Fluorescent Lighting: DC application

All of my knowledge on fluorescent lighting fixtures lent me to believe that a ballast for regulation and a high voltage power source for initial arc were required in order for lamp function.

I came across an old fluorescent bulb lantern recently and it has two eight inch, six watt fluorescent tubes. (T5) It had two 6volt alkaline batteries for power. I understand that because it is using DC it would not necessarily need a ballast for current regulation purposes but this doesn't always seem to be the case as in many RV fluorescent DC applications that still use a ballast. First, when and why/(why not) is a ballast necessary when using a DC power source?

Second, I was unable to take the lantern completely apart without breaking it, but curiosity is soon to get the best of me. It seems as if the power is connected directly to the sockets and hence the lamps. How can this be? Is there not around at least 100 volts needed to arc the gas initially? Finally, how is this done or would be done the best way in DC fluorescent applications... inductor, capacitor, etc?

I hope to find out more before I rip this old family lantern apart... Thanks in advance for the help.

Connor

In addition to the comments by Colin, I offer the following expansion. A (gas discharge tube) fluorescent bulb is such a tube has a very low impedance when lit or struck. However its impedance is very high when not lit. A high voltage pulse (possibly hundreds of volts) is needed to kick it into action, Once strucck it only needs about 90 volts to kep going. The inductor is required to give some impedance in series with the tube to limit the current as the tube behaves much like a high voltage zener diode and a small increase in voltage (above approx 90V) will cause a very large current increase within the tube without the inducror being present. Now I think it necessary to mention here that there needs to be a DC to AC converter to change the 6 volts to 90 volts and thiis is usually done with a single transistor oscillator. The high voltage starting pulse is a natural function of the oscillator.

Hi Connor,

I spent many years working on designs for both fluorescent and Xe discharge tubes. In all these cases if a continous light is required then some form of ballast is required.

The reason for this is that a plasma arc exhibits negative resistance. This may sound bizzare and is best understood by considering the value of dV/dI (rather than the usual V/I) to equal resistance at a given point, in which case when struck, the voltage accross the arc goes DOWN with increasing current (c.f. the voltage accross any normal resistor goes UP with ncreasing current), hence the slope is the other way and the resistance (in that region - hence the use of dV/dI) is negative!

It's not difficult to se, that with any supplied voltage, the current will just increase and the voltage fall, forming an equilibrium with the supply, and unchecked would involve huge and damaging currents in the system. In otherwords, a negative resistance is indeed worse than a short circuit as it subtracts from the internal resistance of the power supply and will really do some damage.

Hence the ballast adds a positive resistance (or impedance), allowing a regulated control from a power supply as required. It will, if resistive, of course; dissipate power, and inductive solutions are thus better.

It is also true that for smaller loads, this ballast can effectively be built into any SMPS output transformer, thereby giving the illusion that it not be required. Nevertheless, the inclusion of a ballast resistance will have featured into the design somewhere - if not the system will have limited lighting ability (one flash!).

Hope this hasn't confused too much - have fun with your design.

Jeff

Thats a very good explanation Tell me why is it then you can power neon lights using an Rf tramsitter whos arial is held close the neon fluorescent light with out any physical connection and why too has this not been resarched to with view to connectionless fluorescent lights What a saftey feature that would be and how easy too to implement think also just how much money that would save as well as from a low voltage point of view Simply push the light in withpout having to power anything down at all All inductivley coupled

When I demonstrated this in a shop operating from a 9volt battery pp3 and showed them a six foot tube igniting they laughed thier heads off and thought I was Mad although I thought how clever could this be

Imagine no 240 volts Imagine low voltage low current lighting operated from an Rf Source from as little as (9 volts at no more than 50 ma ) and if you can get the molecules of a flouresent to vibrate with RF such that the tube emits light, how far are we away from cold light and if so what gas would the tube be filled with I wonder

the original inverntor of cold light was bought out and subsequently silenced He worked for phillips UK

The original inventor of cats eyes in the roads that you see so many of today never made a penny out of his invention yet they must have made fortunes out of his ideas Also true Also British (Fact not Fiction )

Truly amazing were my thoughts at the time although many other people said it wasnt possible till I proved this to them ( IT is and I know it is )

Like to hear ideas on this

Join the club! Invention is very diffcult to market. Greedy investors and other rip off merchants will not play ball with you (personal experience). You see, unlike copyright, stealing an invention is not a crime, so an inventor needs to go the civil litigation way if needed, and of course it is usually not possible due to a lack of money to do so. Governments can acqure patents they feel is in the national interest, and not pay a penny to the inventor. (As experienced by an associate of mine). My suggestion to you is to go the way I am going to go from now as I have bfore. Just do your experiments, have fun and publish in the press such as Pulse Magazine. I am a prolific inventor, and if industry and investors were fair, both they and myself would be much wealthier,
Now as for the glowing tube. The tube needs high voltage to light, and this comes from the end of an odd number of quarter wave lengths of a wire antenna, or ina a case where I wanted to impreess the secretaries of the design office, I used an antenna tuner and a 100 watt transmitter and held the end of a fluorescent tube, it looked like a light saber from Star Wars. I am not too sure of just how much light was emitted however. I wonder if an equivqalent amount of light will come from a non contact RF input as would be generated from connection to the mains.