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Thread ID: 98857	2009-04-10 08:17:00	Electronics question: Parallel backlight inverters	george12 (7)	PC World Chat

Post ID	Timestamp	Content	User
763784	2009-04-10 08:17:00	I have been tasked with (if possible) replacing the faulty backlight inverters on three TFT security monitors - one 17" and two 19". Original replacements are not available. The inverters aren't repairable. In all of them a surface mount chip is burnt out. The CCFL tubes are fine though. I have an abundant supply of generic CCFL inverters for 30cm tubes. These light up the display (one for each tube), but not brightly enough and the light doesn't completely fill the screen. There are two tubes in each monitor - one on the top and one on the bottom. Is there any way I can put two inverters in parallel or series for each tube to get it brighter? My understanding is that the output must be fairly unbiased AC. I think it's around 700V. I have a pack of 1000V diodes here which should handle the voltage, but I'm not sure how I could arrange them, if it's possible at all. Would it help if I bridged the bases of the switching transistors on the inverter boards to make them switch in sync? One could control the switching and I could disable the switching circuitry on the other. Any tips on whether it's likely to be insufficient voltage or insufficient current? Cheers George	george12 (7)
763785	2009-04-10 08:42:00	Are you able to salvage inverters out of other 17/19" monitors?	somebody (208)
763786	2009-04-10 08:55:00	ok, with neons and CCFL tubes they have a strike voltage and once struck they have a rather low resistance, the one that strikes first essentially shorts out the other tube, to cure this either use seperate supplies or hook the lamps in series and perhaps bypass the capacitor or PTC resistor that limits the tube current when placed in series to increase the current going to the tube(s) never attach tubes in parallel as - i know im repeating myself - when one tube strikes the other tube may not	williamF (115)
763787	2009-04-10 09:07:00	I sold all my faulty 17" LCDs :( Got laptop ones but they won't be nearly powerful enough I suspect. I'm using a separate inverter for each lamp. However it's still not enough to power it. Interestingly when I attach a 12" laptop panel to the inverter it lights up fully and draws 350mA at 12VDC. However when attempting to light this panel, the inverter only draws 100mA from the DC supply. I take this to mean that the inverter isn't putting enough voltage out to cause the required current draw to fully light the tube, but can supply enough current. So I need to put two inverters in series. Thus I seem to have answered half my original question (parallel or series). But how do I do this? Some combination of diodes I take it, or shall I try to get them switching in phase? Edit: To clarify, I have no intention of putting the tubes in series or parallel - just the inverters to acheive the required voltage. In the original board it had one transformer for each tube.	george12 (7)
763788	2009-04-10 09:12:00	or the tube(s) have failed. failed tubes will draw less current. increasing the output current/voltage by linking the current limiter on the output (usually a capacitor or PTC resistor) will help but you will need to eventually need to replace the tube or monitor you can try hooking several inverters in series and see how it goes eg two inverters per lamp	williamF (115)
763789	2009-04-10 09:17:00	Whatever you do, be very careful. If this is for a business, it may pay to check their insurance policies, as they may not cover any damage (i.e. fire!) caused by electrical equipment that hasn't been serviced by qualified professionals.	somebody (208)
763790	2009-04-10 09:21:00	or the tube(s) have failed. failed tubes will draw less current. increasing the output current/voltage by linking the current limiter on the output (usually a capacitor or PTC resistor) will help but you will need to eventually need to replace the tube or monitor you can try hooking several inverters in series and see how it goes eg two inverters per lamp Thanks. I bypassed the transistors on one inverter and ran it off the other one, so I have two transformers in phase, and put them in series. Now it draws 300mA from 12V and lights up much better. It still needs to be a bit brighter though. Tell me more about the current limiter? There is a capacitor in series with the output, so will I raise the voltage by bridging it? Is this OK for the tubes and the inverter? 6 failed tubes and 3 failed inverters in 3 monitors seems unlikely, surely?	george12 (7)
763791	2009-04-10 09:23:00	Whatever you do, be very careful. If this is for a business, it may pay to check their insurance policies, as they may not cover any damage (i.e. fire!) caused by electrical equipment that hasn't been serviced by qualified professionals. Thanks, I'll look into that. The inverter is enclosed in a solid metal shielding, which certainly helps, and the input to the whole screen is 12V, so it's not as dangerous as it could be!	george12 (7)
763792	2009-04-10 09:29:00	unmodified inverters in series that or find the base/gate of the main driver and cut the base/gate on the other one and solder a wire from the unmoded one to the base/gate of the one with the cut base/gate line. the base/gate line should be the thin track on the driver transistor/mosfet the other two being drain source or collector emitter	williamF (115)
763793	2009-04-10 09:30:00	WOW Bypassing the capacitor means one tube now draws a whole amp and fills the entire screen brighter than when new! So now all I have to do is experiment with the capacitor values and I'm sorted. The transistors on the inverters are rated at 1A comfortably and 2A pulsed so no problems there. Do I need to go higher or lower on the capacitance?	george12 (7)
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