[Collins] 872a Lamp, revisited
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Tue, 2 Sep 2003 15:01:36 -0400 (EDT)
Jerry, I do understand how ballasts work. The resistor is a KISS approach
to limit the current as you said may be necessary when I first suggested
using a ballast...
> Since a fluorescent light is based on mercury vapor, the ballast idea is
> a good one. There are ballasts rated at 1/2 amp through the tube.
> Voltage drop in the 872 should be lower than in a long fluorescent
> though which might fry the ballast if its current limiting isn't all in
> the coil and core.
So, I don't understand what you're getting at. A cap is fine for AC
current limiting as well. You said it yourself in your example of raising
and lowering the voltage across a fan motor. One just has to understand
what is going on in the particular setup.
I've been giving some additional thought to this and have an idea that the
872A shoudl work great with a ballast designed for two 20W tubes (40W
ballast). Even though the 872 is rated for about 1.5A continous , this is
as a rectifier and as a rectifier, current will only be flowing during a
half cycle, so the tube should be fine. OTOH, perhaps the 872A could be
operated more like a lamp with the full cycle used for excitation. Would
we have to limit the current to 20Watts? I don't think so, because once
fired, the heater could be shut off and the total heat being generated in
the tube will be abotu the same as the half wave rectifying case.
I'm also thinking that a 20 W or so incandescent lamp in series with the
tubes heater would make it a better match for preheating with the ballast.
Guess it's time to root around and see if I have any magnetic ballasts in
the junk pile...
-Bob
On Mon, 1 Sep 2003, Dr. Gerald N. Johnson, electrical engineer wrote:
> 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
>
>