I use an isolation transformer in line with my scope's mains power, as Ritchie suggested. They are a cost effective protection measure which I highly recommend, for the safety of you and your equipment.<br><br><div><a href="https://overview.mail.yahoo.com/mobile/?.src=Android">Sent from Yahoo Mail on Android</a></div> <br> <blockquote style="margin: 0 0 20px 0;font-family:Roboto, sans-serif; color:#6D00F6;"> <header> <div>On Thu, Dec 10, 2015 at 11:21 AM, Doug Forbes</div><div><dougforbes@dslextreme.com> wrote:</div> </header> <div style="padding: 10px 0 0 20px; margin: 10px 0 0 0; border-left: 1px solid #6D00F6; color: initial;">
<div class="moz-cite-prefix"><font face="Arial">How about just get a
digital multimeter from Harbor Freight for $6 or for free when
they have a coupon ?</font><br clear="none">
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<div class="moz-signature"> <a rel="nofollow" shape="rect" target="_blank" href="http://www.harborfreight.com/7-function-digital-multimeter-90899.html">http://www.harborfreight.com/7-function-digital-multimeter-90899.html</a><br clear="none">
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On 12/10/2015 8:07 AM, Richie Burnett wrote:<br clear="none">
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<blockquote type="cite">You need to be careful with grounding if you use a
current sensing resistor and an oscilloscope to measure current
draw from a power supply. For instance, if both the power supply
and oscilloscope are grounded to the mains supply earth, and you
put the current-shunt resistor in the V+ feed (high-side sensing)
to measure the current there, then you can't just connect one
oscilloscope probe across the resistor to measure the voltage
drop. Connecting the scope's ground clip will short the V+ supply
to ground through the scope, possibly damaging either the scope or
power supply!
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The solutions are:
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1. Power the scope from an isolation transformer.
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2. Use a battery powered "Scopemeter" that isn't earthed.
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3. Run the power supply from an isolation transformer with it's
earth floated.
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4. Use a Class-II power supply that isn't earthed.
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5. Use two scope probes to measure the voltage drop in (Y1-Y2)
differential measurement mode.
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6. Move the sense resistor into the 0v line of the power supply
and connect the scope probe ground lead to the PSU ground side.
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Note, method 6 is only acceptable if the current flowing in the 0v
line is the same as the current you wanted to measure. This is
fine for something like a single output +12V power supply, but
measuring the ground current won't work for a multi-output
+5V/-12V/+12V power supply where all of the wires will likely
carry different currents.
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Hope this helps,
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-Richie,
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-----Original Message----- From: Vladimir Pantelic
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Sent: Thursday, December 10, 2015 2:58 PM
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To: <a rel="nofollow" shape="rect" class="moz-txt-link-abbreviated" ymailto="mailto:synth-diy@dropmix.xs4all.nl" target="_blank" href="javascript:return">synth-diy@dropmix.xs4all.nl</a>
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Subject: Re: [sdiy] measuring dc current max/draw of psu with a
scope or multimeter??
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On 10.12.2015 15:48, Dan Snazelle wrote:
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<blockquote type="cite">im trying to figure out if this is
possible witn regular tools
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i know i could buy a dc ammeter but those are expensive
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ive also read about current probes but some of these seem to
saturate at 500ma dc
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so
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i figured there must be a way to test current with a resistor
and voltage
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any ideas??
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</blockquote>
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if you put a 0.1 Ohm resistor in-line with the PSU, then a current
of 1A
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will give you a voltage of 0.1V that you can read/measure with the
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scope. if the voltage drop is too high, you can go to a lower
resistance
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as long as you can reliably measure the voltage. make sure the
resistor
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has a tight tolerance e.g. use a precision shunt.
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