Subject: What are fluorescent light ballasts?

From: [email-masked] (Joel Grinberg)
Date: 6 Sep 1995 18:36:55 GMT

I recently went to buy simple fluorescent light fixtures for my
garage, but was overwhelmed by the selection: no ballast (in shop
lights), cheap or expensive ballasts, electronic ones...
Of course, the salesman were incapable of giving any real information.

What to these ballasts do? How do they work? How can you tell the
quality of a given ballast?

Thanks,
Joel

---

From: sam@colossus (Sam Goldwasser)
Date: Sat, 9 Sep 1995 23:15:50 GMT

>   quality of a given ballast?

Here is a brief summary.

--- sam

Flourescent Lamp Ballasts:
-------------------------

For a detailed explanation, check your library.  Here is a brief summary.

A ballast serves two functions:

1. Provide the starting kick.
2. Limit the current to the proper value for the tube you are using.

In the old days flourescent fixtures had a starter (some cheap ones still do).
The starter is a time delay switch which when first powered, allowed the
filaments at each end of the tube to warm up and then interrupted this part
of the circuit.  The inductive kick as a result of interrupting the current
through the inductive ballast provided enough voltage to ionize the gas
mixture in the tube and then the current through the tube kept the
filaments hot - usually.  You will notice that a few iterations are sometimes
needed to get the tube to light.  The starter may keep cycling indefinitely
if either it or one of the tubes is faulty.  In this case, the ballast
is just an inductor which at 60 Hz (or 50 Hz) has the appropriate impendence
to limit the curret to the tube(s) to the proper value.

Types of iron ballasts:
----------------------

Instant start, trigger start, rapid start, etc. ballasts include loosely
coupled high voltage windings and other stuff and do away with the starter:

1. The ballast for a preheat fixture (using a starter) is basically a series
   inductor.  Interrupting current through the inductor provides the starting
   voltage.

2. The ballast for a trigger start fixture has a loosely coupled high
   voltage transformer winding providing about 500-600 V for starting
   in addition to the series inductor.

3. The ballast for a rapid start fixture has in addition small windings for
   heating the filaments reducing the required starting voltage to 250-400 V.

Starting voltage is either provided by the inductive kick upon interruption
of the current bypassed through the starter for (1) or a high voltage winding
in (2) and (3).

In all cases, the current limiting is provided primarily by the impedance
of the series inductance at 60 Hz (or 50 Hz depending on where you live).

Electronic Ballasts:
-------------------

Electronic ballasts are basically switching power supplies that eliminate
the large, unreliable, 'iron' ballast and replace it with an integrated high
frequency inverter/switcher.  Current limiting is then done by electronic
chopping instead of inductance.  Properly designed electronic ballasts
should be very reliable.  Whether they actual are reliable in practice
depends on location with respect to the heat produced by the lamps as
well as many other factors.  Since they include rectification, filtering, and
operate the tubes at a high frequency, they also usually eliminate the 120 Hz
flicker associated with iron ballasted systems.

I have heard, however, of problems with these relating to radio frequency
as well as optical interference.