Retrofitting Class 37 Directional Lighting using DCC

[Smudge617]

Hi

I'm still new at this. so please bear with me, I would like some advice. I intend to fit Directional lighting (no cab lights) to a class 37 loco, something I've never tried before, I think I have the wiring sorted to/from the decoder, (I'm using DCC) My question is do I need to fit a resistor to each LED or can I use one resistor for each set of lights (making four resistors in total) or does anyone know a different or better way of doing this, I am not electrically minded so anything I need would have to be bought and simple to install/solder.

I know of a company called Illuminated Models who make these type of things, but having spoken to a friend of mine who bought one, he suggested fitting my own LED's as their version sits too low and to make fit I would have to cut a large section out of the chassis to make room, also they do not have a template I could use to mark out where to drill, making that process a bit hit and miss.

Any thoughts would be appreciated.

[Chrissaf]

Directional lighting uses both Red and White coloured LEDs. Different coloured LEDs have different 'Forward Bias' voltages that need to be managed. Thus the location of the resistors needs careful planning else it is possible to have red LEDs that do not light up.

The most technically efficient way is to use a separate resistor for each LED, or at the very least a separate resistor for each colour. If you use the one resistor per colour option [i.e a total of 4 resistors] then each resistor will try to pass the current for both LEDs, thus the resistor value then needs amending. The 8 x 1KΩ resistors in the drawing below would then typically become 4 x 470 Ω resistors.

The image below is a generic directional lighting scheme for the conversion of locos that have no factory fitted lighting circuit boards.

[Smudge617]

Thank You Chrissaf. Just what I needed

[ColinB]

I did my ex Lima class 47 with individual lights. I either use a 1 k ohm SIL commoned resistor pack, look on EBay you will see them, where 1 pin is the common and the other pins connect to the resistor. So if you have a 9 pin SIL you have 8 resistors commoned at one end, where you put the blue wire. Alternatively the guy on EBay that sells LaisDCC decoders does a small pcb that has 8, 1 k ohm surface mount resistors, again a neat solution. I used tower leds they are 2 mm diameter.

[96RAF]

Be aware that you need to keep the led load current within the decoder function output limit, typically 100mA.

I found with an Express Models Class 37 kit - three x front plus two x rear leds was too much for a TTS decoder so I had to fit another 1K ohm resistor in the blue lead as well as those provided by Xpress Models pcb to drop the overall current.

If using your own leds, then check their spec sheet and apply Ohms Law to find out the current as you add 1K ohm resistors which are more or less a standard value for DCC led lighting.

[ColinB]

I must admit if you use the tower leds, for some reason they are incredibly bright. I found I had to use 4.7 kilo ohm resistors otherwise they look like halogen headlights.

[Smudge617]

Thanks to everyone for their help and advice. Fitted 3mm LED's too big I know but hands too shaky for anything smaller, using a directional lighting board, courtesy of ebay, but yeah first attempt, all works. Thanks again

[ColinB]

Next time you do it use the 2mm tower leds, although the width of the lens is 2 mm the rest of the led is quite chunky. You just drill a 2 mm hole and push it through. I think the base part of the leds where the leads are, is bigger than a normal 3 mm led.