[SI-LIST] Re: Current Sense Resistor Measurement
- From: DAVID CUTHBERT <telegrapher9@xxxxxxxxx>
- To: Wolfgang.Maichen@xxxxxxxxxxxx
- Date: Wed, 30 Nov 2011 07:06:37 -0700
It is a very common practice to float an oscilloscope using an isolation
transformer or even by removing the AC power cord ground prong. And of
course a battery powered oscilloscope is even better.
Dave C
On Wed, Nov 30, 2011 at 1:21 AM, <Wolfgang.Maichen@xxxxxxxxxxxx> wrote:
> IMHO, this is an extremely risky suggestion, and I would strongly caution
> against ever using this approach. If anything goes wrong the whole scope
> and anything connected to it becomes a potential death trap. There is a
> good reason all scopes use a three-prong plug with ground to make sure the
> chassis can never be far from earth potential. The scope is a piece of
> equipment that is connected to line voltage so the danger persists
> irrespective of the voltage level you want to probe. The correct way of
> doing the measurement has been outlined by others - use DC blocking caps,
> use a differential probe with sufficiently high common mode capability, use
> a contact-less current probe, one could also build resistive voltage
> dividers to reduce common mode to a measurable size, and so on. But NEVER
> circumvent crucial safety precautions as a quick fix!
>
> My 2 cents
>
> Wolfgang
>
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Alfred Lee
> Sent: Wednesday, November 30, 2011 6:11 AM
> To: ERIK KUNDRO; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Current Sense Resistor Measurement
>
> With appropriate precautions, float either your scope or your DUT, then
> double check to be sure you can safely connect the ground of the scope to
> the high side of your supply and make the measurement? Watch out for
> unexpected high voltage on floated equipment! All warnings and disclaimers
> applied:)
> Regards,
> Alfred
>
> --
> Sent from my Android phone with K-9 Mail. Please excuse my brevity.
>
> ERIK KUNDRO <kundro85@xxxxxxxxxxxxx> 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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