[SI-LIST] Re: Current Sense Resistor Measurement
- From: ERIK KUNDRO <kundro85@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, steve weir <weirsi@xxxxxxxxxx>
- Date: Tue, 29 Nov 2011 20:00:42 -0800 (PST)
Steve,
Thanks for the quick response. My only issue with your suggestion is that by
adding DC blocking caps I am removing the steady state value of the current and
only viewing the AC/transient portions of the waveform. I need to know both
steady and transient currents.
So for example, if my circuit draws a steady 4A but has a 300mA fluctuation
about that 4A... all I would get on the scope is the 300mA fluctuation waveform
and not be aware of the 4A continuous current since the DC blocking caps would
remove it. Basically I would see a peak current of 300mA in this example, when
really the peak current is 4.3A. Correct? or did I misunderstand your
suggestion?
Erik M. Kundro
--- On Tue, 11/29/11, steve weir <weirsi@xxxxxxxxxx> wrote:
From: steve weir <weirsi@xxxxxxxxxx>
Subject: [SI-LIST] Re: Current Sense Resistor Measurement
To: si-list@xxxxxxxxxxxxx
Date: Tuesday, November 29, 2011, 9:33 PM
If it is not too late to alter your circuit board, then you can put down
DC blocking capacitors on either side of your sense and terminate those
to common. Then you can use just about any differential probe around.
The only limitation will be on the low frequency end. You can jig such
a thing on a small circuit board as well.
With straight coax, you should use the 50 Ohm input setting on your
scope and DC blocks. Unless you are mechanically gifted, Tek P6150A, or
Agilent 54006 probes are a lot better than what you can put together
yourself.
Steve.
On 11/29/2011 7:03 PM, ERIK KUNDRO wrote:
> 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
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