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By MichaelN
#96466
jrisom wrote:I might get some smps drivers though since it might be simpler overall. But dealextreme's extremely light on details. Is 3*1W supposed to be three 1W LED's in parallel or series?
You need to read the reviews and comments, since the title description isn't always accurate:

http://www.dealextreme.com/details.dx/sku.13557 Despite the title, it looks like this can drive one or 2 LEDs in series at 650mA or so (ie, 2W per LED). You should be able to drive 2 identical 1W LEDs in parallel (or 2 strings of 2) although it's a good idea to include small "balancing" resistors if you do this. If the LEDs are identical the balancing resistors aren't really needed though. You may be able to drive even more in series, since most current-regulating "buck" converters can output almost the same voltage as the input (minus a bit of overhead).

http://www.dealextreme.com/details.dx/sku.13555 This one looks like it actually outputs about 350mA (ie, for 1W LEDs), and can drive 3 LEDs in series.

http://www.dealextreme.com/details.dx/sku.13552 It looks like this is designed to only drive a single LED at 650-700mA (ie 2W nominal, despite the title saying 3W). As above, you could drive 2 X 1W LEDs in parallel, but it is a good idea to include low-value "balancing" resistors. If the LEDs are identical the balancing resistors aren't really needed though.
By jrisom
#96545
I did forget to consider one thing for the LM317. By National's datasheet, at about 350 milliamps, the LM317 will drop about 1.75 volts average at room temperature. Then the resistor has to drop 1.25 volts giving me a three volt drop before the LED's. There would be so little room for tolerance that it wouldn't work without an LDO variant of the LM317, at least not with three LED's which would be the focus for efficiency.
long long title how many chars? lets see 123 ok more? yes 60

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