Fluorescent lighting

[Dan Beck]

In the typical set-up, what is the purpose of the "ballast"? Is the
"starter" simply a capacitor? Does the "ballast" ever wear out? If the
"starter" is a capacitor, I would imagine that can wear out?

I tried Google to research this first, and it told me this newsgroup no
longer archives :-(

Thank you for reading,
Dan

 

[Joel Kolstad]

In the typical set-up, what is the purpose of the "ballast"?

To limit the current through the tube so that it doesn't self-destruct as
well as helping to start the tube. Traditional ballasts are just big
inductors, so there's a series impedance of j*omega*L with the tube.

Is the
"starter" simply a capacitor?

No, it's a little bi-metallic strip that slowly heats up and, after a few
seconds, opens up. This interrupts the current flow through the ends of the
tube (each of the two pins on either side of the tube are a filament...
running current through it heats up the filament so that many electrons are
boiled off). SInce the ballast is a big inductor, the interruption also
generates a high voltage pulse that creates the arc in the tube. Once the
arc is struct (the gasses in the tube are ionized), the voltage required to
maintain the arc is relatively low compared to the starting voltage. Still,
your available voltage limit how long the tube can be, generally speaking --
in the U.S., with only 120V, I don't believe one could do better than 4'
tubes. Commercial installations have higher voltages available, though.

Does the "ballast" ever wear out? If the
"starter" is a capacitor, I would imagine that can wear out?

Starters wear out pretty commonly, being a quasi-mechanical device.
Ballasts are much sturdier -- many seem to go for years without problems. I
don't know their typical breakdown mechanism either.

I tried Google to research this first, and it told me this newsgroup no
longer archives :-(

Keep in mind that, these days, many so-called 'ballasts' and 'starters' are
all-electronic affairs. These are effectively switching power supplies that
output a regulated voltage and can provide benefits including instant
starting, higher reliability, more efficient operation, and dimming.

---Joel Kolstad

 

[Steve Dunbar]

In the typical set-up, what is the purpose of the "ballast"?

See Sam Goldwasser's Fluorescent FAQ at
<http://members.misty.com/don/f-lamp.html>.

 

[Michael A. Terrell]

In the typical set-up, what is the purpose of the "ballast"? Is the
"starter" simply a capacitor? Does the "ballast" ever wear out? If the
"starter" is a capacitor, I would imagine that can wear out?

I tried Google to research this first, and it told me this newsgroup no
longer archives :-(

You read it wrong. Google no longer archives
It still archives

 

[w_tom]

Lamp has two modes. First a high voltage (little current
required) must convert gas to plasma. Second circuit must
change from a voltage source to current source - to limit the
amount of current through a near short circuit created by that
plasma. As long as sufficient current is maintained, then
plasma will remain as plasma, conduct the current, and emit
photons.

 

[JeffM]

what is the purpose of the "ballast"?
Good answers, all, from Joel, Steve, and Michael.
Here's a simple drawing to boot:
http://groups.google.com/groups?hl=...e=off&q=jeffm_+Fluorescent&btnG=Google+Search

No, it's a little bi-metallic strip that slowly heats up
and, after a few seconds, opens up.
Joel Kolstad

Typically the thermostatic switch is inside an envelope (a bulb)
which contains a noble gas (argon or neon).
When power is applied, the neon glows and creates heat.
The thermostat gets warm and closes, shorting out the gas-filled gap.
With the neon no longer lit, it cools and the thermostat opens.
The shunt path offered by the now-ionized mercury vapor
inside the fluorescent tube doesn't allow the neon to strike again.

 

[Dan Beck]

Thank you all, it was very informative!

Now, to satisfy the chemistry geek in me, does anyone have the physical
chemistry/engineering background to explain the relationship between Ar
atoms, (or Ne) with Hg atoms, electrical energy, and the resulting UV
photons that light the phosphors? I probably should post this to a
chemistry group, I realize; I just thought I would try here.

Thank you again.
Dan

 

[w_tom]

For more details of the concepts, seeks books on plasma
physics. Typically requires an engineering library with a
good supply of books published on or before the 1950s. Also
IES handbook (industry bible) may provide good background.
Again, only a better library would have this.

Conversion of inert gases to plasma is a function of
voltage, pressure, the gas (or composition), and other lesser
factors such as temperature and material of electrodes.
Generally the tube is a low pressure inert gas or combination
of inert gases - depending on light frequencies desired. These
are UV frequencies which is why a moderator inside the tube
converts UV light to visible light.

Trace amount of mercury is added to assist startup. Other
techniques include keeping the glow filaments slightly warm to
last longer. These are details that refine a design. Hg is
not essential to basic operation. Gas composition and its
pressure more determine tube operating characteristics.
Unfortunately, trace amounts of Hg improved operation.

Related example is a vacuum tube called 0B2 which provided a
regulated 105 volts. Again, plasma physics.

 

[Doug]

Thank you all, it was very informative!

Now, to satisfy the chemistry geek in me, does anyone have the physical
chemistry/engineering background to explain the relationship between Ar
atoms, (or Ne) with Hg atoms, electrical energy, and the resulting UV
photons that light the phosphors? I probably should post this to a
chemistry group, I realize; I just thought I would try here.

Thank you again.
Dan

Also look at posts in sci.engr.lighting there are some very knowlegable
people there.