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Step-up transformer built from two standard types?

Discussion in 'Electronic Components' started by Lostgallifreyan, Aug 27, 2007.

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  1. Can anyone advise if this is safe?

    I have an idea for a laser pulse drive using an 850V capacitor.
    If I feed 15VAC into the secondary of a 240V mains 6V output transformer,
    I get a calculated 600VAC out, which after a bridge rectifier (*1.414)
    gives me a 848.4V peak (no-load) voltage.

    That looks ideal, and cheap, so I want to try it, but what other
    calculations should I do to set a safe VA or current rating for each
    selected transformer, assuming a maximum continuous current of 20 mA on the
    600V output current. (There will be a brief surge as the capacitor charges,
    but I will limit that with a series resistor).

    I'll come back to this after hunting for a small 600V output transformer,
    but I don't hold out much hope of finding any, let alone as cheap as a
    combination of two small standard transformers.
  2. Hal Murray

    Hal Murray Guest

    600V center tap used to be common back in the days of tubes.
    They may not have been as small as you want. Fist size or larger
    is the right ballpark.

    You might be able to find one is a surplus/junk place. When I was
    a kid, my junk box had lots of them. Maybe check out some junked
    modern tube audio gear.
  3. Nice post. I hadn't considered the saturation issue but I had considered a
    higher frequency, and that led me to find a few DC-DC converters that will
    do ok, saving me a lot of trouble.

    I might also consider the voltage multiplier, but it looks like the DC-DC
    converter is a nice easy cop-out. :) The cost is low enough in the cases I
    found to justify it too.
  4. (Hal Murray) wrote in
    Not got access to it, or I might (it all seems to be on the web now
    anyway)... Also, if I found something good I'd want to be able to repeat
    it. I did find that Maplin (who I'd long since thought had fallen into the
    pit and subsided) still sell a valve power supply transformer, but it's
    240-0-240, not enough. Might be useful enough to get a couple anyway

    I decided to go with DC-DC converters fed by standard low-volt transformer
    and rectification and smoothing. The total cost is still low, and it's
    repeatable and safe.
  5. Chris Jones

    Chris Jones Guest

    Unless you increase the frequency, you cannot increase the voltage of the
    transformer much above its nominal value without making the core saturate.
    (saturation makes the inductance suddenly become very low and so a large
    current flows and blows your fuse, assuming you have one, or burns out
    something else if you don't.) This is because the rate of change of
    magnetic flux is proportional to the voltage, so the total magnetic flux is
    proportional to the integral of the voltage over one quarter cycle. It's
    quarter because in one half cycle of the sine wave, the flux goes from
    negative maximum flux, to zero, and on to positive maximum flux. If the
    cycle takes longer (lower frequency) then the integral of the voltage over
    a quarter cycle will be a greater number so the flux will be greater, and
    may saturate the core.

    If you want 600VAC from a 240V transformer then you will have to more than
    double the frequency (which you could do with an inverter). (You should
    also make provision for the possibility that the insulation is not up to
    the job in the long term.)

    I would suggest investigating the Cockcroft-Walton voltage multiplier, so
    that you can use the transformer at its nominal voltage, or consider
    connecting several transformers in series to achieve the voltage that you

    A small flyback converter might also do what you want.

  6. LVMarc

    LVMarc Guest

    For real step up voltage fun, I like a 36 foot neon sign transformer.
    output > 30 kV. Alternatives include, transformer from microwAve oven,
    and oil burner starters. These are the classic three for making
    relatively high 1 to 42 KV...Happy sparking



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  7. Hal Murray

    Hal Murray Guest

    Is that correct?

    Doesn't the input current depend on the load as well? Wouldn't it
    still work (aka not saturate) if the load was reduced?
  8. Chris Jones

    Chris Jones Guest

    Yes, the input current depends on the load current, but the magnetic flux
    density in the core is almost unaffected by the load current. From the
    core's point of view, any change in the load (secondary) current is
    practically cancelled by the change in the current in the primary winding.
    This means that even with no load, there is a limit to how much voltage you
    can apply to the primary winding before the core saturates. You can try
    this if you get a transformer and a Variac and an ammeter. It is an
    interesting experiment.

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