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my last transformer post, I promise

Discussion in 'Hobby Electronics' started by gav, Sep 21, 2003.

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  1. gav

    gav Guest

    hello again,
    I've been searching and trying to understand as much as I can with my
    limited electrical experience but I really do have to ask some more
    questions about my experiment.

    I plan on experimenting with winding my own toroidal power transformer.
    I require a 1:2 step up at 20V 150hz supply aiming at 150-300VA or so.

    I have found a general ballpark equation to calculate primary turns for an
    EI type transformer which goes like this:
    is N = (E · 10^8)/ (4.44·f·A·B),

    N= number of pri turns
    E= supply voltage
    f= frequency
    A= Cross sectional area of core (I think in square inches)
    B=max flux lines = rough figure of 60,000

    Using a toroidal core with say a CSA of 1sq" it gives me 50 turns for the

    Is this equation correct and should it work for a toroidal?

    Is the value of B appropriate for say a Mains 300VA toroidal GOSS (Phil)
    core off the shelf at Jaycar?

    I think I understand that too few turns on the primary will cause saturation
    and large currents. Is there a severe disadvantage to putting a few too
    many turns on the primary apart from increased copper losses? I only ask
    because someone knowledgeable may be able to advise me X turns on the
    primary will work and I'll be able to forget about the theory for now.

    Does it have to be more precise than this. I don't mind the thing burning up
    if it's inefficient when I put a load on it. Just means I'll have to

    If anyone can help me answer these questions, or point me somewhere that
    can I would be very grateful.

  2. Mark Harriss

    Mark Harriss Guest


    Just Kidding Gav!!

    Have you got any access to ARRL handbooks at the library?
    they have very practical advice about transformer winding
    mostly in the 1970's editions.
  3. Mark Harriss

    Mark Harriss Guest

  4. Phil Allison

    Phil Allison Guest

    ** A 50 Hz toroidal needs at least 4.5 turns per volt for 1 sq inch of
    core. So for 150 Hz only 1.5 turns per volt are needed. For 20 volts that
    means only 30 turns .

    Max B is 1.1 Teslas for the grade of GOSS normally used in 50 Hz
    toroidals. ( B can go as high as 2 Teslas when exotic materials are used.)

    ** No - a few extra will not hurt.

    Your problem may be to get enough volume of copper into a mere 30
    urns - for reasonable efficiency the space available for windings must be
    well filled, half the space with the primary and half with secondaries.

    Probably you need to wind several layers of 16 G wire for the primary
    and then parallel connect them - same goes for the secondary. I hope you
    have strong fingers.

    ............ Phil
  5. Peter Howard

    Peter Howard Guest

    Can't help you Gav, I know as much as you, but here's a dumb question for
    the group.
    How DO they wind toroidal transformers or chokes commercially? I know it
    can't be rows of strong fingered and patient ladies winding the turns on by
    hand. I have wound the occasional hf toroid by hand and can't even begin to
    imagine what sort of machine would pass a tiny shuttle loaded with a tiny
    reel of wire through the hole and around the outside time and time again.
    Anyone ever seen such a machine in action?

  6. Hello Gavin,
    I missed your previous questions. Please tell me again,
    what is that you are ultimately wishing to do. I remember
    something about running a transformer from a car alternator
    for some reason. I can't remember, tell me again.
    What equipment do you wish to operate in your car?
    Just Interested.
    John Crighton
  7. Phil Allison

    Phil Allison Guest

    ** A doof doof machine of course !!!

    .......... Phil
  8. Hello Phil,
    I have no experience in these things. I thought
    they operated off plain ordinary 12 Volts D.C.
    from the car battery.
    Does Gavin wish to operate a special amplifier
    from a higher DC supply voltage? How High?
    How much current?
    John Crighton
  9. Phil Allison

    Phil Allison Guest

    ** Sensible ones do.

    ** Yep.

    How High?

    ** +/- 60 volts.

    ** 4 amps DC average per rail.

    .......... Phil
  10. Hello Phil,
    cripes, that is awkward!
    A DC to DC converter would be neat, but to supply
    plus and minus 60 Volts at 4 amps would be a big
    project in itself. A good one though.

    Getting back to the transformer.
    How was that going to work with an alternator?
    A second alternator modified for single phase
    output, maybe? Sounds interesting!
    John Crighton
  11. Phil Allison

    Phil Allison Guest

    "John Crighton" <

    ** Well, that is HIS problem.

    He is the sort of "individualist" who thinks it is worth a go.

    Buckley must be his patron.

    ............. Phil
  12. Mark Harriss

    Mark Harriss Guest

    I asked a guy at Harbuch that question, he said he can't
    explain it but that it's really obvious once you see a machine.
  13. gav

    gav Guest


    John, Phil has pretty much summed it up
    Second alternator, modified for single phase AC output.
    A Tranny to do 1:2 step up.
    Rectified and fed back to modified regulator circuit to hold at 60V
    Use this as your power supply for any amp module from jaycar requiring 60V

    It would be cheaper to go buy a normal car amp. But not as fun.

  14. Hello Gavin,
    how about a second dedicated electrical power system of 24V.
    The second 24 V alternator (used one from wreckers) charges
    up two 12 volt batteries in series tucked away somewhere.
    Now you would have to find or build an amplifier that ran off 24V.
    but you would have more DC power at your disposal.
    The car engine not running wouldn't be a problem either.

    High power 12 volt amplifiers are readily available second
    hand from cash converters or wherever. Maybe half a dozen
    amplifiers with half a dozen speaker would give lots of
    noise for the least effort, running off a second dedicated
    12 volt electrical system off course.

    You could build your own amplifiers using individual
    components or try the high power audio module types.
    That would be fun!

    Just a thought.
    John Crighton
  15. gav

    gav Guest

    Hello Gavin,
    I've gone too far to turn back now.
    Truck alternator is the next upgrade though. Bet they put out some power.
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