Matching 700mA constant current driver to 20mA LEDs? Answered
Ultimately I'm trying to drive upwards of 100 3mm white LEDs in an art project with dimming and a connection to 120v AC power (this is for a chandelier).
I've been playing around with some constant current IC driver like the Supertex CL2 as well as 5 and 12v power supplies but I've been looking for a dimmable solution with a small form factor. I picked up a Robertson constant current LED driver but it outputs at 700mA, I assume because it's intended for 1W or 3W LEDs needing the higher current.
Is there a circuit design that I can use like a current divider to drive smaller loads with this supply?
My current thinking is as follows:
1) I could simply load 35 parallel strings of LEDs since 700/20 = 35 (of an appropriate voltage drop probably between 9v-15v) and rely on the equivalent resistance of each string to act as a defacto current divider.
2) I could do the same thing but with an in series resistor of some value for each string, the constraint being that increasing the resistor will reduce the number of parallel strings and I may need relatively high wattage resistors if I'm driving 9+volts of LEDs on each string.
3) I could do a smaller parallel current divider, but I'm not confident in my math analyzing the ratios of the resistors to achieve this (for instance if I had only two parallel loads and one was drawing 20mA and the other the balance of 680mA the resistors would have to have the same (inverted) relationship i.e. 34:1...to say nothing of the wattage through the 680mA line, which might be as high as 6-8 watts depending on voltage. Is that right?
Are there any other clever solutions I'm not thinking of? Obviously I could buy a lower current driver (and I may ultimately) but even the lower current options are at 350mA, so the same problem will exist at a smaller scale.
Thanks everybody for thinking about this, I look forward to seeing your thoughts!