Using surface mount resistors for LED headlamps

[rhiadon]

In my install of a DCC sound decoder in the Kato NW2, I attempted to place stick-lead resistors with micro connectors from SoundTraxx. Turns out, they're just too big.

Because of this, I'm exploring other ideas and would like to pick the brains of those more experienced than I.

The idea that I have is to use a set of surface mount resistors on a tiny circuit board. This board would have pads for soldering the LED wires to. This would make the connection semi-permanent but still allow for the shell to be fully disconnected if necessary.

First off, can I use tiny surface mount resistors for this application? I imagine there's not much current going through them, but they do have to drop most of the voltage

Any body have other ideas?

Here is where I was thinking to mount the hand made circuit board. The polygon in red shows the location.
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I figure I could find some really thin single sided copper board and fashion a tiny board for 0603 resistors.

Thoughts?

Thanks,

Kris

[mmagliaro]

You certainly can use surface mount resistors for LEDs.  Lots of the commercial boards you'll see inside locos do just that
for the LEDs, and I have personally done it.

You need to select your parts carefully.   
At worst, you could have 10 mA for an LED, and if it were driven at 12 volts, that would be about 0.12 watts (around 1/8th watt)
There are 0603 surface mount resistors that can handle that much power, and even more.  Be sure you check the specs on
the ones you buy.  Not all of them can handle that much.  Some can only handle .03, .04 watt. 

I think that would work and would be nicely conservative.  In practice, you can probably use a 2k ohm resistor because those
little SMD LEDs light up quite brightly even with only 1-2 mA of current.  You don't need 10.  That would get your power  down to
more like .02 - .03 watt.

[jdcolombo]

Agree with Max.  Use a resistor value of 2K or more.  I've used 1K resistors with my white surface-mount 603-package LED's, and honestly, they are way too bright and probably will shorten the life.  Something between 2.2 - 3K would have been better.

John C.

[pdx1955]

Ngineering sells a "LED/resistor" board that has a connection points and pads for soldering SMD components to. You wouldn't have to make your own.

I was thinking about the lights with this install this afternoon and you could so something similar as tehachapifan did with his SD9 installation. You could glue a a piece of styrene to the top of the sound chamber that would have the SMD LED glued to it. The resistor could fit easily in the cab too more or less nearby. A SMD glued to a piece of styrene with a "handle" piece sandwiched between the frame halves might fit in front with  the resistor nearby. Wiring could be accomplished by cutting a groove low along the sides (avoiding the frame bumps) back to the decoder. This would seem to work - there would be room in the rear , front is more iffy but then it would keep everything on the frame and no need for connectors, etc.

Peter

[rhiadon]

Ngineering sells a "LED/resistor" board that has a connection points and pads for soldering SMD components to. You wouldn't have to make your own.

I was thinking about the lights with this install this afternoon and you could so something similar as tehachapifan did with his SD9 installation. You could glue a a piece of styrene to the top of the sound chamber that would have the SMD LED glued to it. The resistor could fit easily in the cab too more or less nearby. A SMD glued to a piece of styrene with a "handle" piece sandwiched between the frame halves might fit in front with  the resistor nearby. Wiring could be accomplished by cutting a groove low along the sides (avoiding the frame bumps) back to the decoder. This would seem to work - there would be room in the rear , front is more iffy but then it would keep everything on the frame and no need for connectors, etc.

Peter

Peter,

This is a really good idea. If doing this, there would be no need for connectors at all. This is very clever.

I'll also look at Ngineering to see what this board is about.

Thanks for the advice about using a 2k resistor, John.

I'll look into tehachapifan's install.

[peteski]

Kris,
You know, this type of question would be perfect for the DCC/Electronic section  of the forum. 

I use SMD resistors almost exclusively for any N scale work.  I have even soldered small diameter (magnet) wire directly to the plated ends of those resistors then encapsulated it in either a piece of heat shrink or liquid masking tape.  But in most cases I etch my own PCBs (Printed Circuit Boards) to house the resistors and LEDs.



See https://www.therailwire.net/forum/index.php?topic=31615.msg351219#msg351219  for more info.



See http://forum.atlasrr.com/forum/topic.asp?TOPIC_ID=70338 for more info.

I also recently posted info in your Atlas Shay decoder install thread showing my decoder install in that loco. There I also used a PCB with a SMD resistor and LED.
It was in this thread: https://www.therailwire.net/forum/index.php?topic=37758.msg454306#msg454306




So, your idea is quite viable.  Just make sure to use resistors which can safely dissipate the power wasted in them (without overheating).  With 12V supply and resistor values of 1-2K ohms you should be safe using 0805 or larger size resistors.

As far as the PCB goes, I often get mine from http://www.goldmine-elec-products.com/products.asp?dept=1034

[sizemore]

I had to run two LED's for the Super-Berk, used magnet wire and SM Resistors without issue. Should work in this application too.

The S.

[rhiadon]

peteski,

I knew I'd seen this info before. I completely spaced it. Also you are right, I think. This does belong in the DCC forum. Is there a way to move it?

[Lemosteam]

I was able to simply use an exacto knife to cut the traces out and remove the excess material by peeling it off the board.  Draw it in paint, print it and tape it to the board and follow the edges.  worked great on four identical boards used for my polar express where SMD components were used.