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

[Chris Juried]

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.

Sent from Yahoo Mail on Android 
 
  On Thu, Dec 10, 2015 at 11:21 AM, Doug Forbes<dougforbes at dslextreme.com> wrote:   How about just get a digital multimeter from Harbor Freight for $6 or for free when they have a coupon ?
  
    http://www.harborfreight.com/7-function-digital-multimeter-90899.html
 
 
 
     On 12/10/2015 8:07 AM, Richie Burnett wrote:
  
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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