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A failing converter unit.

  1. Aug 29, 2011 #1
    Hi all, i'm trying to build a transformerless converter unit, that will take an input of 240v a.c, and will output 12v d.c. I used a voltage divider to get a 16v from the a.c supply of 240v and connected my rectifying circuit across the 16v. I got an output of 14.59v. But on connecting the capacitor, the voltage went up to 22v. WHY? Another thing is that, on connecting the zener diode, i was unable to get any reading. I'm confused.
  2. jcsd
  3. Aug 29, 2011 #2
    14.59 v is the rms value that you measured without a filter, since there is a ripple in the signal.

    After you place a capacitor, the average dc voltage increases to almost the peak value, then v = sqrt(2)*14.59 = 20 v

    Zener diode should be connected between the output voltage and the ground, show your connection.
  4. Aug 29, 2011 #3


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    Staff: Mentor

    Welcome to the PF.

    What you are doing sounds quite dangerous. Your are working with 240Vac, non-isolated, and you do not seem to understand what you are doing.

    Can you please tell us about your background and training? And why are you building this circuit? What safety measures are you building into this AC Mains powered, non-isolated power supply? What is the application?
  5. Aug 29, 2011 #4
    i'm a student. A beginner. I want to use the circuit to power a relay, part of an experiment.
  6. Aug 29, 2011 #5
    You really need an oscilloscope. Add one part at a time, checking your waveforms with each addition. Isolation transformers are always good because it keeps 240VAC from "electricfying" the case of the converter you are building. 240VAC both leads are typically "hot."
  7. Aug 29, 2011 #6


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    Staff: Mentor

    This is a very dangerous circuit for you to be trying to build as a beginner. As dink says, you really need an isolation transformer to keep your output circuit more safe. Why do you want to use a non-isolated power supply? Can't you just use a regular power supply to power your relay?

    Do you have a faculty advisor overseeing this work? Is he/she familiar with AC Mains safety?
  8. Aug 29, 2011 #7
    If you go to measure with a scope, don't forget that most scopes have their signal grounds connected to protective earth - touching a live circuit can do very bad things.

    I'm with Berkeman though - you don't sound very experienced. If you absolutely don't need a non-isolated power supply, you could just use a wall-wart to get your 12V until you can have somebody experienced help guide you. Don't take this as a personal attack, we all have to start somewhere.
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