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

1. ### Roland KentGuest

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

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. ### whit3rdGuest

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. ### Paul Hovnanian P.E.Guest

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

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/WB2TTGuest

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.

Tam