Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Circuit problem

  1. Nov 5, 2011 #1
    1. The problem statement, all variables and given/known data
    Here is the problem: http://img507.imageshack.us/img507/6488/unled2b.jpg [Broken]

    What is the open circuit output voltage, Vout?
    Estimate the short circuit output current

    Can someone explain to me how to do these.
    2. Relevant equations

    Honestly... no idea. I know V=IR and all that but it doesn't seem to help here.

    3. The attempt at a solution
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Nov 5, 2011 #2


    User Avatar

    Staff: Mentor

    You'll have to look up the characteristics of the LT1084C regulator to see how it operates. Perhaps download a datasheet.

    The transistor Q1 looks like it might be for overload (short circuit?) protection. Its base bias resistors are in the path of the load current (returning back to the bridge rectifier). After some load current threshold it's going to turn on and pull the regulator's Adj pin low, dropping the output voltage. You should probably look into what load current will cause the transistor to turn on and start to pull the output voltage down.
  4. Nov 6, 2011 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    For OC output voltage Vout, all you need is the values of R1, R2 and the mfr's spec on the relationship between Vout and the voltage at the ADJ pin.

    For SC current, I'd look at the spec sheet for that info.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook