SMD 603 LED question

[emkay_777]

Greetings All,

 

I am converting my one and only Kato HO locomotive (EF510-500) to DCC with a LokPilot standard decoder.  The motor control was a breeze but the lighting is driving me up a wall.  So in DC mode, one forward white LED and 2 rearward red LED's come on depending on direction so there are 6 total LEDs on the board.  This is done with diodes and resistors on the stock Kato board so if you head in one direction, 3 come on, and the other direction, the other 3 come on.  Pretty simple . . . so I thought.  So I endeavored to replicate this using DCC (the yellow and the white wires for headlight direction).  I watched a youtube video by someone who did one very similar and he soldered a wire to each of the SMD LED connections to attach to the decoder - okay, makes sense to me.  So I did that and wired them in parallel, and hooked the white and yellow wires, each with 1000 ohms of resistance, and put it on the test track. This is the part that has me baffled - the 2 red LED's lit fine, but the white LED did not.  First thing I thought was, oh crap, I burned them out but they all worked fine on the bench with the 9V LED tester,  So I illuminated the single white one, then attached the 2 parallel red ones.  The red ones came on and the white one went off.  So they all come on, but not at the same time.  At first I thought that the white LED wasn't getting enough power with the red ones being illuminated, but I can light all 4 red LED's at the same time.  This leads me to believe that the white LED's require more juice than the red ones.  Would greatly appreciate any insight anyone can give me on this.  Thanks,

 

Evan

[Kiha66]

Interesting problem evan, could you take some pictures of the arrangement?  It is a little hard to see what is happening in text, pictures would be a little easier. 

[mrp]

Unless the LEDs have matching foward voltages, wiring them in parallel without individual current limiting resistors won't work.  Red 0603 LEDs usually have a forward voltage of between 1.8 and 2.2 V, whereas white LEDs are often a lot higher - between 3.0 and 3.4 V.  It looks like just the two red LEDs with the lower Vf are turning on.  The best solution would be to use individual current limiting resistors with each LED.

Edited May 7, 2017 by mrp

[emkay_777]

Thanks for reading my post.  MRP, you hit it on the head - I used individual limiting resistors and now everything works as expected.  Wish I'd have known that before I began . . .