Wednesday, November 9, 2011

Fluorescent Starter


I recently changed a fluoro starter at my sisters place and was amazed to see it was made in Costa Rica.  Don't get me wrong, I have nothing against Costa Rica, but I had never seen anything made there.  I suppose that I have become accustomed to everything electronic being made in China.  Anyway, that aside, it got me thinking, I must have changed 100 or so of these things in my life and never even wondered how they work.  So I thought I crack it open and have a look.

Fluorescent Starter Innards

I don't know much about them, but I know the newer ones are more complicated and have more electronics, but given the obvious age of this one it had to be the older kind, that I now know is called a glow starter.  There isn't much to it, just a capacitor and what looks to be some sort of neon light.

Schematic

The schematic above give a clear view of how the lamp (A), ballast (G), and starter (C) are all connected, and makes things a bit clearer.  To start with there is no power in the system, then power is applied to the terminals at (B), but the there are two problems, there is no current able to flow through the filaments (F) at each end to "boil" off electrons and create a conduction path for the arc, also there is also not a high enough voltage across the tube to strike an arc.  This is where the little glass tube (D) plays its part.  The glass tube is actually a gas discharge lamp with a bimetallic switch in it.  The full mains voltage is applied across the neon tube, via the filaments and ballast, and causes it to glow and give off heat.  This in turn causes the bimetallic switch to close which allows current to flow thought the filaments and preheat them, causing electrons to be liberated.  Because the bimetallic switch is now closed, the heat associated with the neon glow is no longer there, which then causes the switch to reopen. This causes a rapid change in the current flowing thought the inductive ballast which causes a voltage spike (remember V= L di/dt ?).  This voltage spike is large enough to strike an arc in the tube, turning the light on.

Once the tube is started, everything takes care of itself, the discharge in the fluorescent tube is enough to keep electrons liberated, and the voltage across the operating tube, which is in parallel with the neon tube, is low enough to prevent the tube glowing and re-closing the bimetallic switch.  The capacitor (E) in the starter doesn't have much to do really, it is there as a snubber to protect the bimetallic contacts from arcing when they open.  By doing this, the life of the contacts is extended.

This is a basic look at how a starter works.  Most starters and ballasts are now electronic, and therefore more complicated, but the basic purpose of a starter remains the same.  Firstly, preheat the filaments, and then create a voltage spike for ignition of the lamp.

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