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Rewinding microwave oven transformer

Discussion in 'Electronic Basics' started by [email protected], Nov 30, 2005.

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  1. Guest

    I am thinking of rewinding an old microwave oven transformer to drive a
    car amp to produce some bass in my bedroom. The amp will continuously
    draw about 30A and will draw a surge current of 150A. I was thinking of
    using copper wire to rewind the transformer but getting copper wire in
    the required thickness was going to be a problem not to mention working
    with the stuff. What I thought I would do is use the wire from some old
    car jumper cables (with the insulation left on) and rewind the
    transformer using the jumper cables. Is there any problem with using
    multicore cable to rewind a transformer? I am aware of the fact that
    the insulation might melt...
     
  2. ehsjr

    ehsjr Guest

    If it's a car amp, it is setup for 12V DC.
    A transformer won't provide DC.

    Ed
     
  3. Art

    Art Guest

    May be easier to just buy a commercial 12VDC Supply, there are some nice
    ones that will put out way over the required 150 Amp constant power. May
    cost you a quid or two tho!!
     
  4. Jamie

    Jamie Guest

    Hmm, ?
     
  5. Guest

    I understand that a car amp needs dc to opperate. I am going to use the
    diodes from an old alternator to get the required dc voltage. It would
    be much more easy for me to just buy a large power supply but short of
    an arc welder i don't think I will be able to find one that can supply
    100A Also I don't want to pay anything for it. I allready have the
    transformer, the diodes and the jumper cables But I don't want to wire
    it all up until I know if it is ok to use multicore cable to rewind a
    transformer.
     
  6. Not unless you get off on the stench of burning plastic! Your idea's a
    non-starter.
     
  7. Jasen Betts

    Jasen Betts Guest

    No problems with multicore cable in a transformer, the only reason solid is
    used is that it's easier to insulate and cheeper.
    You must have some pretty flimsy jumper leads there if you're expectring
    to get 10-20 turns on the transformer.

    microwave oven transformers aren't built for continuous running without a
    cooling fan blowing over them, they can be modified by adding some extra
    turns to the primary,

    Don't forget to fuse it, something like that could start a fire.

    Bye.
    Jasen
     
  8. Rich Grise

    Rich Grise Guest

    The problem isn't using stranded wire - in fact, in those "make a spot
    welder with your MOT" articles, they recommend welder cable, which is
    stranded.

    The problem is a MOT won't give you 100A either, at lest not for very
    friggin' long. And they're rated for intermittent duty, and _that_'s
    with forced air cooling!

    Very very bad idea - you'd be better off to look at something like
    surplus BMF transformers, or an industrial battery charger.

    Good Luck!
    Rich
     
  9. Brian KP

    Brian KP Guest

    If you would really like to use a car amplifier, I would suggest
    getting an old car battery and an old car battery charger. The kind you
    use to charge a dead battery inside your house, they plug into
    household 120V outlets and continuously charge the car battery. (Lead
    acids like being charged constantly). If you haven't lost me here is
    the method to my madness:

    More than likely, the car amp only uses 150A when it needs to drive a
    really hard bass hit at full volume. Usually it is only using a
    fraction of that. No transformer that isn't $$$ is going to provide
    that. However, hooking your amplifier to a car battery that is being
    continuously charged can handle that. When the amp draws alot of power
    the battery acts like a huge capacitor and provides it, otherwise most
    of the power comes from the battery charger and a little from the
    battery. Of course in the end the net charge into the battery must be
    greater than the net discharge of the battery but the battery will be
    recharged whenever you're not using the amplifier, which if your normal
    will be at least 8 hours a day.
     
  10. Guest

    Thanks for all your help it has given me a few good ideas. I gave up on
    the jumper cables as they where to large but I am going to try
    rewinding the transformer with 80A car wire. I will let you know what
    happens. The amp is only 300w so I don't think that the microwave
    transformer will have any problem powering it.
     
  11. Guest

    I set it up as above and it appears to be working fine but after
    running for 1/2 an hour or so the transformer gets very hot. I have
    kept this under control with a couple of fans but I am wondering if it
    is damaging to the transformer running it like this? What could I do to
    get the heat down other than adding more fans? Would replacing the
    magnetic shunts stop it heating up or does it need some form of current
    limiting?
     
  12. Tim Williams

    Tim Williams Guest

    As long as temp stays below, say, 100°C, it should be fine.

    Technically. Personally, I would try to improve that!
    Nah. Here's what you have to do. Look at the size of the primary winding's
    wire. Find some laying around of the same size (I'm guessing 14 to 16AWG).
    Put on about 20 turns (yes this will take up some of your secondary's space,
    you'll have to live with that) and connect it in series with the primary.
    Make sure to match winding direction, otherwise you'll overload it worse.

    You'll need to add a few more secondary turns.

    Alternately, Google "buck transformer" and drop primary volts by 10-20V or
    so. Same idea, you need more secondary turns.

    The problem is, MOTs are wound with as little copper as possible. This
    overloads (saturates) the iron core, causing high peak currents in the
    primary (the secondary load doesn't matter!). High currents means more heat
    in the primary copper, causing it to heat up. A rather unhappy situation,
    so by adding primary turns (or reducing the voltage), you give the iron core
    an easier life and everything is happy in Transformer Land.

    Yes, to fit more turns probably means smaller wire means less current
    handling. That's the price you pay for electrical happiness.

    Tim
     
  13. Removing the magnetic shunts and using some of that space to put some
    additional primary turns would help a lot.

    A way to effectively double the primary winding is to use two
    identical transformers, but put the two primaries in series and the
    secondaries in parallel.
     
  14. Guest

    Thanks for all the help. I think I will try putting two transformers
    together with the primaries in series and the secondarys in parallel.
    However I don't have two MOTS the same. Will it matter if they are a
    little bit different? One is a bit smaller than the other.
     
  15. Jasen Betts

    Jasen Betts Guest

    Add more turns to the primary, (this will reduce the current in the primary,
    and therfore the heating...) and also the voltage in the secondary, so you
    may need to add turns there too)
     
  16. Guest

    Thanks to everyone that posted replies. I found a similar transformer
    to the one I all ready had at work and I set the two up with the
    primaries in series and the secondary in parallel and with this setup
    the transformers will happily power a 100w bulb continuously with only
    a little heat output from one transformer and no heat output from the
    other one. I think I could improve this by using two identical
    transformers but I hardly ever see two identical microwaves.
     
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