Sid,Roman,Peter..
Thanks for you all for your useful explanations.

Tal

-----Original Message-----
From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Peter L. Peres
Sent: Sunday, January 05, 2003 8:29 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Ballast


A fluorescnet ballast is something that allows a discharge tube to be
connected to an ac (or dc) source. For ac mains it is usually a coil or
choke, which is deliberately lossy. An electronic circuit can replace it.
Then it will also work in dc.

The simplest 'choke', is a high power resistor. Its purpose is to limit
the current into the tube, which will otherwise rise until the tube or
something else will be damaged. The resistor will dissipate a lot of heat
(up to 80% of the input power for 220V in and 14W tubes).

In ac one can use a coil instead of a resistor. This will reduce the power
dissipated in the 'resistor' (now a coil) with the same effect.

An electronic ballast essentially does what the coil does, as it also uses
a coil, but a smaller ferrite one, and it is more efficient and starts the
tube faster.

A capacitor can also be used for the same purpose, but only for ac
circuits. Then special arrangements are needed to ignite the tube. Roman's
circuit is an example.

Peter

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