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

[Tim Ressel]

You can do worse than a Fluke.  Which one are you getting?

--TimR

On 12/10/2015 12:47 PM, Dan Snazelle wrote:
> Thanks guys
>
> I just ordered my first fluke
>
> Sent from my iPhone
>
> On Dec 10, 2015, at 2:37 PM, Jason Proctor <jason at redfish.net 
> <mailto:jason at redfish.net>> wrote:
>
>> 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 <mailto: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 <http://SnazzyFx.com>
>>     and find us on twitter, facebook and instagram
>>     for tons of my music and some snazzy fx sounds too,
>>     soundcloud.com/loss1234 <http://soundcloud.com/loss1234>
>>
>>     On Dec 10, 2015, at 11:24 AM, "Doug Forbes"
>>     <dougforbes at dslextreme.com <mailto: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
>>>>     <mailto: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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>>>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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