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

[Jason Proctor]

fwiw, my Fluke 115 has a special terminal for measuring up to 10A.



On Thu, Dec 10, 2015 at 11:30 AM, Dan Snazelle <subjectivity at hotmail.com>
wrote:

> thanks for all the advice
>
> most multimeters cant measure above 1amp dc correct?
>
> i own a few and they only go up to 1amp
>
>
>
> Dan Snazelle
>
> SnazzyFx.com
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> On Dec 10, 2015, at 11:24 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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