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Experiment with Florescent Lights, suggestions invited

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GammaRay

Mon Mar 25 2013, 03:18PM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
Location:
Posts: 104

As a fun experiment, I charge up a large value HV capacitor and then discharge it through HV resistors ranging between 25megs - 500 megs to illuminate between 4 foot - 32 feet of 40 watt Florescent light tubes connected in series. The higher the resistor value, the longer time the bulbs stay illuminated, but at the cost of dimmer light output. Much of the HV capacitor's energy is wasted in the resistor, thus the purpose of this post.

With the goal of using a HV capacitor to illuminate the Florescent bulbs for as long as possible (on a single charge of the HV capacitor), I wonder if anyone in this Forum can suggest an off-the-shelf (Demo/Evaluation PCB) High Voltage PWM that can accept the source voltage from the HV cap >1kV to drive the Florescent tubes? Alternatively, if there are no off-the-shelf suggestions, I would be appreciative of recommended circuit designs for me to fabricate to try in this experiment. Thank you.

Ash Small

Mon Mar 25 2013, 04:00PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Maybe use a vacuum tube? A simple timer circuit could be used to drive it.

Fraggle

Sat Apr 13 2013, 11:00AM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

How about an inductor as ballast?

...and the beauty of that is, as the cap discharges and the dI/dt drops so will the reactance of the inductor so the brightness will be more constant throughout.

Dr. Dark Current

Tue Apr 16 2013, 07:20AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You might rather want to charge a lower voltage higher value capacitor with the HV one by means of inductor (and reverse diode on the HV cap). You can then use a normal low voltage switcher.

GammaRay

Wed Apr 24 2013, 01:51PM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
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Posts: 104

Dr. Dark Current, I'd like to try your suggestion. Can you enlighten me with a bit more detail, or forward me to a link that might educate me a bit more as to your recommendation? Thanks in advance.

Sulaiman

Wed Apr 24 2013, 06:09PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Almost all gas discharge lamps/tubes have two main problems;
1) The voltage required to start a discharge is much higher than that required to run it
2) During discharge the lamp has a negative resistance characteristic,
increasing current reduces the required voltage

You have been using a very high voltage
(required to initiate discharge)
and a high value of resistance
(much greater than the combined negative resistance, stabilizing discharge current)

A typical fluorescent lamp used a series inductor to provide a high initiation voltage
and provide positive impedance greater than the lamp negative impedance,
modern ccfl lamps provide high voltage by resonance,
and impedance by using under-rated components ;)

Neon sign transformers (more like your requirement) provide high voltage by transformer turns ratio and high output inductance as series impedance.

You could consider a flyback (T.V./crt LOPT or automotive ignition coil)
as they can be very good at driving discharge tubes,
high voltage 'spike' to ignite the gas, then constant power output.

Powering series lamps from a capacitor ......
as your setup, find the lowest voltage that will initiate discharge
(if you haven't got a safe h.v. probe don't even try, we need Members)
you may be able to parallel what were series capacitors for maximum stored energy
then it's just as you said, higher resistance = longer time = less bright.
You could use electronics to improve efficiency but the cost of high voltage electronics makes it not worthwhile unless you MUST operate at high voltage.
A large value low voltage electrolytic capacitor powering an inverter would be more economical.

Tony Matt

Thu Apr 25 2013, 01:39AM

Registered Member #3700 Joined: Sat Feb 19 2011, 12:59PM
Location:
Posts: 107

Why do not use LEDs ?

Conundrum

Thu Apr 25 2013, 11:05PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

IIRC they parallel connect CCFLs on some older LCD TVs. though normally there is also a current limiter integral to each tube in the form of an HV capacitor.
Some of the more recent ones use low side limiting with a current feedback so the tube brightness doesent vary with time.

radiotech

Mon Apr 29 2013, 06:19AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The 25 to 100 megohm balast you are using will just give you the light that is
released by the filler gas and the blue glow found in the top of old barometers.
The 254 nM line of the Hg will never happen unless you pass rated current
down the T12 bulbs. You probably would get more light with a static machine.

Lots of experiments you could do. Try and use a lamp as a rectifier to produce DC.
Google RF type fluorescent lamps circa 1940.

You could power up a germicidal lamp and then insert it into a T!2 tube
with both ends carefully removed, and activate the phosphor through the air.

Dr. Dark Current

Mon Apr 29 2013, 09:30AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

GammaRay, here is the circuit I was talking about.

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