Re: Fluorescent Lamp Heater Filament Current

"Don Klipstein" <don@xxxxxxxxxxxxxx> wrote in message
news:slrnfohs61.5sn.don@xxxxxxxxxxxxxxxxx
In <478892a4$0$32673$afc38c87@xxxxxxxxxxxxxxxxxxxx>, Steve Carroll wrote:

Does anyone know the typical current and/or applied voltage through a
fluorescent tube's heater filaments, during startup and continuously
through
'rapid-start' types, or know of a good source of this info?
(I'm mainly interested in compact fluoros and small, (F4-T5, F8-T5,
G4-T5,
G8-T5), standard fluoros.)
In preheat lamps, the starting current is a little more than the normal
operating current. For those lamps, typically the filaments have current
sent through them as opposed to having a voltage source applied to them.
In F40 rapid start lamps, the voltage applied to the filaments is about
8.5 volts, and is not much different after starting than before.
Avoid applying near or over 10 volts to a fluorescent lamp filament
unless current is limited. At about 11-12 volts, an arc usually forms
across the filament. This often occurs in preheat lamps, but that is OK
since current is limited.
There is a variation of "rapid start" known as "trigger start". That
involves a ballast with filament windings that produce voltage that
decreases once the lamp starts. Such ballasts are intended to run preheat
lamps.
One source: Sam Goldwasser's F-Lamp FAQ. A recent version is somewhere
in http://repairfaq.upenn.edu or in http://www.repairfaq.org (from
memory). I have an older version at:
http://members.misty.com/don/f-lamp.html
That document mentions voltages from a common rapid start ballast.
- Don Klipstein (don@xxxxxxxxx)

Hello Don, Thanks for all the info. I've already read f-lamp.html, (I
have a local copy of most of Sam's FAQs), including your page on discharge
lamp theory. It was very informative. So was the link provided by Wim.

I'm designing a couple of inverters to run a 4W and a 20W tube from 12VDC,
ideally in modified 'rapid-start' mode. (The heater current is reduced or
removed after the tube ignites.)

It appears that most tubes use 3.4 to 4.5V, while some need about 8V, as you
mentioned. I have two tubes in front of me, a 20W coil type, taken from a
compact fluoro and a 20W T10 black light tube.
The compact fluoro coil tube filament has a cold resistance of 4 ohms and
lights up dimly with 3.5 to 4V (DC) applied.
The black light tube filament has a cold resistance of about 8 ohms and
lights up dimly with 8V (DC) applied.

I'll do some hot filament current measurements next.
First, though, a couple more questions, if you don't mind.

1. Do you know if it's better to run the filaments so that thay're visibly
glowing, or just short of that point?

2. Most small inverters built for this purpose, especially 4W to 8W tubes,
use a single-transistor oscillator with positive feedback derived from a
feedback winding on the inverter transformer. This results in a pulsed
output. Does this mean reduced brightness and if so, can it be compensated
for by adding extra turns to the HV secondary, or is a 50% duty-cycle
complementary drive better?

.... Steve


.

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