has anyone came across flourecent lights that are dimmed down to a set level with a 24 volt dc supply?my problem is when i diconnect the dc supply from one fitting the rest of the fittings in the room go bright,which is a problem given the type of enviroment they are illuminating.my idea was to find the resistance imposed across the dc supply and replace the fitting with the same value resistor enabling me to take the fitting out of service for maintenance with the remainding fittings dimmed.
any info would be much appreciated

James,
could you give a bit more information mate, what type of fittings, control gear manufacture etc, are the fittings all on the same dim pack ?

britspark

Second the questions. What kind of dimmers are in use? Are they just varying the voltage or are they some kind of PWM (Pulse-Width Modulation) controller?

If it's the electronic type of ballast with remote dimmer you're describing, the dimmer is just a typical potentiometer, so yes it could be replaced with a fixed resistor.
These ballasts rectify the incoming mains, with the resulting 340V DC fed into a high frequency (~30-100KHz) ferrite transformer via high voltage switching FET's or transistors. There's a controller IC that sets frequency and pulse width of the drive to these transistors, as well as incorporating other features like shutting down if the tube is too worn out to fire etc.
It is a more efficient way to run a fluorescent tube than an iron cored choke partly because fluoro tubes work better at hight frequencies, and also higher frequencies means less wire (ie. less voltage drop) wound on the inductive components. A further advantage is you eliminate the danger of the strobing effect near rotating machinery. The downside of these ballasts is that they're much more susceptible to damage by high voltage spikes on the mains.
The old way to dim flourescent tubes is to run them via a normal triac light dimmer and iron cored choke, but with the tube heaters permanently energised via a separate filament transformer. The range of control isn't the best...problem being that unlike an incandescent bulb, a fluoro tube needs a minimum voltage for the mercury vapour to ionise. By using the electronic ballast method you can keep the voltage high but just vary the RMS current through the tube by altering the pulse width.

[This message has been edited by aussie240 (edited 08-15-2005).]

240,
Great comments there mate!.
I agree, the older form of dimming was probably more wasteful, with respect to energy use than anything we use today.
The best thing about HF Ballasts these days is the lack of Strobo-scopic effect, as you mentioned.
Where as a few years ago, you had to have a few fittings on different phases to prevent the same thing.