Been a while since I posted but here is my question. I have two very small signs i'm trying to build one the first one will have 49--- 1.8 mm red leds and the second will have 38--- 1.8 mm red leds there will also be two smaller signs with 9--- 1.8mm red leds each but on two different circuits. Can I light the two larger ones with a 12V power supply and if so how do I do it??? I'm prob goin to get them from m.lilyleds.com

Thanks for your help, Jim

 

sure.. with a combination of multiple series strings in parallel and a 12V supply capable of providing enough current.
Each series string MUST have its own resistor.

The number in each string is 12V/vf rounded down to the nearest whole number where vf is the forward voltage of the LED (assuming identical LEDs)

Now see if you can find the 2 formulas to calculate the resistors resistance and power rating for each string.. (note multiply the calculated power rating by 2 or 3 for a safety factor to ensure they don't run too hot)

 

sure.. with a combination of multiple series strings in parallel and a 12V supply capable of providing enough current.
Each series string MUST have its own resistor.

The number in each string is 12V/vf rounded down to the nearest whole number where vf is the forward voltage of the LED (assuming identical LEDs)

Now see if you can find the 2 formulas to calculate the resistors resistance and power rating for each string.. (note multiply the calculated power rating by 2 or 3 for a safety factor to ensure they don't run too hot)

Its worth adding; the more of the voltage you use up with series Vfs, the smaller (stiffer) the current limiting resistor will be - and the less tolerant the LED chain will be to poor voltage regulation. If the PSU regulation is good; you can cut it fine without serious consequences.

A poorly regulated PSU means the resistor needs more "give" to accommodate the highest anticipated voltage - but that means a higher resistance and less LEDs per chain, which means more components and higher wasted power.

 

Just assuming a Vf of 2.2V, 13 strings of 4 LED's & another of10 strings at 15 mA= 345 mA or at 20 mA = 470 mA.
A SMPS ,12V, 1 A should be no more than US $ 8.00.