):


It is a Cortex M3 micro, and it looks like the current enters the device from the 4 corner VCC/GND connections (faint thermal line seen in each corner)

So right now I am guessing it is a higher current due to a floating pin. So will try to connect each pin in turn to VCC with a 1k resistor and see if the power dissipation is reduced

This board is manufactored at a EMS, but the soldering is bad:


I would like to find the fault before re-soldering all pins, which could cause a short and a dead device

Some follow-up questions:

  1. Do you have a schematic to see which pins are supposed to be floating and which are supposed to be connected?

  2. Do you have a “known good” module that you can use as a reference (thermal, power signature, etc)?

  3. Which Cortex 3 microcontroller are you using? I can review the datasheet to see how power is routed and to see which pins are not to be left floating.

It’s a STM32 Cortex, and sorry, I am not allowed to share the schematics ;-(

I did use a current probe on the 3.3V VCC, and found that after 0.2ms a large current is running:

So, I have asked the SW guy to single step and find the source of the current that way. Then I can isolate that circuit on the board

Sometimes you can use a millivolt scale voltmeter for DC or low voltage oscilloscope range and look for a voltage drop across a trace or at a pin.


Interesting idea. Or if it’s steady state DC, just put the DMM positive on the rail and positive on the top of the VCC pin to see relatively how much drop across the PDN there is on each one.

I’d be willing to bet that’s just the shape of the leadframe conducting more heat out of the core:

I think it’s pretty common for leadframes to have stubs of metal out to the corners for mechanical handling during manufacture, and if so, those would be the most thermally coupled to the core.