# Flourescent Light Transformers = Ballasts

Discussion in 'Electronic Basics' started by BretCahill, Sep 24, 2003.

1. ### BretCahillGuest

How did the high voltage transformers in flourescent lights get the name
"ballasts?"

Bret Cahill

2. ### Fritz SchlunderGuest

Letsee here. My Websters New World dictionary defines "ballast" as
"anything giving stability and firmness to character, human relations, etc."

So it sounds like adding a ballast to a system will make it more stable.

In terms of fluorescent (by the way, it is not spelled FLOURescent, though a
very common misspelling) lamps the "high voltage transformer" you speak of
is really performing a ballasting function more so than a voltage
transformation function.

Simple transformers merely change voltages, they do nothing in terms of
limiting current. An electric arc however will not respond properly to a
constant voltage. The more the arc conducts current the more conductive the
plasma becomes, thus the lower the resistance of the plasma, and thus the
lamp draws more current from a constant voltage supply. The situation soon
degenerates to a point where either the lamp explodes or a circuit breaker
or something trips to interrupt the current (at least in extreme cases).

So in the case of fluorescent lamps the "high voltage transformer" you speak
of really is a ballast. That is, it makes the system more stable by
introducing a substantial series inductive reactance (something not
exactly/precisely true of regular voltage transformers) which limits the
current through the lamp.

3. ### BananaPannaPoe-Guest

Since a fluorescent lamps don't really need ballast, (I have had several
that did not have ballast boxes) It's probably to reduce the voltage from
208 to 110 volts. I have only seen ballasts on 208 volt building systems.
Another explanation could be that Mr. Fluorescent had a brother-in-law that
made ballast boxes and wanted to help him get rich.

4. ### Fritz SchlunderGuest

What led you to the conclusion that fluorescent lamps don't need ballasts?
I can assure you they do need them whenever they are going to be operated
from stiff voltage sources (which the AC mains is an excellent example of).
If the voltage source isn't stiff then you can sometimes get away without a
separate ballast (with very poor regulation though) since the voltage source
itself will perform the ballasting function.

If you have had lamps that didn't have ballasts, and they worked properly,
then you simply didn't see the ballast but it was there. Sometimes they are
made small and integrated into things so they are harder to find/see. Many
of the new compact fluorescent lamps (with basic medium screw bases on them)
are good examples of that. The ballast is small and electronic and is
integrated into the base of the lamp.

The only electric arc based lighting that doesn't need a separate ballast
that I am aware of are the high pressure mercury vapor "self ballasted"
lamps. They use a resistive tungsten filament to perform the ballasting
function and are built into the lamp itself.

5. ### JeffMGuest

/
/ START
+---------------o ---------------------+
| |
| ================================ |
+--I -+ +--I----+
I \ / I
I / \ I
---o --------I -+ +- I----+
\ ================================ |
\ OFF |
Vin |
_________________________ __________________|
UUUUUUUU
========

HOW A FLUORESCENT LIGHT BALLAST WORKS

When the starter switch is closed, the filaments at each end of the bulb
start to heat up and the ballast inductor develops a magnetic field.
The glowing filaments ionize the gas inside the tube.
When the Start switch is released,
the magnetic field in the inductor collapses
and the resulting high voltage helps to further ionize the gas,
making a better electrical path through the tube.
Once the gas is ionized, it conducts with just the normal line voltage
and the ballast limits the current.

6. ### Fred AbseGuest

'Cos they ain't transformers, they're series chokes used to drop the
supply voltage without dissipating (much) power.

I've seen some old fittings with a tungsten lamp as ballast. Ugh! how
wasteful!

BTW, the correct spelling is "fluorescent"

7. ### Dimitrij KlingbeilGuest

Good explanation, Jeff. Actually the only reasonable one.

You only forgot to mention the fact that when the starter closes the
ballast is virtually the only series resistance in the circuit as the
filaments' internal resistance is pretty much negligible. Without the
ballast the lamp (and the starter) will most likely explode when powered
on even before the lamp actually starts.

Mr. 'BananaPannaPoe', you see, that sort of thing won't work without
current-limiting on a fixed-voltage source. P.S. What about a real name?

Dimitrij

8. ### SHAUNGuest

Can this explain when you like hit the light switch and the light will not
come on, but if you keep turning it off and on it then finally come on?

Shaun

9. ### JeffMGuest

It's harder to ionize the gas in an old bulb
but more tries may give a greater chance of success.

10. ### SHAUNGuest

So in other words try replacing the bulbs first, then if this does not work
what should I try next? Thanks for the previous reply Jeff!

Shaun