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KB2VXA > TECH 30.11.03 00:46l 55 Lines 2310 Bytes #999 (0) @ WW
BID : 16194_WT3V
Read: DB0FHN GUEST OE7FMI
Subj: Re: M1BYT > DC floro tubes
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Sent: 031129/1942 16194@WT3V.#CNJ.NJ.USA.NOAM $:16194_WT3V
From: KB2VXA@WT3V.#CNJ.NJ.USA.NOAM
To : TECH@WW
Hi Harry and all,
With all this talk about BC resistive ballasts I'll toss out some food
for thought and let you guys chew on it. I'll start by saying that
resistance limits current by dissipating the greater portion as heat,
wasteful. Think about it a moment....
now move on.
I have peeked inside the power supplies for movie theatre projectors out
of curiosity to see how they derive power for the carbon arc and found
something not the least surprising. The box contains a three phase
transformer, full wave rectifier and reactor, nothing more. It's simply a
DC power supply you're all familiar with, minus the capacitors.
Now think about that supply for a moment, the only departure from the
familiar really is the three phase supply vs. single phase in the ham
shack. BTW, high power broadcast transmitters use a three phase supply
for current considerations, but the major by product is far less ripple
so smaller capacitors may be used to smooth it out. OK, the poin is
RIPPLE.
So in reasoning it out, one may conclude that those carbon arc supplies
(and DC arc welders) get away with a reactive ballast rather than the
wasteful carbon stack resistor that was employed in very early supplies.
Now consider that DC mains were supplied by generators or rotary
converters in some unusual cases which have considerable ripple content.
What, did you think they came from really big batteries? (;->) OK, now
you have something to nibble on!
Oh, now if you somehow can't quite put your finger on it here's a tidbit.
Think "split core reactor" or "split core transformer" and you've got it.
How do they limit the current through the magnetron in a microwave oven?
Peek inside and you'll see a voltage doubler circuit with no load, what
appears to be a short circuit to the uninitiated. Clue: Split core
transformer. Still confused? One last clue, core saturation.
OK you electrical engineers (boffins included) GET ON IT! (;->)
73 de Warren, KB2VXA@WT3V.#CNJ.NJ.USA.NOAM
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Message timed by cesium laser: 19:50 on 2003-Nov-29 GMT
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