[Collins] 872a Lamp, revisited

[Dr. Gerald N. Johnson, electrical engineer]

Sorry Bob, coffee or no, you don't understand the details of the
ballast. Its all inductive and magnetic. Its complex, the high voltage
output of the fluorescent ballast is generated by a transformer with a
saturating section between the primary and the secondary that makes it
very much a constant current no matter whether the load terminals are
short circuited or nearly open. Like a welding transformer that shows
the same current whether shorted or arcing. Some welding transformers
move the position of the magnetic shunt to change the current. In the
old two lamp trigger start ballast the current through the four
filaments is practically the same when the starter has them all hooked
in series across the output as the current is through the fired lamps.
In that system, when the trigger starter opens, there is an inductive
kick that gets plenty of voltage to the lamps for starting the arc. One
need not add R for making the 872 glow, the ballast will take care of
that by the core saturation.

A cap may well resonate with some condition of the ballast's inductance
and INCREASE the current. I've used that effect with a 220 volt muffin
fan on 120 volts. Just the right C approached resonance enough to give
220 volts on the fan coil terminals. In another case I used a much
different C to lower the voltage on the fan terminals to make it run
slower. Both are possible, sometimes with disastrous effects when the
resonance isn't needed.

73, Jerry, K0CQ

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.