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. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu