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Dual supply using 2 transformwers

Discussion in 'Electronic Basics' started by John, Jan 24, 2004.

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  1. John

    John Guest

    Hi.
    I have two 12v-0v-12v one 230VA other 240VA transformers, can I connect the
    two transformers in series to give me a 24v-0v-24v supply (24v from
    12v-0v-12v). Then, feed these via rectifier to well sunk LM317 / LM337T's.
    Would I have any issues.
    I would also like to add a couple of digital meters, either off the shelf or
    using PIC, I could use a little guidance concerning how to power them e.g.
    what issue can I expect as the meters will have common supply/ground with
    the PSU proper and each other, what if I use a third with a shunt for
    current.
    Thanks.
     
  2. You cannot connect the secondaries of two transformers in series to
    form a single center tapped winding to be rectified into a pair of
    stacked DC supplies. This is because each of those half windings
    carry DC adn this cancels only if both windings are on a single core.
    A better way to use your two transformers is to use a full wave bridge
    rectifier on each full secondary, and then connect the outputs of
    those two bridges in series to form a positive and negative raw
    supply.
     
  3. The bridge rectifier version is certainly better than the 2 diode
    rectifier version, but there is still a chance of DC in the cores if
    the load is not connected only across the positive and negative bus.
    Any load that flows from either bus to common produces DC current in
    both transformers. I also doubt that the two transformers will share
    the load properly if they are not identical transformers.
     
  4. John Fields

    John Fields Guest

    ---
    Are you sure, John?

    Although the argument could be made that they're not really in series,
    they're not really in parallel either and with the flux reversing in
    both secondaries (since there are two oppositely wired half-wave
    rectifiers in each one) It seems like this would work:


    ACIN>-------- ----+--->|-------+--->+OUT
    )||( | |
    )||( +---|<---+------->-OUT
    )||( | |
    )||( | |
    ACIN>-------- ----+--[+C]--+ |
    | | |
    +---------------->COM
    | | |
    ACIN>-------- ----+--[C+]------+
    )||( | |
    )||( | |
    )||( +---|<---+ |
    )||( | |
    ACIN>-------- ----+--->|-------+
     
  5. John

    John Guest

    Hi Guys.
    Thanks, shame, it would have been good to put them to good use. As it
    happens I have also come across a pair of 12v 500VA transformers as well,
    never rains etc....

     
  6. You can certainly use them by the method I first described with only
    an extra bridge rectifier. Not much extra cost compared to the
    transformers.
     
  7. John

    John Guest

    Sorry guys, lost the tail end of the thread, can you explain to me what
    effect the DC portion that you refer to is and how it gets into the core.
    Am going to try the two transformers, and use two bridge rectifiers in
    series and see what happens with a load! what should I be looking for if
    there is an issue.

    Another question I have a transformer with a 115/230 primary 15/30 secondary
    (210VA), what would happen to the secondary voltage if I used the 115
    primary at 230V and what would be the effect on the VA rating.
    Thanks.


     
  8. There is no issue with that arrangement.

    The problem occurs if you connect two separate secondaries (from two
    cores )in series as if they were a single center tapped winding, with
    a single bridge to create a positive and negative supply. If you load
    either output (or have mismatched loads) each secondary is effectively
    feeding a half wave rectifier, passing DC through the winding, and
    risking saturation on the core.
    If you apply 230 volts to a 115 volt winding the transformer catches
    fire. If you connect 115 volts to a 230 volt winding, all secondaries
    produce half of their rated voltage at rated current, so the power
    rating of the transformer is effectively cut in half. But it will run
    very cool due to the low core magnetization and produce much lower
    (less than half) stray magnetic fields.
     
  9. John

    John Guest

    Thanks John, got it now, LOL.
     
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