DC directional lighting, multiple LEDs, DCC in the future

[craigolio1]

Hello all.

I have a loco that I'm ready to finish. The last step before I reassemble is the lighting. I'm running DC for now but in the future it will be upgraded to DCC. I need some help choosing LEDs.

It's an Atlas RS11 chassis and will have ditch lights and a head light at each end which will total 5 SMD0402 LEDs at each end. I initially planned to connect the headlights to the appropriate connections on the factory light board, and then the four ditch light LEDs on their own resistor for easy connection to DCC in the future.

I want to use just one SMD (1/8 watt?) resistor as space is a big issue. I know series is the safer bet but if one LEDs dies then it'll be like finding the world's smallest burned out mini Christmas tree light. Parallel has the higher risk if blowing them all up if one goes, due to the current being fed through the remaining surviving LEDs.

I would prefer parallel. Does anyone see an issue with putting in a large enough resistor to limit the current to the 20ma, or what ever it is, for the worst case scenario of all but one LED failing? Will the current used through all four be enough to light them?

I don't need to light the room with these. In some passenger car lighting I did a whole back I recall using a much larger resistor to keep the brightness to a non-welder like level. The LEDs in my current project are cast into the four ditch lights and the viewer will see absolute maximum light output.

I don't have time to bread board it for a few days, but I do have time to type this so I thought I would ask.

Thanks for your input.

Craig

[craigolio1]

I just had a thought. I could make a little bank, or manifold so to speak, of resistors on a piece of .005" styrene. I think it would be small enough that I could mount it inside the nose under the headlight and under the rear light. The four magnet wires would terminate there and that would allow me to bring just two wires back to the board instead of 5. Less clutter. Less wiring. Less opportunity to break one of those tiny magnet wires.

Craig

[peteski]

You know, this should really be posted this in the Electronics section. 

For white LEDs in this application 20mA is way too high of a current - it would make the LEDs blindingly bright.  Since the way you are installing these will make the LED directly viewable (none, or very short light pipes) they will be plenty bright at anywhere between 1 and 5 mA.  That also reduces the power dissipation of the current limiting resistor.  That is god as you can use smaller size (wattage) resistors.

Why do you need 5 LEDs per end?  2 for ditch lights, 1 for headlight and the other 2 are for what? Number boards maybe?

But I would not go the single resistor route.  Your "manifold" idea is excellent.  One of the biggest benefits is that you can control the brightness of the individual LEDs. Or you can use mixed hookup where for example both ditch lights are in parallel with a single resistor (like Jim R recently demonstrated) then the headlight would have its own resistor.  That way you can tune the brightness of the headlight and ditch lights independently.

How about alternating ditch lights?  In that case each should have separate resistor.

As far as the "manifold" goes, you should make it out of copper-clad PC (printed circuit) board.  That will dissipate the heat better and will also provide solder pads to which the resistors and wires can be easily soldered.  Copper-clad fiberglass-epoxy circuit board material is readily available (even from electronic surplus suppliers).  I usually use very thin board which can be cut with scissors.

As far as the copper cladding goes, best way to make solder pads it to etch them.  Here is an example.


For small one-of jobs I simply hand draw the traces on a well-cleaned copper-clad side using an old technical pen and some slightly thinner Testors PLA Enamel hobby paint (small square bottles). Ince the paint dries I dunk it in the ferric chloride etchant (should still be available at Radio Shack).  Once etched (about 20 minutes) I rinse the PC board and clean of the paint.  Then solder the resistors on before installing it in the model.


I usually get me blank PC board material from http://www.goldmine-elec-products.com/products.asp?dept=1034 . They seem to be out of single-sided stuff, but you can use the double-sided boards - just let the etchant eat away the copper on the other side.  Or if you search the Web you can find other suppliers of single-sided thin PC boards.

[craigolio1]

Peteski, I think you are probably correct.

The locomotive I've modelled is pictured in this link:

http://www.rrpicturearchives.net/showPicture.aspx?id=633761

It's got 4 ditch lights on each end, with 2 each mounted on posts. I made the parts and cast the LEDs inside clear epoxy so no light pipes here.

I may do a resistor per pair, and then one for the head light. If one LED fails I'm replacing two for sure so I may as well save the space.

I like your circuit board idea. Very neat and easy to build on the bench. Also I happen to have all of the required supplies. I've never done my own etching, so why not now?

Thanks.

[craigolio1]

Oh, and what do you think about a protection diode in series?  Since I'll be using DC at first I'll be relying on the diodes them selves to provide the directionl lighting. Is it wide to put a diode in series instead of relying on my meticulously hand crafted ditch lights to do it?

[peteski]

Oh, and what do you think about a protection diode in series?  Since I'll be using DC at first I'll be relying on the diodes them selves to provide the directionl lighting. Is it wide to put a diode in series instead of relying on my meticulously hand crafted ditch lights to do it?

Protection diode is a very good idea.  One in series with all the lights for each end.

As for the double ditch-lights on a post - are they already wired in series or are there individual leads (4) coming out of the post?

[craigolio1]

If one diode is enough then that's perfect. If I recall, there is one on the atlas light board so I can just attach there.

Each post has four leads coming out. Are you thinking each post in series with a resistor for each, then those pairs in parallel?

[peteski]

If one diode is enough then that's perfect. If I recall, there is one on the atlas light board so I can just attach there.

Each post has four leads coming out. Are you thinking each post in series with a resistor for each, then those pairs in parallel?

I don't remember how the Atlas protection diode is wired on the light board, but it sounds like it will work.

Hooking both ditch light LEDs in series then adding a single series resistor would be the best way (and minimal wiring). No problem when using DCC, but in DC those LEDs would not start to glow until the track voltage was around 6 volts. 

If you hook them up in parallel with a single series-connected resistor then they will come on around 3 volts.

[craigolio1]

Awesome. Thanks for the info.  I think I would go with series. The end goal is DCC and I don't plan to re-wire the LEDs at that time. I have the DCC system, and some of my locos do have decoders. Perhaps this should he te next one to get one just to keep it simple.  Do you still recommend the protection diode with DCC? I havn't in the past and can't really see a reason to. They would never get reverse current.

Craig

[peteski]

Do you still recommend the protection diode with DCC? I havn't in the past and can't really see a reason to. They would never get reverse current.

Nope - for the reason you stated.

[craigolio1]

Awesome. Thanks.