[SOLVED] Questions about transformers and diodes

Discussion in 'General Electronics Chat' started by l0vot, Jan 16, 2016.

  1. l0vot

    Thread Starter Member

    Apr 29, 2013
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    I have two questions about transformers and diodes, the objective is to make a power supply for an electrolyser, which needs high amperage (probably at least 100A) low voltage DC to operate (the potential difference across the electrolyser is less than 3V with the current power supply, I have no way to find the amps until my amp meter comes in, but it's going to be pretty high)

    The first question is: if a full wave rectifier (no capacitor) were attached to the primary of a transformer would the transformer operate as if it were operating on mains current, except with pulsed DC? It's much easier for me to find high voltage low amperage diodes than high amperage low voltage diodes.

    Second question: If a transformer has multiple secondaries, each with the same number of winds and a full wave rectifier attached to each, could the outputs of the rectifiers be placed in parallel to make the diodes work together? I have been told that diodes will not normally work together in parallel very well as the diode with the lowest forwards voltage drop will carry all the current, and burn out if it's capacity is exceeded.

    I ordered diodes for my Lincoln 225, which will have the advantage of turning it into an AC/DC Lincoln 225, but that isn't an ideal power source for an electrolyser, and could fry the welder because it isn't designed for this sort of thing.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    1. No.

    2. No.
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    First question - I am little confused. A power transformer operates on AC sine waves, in and out. Any wave form other than a sine wave and the transformer is in trouble. Ask your first question again a different way, maybe I missed something.

    Second question - Mostly yes, you can parallel identical secondaries with rectifiers. The internal resistance of the secondaries acts to balance the current. The lower the voltage the more critical things become and the answer gets closer to No than Yes.
     
  4. #12

    Expert

    Nov 30, 2010
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    1) No. Running a transformer always in the same direction warps the magnetism of the core.
    2) Yes. Diodes aren't all that bad about matching, especially at high currents. Look at the Vf/If curves and see that as one diode loads, its voltage increases and that kicks on the next best diode. They might not be equal, but a little bit of resistance added in series with each diode will even the current out. In addition, the act of creating current loads down the winding in the transformer, favoring the unloaded winding kicking in.

    yeah. Lestraveled just said that.
     
  5. l0vot

    Thread Starter Member

    Apr 29, 2013
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    A full wave rectifier on 60Hz mains current will produce 120Hz pulsed DC, which will have the same curves as mains current, except the bottom half of the sine wave will be flipped on top.

    For the second one, if the secondaries are current limited using magnetic shunts in the core, will that help with load balancing? That way the open circuit voltage can be higher, but as soon as the secondary connects to the load it's voltage drops. The shunts might limit the amount of secondaries to two per transformer (one above the primary with shunts separating it from the primary, one below with the same configuration), but could ensure load balancing.
     
  6. l0vot

    Thread Starter Member

    Apr 29, 2013
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    Thanks, I was sort of skeptical about the first one, but it was worth a shot.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Correct, but the transformer requires the opposite polarity sine wave to un-magnetize the core. As far as the transformer sees, you are feeding it with chopped DC and not the alternating Positive and negative sine wave it wants to see. Go ahead a try it but keep a fire extinguisher around.
     
  8. l0vot

    Thread Starter Member

    Apr 29, 2013
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    I will, just for shits and giggles using a current limited source so the breaker doesn't flip. I would probably need an air gaped core so the field collapses without the other side of the wave. I will post the results, even though you know those diodes are probably getting fried.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    I betting on the transformer burning up first.

    This would be a great application for you to wind your own transformer. If you find the right 'starter' transformer it would be easy.
     
  10. profbuxton

    Member

    Feb 21, 2014
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    I actually have a regulated 150AMP 5v power supply that can from a main frame computer I could sell you but the shipping cost would be excessive and its 240 mains input.
     
  11. l0vot

    Thread Starter Member

    Apr 29, 2013
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    Nothing burned up, I used a 12V battery charger to supply the pulsed DC to a MOT I rewound, the primary got hot, the secondary did nothing, same results with and without an air gap. I think it's because inductors act completely different when used in an AC or DC environment, with DC the inductor simply tried to smooth out the pulses.

    I have a lot of trouble finding big enough transformer cores to do anything with, MOTs are as big as I can find where I live, unless I steal the core out of a welder.
     
  12. l0vot

    Thread Starter Member

    Apr 29, 2013
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    The electrolyser would straight up fry that power supply, at 5V it would keep drawing more current until it burns something out, especially since the electrolyser's internal resistance drops the hotter it gets. The power supply must be power limited, which is why I am doing research on iron core stuff.

    That would probably be bullet proof enough to put in my sister's computer which keeps having trouble with it's 5V rails though. Thanks for the offer though.
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    @l0vot
    Vicor makes a power supply called a "MegaPac". They are based on the power conversion modules they make which are adjustable from 110% to 50% of rated voltage. They have been around for many years and you can get them used, cheap and extremely powerful on Ebay. Check this one out.

    http://www.ebay.com/itm/Vicor-MegaP...488969?hash=item4af6296949:g:cn0AAOSwQJhUgNHt

    It has eight 5V at 39 amp modules strapped together, equaling 312 amps and with a twist of a pot you can adjust it down to 2.5 volts.
     
  14. l0vot

    Thread Starter Member

    Apr 29, 2013
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    The ones I see with the capabilities I require are sort of steep, I will see what can be done with a bunch of MOTs (I can usually get those cheap at the junk yard, and they are shunted, so load balancing wouldn't be as difficult) and some diodes (which are cheap on eBay). It's about time I learned to make the transformer equivalent of a Beowulf cluster anyways.
     
  15. l0vot

    Thread Starter Member

    Apr 29, 2013
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    Thanks for all the help! Marking as solved.
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
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    What is a MOT??
     
  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Microwave oven transformer
     
  18. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If you are looking for a bigger better alternative to MOT units pick up a few large HID lighting ballasts from the scrap yard. They are designed to run continuously in high ambient temps and thusly can take a substantial overload when rewound to work as a normal custom transformer.

    As with the typical MOT most ballasts are built on a easy to disassemble modular two or three piece cores so taking one apart to rewind is pretty easy.
     
  19. l0vot

    Thread Starter Member

    Apr 29, 2013
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    I'll look into it, but I sort of doubt the junk yard has those.
     
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