DMM Measure Current: What's Inside?

[DmytriE]

Good Morning,

I was using the Digital Multimemter (DMM) a couple days ago and was wondering what inside the DMM (Circuitry) enables it to measure the current running through my circuit.

I thought that it was a resistor with a preset resistance where the DMM measures the voltage potential across its internal resistance and then divides by the resistance to obtain a current reading. Am I on the right track in my thinking?

 

[milesyoung]

Definitely on the right track. The basic operation of a DMM for current measurement is to sample the voltage across an internal resistor (of very small resistance as to not disturb the circuit being measured) using an analog-to-digital converter.

For the cheaper ones, you manually switch in different resistors depending on the range of currents you're working with.

 

[Graniar]

May be some of old models uses resistors. But, as I know, modern ones uses Hole effect. It's easier to get more precision on current, whether than voltage.
Heard also about very expensive models, which uses Josephson junction to get more precision on current

 

[milesyoung]

I've used Hall sensors when I needed to make current measurements while galvanically isolated from the circuit in question. This usually isn't a concern in a battery powered DMM as its terminals are floating. Also, bias, linearity and bandwidth are usually a concern with the cheaper Hall sensors.

Do you have a reference for a multimeter using a Hall sensor for current sensing?

 

[Graniar]

I'm a bit confused, cause wikipedia mention resistors: http://en.wikipedia.org/wiki/Multimeter
Searfing inet gives this example for Hall's multimeters: http://www.batteriesinsight.com/index.php?option=com_content&view=article&id=9&Itemid=18

My UT70B multimeter manual says nothing about, but regarding it's precisions (100mcV,100 nA), I never thought it would uses resistors. At least 1KOhm to measure 100nA with 100mcV precision.

Right now I don't have experimental setup, so did a dummy experiment trying to get low current with low internal resistance: Inserted termocouple to mcA measurement. Even burning it with the lighter didn't give any sense (but this made 30mV for voltmeter).
What a c*p, paid $200 for such a b***t!
Long time ago I rely some experiments on it, assuming it wouldn't have such big internal resistance. Should be checking equipment first.

Thank you for a tip.

 

[milesyoung]

As you say, for measuring currents that are that low in magnitude, you usually do not use a shunt resistor. See here for instance:

http://www.myflukestore.com/crm_uploads/fluke_multimeters_-_accurately_measuring_nanoamperes_using_the_fluke_8808a_multimeter_application_note.pdf

I'd suspect, though, that you would find it much more difficult measuring current accurately in that range using a Hall sensor.

That 'Penta Multimeter' is designed for reading external current sensors (as far as I can tell) and is not your typical DMM.