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I built myself a power monitor a few years ago that works and I was trying to simplify mine by cutting out an expensive and hard to find 12V ac:ac adapter. I am familiar with a voltage divider and it's operation makes perfect sense to me in the context of a DC circuit. Ie the voltage drop across elements is proportional to their resistance vs the series resistance. But I realized that I actually built a AC voltage divider (a 10:1 transformer with a voltage divider). I was surprised when I looked up an AC voltage divider and found it is exactly the same for resistive dividers.

In my head the voltage comes from one direction in an DC circuit. It starts high (we'll say 12V) and drops say 10V as it runs through the first resistor (10kohm) and drops the last 2 volts as it goes through the last resistor (2kohm). But in an AC circuit, as the phase goes past 180 degrees the voltage on the 2kohm side would now go to 12V (lets just talk absolute amplitude to keep it simple), so I would expect that it would drop 2V and the voltage divider to measure 10V now, and the last 10V to drop as it goes across the 10kohm resistor. The one on my power monitor makes sense to me because I add a DC bias which prevents it from ever changing directions, but none of the online resources I have seen indicate that that is necessary.

Can anyone explain to me why the direction of current flow doesn't matter for the AC voltage divider? I mean if I flip the polarity on a DC voltage divider, it makes a different, so why not for an AC divider? Or does it make a difference and I am just missing something in my search online, like they are only considering voltage at 90deg or something like that?

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Resistive voltage dividers divide voltage. It does not matter what the voltage is or does it change, it is always proportional to the instantaneous voltage. And AC voltage is nothing more than voltage than changes and if you stop the time at some instant you can view it as DC.

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  • \$\begingroup\$ Using circuit lab I realized that the ground point makes the difference. With a DC circuit, your ground point is always the negative terminal so when you flip the battery around, you are always working with positive voltage. But when you introduce AC into the DC circuit, you can get negative voltage and It seems negative voltage does not behave like positive voltage. Still not intuitive to me, but this does make it seem important to me that I need to have an isolating transformer in the middle of my voltage divider. \$\endgroup\$ – jeffpkamp Jun 12 '20 at 19:26

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