Connect with us

Elementary AC circuit analysis

Discussion in 'Electronic Basics' started by Theo Markettos, May 18, 2009.

Scroll to continue with content
  1. 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:

    | | |
    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.


    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.
  2. Thanks. That's what I thought I'd worked out from first principles, but my
    brain was having a day off :)

  3. Guest

    Cool circuit, If I make RL an LED it looks like a simple modulation
    (in which case I'd like to add a bit of inductance to R2. No sense
    wiggling the battery around at AC.)

    ">In DC analysis, voltage sources are treated as having infinite
    Opps, DC or AC ideal voltage sources have zero impedance.

    George Herold
  4. Jasen Betts

    Jasen Betts Guest

    Or you could replace R2 with a current source if the inductance needed
    proves too bulky.
  5. Well spotted, that's exactly what I'm doing! (but at higher frequencies)
    That's an interesting idea... 'wiggling the battery around' was indeed what
    I was trying to avoid, especially with power supplies that are chock full of
    smoothing capacitors.
    That was my brain fart. And the more I stared at it the more my head
    started going round in circles :)

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day