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Load match HV secondary

Discussion in 'Electronic Design' started by Roland Kent, Apr 23, 2007.

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  1. Roland Kent

    Roland Kent Guest

    I am using two 120V-12V power transformers back-to-front to step up a
    120Hz sinewave from 6V (1A available) to around 100V. The primaries
    will be wired in parallel, with a small value series resistor if
    required. The secondaries are wired in series. They will be driving a
    resistive load.

    Can anyone tell me how to calculate the optimal load resistance?

    IOW a load resistance value that is not so low as to degrade the
    waveform, or overload the primary circuit, but not so high as to
    unnecessarily attenuate it.

    Thank you kindly for any advice.

    Roland Kent
  2. whit3rd

    whit3rd Guest

    Well, power limit on the input is on the order of 6V * 1A = 6 W

    To get the same power on the output, compute the resistance
    that draws 6 watts of power at 100 V (or whatever).

    Probably that's a minimum resistor, not an optimum. And, of course,
    6W resistors are big, pricey, and get hot. And 100V is
    high enough that there's a bit of electric shock hazard. You
    REALLY don't want to melt your insulation.
  3. No series resistor will be required on the primary (6V) side.
    Optimal for what? If you know what kind of load the 100V side will be
    driving, that will be what your 'load resistance' will be.

    If you are trying to calculate the maximum load (minimum resistance) on
    the 100V side, that will be the resistance that draws the rated current
    of the secondary windings. You mentioned 1A available, but I'm not clear
    on whether this is the capacity of your 6V source, the primary windings
    or what.

    With the connection you propose, 1A into the paralleled primaries (6V)
    side will provide 50 mA at the secondary.
    You don't have to worry about a distorted waveform (due to the
    transformers). Its high voltage that causes this kind of problem and it
    appears that you are using these below their nominal voltage ratings.
  4. Tam/WB2TT

    Tam/WB2TT Guest

    I have read this 5 times, and still do not follow what you are proposing to
    do. You want to go from 6V AC to 100V AC, right? Ideally, you need a 6V to
    100 V transformer. This is not at all far fetched. You should be able to
    find a 120V transformer with a 100 V (common in Japan) tap on it. If you
    can't find one , get a 6V to 120V transformer and add turns to the 6 V
    winding until the 120 drops to 100. What you end up with is a 120 to 7.2 V
    transformer. You don't have to do anything to the existing winding, just
    make sure there is enough room to get something like 10 turns of wire on
    there. If the voltage goes in the wrong direction reverse the winding.

    Yes, I have done this. Made a 12 V winding into a 14 V winding. As I recall,
    took less than 10 turns of wire.

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