🚀 go-pugleaf

In article <Bz1z7y.Byn@wrs.com> miked@wrs.com (Mike Deliman) writes:

>I've been trying to stay out of this one, but can't resist :-).
>John is correct. The ballast converts the voltage to a range which will
>excite the gasses in the tubes. The starter on the other hand, preheats the
>cathode and annode so the current (miniscule) will flow; the heated elements
>more actively shed electrons. What the starter does is make a circuit which
>flows through both "fillaments" inside the bulb until they are hot, at which
>point the circuit is broken, and the current flows through the gas inside.
>Some ballasts create high enough voltages and/or frequencies to not need a
>starter. Some older modles eliminated the starter, and instead used a
>momentary switch (such as the type (1) mentioned above).
The starter heats the filaments.  The filaments vaporize a bit of
the internal mercury, starting conduction, as well as let off a
few (quickly captured, it's hardly a vacuum in there) electrons.
This provides a bit of current down the tube, creating more
ionized mercury vapor, creating more current, etc, until the 
tube ignites and enters normal conduction.  The flickering and
swirling of old lights come from an inability to establish a
good, steady conduction pattern.

>Most "starters" are filled with (I think) mercury gas, and are encased in
>metal housings as they may give off radiation. The function as a sort of
>capacitor; as long as the resitance across the tube is high enough, the
>charge will build in the starter until it can jump across the annode &
>cathode within, making the circuit to preheet the tube's annode and cathode
>at which point the resitance in the tube drops to where it's connection is
>made and the flow through the starter is interrupted and the light
>illuminates.
Noooooooooooo.......

Starters consist of a small heater, a normally-closed thermal switch,
and a neon bulb, all in series.  When the lamp is NOT lit, the neon
bulb will strike. This pulls the current that would normally go through
the tube through the filaments.  They heat up. The heater also
heats up.  It opens the thermal switch. The flyback effect puts a 
substantial voltage spike across the tube.  The combination of the
warmed filaments and voltage spike lights the Tube. Usually.
When the tube is lit, the neon bulb won't strike, so the
starting process doesn't occur repeatedly (bad neon bulbs
are one classic symptom of a bad starter.  tube lights.  goes into
start mode. lights.. repeat until filaments burn out)  If the tube
fails to light, the voltage across the tube is high enough to restrike
the lamp when the thermal switch closes again, and the cycle repeats.

The other failure mode for a starter is "nothing happens". This means
either the heater is burnt out (i.e. open) or the neon bulb won't
strike. This can be due to either ballast undervoltage or to 
a bad neon bulb.  In any case, replace the starter first.

Newer ballasts use several methods. Fancy electronic ballasts
detect an extinguished lamp via other methods, and heat the
filaments when this happens.   They have a similar voltage-current
behavior to the standard ballast, but implemented a more efficient way.
Cheap electronic ballasts use an RF ignition circuit, they put a LOT
of voltage across the tube, and strike the tube that way.  These ballasts
work pretty well at room temperature, but can often fail to get
all of the mercury volatalized, so they often run over-current to prevent
this, greatly lessening the life of the tube.  Some, instead, don't
ruin the tube, but take hours to get to full output.  These ballasts
require the "grounded shield" in order to work. Some of the better
electronic ballasts also require the "grounded shield", unforunately,
so it's not a cut-and-dried test.
-- 
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