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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?

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  • \$\begingroup\$ So I measured the power supply voltage for the tube How did you measure that, did you use a device that can accurately measure a a 40 kHz sinewave? Cold-cathode tubes are usually powered by a high frequency in order to increase efficiency and to be able to use a small (high frequency) transformer. which sometimes requires as much as 10,000 volts! Where did you get that "wisdom"? Usually a few hundreds of volts is enough. 10 kV would require serious insulation which is expensive. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 13, 2020 at 12:33
  • \$\begingroup\$ Have the tubes been used for many hours? Then they are very likely simply worn out. Do yourself a favor and stop making false conclusions based on an incorrect understanding of how things work. \$\endgroup\$
    – Bimpelrekkie
    Commented Oct 13, 2020 at 12:36
  • \$\begingroup\$ @Bimpelrekkie "did you use a device that can accurately measure a a 40 kHz sinewave?" No I used a multimeter, It successfully read out the input 110 +- 3 volt, so assuming the output working at the same frequency, it should be accurate unless of course the frequency changed. \$\endgroup\$
    – cream_pi
    Commented Oct 13, 2020 at 14:17
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    \$\begingroup\$ @Bimpelrekkie "Have the tubes been used for many hours?" Yes they have been! But both tubes, the bottom and the top one quits at the same time? Seems unlikely. \$\endgroup\$
    – cream_pi
    Commented Oct 13, 2020 at 14:21

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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.

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  • \$\begingroup\$ Thank you! So, if you were to measure, say the driving output of the power inverter board and read only 7 volts with the multimeter, it is actually pretty normal? (As multimeter works on some sort of equivalent principle and couldn't measure the voltage correctly if frequency is different than 50 or 40 Hz?) \$\endgroup\$
    – cream_pi
    Commented Oct 14, 2020 at 0:00
  • \$\begingroup\$ The output of the inverter is high frequency and very low current. It is not something that can easily be measured with normal equipment. I don't think you got a meaningful measurement. The only way I can think of testing one is to use an oscilloscope and measure the low voltage waveforms at the input of the transformer. Or put the scope probe close to (bit not touching) the inverter output. Any circuit loading (even touching with a probe) will dramatically alter the output. \$\endgroup\$
    – Kevin White
    Commented Oct 14, 2020 at 0:35
  • \$\begingroup\$ Got it! Thank you! \$\endgroup\$
    – cream_pi
    Commented Oct 15, 2020 at 0:50

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