PWM question

Discussion in 'Off-Topic' started by mines, Jan 9, 2014.

Thread Status:
Not open for further replies.
  1. mines

    Thread Starter New Member

    Feb 9, 2011
    16
    1
    Hi I am building a high current PWM for H2=O generation. I'm using a linier power supply that I built from rewiring a microwave transformer, a bridge rectifier and a couple of caps. I've got 17 volts at 30+amps. I'm not at all good at design yet. I found a nice schematic for HHO generation but have a couple of questions bugging me. Down stream of the load he is using bank of Mos-Fets for the modulation. I've built a pretty good driver using a 555 that I'm able to vary both the duty cycle and on off time. I intend marry this in to the Mos-Fets he uses to actually pulse the load. He advises the use of Schotty diodes in parallel to the Mos-Fets. My question is why are they there and how do I calculate the size of them? My text books tell me what they are but not how to apply this knowledge. [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Just use mosfets that come with the diodes in them.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    PWM works great when you want to do things like dimming lights, heat control, motor control, etc but not so well for your purpose, when it's just pure PWM. There's nothing to limit the maximum current except for the resistance of the wiring and the MOSFETs when they are turned ON, and when they are OFF, there is no electrolysis action; so with PWM you are toggling between two undesirable states.

    You really need a current-mode PWM controller with a heavy-duty inductor that keeps the current relatively constant.
     
    THE_RB likes this.
  4. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Thanks Wookie. The nuances of HHO production escape me.
     
  5. mines

    Thread Starter New Member

    Feb 9, 2011
    16
    1
    OK Sargent As I have gotten this far can you point me in the right direction to fabricate a good heavy duty inductor? And also Why the heck are those Scotty diodes there?
    I'm so glad that you guys are here and willing to share your knowledge with me.
     
  6. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    The Shottky diodes are to protect the mosfet from having the wrong polarity applied to it. Where you're going to get the wrong polarity out of a 12 volt battery also escapes me.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sorry, I'm not on the Board much recently; my life has become quite busy in the last few years.

    You haven't given a target current yet; what kind of current do you want to maintain through your load?

    For lots of reading material on switch-mode power supplies, go here:
    http://www.smps.com/

    For something rather simple/basic, which also serves as an excellent tutorial, have a look at Ronald Dekker's site:
    http://www.dos4ever.com/flyback/flyback.html

    Now, a flyback converter is not what you need for YOUR project; they're only good for up to around 150 Watts. However, it gets you started on switch-mode power supplies that you can build relatively easily, and you gain valuable experience by making your own inductors/transformers using salvaged (or new, if you wish) materials. The thing is, without such experience and knowledge, you will be quite lost when trying to design/build/troubleshoot a current-mode SMPS.
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    HHO is a code word for overunity, and is not allowed on AAC.

    No more HHO, overunity, or Meyer

    If your question is strictly about electrolysis it would be allowed, but that is not how I interpreted the thread.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    The Schottky diodes are there to protect the driver transistors from back emf if your load has inductance. If the load is purely resistive then you would not need the diodes.

    Also many power MOSFETs have the diodes built in.
     
Loading...
Thread Status:
Not open for further replies.