How does a DMM measure voltage, current, and resistance exactly?

[teddybear211]

I want to make a software program which emulates a DMM, but have no clue as to how the DMM actually measures these 3. I know Ohm's Law is used.

For measuring voltage I was wondering if the DMM probes have a huge input resistance? For current I was wondering if the DMM probes then change to a small input resistance. I have no clue as to how resistance is measured.. Maybe with a constant supply of current and voltage? I'm not too sure. Help please!

 

[tgootee]

Basically, they actually measure only voltage. In order to measure current, they measure the voltage across an internal "current-sensing" resistor, which is usually a low value but would probably depend on the range selected. To measure resistance, I think they apply a small voltage across the probes and measure the resulting current by measuring the voltage across a small resistor (again, the value probably depends on the range selected). Then Ohm's law is used.

Someone else will probably come along with some more-precise information. or you can look up a circuit somewhere and see exactly how it's done.