I ran into this old cockpit/pilot room for heavy mining machinery in which there was an LCD display that's not lit properly, that is, the image was there but the backlight clearly was gone.
Upon careful disassembling the display, I found out that the backlight was indeed not properly working. However I couldn't tell if it was the fault of the fluorescent tube, or the power supply.
So I measured the power supply voltage for the tube, the voltage naturally was in AC instead of DC, and the output was around 7 volts.
This led me to believe it probably was the fault of the converter board instead of the tube, because no way you could light up a fluorescent tube with a meager 7 or so volts, which sometimes requires as much as 10,000 volts! However I have no specialty in this and relied solely on my common sense, I could be wrong, as there could be for example circuitry that brings up the voltage in the LCD assembly?
But that is unlikely as I should probably point out that the display was, and I say it with great confidence, a run-of-the-mill 17 inch commerical lcd display, nothing special about it, other than it was embedded in the control panel of the pilot room for mining machinery.
So… was my judgement correct? What is the typical voltage for driving LCD fluorescent backlight tubes?
Best Answer
Cold-cathode backlights do need a couple of thousand volts peak at a few 10s of kHz AC to ignite.
Once running the voltage drops to a few hundred volts. Typically they use a small capacitor (a few pF) as the ballast in series with the tube.