LV Fluoro-tube driver


Fluorescent tubes. Who hasn't seen one? These guys come in so many shapes and sizes nowadays from a few watts to hundreds of watts, and from tornado spirals to donut shaped. The light they produce also varies, from 'warm' varieties at about 2700K, up to the daylight types at 6500K.

You also probably know that these tubes that you see everyday are being powered by mains power, which in Australia is 240V.

Image courtesy of Sela-Light

Image courtesy of HowStuffWorks

Inside each fluorescent tube fitting is something called a starter and a coil of wire known as a ballast. Then for it to work...

1. The electrodes are heated to emit atoms.

2. A high voltage is delivered into the tube, igniting the gas.

3. The current fed to the tube must be automatically limited, as fluorescent tubes are negative resistance devices and they will attempt to draw infinite current from a source if allowed.

The LV Fluoro-driver MARK I

So what if you want the tube to run off a low voltage, such as from a battery? Then there must be complicated semiconductor ignition circuitry to achieve steps 1 to 3.

Basically the idea is to convert low voltage DC into higher voltage AC by feeding it into a transformer. This transformer requires 7 outputs:

4x 240VAC tube supply
2x Filament supply
1x High voltage ionizing supply (~2000V)

Mark I fluoro-driver setup

To achieve all seven output requirements, the transformer is tapped at different points. The circuit works by effectively "chopping up" the DC input into something that roughly resembles AC (but square wave). This is then fed into the output transformer that ramps up the voltage and provides the appropriate filament power. I remember the efficiency of the circuit to be up around 80%.

Here is my MARK I fluoro-driver lighting up an 11W green fluoro tube.

The LV Fluoro-driver MARK II

With the Mark I driver, the light given off by the tube was quite dim, because the circuit could not provide enough power. The Mark II version employed modifications to increase the output power, including running the transistors at a faster rate through high power resistors.

These resistors had to be fan and heat-sink cooled or else they would start to shoot bits of ceramic around the room. Never a good thing.

Mark II fluoro-driver setup

A close up of the heat sink, fan, and resistor assembly. A real job would have meant bolting down the fan instead of the blu-tac job seen here.

And, just proving that it works. Here is a 20W pure white tube lit up fairly brightly, although nowhere near its maximum potential.

The circuit can power up the 11W green tube to maximum brightness, but struggles to cope with this 20W tube. So I then went on to design a better circuit...

The LV Fluoro-driver MARK III

Again, I made a couple of modifications to the original driver. This time I used higher speed transistors that were capable of much higher currents.

As you can see, the heat sink and fan have been disposed of, as originally they wasted too much power from the battery, and thus lowering efficiency. The higher current handling capacity of the transistors allows them to run much, much cooler than in previous versions.

Mark III fluoro-driver setup

And the Mark III lighting up the same 20W tube as before. Notice how saturated this photo is. The same exposure was used on both photos.

This setup draws 2A at 12V, which is 24W. Assuming the 20W tube is taking in exactly 20W, the efficiency is around about 83% as a ballpark figure. Ahhh...not too bad I guess.

And just for fun, the Mark III lighting up a 10W green fluoro tube.

So there it is, a very harshly assembled, but fully operational low voltage (12VDC) fluorescent tube driver circuit. This would make a suitable portable room light for camping. However I'd like to make a case for it first...




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