[SI-LIST] Re: Current Sense Resistor Measurement

  • From: "Barry Rowland" <bfryvr@xxxxxxxxx>
  • To: ERIK KUNDRO <kundro85@xxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Wed, 30 Nov 2011 08:32:04 +0200

I do this all the time, but do not recommend using a series resistance, because 
it influences the dynamics of the DC/DC ... Beg, borrow, rent a TCP0030 current 
probe (120 MHz 30 A max )!

Barry Rowland

-----Original Message-----
From: ERIK KUNDRO
Sent:  30/11/2011, 05:05 
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Current Sense Resistor Measurement


I am trying to measure the current going into a DC-DC converter by removing a 
series fuse and placing a small value resistor in its place. I want to make the 
measurement using an oscilloscope so I can get a plot of the actual current 
waveform, not just an rms number like with a DMM (I need info like peak current 
and the current waveform shape).
I'm measuring in system, so I'm in a tight space and single ended type probes 
are too large. The input to the DC-DC is powered by 12V, but all the 
differential probes I see do not have a common mode spec anywhere near 12V 
(most seem to have a max common mode of about 3V). So it doesn't look like I 
can use a differential probe to measure across the resistor since it sits on a 
12V line.

I tried using two coax cables, each with one end cut and soldered from power to 
ground on my PCB on both sides of the resistor. Then I used the scopes math 
function to subtract one channel from another to obtain a current waveform. But 
for some reason the coax cables seem to be causing some ringing or resonance 
like oscillation that is greatly distorting the measurement.

I am running out of ideas here... anyone make a measurement like this before? 
Any thoughts or suggestions? I'm getting desperate here. Seems like it should 
be an easy measurement to make, but I can't seem to be able to do it.






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