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Best LED driver? Answered

I need to driver (5) 1watt white LEDs (forward voltage of 3.2~3.5v) and current of 350mA. I guess the best way would be to connect them all in series and somehow give them power. This is meant to be powered off of the wall socket (whether through SMPS or a capacitor or a resistor or a transformer or anything.). My question is what's the best way to drive them? I've never really used high power LEDs before. I'd also prefer to be able to adjust the brightness...

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guyfrom7up

9 years ago

my brains going dead... can someone calculate what R2 and the capacitor would be if R1 is a 100k potentiometer and I don't care what the capacitor is (I have every standard ceramic value) I have barely slept for almost 48 hours now, me and my friends had an intense coding session, lol. *staring at dwg300, even though he probably won't read this cause he's a lazy bum*

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guyfrom7up

9 years ago

the lm317 isn't working out, I might have to go to a 555 timer... unless anyone knows any other circuit?

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guyfrom7up

9 years ago

oh, quick question with the lm317 circuit, does it matter what the voltage is (as in can I go higher?) fore example: lets say I have 2 of the LEDs in series (so minimum of 6.4 volts and max of 7), would it harm them if I feed 12 volts in, drops down to 9 via stuff in the regulator, and then go through the resistor and LED? Would this damage the LEDs? I'm wondering because depending on the current I set, the cheap wall wart will be at different volts (unload like 18, loaded like 9) Will this be a problem?

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11010010110guyfrom7up

Reply 9 years ago

when leds are 3.2 to 3.5 it means that their minimum voltage is in between above this voltage a tiny change in voltage is a huge change in current. even few 0.1's of a volt can kill a led a resistor is like a ballast for the led. with correct resistor the led 'feels' what it needs and takes only that. dont skip on a resistor (or something with similar functionality) if you use a lm317 i think its output should not depend too much on input voltage - just like with 7805. set it to some voltage and connect the leds to it with resistors for that voltage

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guyfrom7up11010010110

Reply 9 years ago

ok, that makes much more sense, I always thought, well, what I thought before was wrong, lol. I was reading about driving high power LEDs (I'm good with electronics, just never really needed to) and I read that the light was totally dependent on current, and I was like, alright, so then whats up with the voltage range? LEDs work much above with the appropriate resistor. So yeah... thanks for clearing that up. Full steam ahead! And here I was trying to find an exact wallwart.

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guyfrom7up

9 years ago

oh, and another question, what would be a good diffuser to put on top? Preferably something pourable. I was thinking maybe some sort of clear resin, but how can I make it foggy? The LEDs by themselves are a little intense, even though they are over a wide area, I need them to be a little more mellowed out. I was thinking of adding sparkles to the resin and mixing, but the sparkles might clump and be ugly, then I'm screwed... any ideas?

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11010010110guyfrom7up

Reply 9 years ago

for supply - capacitor and resistor --> 4 diode bridge without second capacitor --> leds. thats gonna flicker at 100 or 120 hz. flicker free design is more complex 1 W leds do make some heat which may build up and get to high temperature. check online how to deal with it for diffuser light the leds at a reflective surface that diffuses like white paper use scratched bit of plastic dip them in silicone sealant or hot glue (may make them overheat when they work)

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guyfrom7up11010010110

Reply 9 years ago

do 1W LEDs really overheat if they're in a solid enviroment (like, say kept in hot glue)?

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kelseymhguyfrom7up

Reply 9 years ago

Well, they're absorbing and re-emitting 1 watt of power. The visible-light portion will be partially emitted through the material, but the scattered visible-light will contribute to heating. And all of the non-emitted power will eventually become heat.

For all intents and purposes, you're probably dumping around 500 to 700 mW of heat per LED into the silicone block, or a total of 1.5-2 W. Since silicone doesn't conduct heat well, that's all going to stay in the vicinity of the LEDs. So, yes, the LEDs will overheat eventually.

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guyfrom7upkelseymh

Reply 9 years ago

Should I be concerned? Would if I mounted them on an alluminum pipe and ran the pipe out of the silicone block? Would that solve anything?

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guyfrom7upguyfrom7up

Reply 9 years ago

oh, and would this become a real problem? like if this light was on for lets say 10 hours max at a time, would the LEDs overheat and die? Cause I would be really PO if these LEDs die while in a chunk of silicone.

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kelseymhguyfrom7up

Reply 9 years ago

I suspect that Binary Guy knows more quantitatively about this than I do. Do you have spec sheets for your LEDs? It should have something about their operating temperature range. The spec sheet for your silicone (or looking it up on Wikipedia) should get you both the heat capacity (how fast it warms up) and heat conductivity (how fast it dissipates heat). With those, and the power output above, you should be able to estimate the temperature rise, and whether it'll equilibrate (if the dissipation matches the input).

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guyfrom7upkelseymh

Reply 9 years ago

well, I bought the LEDs on ebay, and I don't know what brand they are, but I'm pretty sure these are the max temp specs: LED Junction Temperature (°C) 135 AluminumCore PCB Temperature (°C) 105 I'm not sure about the silicone/resin I'm using, the silicone is in a caulk tube (the guy at Lowes recomended it) and the resin is just craft resin I have. It shouldn't get past boiling temperature inside there, should it?!?

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11010010110guyfrom7up

Reply 9 years ago

leds and electronics in general work better and live longer when they are away from their max temperatures aluminium is great. just find a way to connect the leds to it so that they can dump heat into it - maybe use the white thermal thing as in computer cpu cooler. i'd go for pressing them to the aluminium in some mechanical way like cut piece of plastic and screws dont trust the isolation on the back of the leds. dont let any parts of the electrical circuit touch the aluminium and dont touch it yourself if its connected to ac without isolated low voltage transformer aluminium cannot be soldered easily. dont overheat the leds with soldering attempts you can place something transparent between the leds and the hot glue so that you can remove it from the leds. i think piece of PET (what plastic bottles are made of) will work great try it with somehing else before trying on the real leds put a piece on the leds and spill very hot (but not boiling) water on it. it'll warp and get to the shape of leds. repeat until it got the wanted shape but leave it strong enough (so it'll not crack with the hotter and heavier hot glue). let it dry. now cast hot glue on it. if it succeeds you get a diffuser of the exact needed shape and it is easily removed from the leds

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Sandisk1duo

9 years ago

variable voltage regulator, 317 then get two trimmer pots, adjust