[sdiy] measuring dc current max/draw of psu with a scope or multimeter??

[Richie Burnett]

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!

The solutions are:

1. Power the scope from an isolation transformer.
2. Use a battery powered "Scopemeter" that isn't earthed.
3. Run the power supply from an isolation transformer with it's earth 
floated.
4. Use a Class-II power supply that isn't earthed.
5. Use two scope probes to measure the voltage drop in (Y1-Y2) differential 
measurement mode.
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.

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.

Hope this helps,

-Richie,



-----Original Message----- 
From: Vladimir Pantelic
Sent: Thursday, December 10, 2015 2:58 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] measuring dc current max/draw of psu with a scope or 
multimeter??

On 10.12.2015 15:48, Dan Snazelle wrote:
> im trying to figure out if this is possible witn regular tools
>
> i know i could buy a dc ammeter but those are expensive
>
> ive also read about current probes but some of these seem to saturate at 
> 500ma dc
>
> so
>
> i figured there must be a way to test current with a resistor and voltage
>
>
> any ideas??

if you put a 0.1 Ohm resistor in-line with the PSU, then a current of 1A
will give you a voltage of 0.1V that you can read/measure with the
scope. if the voltage drop is too high, you can go to a lower resistance
as long as you can reliably measure the voltage. make sure the resistor
has a tight tolerance e.g. use a precision shunt.


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