[SI-LIST] Re: Current Sense Resistor Measurement
- From: <Wolfgang.Maichen@xxxxxxxxxxxx>
- To: <telegrapher9@xxxxxxxxx>
- Date: Wed, 30 Nov 2011 14:11:58 +0000
"Common practice" maybe, "safe to do" absolutely not. A mains powered scope is
NOT intended to be used freely floating! Doing so is highly dangerous. You
can't compare this to a handheld instrument (DMM or handheld scope) which has
fully insulated housing and is intended for such applications.
Wolfgang
From: DAVID CUTHBERT [mailto:telegrapher9@xxxxxxxxx]
Sent: Wednesday, November 30, 2011 3:07 PM
To: Maichen Wolfgang (IFAG OP BE R THA 2)
Cc: alfred1520list@xxxxxxxxx; kundro85@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Current Sense Resistor Measurement
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<mailto: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>
[mailto: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<mailto: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<mailto: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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