[SI-LIST] Re: Current Sense Resistor Measurement
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 29 Nov 2011 22:49:48 -0800
The Tek TCP current probes are nice enough to warrant their considerable
price. The wire loop to go through the probe introduces up to many nH.
That is often much more intrusive than a 10mOhm current sense resistor
that if mounted decently will be in the 1nH area.
Steve.
On 11/29/2011 10:32 PM, Barry Rowland wrote:
> 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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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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