Re: ballast connected in parallel?

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Original poster: "Sean Taylor by way of Terry Fritz <twftesla-at-qwest-dot-net>" <taylorss-at-rose-hulman.edu>

> > basic question...
> > current limited ballast connected in parallel on
> > primary of HV xfmr?
> > as connecting in series will produce a voltage drop.

Hi Larry,
A typical ballast would be connected in series with the load to increase the
impedance, thus lowering the current draw, V = I*R, I = V / R.  The voltage
is 240 (most likely . . . I'm just guessing at what you're using), and the
resistance is the total resistance (or impedance) of the load.  With a
ballast in series, the two impedances add, and you have less current draw.
You are right about the voltage drop, but that is how the current draw is
decreased - the HV transformer must obey ohm's law also, you can decrease
the current without lowering the voltage across the transformer.

What you propose, about putting the ballast in parallel with the LV winding
of the transformer would actually increase the current from the source.  You
would have the impedance of the HV transformer and of the ballast in
parallel, and the impedance would decrease, thus more current.  As for
Harvey's statement of increasing the impedance . . . if you have an
inductive load (the HV transformer) and place a capacitive load in parallel
with it (IE PFC), the impedance will increase as the capacitive and
inductive impedances will cancel somewhat, totally if the right capacitance
is used.  But, with a ballast, you're most likely using an inductive
impedance so no impedances will cancel.  Hooking anything in parallel with
the HV transformer won't limit the maximum current, but instead will reduce
the current draw if the right values of capacitance are used.

Hope that wasn't too long winded :-)

Sean Taylor