Re: [VAL] 12 Volt Flourescent Fixtures?

[root <geraldj@xxxxxxxxxxx>]

There NO gas discharge devices that will sustain an arc or discharge
below about 60 volts. And I know from my own experiments in running
fluorescent tubes from DC several decades ago that reliable starting was
much better with a 200 volt supply and series current limiting
resistance (ballast if you will) and some inductance for applying a
larger inductive kick voltage for starting.

You said there was a glow? Was that from the end of the tubes only and a
bit orange or was the glow the length of the tube? There have been long
straight incandescent lamps with diameter about the same as fluorescent
tubes. I don't know of any rated for 12 volts, though that is a
possibility. Generally the end terminals on the long double ended
incandescent lamps are dimpled at the middle, otherwise relatively flat
disks. Glow only from the ends tending to be orangish would be using the
filaments of the fluorescent lamp only. Not energy efficient, nor a
great idea for the environment when the lamp breaks from the excess heat
at the ends and spills phosphors, glass fragments, and a bit of mercury.
A couple automotive dome lamps would be safer and more energy efficient.
Today a couple light emitting diodes would be even better.

The simplest fluorescent lamp circuit (one lamp) takes the lamp, an
inductor (two wire ballast) and a starter. Either a plug-in starter or a
manual start switch, as on many desk lamps of the 50s and later. Power
is wired to one filament though the inductor, the other terminal of that
filament is connected to the filament on the other end of the tube
through the start switch or the starter, then the other side of the
second filament returns to the other side of the power source. To start
you close the switch until the filaments heat, then you (or the starter)
opens the circuit and the inductive kick from the inductor triggers the
gas discharge. The current is limited by the inductor while starting and
while running. The gas discharge does not limit current, if fed from a
stiff voltage source it will draw as much current as the source will
supply and will self destruct in the process.

If that simple circuit is connected directly to 12 volts, the filaments
will glow while the start switch is closed, and there probably will be a
flash as the energy stored in the inductor creates a voltage great
enough to make the fluorescent lamp discharge begin, but as the voltage
falls back towards 12 volts the discharge will quit. Every time.

The earliest solid state ballasts I ever noticed were in city buses in
St. Louis in the early 1960's. They ran by inverting the DC to an ac
voltage at an audible frequency (an unpleasant squeal). Fluorescent
lamps give better efficiency at higher frequencies than 60 Hz power and
tend to blacken one end of the tube when run on DC. knew about

There are other fluorescent lamps and lamp circuits besides the trigger
start. Rapid start and instant start depend on the ballast creating a
high enough voltage (several hundred volts at times) to start the
discharge without heating the filament as hot in the rapid start and
with no filament at all in the instant start lamps.

The two lamp rapid start lamps actually run the lamps in series but
start them one at a time without any starter switch. I've been into knew
about those down to the copper and laminations level while serving as an
expert witness in a fire case caused by the failure of such a ballast. I
was privy to the ballast maker's most private internal design and
production documents during that case. And by abusing a ballast, I was
able to reproduce the source of the fire. There had to be failure of the
external lead insulation at the same time there was an internal failure
that caused the ballast to heat and exude asphalt and asphalt fumes. The
odds of the two happening together were very small but not zero. At that
time, the reports were about 10 per year with about 10 million new
ballasts made each year by that manufacturer and for that particular
type of lamps.
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Today a rope light made to run off a transformer is probably 12 volts
and would work well in the trailer running off 12 volts for mood
lighting.

The modern electronic high power factor ballast made for running on 120
volts AC may run on 12 volts as mentioned here already. What it has is
an input stage that takes the variable voltage of the alternating
current power source after rectification steps that voltage up to an
intermediate DC voltage varying the step up ratio through out the AC
cycle to draw current proportional to the applied voltage. That makes
knew about it look nearly like a resistive load. The earlier electronic
ballasts and indeed the magnetic ballasts all drew a short pulse of
current at the voltage peak which caused a lot of harmonic current in
the AC power system leading to wiring and transformer overheating. I've
not seen any high power factor ballast circuit applications notes saying
it would run on DC since to give enough power to the lamp, the input
current at 12 volts would have to be ten times the input current at 120
volts and during the AC cycle, the current at 12 volts would have to be
one tenth the current at 120 volts to make it have a high power factor,
or the current exactly instantaneously proportional to the voltage. So
as noted here this afternoon, its likely that the high power factor
electronic ballast would burn up the input section after running on 12
volts DC for a while. If it didn't that sure would be an interesting
application because the other 12 volt ballasts that are good are a whole
heap more expensive.
 knew about
To run the fluorescent lamp from a 12 volt supply, the voltage has to be
stepped up with an inverter to at least 80 volts and there has to be a
pulse of higher voltage for starting. Then there has to be some circuit
provision to limit the current through the lamp because the lamp won't
limit the current. That's the nature of the fluorescent lamp.

But the fluorescent lamp won't have a discharge with only 12 volts
applied to the lamp.

Gerald J.