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Combining Transformers for PS

Discussion in 'General Electronics Discussion' started by Gristle McThornbody, May 13, 2012.

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  1. Gristle McThornbody

    Gristle McThornbody

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    Mar 14, 2012
    I'm building a fairly high currrent (12 amp) 22 volt regulated power supply and have two identical transformers that I would like to use together to get that amount of output current. Each by itself can handle about 7 or 8 amps comfortably. I know that trying to combine power sources like this is generally a no-no, but I'm wondering if there is a reasonably simple way to make this work. I've read of "balancing" the sources using an array of very low value resistors, but I don't know much about this approach.

    I can pair the transformers at their outputs or I can build rectification and filtering for each and combine DC after if necessary.

    To get a single transformer with the necessary capabilities would be difficult and very expensive. The output voltage of the transformers I have is just right for this application, i.e., it saves me from having to dump a lot of excess voltage across the pass transistors, so that makes them appealing. I'd really like to use these together somehow. Any recommendations?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    How well do the transformers match? Have you measured the voltage a t the transformer's outputs under the same load? How much di they differ? If not by more than e.g. 1%, combining the transformers should be no problem.
    I suggest you combine the rectified DC. The internal resistance of the transformers will help balancing the load plus each rectifier sees only 1/2 of the current.
    Also if you combine the trnasfomer's outputs on the AC side, you risk dangerous voltage on the primary in case one transformer's primary connection gets loose because in that case this transformer would transform the low secondary voltage of the other transformer back to mains level.

    Harald
     
  3. Gristle McThornbody

    Gristle McThornbody

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    Mar 14, 2012
    I've checked the transformers under both light and heavy loading and the output voltages are essentially identical. I connected the primaries together to make sure that they were seeing the same input voltage and measured secondayy voltages back and forth between the two to eliminate the effect of any momentary fluctuations in the mains. I get values that are within less than 0.1 vac at about 45 vac secondary output, equivalent to 0.2 percent. I also checked the center tap voltages and these fall into line. It looks like these transformers are as closely matched as can be. BTW, they are new (old surplus) and marked identically, so I guess this shouldn't be a surprise. I'll take your advice and connect them after the rectifier and before the filter caps.

    BTW, if the transformers weren't matched closely enough, what sort of problems and/or symptoms would there be?

    One question, though. Each of these transformers has two center tapped secondaries. For my project, I'll use the bigger secondary and ignore the smaller. In addition to all those leads, there is a bare stranded wire emerging from the hole in the case along with the other wires. Through the hole, I can see that it goes down into the windings with the other leads. It is identical in both transformers, i.e., it is part of the original design rather than added on. It does not seem to be connected to anything, not even the case. My first thought was a case ground, but, again, the ohmmeter shows no connection to the case at all. Under power, it shows no voltage relative to any other leads. So if it's not power and not a ground, what could this lead be?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    add1 ) If the transformers don't match wll enough (your's seem to match), the one with higher output voltage will take more load, thus distributing the power (and thge losses) unevenly between the two. Depending on the degree of mismatch, this may cause overheating and finally destruction.

    add2) I would have suspected that the single wire is connected to the transformer core. I'm not sure what it means that your Ohmmeter shows no connection to the case. Is case connected to the transformer core? If the connection goes to the core, it could be a safety feature if you connect the wire to protective earth. In case a primary winding has a short circuit to the core, the fuse (you do have one, don't you?) will blow and protect the rest of the device including an unsuspecting user.
    Another option for such a wire would be the connection to an internal shield which may be present to isolate EMI/EMC between primary and secondary windings - although I think that would be rather unusual for a power supply transformer. In this case, the wire should be connected to earth, too.

    I have to admit that I'm no specialist in transformers. Either someone with more knowledge jumps onto this thread or you can find a datasheet of the trasnformer giving more detail.

    Harald
     
  5. weird_dave

    weird_dave

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    May 9, 2012
    This would be my preference, but only because you already have the transformers.
    I'd make s simple switcher using FETs that swaps the between the two DC outputs. Beer isn't giving me a circuit diagram right now tho... ;)
    Can you give us a bit more info on the project? You mention dropping volts across pass transistors, I'm starting to think a simple custom switch mode circuit with two inputs might do well here and get your volts closer to your actual requirements.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,388
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    Nov 17, 2011
    I don't think switching between the transformers is what Gristle is after. He wants to combine the two transformers so he can have the added power of both available at one output:
    [​IMG]

    Harald
     

    Attached Files:

  7. weird_dave

    weird_dave

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    May 9, 2012
    The switching plan is at a reasonable frequency (over 20KHz), the power would be pulled evenly between the two supplies. The caps would average out the current :)
     
  8. Gristle McThornbody

    Gristle McThornbody

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    Mar 14, 2012
    Thanks, Wierd Dave, but I'm going with Harald's suggestion. I've already coupled the transformers and tested them under significant load. Works great. In this case, I'll just stick with the KISS principle. Thanks for all the help, everyone.
     
  9. weird_dave

    weird_dave

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    May 9, 2012
    My plan was a little flawed anyway ;)
    I guess the simple solution works due to the coil resistance allowing them to load share reasonably well.
    Are you able to measure the current from each of them separately and at the same time? I'd be quite interested to see how well they are load sharing.
    There is one small snag with the simple solution, if one supply isn't on for some reason, the other might feel a bit of pain (unless you have some detection circuity you've not told us about :) )
     
  10. Gristle McThornbody

    Gristle McThornbody

    31
    0
    Mar 14, 2012
    The primaries of the two transofrmers would be hard wired together so the only way for one of them to be off would be for the transformer itself or the rectification to fail.

    Testing the current draw for each transformer under load is a good idea. It wouldn't be that tough for me to do and it would serve as a final check to make sure that both are sharing the load equally. Thanks for the idea.
     
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