Basic current measurement question

[jgalak]

I have a breadboarded circuit for which I want to measure the total current draw with my DMM.  Am I right in my belief that it makes no difference whether I measure high side or low side?

There are lots of articles out there about the advantage of high side sensing, but those mostly seem to be about in-circuit measuring, and dealing with changing operations.  I don't think any of that applies to this, but am I missing something?

[danadak]

If you are measuring current thru a shunt, with a DMM, in high or low side
no difference.

But if measurement is being done that cannot handle floating measurements,
eg. high side, then specific circuitry has to be used to enable high side
measurements. Note there are chips that can handle this, such as


https://www.maximintegrated.com/en/app-notes/index.mvp/id/746


http://www.analog.com/en/analog-dialogue/articles/high-side-current-sensing.html



Regards, Dana.

[jgalak]

I'm just looking to connect my DMM in series with the circuit, via the A/mA jack.  The shunt would be the one internal to the DMM.

[rstofer]

If, for example, you used low side MOSFET switching, raising the source voltage by the amount of voltage drop in the shunt resistor will also affect the required gate voltage.  In some applications, there simply isn't enough gate voltage to allow for this.  In any event, the switching threshold moves around with the load and this can't be good.  So, that's why we use high side sensing.

[jgalak]

Ok, that makes sense.  In this case, I'm mostly just powering a few ICs from a common power rail.  The other components all either draw their current from the ICs or are things like decoupling capacitors. 

Regardless, high side is just as easy as low side, I was just trying to figure out why it would matter, since it seems symmetrical to me.

Thanks!

[w2aew]

Either will work.  Usually you sense on the high side because ground may be common to other parts of the circuit or other connected devices.  At low supply voltages, the burden voltage of the ammeter may require some consideration.  You can minimize burden by using the highest range on the ammeter that still gives you the resolution you need, or use remote sensing on the power supply side.

Here's a video I did on ammeter burden voltage:

[jgalak]

Thanks!  I've seen the video, it was quite helpful, along with Dave's on the subject.

I think I'm ok - I'm expecting a draw in the dozens-to-hundreds of milliamps, and right now mostly want to use the data to size the regulator for powering the circuit.  I'm going to oversize it a bit regardless, so high precision isn't critical, just general feel for where I'm at.

I'll probably get a uCurrent at some point, for more precise measurements.