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Building a transformer

Discussion in 'Electronic Design' started by Mark Fortune, Aug 8, 2006.

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  1. Mark Fortune

    Mark Fortune Guest

    Greetings to the collective.

    For my next project I want to build a bench power supply and do away
    with this old switch mode AT computer PSU that i'm currently using.

    The design I have in mind will be pretty beefy, giving a wide range of
    fixed and variable output voltages (i'm thinking from -50v up to +50)
    and deliver up to 5amps of current. if my estimates are right i'll need
    a 600va+ (100v * 5a + overhead) transformer to do the job well. Now i've
    had a look at some transformers in this range, and they're a little out
    of my price range. so now i'm considering building my own.

    The specifications I need are as follows:

    primary: 0-230v @ 50Hz
    Secondary: 60v-0-60v
    secondary output current max: 5A

    I have an intermediate understanding of transformers, but have never
    actually built one, hence I am seeking advice from those with experience
    in this field.
    So where do I start? what type of core is best for this application? - I
    have a friend who's blasting off that toroids are best, but i'd like a
    second opinion, and what kind of wire to use for the primary and
    secondaries? what are good safety practices regarding insulation? Is
    this even a feasable project to undertake (moreso on the money side than
    timewise). Any other advice also greatly appreciated.

  2. t.hoehler

    t.hoehler Guest

    Not really worth the effort, IMHO. However, take a look at All Electronics,
    Hosfelt Electronics, MCM Electronics. There are others also. They have over
    run surplus trannys with really good prices. The other alternative is try to
    find an old high power am fm receiver which is kaput and not worth fixing.
    They have a transformer with the approximate outputs you seek. You're right,
    if you have the time, building your own power supply is fun and saves lots
    of loot.
  3. By the time you have bought the core, be it toroid or E I lams, bought the
    bobbin (if you use E I) bought two lots of wire, insulating fabric tape etc etc
    etc., you may as well go and buy the right thing.

    We make our own up to 3kVA, and we have all the bobbins, wire, varnish, tapes
    etc., but before you ask, we don't do one-offs. Most of ours are specials that
    we have designed by an outside consultant. Above 3kVA we go outside for them as
    it isn't worth keeping all the steel strip, special wire and bigger winders,
    varnish dip etc etc.

    We have a pair of winding machines plus a hand winder which we can wind up to
    3mm copper wire or small rectangular tape.

    You may have to look around for what you want, but I promise you it will save
    you a lot of time and trouble.

  4. Gerard Bok

    Gerard Bok Guest

    No, it won't :)
    Your math is right. But what do you expect if you decide to draw
    1 Volt at 5 amps from that beast ?
    Who / what is going to dissipate the 600 - 5 = 595 VA for you ?
    Here's your advice: don't even think of starting at 600 VA if you
    want to roll your first transformer :)
  5. Modat22

    Modat22 Guest

    I had 3 toroids made by Tortran a couple years ago for a homemade
    audio amp, they where very reasonable. I recommend you tell them to
    give you a non-thermo protected primary.
  6. You need more than 5 A AC to get 5 A DC. The rough rule is that a bridge
    rectifier needs a factor of 1.6 times, but with modern huge filter caps,
    think of at least 1.8 times. That's a BIG transformer, with THICK
    secondary wire. Not a project to learn on, I would say.

    It's not even easy to buy parts for transformers now. Maplin used to do
    kits - bobbins, laminations etc. , but not now.

    To learn about transformer design, you need a textbook and one of the
    best is a classic - Radio Designer's Handbook, by F Langford Smith,
    which was reprinted by Newnes (Butterworth-Heinemann/Reed) a few years
    ago. ISBN0 7506 3635 1.

    Modern core material (silicon-iron) is better than the materials
    mentioned in the book. You may be able to get the information on current
    materials (maximum recommended induction, core loss power/kg,
    magnetizing VA/kg) from the web.
  7. By a lot of cheap toroidals on eBay, 5A at some low voltage that doesn't
    sell well. Outputs in series, inputs in parallel. Switch-select to get
    coarse adjust, variable regulate over one output's voltage range to get any
    voltage over a large range with little loss.
  8. A bit difficult for a person based in UK, perhaps. And 60-0-60 V at
    around 9 A is a BIG receiver or amplifier. Around 300 W/channel.
  9. Eeyore

    Eeyore Guest

    Toroids are relatively lightweight and more efficient that most E-I designs.
    They also have relatively low stray flux which is nice if that's important to

    For an uncomplicated requirement like that, simply buy one off the shelf. You
    have no chance of competing on price, never mind your transformer's unlikely to
    meet safety regs if you don't know what you're doing ( which clearly you don't
    since you're asking basic questions ).

  10. Guest

    I would recommend reading up from every source. I have built torroids
    using the winding which was orginally a variac. These were 110 volt
    units. You automatically have a primary so all you need to do is wind
    the secondaries on top. I have also done the other type of transformer,
    schelacing.the iron plates. I would also recomend what others have, buy
    surplus on the market. Combine transformers if you can't find them.

  11. Jim Thompson

    Jim Thompson Guest

    Sounds about right.

    Years ago I had a neighbor kid who needed tutoring in Algebra.

    I couldn't get him really interested until I sweetened the pie by
    offering to help him build a 400W guitar amplifier IF he got an "A" in

    He did get the "A".

    We built this beautiful thing (in 1975) that was so heavy that it took
    two people to carry it, and I had to use fans on the heat-sinks ;-)

    Put out 400W RMS into 4 x 16 ohm speakers in parallel.

    For my effort his father, President of Anthony Pools, built me a
    swimming pool at a bargain price.

    ...Jim Thompson
  12. What primitive semiconductors did you use in the output stage, or was it a
    toob device? Did we have 2N3055s in '75? I don't memember.

  13. Do you have lots of time and money and power transistors to blow?

    It's not too hard to build a power supply to your specs. It's a lot
    harder to build one that will have a MTTF of a week or more. Are you
    sure you can design a power supply that can survive the accidental
    short circuit, reverse polarity across a car battery, static
    discharges, inductive kicks, thermal cycles, and all the things that
    bench power supplies have to tolerate?

    In my experience, bench power supplies blow out about once a year.
    Blown pass transistors, driver transistors, yuck.

    You might consider just souping-up a PC power supply. Put a LM350 and
    a foldback current limiter, switchable to the +5 and +12v outputs. Add
    a hefty diode and low ESR capacitor to voltage-double the 12 volts to
    give you a 24VC source. You can even add a pot where the
    voltage-sensing wire senses the +3.3 at the motherboard end to get a
    variable voltage power supply. And the PS specs require over-current
    and foldback protection on all PC power supplies.

    Put two in series (with careful separation of the grounds) and you're
    up to+48 volts at many man y amps, at very low cost and weight. For
    example, MPJA has nice brand new 180 watt supplies for $8.95 !!!
  14. Jim Thompson

    Jim Thompson Guest


    I'm trying to remember what semiconductors I used. But, IIRC,
    2N3055's *were* available around that time.

    ...Jim Thompson
  15. Therein lies another nice thought. Lots of people use bench supplies, but
    not everyone who does knows how to repair them. So if you find a decent one
    broken you might buy it for peanuts and get it working. If you do, you'll
    have learned a bit, and saved a lot.
  16. Genome

    Genome Guest

    Hey, stop being a fool.... switch to Slackware.

    Then mum won't have to explain about piss and shit stains becoz you will

  17. ian field

    ian field Guest

    Was it Ge or Si ?
  18. ian field

    ian field Guest

    You can't beat a microwave oven transformer with a secondary transplant!
  19. Jim Thompson

    Jim Thompson Guest

    It was Silicon. I didn't use any Germanium power devices since I
    boosted my car radio back when I was a junior in high school.

    ...Jim Thompson
  20. mc

    mc Guest

    I would look at cheap battery chargers.

    Note that you can put the primaries in parallel and the secondaries in
    series to get a higher output voltage by using multiple transformers. Check
    phasing if you do this.
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