LED Driver


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  1. #1
    Prolific Poster pic-ignorant's Avatar
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    Default LED Driver

    Happy New Year!

    I'm trying to improve a Led driver that I made a while ago.
    The existing one is based around a LM317 regulator in Constant Current mode, (24v @125mA) and it works very well. Even with 2 motors causing PSU fluctuations there is no difference in Led brightness.

    I want to add some more leds to the string, and maybe use higher Vdrop Leds, such as Blues and whites. I've looked at many solutions, but most don't cater for Vin fluctuations. Would a Buck/Boost controller be a good solution? (Board space is tight)

    Many thanks
    John

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  3. #2
    robbed666
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    Default Re: LED Driver

    I used the following, 1 Amp 30 Volt I/P. http://docs-europe.electrocomponents...6b80f8c915.pdf

    It is SMT,, but 98% ish efficiency. No (very little) heat generation even when powering a 6 Watt MR16 at 24 volt in. Got the PCB size down to 10 x 15mm
    Last edited by robbed666; 1st January 2012 at 09:43.

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    Prolific Poster pic-ignorant's Avatar
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    Default Re: LED Driver

    Thanks for that Rob,

    It looks like the output would vary with a dip in Vin?

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    Prolific Poster pic-ignorant's Avatar
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    Default Re: LED Driver

    I'm wondering if a blocking diode and a bulk capacitor close to the LED driver might stop it 'seeing' the voltage dips caused by the motors?

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  9. #5
    robbed666
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    Default Re: LED Driver

    Er........ less than 1% variation of output I with 1 led, and an input variation from 12 to 30 Volts.........

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    Default Re: LED Driver

    I'm reading the DS on my phone. I'll order some and try them out.
    Thanks
    John

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    Default Re: LED Driver

    I just read through the datasheet Rob. If I have a Led string with a total Vdrop of say 21v, and the 24v supply dips to 18v as the motors ramp up, I cant see how the leds are not going to dim slightly.

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    Default Re: LED Driver

    Quote Originally Posted by pic-ignorant View Post
    I just read through the datasheet Rob. If I have a Led string with a total Vdrop of say 21v, and the 24v supply dips to 18v as the motors ramp up, I cant see how the leds are not going to dim slightly.
    Is there any reason you can't just split your string of leds to multiple strings, and use an LM317 (constant current mode) for each individual string?
    A wise man may listen and say nothing. A fool may listen and have nothing to say. Which one are you???

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    robbed666
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    Default Re: LED Driver

    Quote Originally Posted by pic-ignorant View Post
    I just read through the datasheet Rob. If I have a Led string with a total Vdrop of say 21v, and the 24v supply dips to 18v as the motors ramp up, I cant see how the leds are not going to dim slightly.
    You've not left yourself much 'headroom' for compliance.... A LM317 'strapped' as a constant current source is going to need at least 2-3 Volts to get into 'regulation' (1.25V ish 'reference' + series saturation voltage).

    I would suggest : 'upping' the led supply voltage if possible, reducing the number of LEDS, splitting the string, of using a Boost-Buck converter to power the LEDs.

    Going to a switching regulator has serious advantages over linear designs, esp as the power required increases.

    Or current limit the Motors in 'ramp up', so your common PSU remains in 'reasonable' regulation..
    If the dimming is short term, i.e a few mS, then a diode and a 'big' cap on the LED leg of the supply could be enough. If it's longer, then that is some serious regulation of the PSU 'issue'.... i.e, you really need a bigger PSU....

    Just my 2Ps worth.....

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    Default Re: LED Driver

    Hi Rob, Thanks for the reply.
    I have decided to go with a seperate board for the Led driver. A 36v Boost driver will do the job, but won't fit on the pcb.

    Regards
    John

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    Default Re: LED Driver

    I sorted it by using the STCS05 Led driver that I had originally planned to use. Vcc = 24v. Led String Vdrop 19.5v @ 175mA. Vcc can vary by +/- 3v without the output current changing by more than a few mA.

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    Default Re: LED Driver

    The STCS05 is a pretty rugged device. I have been testing one with 24v input, and led string Vdrop of only 12v, to get it nice and hot, so I can test the thermal shutdown. It shuts down and restarts at approx 1 hz, and is still going strong, even wrapped up in a insulation jacket.


    I have also ordered the device you advised above Rob. I liek to have some options.

    Many thanks
    John

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