LED Camera Light - Flickerless Dimmer

Discussion in 'The Projects Forum' started by Thazatoon, Feb 17, 2009.

  1. Thazatoon

    Thread Starter Member

    Feb 17, 2009
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    I’m building an LED video camera light, but I’m having trouble creating a flickerless dimmer to control it. I’ve tried a PWM circuit that controlled a motor, but the flickering was apparent even when the camera shutter was at 1/100th.

    http://www.mfacomodrills.com/speed_regulators/voltage_regulators.html


    The circuit claimed to operate above 1Khz so I was confused as to why the flickering was so apparent at such a slow shutter speed.


    Browsing various circuit forums, I came across this comment:-


    “As has been stated before, LEDs can be dimmed with a continuous
    current. In this case there is no flicker from the LED. The trick is
    to make a variable current source with high efficiency. This is done
    all the time by using a PWM circuit inside the power converter and
    then smoothing (filtering) the current before it is applied to the
    load. You get a continuously variable, high efficiency current source
    that has very little modulation on its output - and hence no flicker.”


    The circuits I have found on-line so far make no mention of placing the PWM within the power convertor, so I’m wondering just how complex a circuit this could become, compared to say this one:-


    http://www.reuk.co.uk/LED-Dimmer-Circuit.htm


    Would it be possible to modify an existing off-the-shelf circuit to do this. If I have to begin from scratch, how would I incorporate PWM into the power convertor.


    I'm using a 120 LED array, which are drawing approx 500mA and run off 12 VDC. Any pointers would be greatly appreciated.
     
  2. RiJoRI

    Well-Known Member

    Aug 15, 2007
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    It looks as if what you need to do is add a low-pass filter to the PWM output of the circuit you are building. Also, putting LEDs in series is Not A Good Idea. Search this forum to see (long) discussions on Why.

    --Rich
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Check the post by CrazyElectroGuy, Projects, similar requirements.
     
  4. Thazatoon

    Thread Starter Member

    Feb 17, 2009
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    Bernard, did you mean this thread:

    http://forum.allaboutcircuits.com/showthread.php?t=18860&page=5

    I read the whole thing but couldn't find any mention of current controlled PWM and flicker wasn't any issue on this project.

    Rijori, I have to use the LEDs in series, as it's all I have. I'm not sure how to add a low pass filter onto an existing circuit as I am a relative novice. If anyone would be able to design it I'd be happy to pay them for it.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    That's the point no flicker, no PDM, simple circuit few parts.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    LEDs in parallel are no good. LEDs in series are fine.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Assuming the device driving the motor{ LED strings ] is a MOSFET , low side drive, & is accesable, adding a low pass filter between driver and MOSFET should work; 4.7kΩ driver to gate,10μF gate to ground. With the MOSFET I tried, W60AN10 , conduction started @ 1.5V , saturation @ 4.5V on gate. Load was 10Ω so about 1 A.
    Do the LED strings have current limiting resistors?
    Is one and of string tied to +12V ?
    If circuitry is not accessable there is another way using a transistor & a MOSFET outboard , or a LC filter on output causing some loss.
     
  8. Thazatoon

    Thread Starter Member

    Feb 17, 2009
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    The power convertor on the motor controller board is CHN 808 with a 1K resistor across it. I could draw up a circuit diagram of this circuit and post it here if it would be helpful. The timer is a 555. It dims the LED chain nice and smoothly, it's just the flicker that's the issue.

    Just glancing at the board there are 2 other 1K resistors, 3 diodes and 3 capacitors and of course the rheostat. There's certainly plenty of room on the opposite side of the board for more components.

    The LEDs are in a chain, all mounted on a flexible circuit board. There is one surface mount component to every pair of LEDs so I assume these are resistors. I can plug either end of the chain directly to a 12Volt battery - or my controller - and it will power up VERY brightly.

    the drawback with this chain is that if you get a break in it, only the LEDs up to the break will light up, from either end of the chain. So I'm assuming that the LEDs are in parallel and not series as I thought. It's hard to see through the casing but I can cut into it and be certain.
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I could find no usefull info. on CHN808, might be a house no ; so please post a drawing, esp. 555 pin 3 and outward toward output " socket".
    I take it that LED string has twin leads and plug on each end ? Battery has polorized socket ? Please show polarities on drawing.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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  11. Thazatoon

    Thread Starter Member

    Feb 17, 2009
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    I finally managed to get the circuit drawn. Apologies for the poor quality and also if I've made any mistakes. I haven't listed the numbers of the 2 diodes as I will have to desolder them to remove them, but they are both largish diodes.

    I've written down the values on the components exactly as I could read them from the components on the board itself. C3 is a bead capacitor and I believe that C1 & C2 are poly capacitors. I hope I haven't missed anything from the circuit.

    Does this help in any way?

    [​IMG]

    Also, I found another circuit which mentions constant current ouput, but the circuit isn't designed for 12V.

    http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html

    If anyone could give me pointers on how to get a constant current circuit on 12VDC I'd be eternally grateful.
     
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I made a quick review of your schematic and do not think it will work as shown, can find no charging path for C1, pin 3 goes nowhere. Might try replacing R1 with 4.7k & add 10μF from MOSFET pin 1 to pin 3[ gnd.]
     
  13. DickCappels

    Moderator

    Aug 21, 2008
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    Quick comment: You can make your life simple - get a monolithic buck converter like the National Semiconductor LM25574. DC output and high efficiency, plus its supported by application notes.
     
  14. Thazatoon

    Thread Starter Member

    Feb 17, 2009
    12
    0
    Thanks for all your help. I can see I made a mistake on the circuit. I can correct it but I think your knowledge and info is too advanced for me to really understand and get the circuit working.

    If anyone is able to design it for me I will offer a good payment for your time. I don't mind where you are based although I am in the UK. Please P.M me or e-mail me if anyone is interested

    thazatoon@gmail.com
     
  15. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I think you did a good job of circuit tracing, one little error does not mean failure. If you want to stay outboard from ckt. try an LC filter between output and LEDs, L any coil you can find, like secondary of power transformer, secondary of filament transformer, try to stay undrer one ohm; capacitor one to ten μF . If it is satisfactory you can replace with real parts later.
     
  16. Bernard

    AAC Fanatic!

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    Talk about mistakes- hit submit button prematurly.
     
  17. Thazatoon

    Thread Starter Member

    Feb 17, 2009
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    Thanks Bernard. Does your suggestion mean that the LEDs will no longer flicker but will still dim evenly? I was thinking of using the coil from a domestic dimmer as I have one here.
     
  18. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Your choke might be a line hash filter, two windings?? not much inductance or current rating. If using RC filtter MOSFET might get hot.
     
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