RE: ELECTRONIC BALLAST?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

 >I would have expected the inverter to be a simple active PFC type
 >with little or no input smoothing capacitance.

Well, one of the selling points of these lamps is that they don't flicker at
60/120Hz. Without the smoothing cap, they would probably flicker just as
badly as an iron cored ballast. To get round this, you'd need an extra PFC
stage in front of the smoothing cap, which is getting a bit fancy for a $5
bulb :(


Here in 240V land, the circuit inside energy saving lamps is a self-resonant
half-bridge inverter using BJTs. The BJTs get their base drive from the
inverter output current through a tiny ferrite toroid transformer. The diac
and associated components are for kick-starting the inverter.

The start/ballast circuit uses a choke in series with the tube and a
capacitor across it. The capacitor is connected through the filaments. At
start-up, the inverter locks to the resonant frequency of this circuit,
generating a high voltage across the tube which usually lights it
immediately. If it doesn't light, the high current in the "tank" circuit
will heat the filaments until it does.

Once the tube lights, the resonance is damped, and the choke acts as the
ballast. The inverter now runs in a ferro-resonant mode with the frequency
set by saturation in the base drive transformer.

It's amazing how they can achieve so much with so few components.

Steve C.