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Transformerless AC to DC power supply

  1. Dec 26, 2008 #1
    Help!! I was trying to build an AC to DC power supply circuit without using transformers. I followed the circuit diagrams given online, which has the 240VAC connected to a resistor at the live and a full wave rectifier whose output was connected to a capacitor and a zener diode. The figure is given in the attachment, everything i use is the same except that the resistor R1 is connected to the live and the value is 2.2k.

    The output waveform i got was a square wave, not a DC straight line as expected (or close to a straight line). I'm wondering how to eliminate the square wave or how to smooth the square into a close-to-straight line.

    Thank you..
  2. jcsd
  3. Dec 26, 2008 #2


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    The capacitor smooths the square wave - whats the circuit?
    The transformer doesn't do anything to change AC-DC it just reduces the voltage.
    The normal design is to reduce the AC voltage to near the final DC voltage with a transformer and then rectify it with diodes.

    PLEASE be careful with 240V ac - it can be dangerous (in fact it can kill !)
  4. Dec 26, 2008 #3
    I'm sorry the attchment is given here...

    Attached Files:

  5. Dec 26, 2008 #4
    The output waveform looks as if the bridge was only half wave rectified. I checked the diodes, they are fine.
  6. Dec 26, 2008 #5


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    The zener gives you square pulses, the size of the capacitor sets the width
    See http://www.allaboutcircuits.com/vol_3/chpt_3/4.html (and the next few pages) for values
  7. Dec 26, 2008 #6
    but we cannot get rid of the zener?
  8. Dec 26, 2008 #7
    is there any way of removing the square wave?
  9. Dec 26, 2008 #8
    You do realize that the entire circuit it hot, right? For all intents and purposes it's at 240 VAC.

    1) Make sure the AC side goes to the squiggely lines of the bridge rectifier.
    2) Is your capacitor connected upside down?
    3) Foremost,Measure the voltage across the capacitor. Connect both leads of your oscilliscope to the capacitor. This means floating your scope, or you will throw some sparks or burn something out. The ground lead of the oscilliscope is going to be hot. The ground lead on the scope is disconnected from earth ground with a cheater. This means your scope is now at 240 VAC. Good luck, and don't fry yourself. Make sure the scope chassis doesn't touch ground somewhere.

    Now get an electrical technician because you're in over your head.

    Need I say, don't handle the scope, 'cept the plastic buttons, until you've unplugged your project?

    A battery scope would be nice to have.
    Last edited: Dec 26, 2008
  10. Dec 26, 2008 #9
    hmm. Reread my post. I'm afraid I added a great deal to it since you posted.
  11. Dec 27, 2008 #10
    Very dangerous..
  12. Dec 27, 2008 #11
    First this is dangerous stuff, as all have said. A cheap transformer should be worth the money for playing it safe.

    If you must not use a transformer, then I guess this could do the trick for you:

    - Prevent the capacitor from discharging into the zener diode. Put a normal diode
    between zener and the capacitor, diode's cathode at the capacitor side.
    - Put an inductor in series with the (+) and then
    - shunt with a high resistor between the (+) and the (-)
    the idea being prevent the capacitor from discharging quickly.

    You can choose any large inductor, larger the better, and a large resistor.
    Here is the diagram.


    What I have described is a passive Low Pass filter (aka the PI filter, as the shape is a PI)

    You may also throw in another large capacitor in parallel with the high resistor.

    Hope that helps, and be very careful.....

    Last edited: Dec 27, 2008
  13. Dec 27, 2008 #12


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    Why are you doing this? As others have mentioned, it is extremely dangerous to work with this kind of a circuit when you do not know what you are doing. What is your end goal?
  14. Dec 27, 2008 #13
    wow, this is a really bad idea. the "ground" will be all over the place and may shock the **** out of you. the line side resistor will be working as a voltage divider, and so you can't just willy-nilly cut it in half without consequence, and even if left as given in the circuit you found, it's value depends on the load from your rectified circuit. at the very least, you'd want to run a circuit like this on an isolation transformer if you planned to probe it or connect it to anything else. seriously, just stop what you're doing.
  15. Dec 27, 2008 #14
    I concur. 240 VAC is especially dangerous once converted to DC, which is known for its ability to deliver a "death grip".
  16. Dec 28, 2008 #15
  17. Dec 31, 2008 #16
    Hey guys Thank you all.. I'm stopping this project now. Will continue using a transformer when i can get my hands on one.

    Thanks again!!!
  18. Dec 31, 2008 #17
    that's good. something else i meant to ask about. is this in the UK and is the 240V neutral actually referenced back to earth ground? because if so, i don't think moving the resistor to the neutral leg was a good idea.
  19. Dec 31, 2008 #18
    The neutral is earthed at the last transfomer and maybe at other points on its way to your house.

    Old fashioned TVs used a single diode rectifier, direct rectified mains... would be considered rather dangerous these days. If the mains was connected one way round the chassis could be live.
  20. Dec 31, 2008 #19
    that makes sense, then. it would move the rectified ground of that circuit just a diode drop above earth with the resistor on the line leg. but the improper wiring scenario is a good point, too. another reason to avoid a circuit like this.

    edit: 240V here in the US is a different beast. we get 240V off the pole, but it's from a center-tapped transformer with the center tap referenced to ground, so what you actually get are two 120V lines 180 degrees out of phase. and if you do use both to power something with 240V, then neither leg is a ground-referenced neutral.
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