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

[Doug Forbes]

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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