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

[Dan Snazelle]

The 115 as recommended ;) 

Sent from my iPhone

> On Dec 10, 2015, at 5:03 PM, Tim Ressel <timr at circuitabbey.com> wrote:
> 
> 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> 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> 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
>>>> and find us on twitter, facebook and instagram
>>>> for tons of my music and some snazzy fx sounds too, 
>>>> soundcloud.com/loss1234
>>>> 
>>>> 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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> 
> -- 
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
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