Hey all,
I've taken apart a few of my DMM's in trying to understand how they are measuring AC current, and scratching my head.
I see that they all seem to take the same approach using a shunt resistor (Constantan 0.1ohm). Looking at my Extech EX330 I see a TLC27M2C op amp that looks like its used for measuring the voltage drop on the shunt resistor. There are some other components before it gets there and I can't identify them all (Q4-Q7) so I don't know what they are doing before it hits the op amp.
The op amp has 5.9VDC at VCC, and I measured the default output of the op amp (on both outputs) and it's 1.5 VDC. When I run a 1amp AC current through it there is no change in output yet the meter is clearly measuring 1 amp. Perhaps this op-amp doesn't really deal with the current measurement but I can't see anything else that obviously does before it enters the MCU. And if it does, I don't understand how it works as the common mode voltage range is only 0.2-3.0V on that op amp and I'm measuring 120VAC. Because its a floating power supply it doesn't matter?
Anyone with more knowledge on the subject? I only half understand it.