Resistors per LED?

[Potrail2378]

Hi, I've acquired a Hornby class 55 Alycidon on DC. I have already converted it to DCC. I have drilled out holes for the head/tail lights and have some LEDs with three legs (a common, and a leg each for white and red). Do I need a resistor for each LED, or will one resistor from the blue wire suffice? I going to have three LEDs at each end, two for headlights and one for a red light.

Thanks in advance. 

[Rog RJ]

You need 1 resistor per LED, connected to the common of each LED.

 

It can be done with just 1 resistor in the blue common but it is not recommended as it can lead to problems.

[ColinB]

You need separate resistors as each diode has a different voltage drop across it, so if you only had one resistor it is possible that not all the leds would shine the same (that is the simple answer). Certain people on this site get upset if I go into too much detail. The easiest way I have found is to either buy what is called an SIL comon resistor pack (a search on EBay will get you one), where you add the blue wire to the commoned pin, and take the feeds off the others, or there is a guy on EBay that does does an SMD resistor based board for about a £1=00 which is small and perfect and easier to fit. I have used both on my old converted Hornby locos. I think people use individual resistors because I don't think they know about the alternatives (well that is the diplomatic answer), although they might be slighly cheaper, but I am not all that sure. They definitely are a lot more work.

[96RAF]

You say you want 2 x whites and 1 x red at each end. You only need 3-legged bi-colour leds if you are having the white and red in the same hole and simply swapping colours with direction. For single colour per hole you only need regular 2-legged leds.

 

For DCC use 3-legged bi-colour leds need to be common anode, whch connects to the Blue (common positive) pin.

 

 

Common cathode bi-colour leds are no use to you. Yellow and white (switched negative) pins connect to the led outer legs.

 

I play safe and put a 1K resistor in each leg. This keeps the brightness down, minimises current draw and maximises led life.

[ColinB]

I forgot to mention. There is a guy on EBay that does a PCB with 8 pin DCC socket and light feeds already wired, all you do is add the resistors. You will find him by doing a search on "8 pin DCC sockets" and they are decent PCBs where the tracks won't fall off when you breathe on them. I have just bought one for my EM2 conversion, it was about £1=60. He also does really decent 8 pin DCC sockets that put the Hornby ones to shame, again "plated through holes" dual layer, quality product. I only wish he had been around when I converted about 20 of my old Hornby and Lima locos.