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Cockroft Walton: Full Wave vs. Half wave?

  1. May 4, 2010 #1
    Hello. For a school project I am building a Cockroft-Walton voltage multiplier designed to be run on 120V conventional household current.

    I am aware that the output is DC, but does the waveform have any peculiarities if I run the multiplier on sinusoidal AC? It seems like I would only get a linear output DC voltage if I fed it a square wave.

    Also, what advantages are there to making a full wave circuit instead of half wave? My only objective is to make a really pretty spark, and I am curious what benefit the full wave version brings.

    Thank you very much in advance.
  2. jcsd
  3. May 4, 2010 #2


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    Staff: Mentor

    Two comments. First, yes, a square wave drive will be more efficient (I don't know about more "linear"):


    Second, there are a bunch of safety issues (both shock and fire) that need to be addressed in a project like this. Do you have an experienced advisor for this project? Are safety issues addressed in your project proposal write-up? Could you post that section of your proposal here please?

    Are you familiar with the UL rules for working with 120Vrms AC Mains voltage supplies? Which ones apply to this project of yours?
  4. May 4, 2010 #3
    Dear Berkeman:

    I can tell you are courting this thread for possibly being locked. Please note that I am in no way asking for advice as to how to specifically create this device, nor am I in anyway advocating this project or explaining how to do it.

    That said, I will freely admit that I do not have the UL rules memorized. I do, however, have substantial experience working with mains current, and I am fully aware of the safety precautions necessary. My supervisor is more than qualified to supervise this project.

    My question is purely theoretical: What does the output DC waveform look like, and what is the difference that a full wave design makes? I would appreciate an answer.
  5. May 4, 2010 #4


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    Staff: Mentor

    Well, I think you slipped up a bit and admitted that you do intend to try to build it:

    On your main question, though, I'm only aware of one C-W multiplier topology (there could be two). Could you post a link to the two topologies? And are there no links with the output waveforms? The link I posted briefly discusses output ripple, but is not very complete. I didn't look to see if there are links out of that wikipedia article that might have more info. The only time I've used a C-W multiplier was for a He-Ne laser tube, many years ago. And yes, it was built to UL standards (but not approved obviously, since it was a 1-off project for me).
  6. May 4, 2010 #5
    with no load, your output voltage will be pure DC, regardless of weather you use a sine wave or square wave source. Also if you are looking to make sparks with it, I would suggest not using as low a voltage as mains. A better solution would be to use the flyback transformer out of an old tv (the AC type) otherwise you are looking at dozens of stages to create the voltage needed to break down air. This is also a safer solution dispite the higher input voltage for a few reasons: 1: TV flyback transformers dont produce very much current, unlike mains voltage. 2: Your circuit is isolated from mains power which means there is less chance of getting a HV to ground shock 3:The flyback transformer operates at high frequency which means you can get away using much smaller capacitors in your circuit, which translates into much less stored energy and a safer circuit. Be careful and if you are not 100% clear with the safety precautions that should be exercised with this circuit, don't build it.
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