I'm an american, so, my "ballast" just has to output X frequency and vary
some amperes to dim the fluorescent light?
Yes, assuming it controls that current in some way and can supply the
current at the voltage needed by the lamp.
For example, if I were to use
some FETs to switch voltage -/+ and output a square wave AC signal, and I
just changed the rails to the switching devices, that would provide a
dimming function?
No! You have described a voltage source not a current source. If a
fluorescent or other discharge lamp is connected to a voltage source
it will draw as large a current as the source allows and either burn
itself out or burn out the voltage source.
So basically in european terms a ballast is some form of
impedence circuit to control the current to the lamp via frequency from a
power supply?
Not sure what you mean by "European terms". Everyone uses a device to
operate discharge lamps that provides starting voltage and operating
voltage and acts like a current source to control the operating
current. On this side of the pond we call this whole device a
"ballast", even though the term ballast was originally limited to the
current limiting element. The Europeans use the term "control gear"
for the same box that we call a ballast, and therefore avoid the
confusion created by the US terminology.
Isn't it a little inefficient to change the current using an
impedence circuit?
No. Perfect inductors have no power loss. Even imperfect ones can be
rather efficient if designed properly.
I mean shouldn't you just have switching transistors
switching the voltage from a switch mode power supply?
You could use PWM to create a current-controlled source, and this has
been done in certain ballasts. However, the lamp current can increase
to destructive levels in far less than 1 msec, even 1 usec depending
upon how much voltage is applied. If current pulses this short, and
with fast rise and fall times to get high efficiency in the switching
circuit, are applied to the lamp, it will generate a high amount of
EMI and be in violation of FCC regulations. You therefore need some
sort of filter on the output of the switching circuit to smooth out
the pulses.
Then you can just use
PWM to control the switch mode power supply to control the dimming. In the
end you would have a PWM switch mode power supply and transistors switching
its voltage (mins and plus so it is an AC square wave....). Then just
changing the voltage the transistors were switching would dim the lamp? This
is also how a rapid start lamp is suppose to be operated correct?
Not unless you have a current limiting device. See above.
--
Vic Roberts
http://www.RobertsResearchInc.com
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