Flourescent Light Voltage

[j611]

Can someone tell me how much voltage is required to power the boards that run the small flourescent tubes used to backlight a laptop screen? Thank you.

[Chris Smith]

It will vary. The theory that I learned when dinosaurs roamed the earth, was it takes a minimum of 69 volts DC to conduct a fluro tube. The heater elements, which are emitters, have to be pre heated and most old common tubes run 6,3 volts or so on these elements. <p>However today, we do things different. High frequency bypasses the need for a heater element, Square wave pulse generators drop the running currents through duty cycles that are very short, and even high voltages up to 1000 volts are used in some cases,... and all if not most run at frequencies from 20 to 60khz pulses. <p>Standard tubes with preheated elements will conduct as low as 70 volts on average, in warm climates.

[dacflyer]

hey since were on subject of these kinds of lights, i have a question maybe you can answer and tellme of a possible fix...
i have a solar light project here..works ok, but also has a problem,,,i have a inverter that i use to power these lights , "Lights Of America" brand
and this unit i use is the 65watt model...very bright..bout like a 150 watt flood..anyway ihave gone thru 2-3 of these already...seems it does not like inverters for some reason..this is a modified sine wave inverter..300watt max...120v output.. i haven't scoped it yet, but its supposed to be at 60hz...
before the lights ballast started failing,i whould have a problem of inverter failure of 140 watt units,,so thats why i upgraded to the beefier one,,and now the ballast in these lights fail...they are electronic ballast....any ideas?

[rshayes]

The lamps usually used to backlight liquid crystal displays are cold cathode flourescent lamps (CCFL). The lamp itself is usually driven with a sine wave of several hundred volts to a kilovolt or so. This is fed to the lamp through a ballast capacitor, so the lamp sees a constant current source. The lamps are unstable when driven by a voltage source.<p>The actual lamp running voltage on the lamp is in the 70 to 100 volt range. The voltage required to start it is much higher, but this higher voltage is only necessary for a short period of time.<p>Sine wave operation is used because it is effficient. The stray capacitance is effectively paralleled with an inductor, so the energy stored in the stray capacitance is transferred to the inductor for part of the cycle, and then used to charge the stray capacitance in the opposite direction later on in the cycle. Sine waves have low harmonic content, which greatly reduces radiation at high frequencies. Even then, shielding is usually required.<p>The boards that you refer to as running the backlight usually have a small high voltage AC inverter and some control circuits on them. The input power to these boards will probably be DC and will most likely be 5 or 12 volts.<p>Linear Technology (www.linear.com) has published several application notes on driving CCFLs. Specifically AN's 49, 55, 65, and 81.<p>[ August 29, 2003: Message edited by: stephen ]</p>

[Chris Smith]

To DAC<p>Fluro tubes show up as a "open circuit" and that is probably why you damage the invertors? Try placing a 15 [?] watt liht bulb parallel in the ac lines with the fluro tubes on the AC output line to make a closed loop. Many devices are not designed to run motors or fluro assemblies. As to the solid state, I couldnt guess but That’s probably not your problem?

[dacflyer]

chris,,, not having inverter problems no more,,its the electronic ballast that are failing..is there something about modified sine waves that electronic ballast don't like..like maybe a hiddin spikes i do not know about??
any suggestions to render this? i will try to contact Lights Of America and see if they have any suggestions,,ok, thanx

[Chris Smith]

Spikes could be bad, if their big enough. Again try putting a resistive load on the line. It does wonders as a "dead short". Light bulbs are great active components. Cheap and easy to experiment with also. Aslo start up surges and peak values you want to keep an eye on.

[sundancer87]

Dacflyer, I live in a solar house so all my power is inverted. I have a Trace 2412, 2400 watts, to run everything in the house. It also is a modified sine wave inverter.<p>As for lighting, all my lights are compact flourescents with electronic ballasts. The compacts that I installed ten years ago are still working fine. I don't want to be a Richard here but, I remember reading something about Lights of America not being worth a damn. Try a Philips or some other brand. Some flourescents just don't like inverted power.<p>[ September 01, 2003: Message edited by: sundancer87 ]<p>[ September 01, 2003: Message edited by: sundancer87 ]<p>[ September 01, 2003: Message edited by: sundancer87 ]</p>

[Edd]

Dacflyer de Jetflyer:
I have used that same L.O.A. unit with the large dia[*whole lamp proper*] fluorescent lamp on pure 60~. No problems with the ballast units just the typical lamp failure….but a bit faster on these small dia [*glass tubing width*] tube designs…due to accelerated ion burns on their electrode terminii.
I average a 3 year lifespan… as averaged on three locations I have them utilized in.
Scope the 120VAC line while connected to the lamp and you will see just how trashy that inverter is. No AM radio DX action with those lamps turned on !
Your most immediate relief would be the utilization of a fused 130V MOV across the inverters power output at the lamp AC socket and optionally also a R/C “snubber “ circuit across the output line. <p>Addenda [**]:<p>j611 de Edd:
As per your initial inquiry as to <<how much voltage is required to power the boards >>(inverters).
In different cases I’ve seen different voltages fed to the inverters, the actual battery voltage of the laptop, or down as low as 5VDC. Most of the units are quite sophisticated circuitry wise in protective features to those lamps that they feed…as you don’t replace those cheaply.
I will submit this surplus site as it might prove useful to yourself or to others:
http://www.excelcomputerinc.com/html/xl ... entory.asp<p>
73's de Edd
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<p>[ September 03, 2003: Message edited by: Edd Whatley ]</p>

[dacflyer]

SUNDANCER....i would use a different brand if i could find something as big and bright as L.O.A.
but no one does make fixtures like that...so it was all i had at hand....Hmmm

[dacflyer]

EDD.....
i will try the m.o.v. on the inverters output...
but i do not understand what you mean about using a snubber on the ballast output...you mean the output that connects to the lamp???

[Edd]

Dacflyer de Jetflyer:
Somehow I overlooked Ur last post entry …two like topics with posts made to both.
The MOV was on the inverters AC output at the lamp proper whereas the other mod was at the primary of the power inverters tranfo.
With some variance in design/parameters in respect to the output devices, and the drive source, one typically finds one fierce overshoot on the leading edge of that inverters pri sq wave signal. For a familiar comparative analogy, akin to 10x the worst case of a waveform you might observe on the 1 KHZ cal square wave fed into an misadjusted/undercompensated scope probe.
At NASA we used a 16 level staircase generated signal for drive, that along with a tuned transformer with their combined ferro-resonant effect, gave a superb result on the generated sine output..<p>73's de Edd
[email protected].........(Interstellar~~~~Warp~~~Speed)
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<p>[ September 08, 2003: Message edited by: Edd Whatley ]</p>

[dacflyer]

oh ok,,, well i hope L.O.A can shed some light too,,,no pun !