Electronic ballast for CFL
compact fluorescent lightbulbs
One of the alleged selling points of CFLs is that they last much longer than incandescents.
Allegedly. My experience does not bear that out. I have quite a few burnt-out CFLs
and I have spent some time investigating why and how they failed. Of course, it could be
that mine are low quality and higher quality CFLs would last longer. Maybe. Maybe not.
These are 220 Vac CFLs.
The electronic "ballast" is a very simple circuit but they fail quite easily due to heat.
The circuit is contained in the base of the unit and gets quite hot so that electrolytic capacitors
and semiconductors tend to fail. I am sure the circuits would last much longer if they were
place outside and well ventilated. .
Here is the basic circuit. It has a diode bridge and a 2.2 ~ 3.3 uF capacitor so that the
circuit is fed at about 310 VDC. The real "ballast" is composed by L1 and C2 which are both
in series with the fluorescent bulb. Two switches (transistors) switch alternating.
Let us have a closer look.
First C2 is charged through L1 and the bulb. The current goes through the primary of T1 which
generates current for the base of TR1 and keeps it conducting. Until the core saturates and
TR1 cuts off.
Now the situation is reversed, the magnetic flux reverses and TR2 conducts so that the charged capacitor is
now discharged, again through L1 and the bulb. Until T1 saturates and the cycle starts over again.
The frequency of oscillation is mainly determined by L1 and C2 and in this example is about 10 KHz.
The capacitor between the bulb filaments allows the current to go through the filaments before the lamp has
started up. This heats the filaments and starts up the lamp. Once the gas is ionized it conducts
and the voltage at the ends of the bulb decreases so less current goes through the filaments and most of the
current goes through the gas in the bulb.
In this photo you can see L1 on the left and T1 is the toroid on the right.
As I say, the electronic ballast often fails before the lamp does. They can be repaired but it may
take more time and effort than it is worth. I have also taken the lamp and connected it to an old
inductance ballast and starter and it would still work well although it would no longer fit in a screw
A couple of CFL tubes fitted with traditional inductance ballasts and starters.