Yes, there are multiple monochromatic colors. I cared about efficiency only because I didn't want to heatsink the=20 regulator (with attendant extra parts). I was poking around Digi-Key and Google looking for a part to use, and=20 considered a simple LM317T constant-current circuit, but again I didn't=20 want to dissipate that much power across the regulator. I also couldn't=20 find any 24V transformers in my parts box. I also didn't want to order=20 so many LEDs for a prototype. So I am just going to use an old ATX power supply for now. With a 3.3=20 ohm 1W resistor in line with 5x 2.15V 400mA LEDs on the 12V rail, and a=20 1.0 ohm 3W in in line with the 3.5V 1500mA LED on the 5V rail. This is=20 a bit larger size than I wanted for now but it's cheap, simple, safe,=20 and expandable. My (likely incorrect) calculations indicate this will=20 draw 12.04W of which 2.72W will heat resistors for an efficiency of 77%. If I do groups of 3 I can run 3x 3.5V LEDs from the 12V rail with the=20 same 1.0 ohm 3W resistor for an efficiency of 87.5%. I am fairly sure=20 I can acquire 12VDC SMPSs for pretty cheap! You mention that a laptop DC-DC supply would be easy to current-limit...=20 I'd be interested in hearing how that might be accomplished. I have=20 searched around on Google for current-limit mods to computer power=20 supplies but found little. Thanks for the reply, Joe On 2011-01-06 22:13, RussellMc wrote: > Please confirm LED forward voltage. > 2.15V suggests some colours and not others and certainly not white. Corre= ct? > > Why do you care greatly about efficiency when protoyping? > > A very low cost very easy solution that seems to meet your need is to > use as many LEDs as are guaranteed to combined have a Vf lower than > the available supply and then add a series FET running as a current > source with a linear regulator. (FET, opamp section, Isense resistor, > voltage reference. ) > > MINIMUM efficiency is about (24-2.5)/24 x 100 =3D~ 90% for the 2.1V > LEDs and could be close to 100%. > And about 85% for the 3.5V LEDS and could be close to 100%.. > > Actual LED Vfs will spread noticeably > If you want simplicity and efficiency do as above but regulate the > nominal 24V supply with a SMPS and run the 2.15V and 3.5V LEDs from > different supplies. > > If you have an unregulated transformer supply as stated, then a buck > converter to the desired string voltage/current is cheap and easy and > gives 90% + efficiency. > > Lowest parts count with good efficiency is a SMPS buck IC with > internal switch and synchronous rectification. > > Cheapest get you going solution is probably an aftermarket "laptop" > 12V to xxx volt supply with Vout tweaked and/or current feedback > added. Example only - $NZ15 or about $US12. 11-14Vin nom to Vout > 15-19V nom - easily converted to N Watt constant current LED driver. > > http://www.trademe.co.nz/Computers/Laptops/Power-adaptors/Universal/aucti= on-344995724.htm > > > Russell McMahon > > > On 7 January 2011 11:04, Joe Koberg wrote: >> I am designing an LED lighting product for personal purposes. No designs >> have been laid down so I am flexible. The LEDs are Osram Dragon Plus >> series costing some $5+ each, so driver chip price point is low priority= .. >> >> I want to drive a string of 2.15V 400mA LEDs. I am thinking 10 in series >> with a driver IC on a 24VDC rail from a unregulated transformer supply. >> I would like to run three strings in parallel. I also want to run 6 or >> so 3.5V 1400mA Diamond Dragons in the same array, leading to my >> arbitrary choice of 24V. >> >> This is idea/prototype stage. Obviously looking for high efficiency. I >> would prefer a driver IC with no or few required external parts, and I >> have lots more capacitors laying around the bench than I do inductors >> (and zero small-value power resistors!). PWM drive is irrelevant, I want >> to run the LEDs at full brightness. >> >> FYI, eventual goal is probably to run off of 120VAC, and I don't really >> have a reason to limit the string length. >> >> Driver module suggestions are also welcome. >> >> Thanks in advance. >> >> Joe Koberg AE5NE >> >> -- >> http://www.piclist.com PIC/SX FAQ& list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .