Low voltage fluorescent lighting
I rescued from the trash a number of emergency lights of the type mandated in buildings in case the main electric power supply fails. These had NiCad batteries and fluorescent bulbs. I thought I would tinker with them and maybe find some use for them. Maybe as emergency lights or maybe as regular low voltage lighting for camping, boat, RV, etc.
The first obstacle I found was that the NiCads provided 4.8 Volts and that was the circuit's design working voltage while I wanted something like 12V. Oh well, I still went ahead with my tests and investigation.
Here is the basic circuit. In a way it is very similar to the CFL electronic ballast in that it has two switches (transistors) conducting alternatively and the oscillation is maintained by the saturation of the transformer core. Let us have a closer look. I found the same basic circuit with a minor difference in how the switching transistors were driven.
The circuit on the left requires two extra resistors while the one on the right needs a center tap y the transformer winding which feeds the bases of the transistors. Otherwise they are identical in the way they work. When TR3 is conducting the circuit is ON. This is the switch used by the rest of the emergency control circuit but it is not necessary if the light is to be turned on whenever there is voltage applied. In the original circuit TR3 conducts when the mains supply failed.
Each transistor conducts in turn. The collector current which goes throught the winding of the transformer induces base current so that both currents grow until the core saturates and base current falls off cutting off the transistor and starting off the same process in the other transistor. These oscillations create a high voltage in the secondary which feeds the lamp. A capacitor C1 is placed in series with the lamp to limit the current serving the same purpose as the inductance ballast serves in low frequency powered fluorescent lamps.
The next graphic shows the voltage measured at the ends of the fluorescent lamp. A noticeable dip happens when the gas starts to conduct.