Wednesday, Jan 30, 2008 at 19:29
OK guys, Fluoros 101:
A fluorescent lamp consists of a sealed tube containing a low pressure gas, usually containing traces of mercury. When a high voltage is applied, the gas ionises and conducts current. The ionisation of the gas causes light to be emitted; different gases give different coloured light; eg, neon gives a red light, argon gives a bluish light. In the case of fluorescent lights, the light is ultra-violet. The inside of the tube is coated with a fluorescent coating, which absorbs the ultraviolet light and re-emits visible white light.
All gas-discharge tubes require a high voltage to start ionisation, whereupon the voltage will drop to a lower value upon ionisation; ie once the gas has been ionised, a lower source voltage can be used to maintain the current. A typical fluorescent light requires about 500v to strike the gas, and then stabilises at about 100v across the ends of the tube. The traditional old fluoro tubes used a starter and ballast to induce the initial high startup voltage, and once struck, the ballast was used to maintain the lower voltage on the tube and limit the current. Without a ballast inductor, the current through the tube would be excessive and destroy it.
In the 1960's it was discovered that operating a fluoro at a high frequency gave better light output than operating the tube at the standard 50Hz mains frequency.
CFLs use electronics in the base of the tube to both generate the high voltage and also generate the high frequency required.
The 240v CFLs rectify the mains, then use a transistor chopper and inductors to increase the voltage and create the high frequency. Internal ballast inductors are incorporated to limit the discharge current through the tube.
The 12v fluoros use an inverter operating at about 20-40KHz, incorporating switching transistors and a transformer to step up the voltage to several hundred volts. The ouptut is nominaly about 200v, with spikes to several hundred volts, which allows the tube to strike. The design of the inverter allows current limiting, and most, though not all, don't bother with a current-limiting ballast inductor.
So the 12v and the 240v circuits are quite different.
As an aside, Oatley electronics supply a kit to run several 240v CFLs off 12v. This is basically a 12v to 350v DC-DC converter, which supplies 350v DC to a rail which can then run several CFLs. 350v is needed, because the peak value of 240v AC is about 340v and the CFLs wouldn't operate properly off 240v DC.
cheers,
Gerry
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