I'm trying to calculate a voltage in a circuit, and having the problem that I can't quite remember simple circuit theory.
I have a circuit that boils down to something like this:
+-C-R1--+--R2--+ | | | AC R3 DC | | | | RL | | | | +-------+------+where AC = an AC frequency source of frequency F, DC = a DC power supply and RL is a complex semiconductor load (where I know the current at a given value of DC, but in general it'll be nonlinear).
Reactance of capacitor C at F is approximately zero - it's simply a DC blocking capacitor. So everything is in phase, more or less.
Now I'm trying to work out the theoretical AC voltage across R3 due to the AC signal. That's no problem for the lefthand loop (I can assume the AC reactance of RL is zero). What I can't quite remember is what to do for the DC source. In DC analysis, voltage sources are treated as having infinite resistance. But what happens in AC analysis? I thought that they were treated as having zero AC resistance (low impedance, hence short circuit). But I'm not quite sure if I remember this right.
So if the impedance of the righthand loop as seen across the midpoints is given by (R3+RL) // (R2+R(DC)), what's a reasonable value to assume for R(DC) - zero or infinity?
What sort of reactance do DC voltage sources have in reality? I'd have thought fairly low, given all those smoothing caps floating around.
Thanks Theo
PS This isn't a homework question, this is the kind of thing you do in school and then never use again. So I've forgotten the vital detail.