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HELP: CCFL (LCD Backlight) Driving Circuit

Discussion in 'LEDs and Optoelectronics' started by Sadlercomfort, Feb 10, 2013.

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  1. Sadlercomfort

    Sadlercomfort Ash

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    Feb 9, 2013
    I'm trying to make a circuit to power a CCFL back-light from an old LCD monitor. I'm using the old transformer from the LCD monitor as well.

    However the schematic I've been referring to shows the transformer with 5 pins on the primary side, but the one I have recycled has 7 pins on the primary? Please see attached for the schematic, and the transformer i'm trying to use.

    I can't find a schematic which shows 7 pins on the primary side of the transformer, and was hoping someone could tell me how to change the schematic to incorporate 7 pins. Also the ccfl lamp I'm using shows 30W on the wires, so i dont know whether this schematic is suitable.


    I'm not too savy in electronics, just finished learning all the basics so can identify almost every type of component. :) Appreciate your time.

    Ashley,
     

    Attached Files:

  2. Sadlercomfort

    Sadlercomfort Ash

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    Feb 9, 2013
    And does anyone know a cheap equivalent transistor to the one stated in the schematic? Or one that may be suitable? That's the only component I don't have yet. Thanks

    Ashley,
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    The 2SC1983 is a 3A, 60V Darlington NPN. Suitable replacements would be BC677/679/681 which are available from Digikey and almost certainly from Mouser, Fry's and others.

    Edit: Oops, you're in the Midlands. That sounds like it's somewhere in the UK. Try Farnell aka Element14.

    CCFL inverters are quite a black art. You can try getting that transformer to work in that circuit but don't expect things to go well unless it's the exact part that the circuit is designed to work with.

    Jim Williams of Linear Technology spent a long time investigating CCFL inverters and has produced a very interesting and detailed application note that would be worth reading. Just Google Linear Technology AN65.

    Re the extra pins. I assume they all have wires going into the transformer? You won't need all of them with that circuit. Identify the primary first - it will have the thickest wires, and there should be three connections to it, assuming it's centre-tapped. (Many CCFL drive circuits use an H-bridge driver which doesn't use a centre-tapped primary.) Then measure continuity between the other pins to see whether any of them can be used as the feedback winding (connected between the bases of the two transistors). Try that winding both ways round; if it's the wrong way round, the circuit won't oscillate.
     
    Last edited: Feb 11, 2013
  4. Sadlercomfort

    Sadlercomfort Ash

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    Feb 9, 2013
    Tested continuity

    Thanks Kris for your reply,

    Okay so i tested each pin and have drawn up a diagram of what i found, I've also measured the sizes of the wires (In millimeters) with a magnifying glass. See attached.

    Pins 1&6 share a winding with a size of approx 0.1mm, so I'm assuming this is the feedback winding? Pins 2&4 and 3&5 each share there own winding's at 0.3mm. I am aware that pins 3 and 4 were connected together on the old PCB, probably for the center tapped point.

    What's interesting is Pin 9 was not showing continuity with Pin 8 or any Pin at all. I did find that Pin 8 seems to be connected with pin 7, which is just about audible on my buzzer. Please note this was tested with my homemade 9v buzzer: I don't know whether pin 9 is indeed connected to nothing or the winding is too long for my 9v buzzer to measure.

    Thanks for the equivalent transistors, but i'm afraid i couldn't find these on Farnell/Element14, Im in the UK. However i did find the BD677 darlington transistor 60v, 4A, SOT-32 Type.. Is this suitable?

    Appreciate your time
    Ashley,
     

    Attached Files:

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Sorry, BC677/679/681 was a typo; BD677/679/681 was what I meant. So you're right.

    Your opinions on the pin allocation could be right; I really couldn't tell you.

    The standard way these CCFL transformers are driven, they only need one primary and one secondary, not even a feedback winding. The drive frequency is generated by a separate oscillator, unlike the tuned Royer oscillator diagram in your first post, which needs a feedback winding because the transformer forms part of the oscillator.

    So if your transformer does have a split primary, which can be wired as a centre-tapped primary, and a feedback winding, then you may be lucky. Perhaps the original driver circuit was a Royer oscillator design?

    Why don't you want to keep the original drive circuit?

    Regarding the secondary, I would expect it to be connected across pins 8 and 9. You should be able to see extremely fine wires connected to those pins. I suppose it's possible that one end is brought out on pin 7 if the output is single-ended instead of differential, but this is not how it's normally done. Perhaps if the original CCFL was relatively small, a single-ended output might be workable.

    I strongly recommend you download and read that Linear Technology app note. CCFL drivers may seem straightforward but they're high-frequency, high-voltage circuits and there are many traps for young (and old) players.

    Edit: Also, try to beg, borrow or steal an oscilloscope. It would be a huge help.
     
    Last edited: Feb 11, 2013
  6. Sadlercomfort

    Sadlercomfort Ash

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    Feb 9, 2013
    I didn't know this was called a Royer Oscillator circuit, but now you've mentioned it I'm finding much more about it. Now I'm almost certain this transformer formed part of a Royer Oscillator circuit when comparing schematics to the old PCB board.

    And the article is very interesting, I'm glad it explains how these ccfl are driven, and am surprised about how much things there are to consider. I'm going to finish reading the majority of the article before continuing with this project. As a beginner I believe I have certainly bitten off more than I can chew, perhaps I should research a little more about this Royer Oscillator circuit too.


    May invest in an oscillator soon
    Ashley,
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Excellent Ashley. All of those comments show that you have a really good attitude. Good luck!
     
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