Surface Mount LEDs and Resistor Values

[diburning]

I am going to have surface mount LEDs for lighting in a locomotive and I was wondering if a 1/8th watt 1K ohm resistor would be ok for the surface mount LEDs. I don't want to burn up the resistors and melt the shell though.

Do you think the 1/8th watt resistors would run too hot for a locomotive that's about N-scale sized? (it is actually HO scale and track voltage will be 14 volts or so)

 

[flh80]

I've been using 1/4w 470 ohm with 3mm led's for headlamps, I would think the surface mt's could use the same, try one out but I'd say 1k is too much, it probbly won't light.

Correction I'm using 750 - 1/4w.

Maybe @ 1/8w a 1k will work?

 

[Larry]

I use 1/2W 560 ohm 5mm for headlites & 3mm for tailites. I also use the same for building interior roof lites.

 

[diburning]

Ok, I looked it up (I probably should have done that in the first place ), Tony's Trains says:

A 1K (1000) Ohm 1/4 Watt resistor will work. If space is tight use a 1.5K 1/8 watt resistor.

When using 1,000 Ohm resistor with 14 volts DC at the Function Output Common, there will be at maximum .0125 Amps or 12.5 mAs to flow through the LED. While this is less than the desired specified current for the LED, it will give plenty of light and the brilliance is only marginally affected. You won't notice the difference. Operating at the lower current, allows for a higher voltage at the track that some DCC systems have, mainly the Entry Level systems.

But, that is for regular sized LEDs. I'd imagine that the surface mount ones would draw the same if not less power.

Also,

Using 1,000 Ohm resistors with your LEDs, as I do, the heat dissipated is .0125 x .0125 x 1,000 = .156 Watts. A 1/4 or 1/2 Watt resistor is suitable.

Two similar value resistors connected in parallel, make a resistor HALF the value of each resistor with TWICE the wattage. E.G. Two 1,000 ohm 1 Watt resistors in parallel makes a 500 Ohm 2 Watt resistor.

So, when I do the install, I'll see how much space I have. If I can fit a 1/4 watt resistor, I'll go back to Radio Shack and exchange these 1/8 watt 1K ohm resistors for 1/4 watt ones. If a thicker 1/4 watt 1K ohm resistor won't fit, I'll use two of these 1/8 watt 1K ohm resistors and hope that 500 ohms is enough.

EDIT: Doing some calculations with an online calculator, it seems that the SMLEDs run at 15 milliamps, 3.4-3.8 volts, so 1K ohm is about right, but I'd need 1/4 watts to be safe for a resistor. Hopefully the 1/4 watt 1K ohm resistor fits!

 

[lmackattack]

just a tip of your intrested in large Qty of LEDs. I bought 50 yelglow LEDs on Ebay last week for about $7 they are prewired and with a resistor for 12V. both in 3mm or 5mm your choice.

I just installed them in my P2K locos and so far so good. biggest issue is the wire size is a little large for what I like to use but I was able to get them in the shell with the wires tucked in nicely.

 

[diburning]

That beats buying the Miniatronics ones I'll look into it.

Usually I don't need LEDs unless I am replacing bulbs or replacing burnt out LEDs on a locomotive. I will probably need a couple when my BLI SD40-2 gets rebuilt (they have 5mm LEDs mounted onto a circuit board, but theres no way to replace the LED on the circuit board so I'm going going to tape it to the shell)

 

[lmackattack]

I was using the minitronic Bulbs and LEDs but I found that even high end stuff seams to fail. I started using red and amber LEDs for the cars and trucks I build, bought them in bulk prewired, saved time and no confusion ow what side the resistor goes on etc...

I dident think I would like the LED in my locos untill I put some in. Now thats all I use and knock on wood they work just as good as the rest.

here is the ebay link.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300516795565&ssPageName=STRK:MEWNX:IT

here is how they look when installed

 

[lmackattack]

I should add the other thing that I dislike about the LED is that the number boards are to bright. I may have to test putting a light coat of paint behind them so the light is not as intense, I do like how the head light look now. you can see the train coming!

 

[TomR]

I posted something in another thread about calculating resistor heat and value:

http://www.modelrailroadforums.com/forum/showthread.php?t=21972

Premature failures in incandescent lamps can be caused by using DC, or of course too much current (from too much voltage). In some bulbs, DC erodes the filament at one end faster than the other end.

One thing that can cause premature LED failure is reverse voltage. If there is significant reverse voltage, such as when an LED is run off alternating current, the junction inside the LED can breakdown.

Also be careful of carbon in resistors. Carbon is a semiconductor, and if subjected to heat can drop in resistance. This caused a lot of TV set fires in the 60's and 70's. If you use carbon resistors be sure to size them conservatively. They are generally unreliable when at more than half the rated dissipation.

A better formula for resistor size is voltage across the resistor and resistance, not current. Few people know what the current really is. Most people can easily measure voltage across the resistor. Just take the voltage squared and divide it by the resistance. If you measure ten volts across the resistor and it is 1000 ohms, it is ten squared (100) divided by 1000 ohms or 1/10th of a watt.

LED's should last a lifetime, so something is outside the rating. Could be AC power, could be excessive current.

Tom

 

[flh80]

I was using the minitronic Bulbs and LEDs but I found that even high end stuff seams to fail. I started using red and amber LEDs for the cars and trucks I build, bought them in bulk prewired, saved time and no confusion ow what side the resistor goes on etc...

I dident think I would like the LED in my locos untill I put some in. Now thats all I use and knock on wood they work just as good as the rest.

here is the ebay link.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=300516795565&ssPageName=STRK:MEWNX:IT

here is how they look when installed

They look good ! I looked at those kind but I'm trying different sizes for mine. They say a 5mm fits right in to a F/E unit so I bought some flat ones and then no lens like I usually do, I suppose you could make those flat easy enough though.

 

[diburning]

I used to use the 5MMs in Athearn Blue Box engines (back when I had them). I'd wire them up with their resistors, then tape them to the inside of the shell with electrical tape so that the light doesn't leak through.

Also, I just found NGineering's site. They sell 1/8, 1/4 and 1/2 watt surface mount resistors that can withstand soldering iron heat for up to 10 seconds (which is more than enough to make a connection). Those surface mount resistors will definitely fit!

 

[TomR]

bought them in bulk prewired, saved time and no confusion ow what side the resistor goes on etc...

The resistor goes on either side of the LED. Doesn't matter to the LED.

 

[diburning]

I generally put them on the common (blue) wire. It makes it easier to have a system.

 

[John P]

Two similar value resistors connected in parallel, make a resistor HALF the value of each resistor with TWICE the wattage. E.G. Two 1,000 ohm 1 Watt resistors in parallel makes a 500 Ohm 2 Watt resistor.

So, when I do the install, I'll see how much space I have. If I can fit a 1/4 watt resistor, I'll go back to Radio Shack and exchange these 1/8 watt 1K ohm resistors for 1/4 watt ones. If a thicker 1/4 watt 1K ohm resistor won't fit, I'll use two of these 1/8 watt 1K ohm resistors and hope that 500 ohms is enough.

You can always use four resistors, two 1K's in parallel, in series with another two 1K's. Total of 1K resistance with 4x the power capability of one of the individual components.

 

[diburning]

Space is at a premium so I doubt that I can fit 4 resistors for each light.

 

[John P]

Aargh! I have been asleep.

Your idea of two 1K resistors in parallel cannot work. Each of them will carry the same current as it would if it were there alone, putting twice the current through the LED. And of course, each resistor will dissipate the same power as it would if it were alone, but the situation will be worse because they'll be side by side, giving the heat less opportunity to escape.

You need to get two resistors of twice the value (2K) and put them in parallel. Then you'd get the same power dissipation as a single 1K.

 

[lmackattack]

The resistor goes on either side of the LED. Doesn't matter to the LED.

Hmmmm. I seam to recall that when I wire up a headlight,rearlight and a beacon the LEDs needed the resistor on the correct lead? if not some leds would not work? may have been bad wireing on my part?
Because of that I also just put the resistors on the side that runs to the blue wire.

The 5mm LEDs are brite and I had to shave them down to fit in the P2k GP18 light housing in the frame. I use electrical tape to cover the roof fans and then put tape across the interior cab sides so you cant see light leaking out. on my C&NW locos I had to paint the inside of the shells Black because the the Yellow parts of the shell was becoming transparent with the brite LEDs.



Trent

 

[TomR]

Hi Trent,

Hmmmm. I seam to recall that when I wire up a headlight,rearlight and a beacon the LEDs needed the resistor on the correct lead? if not some leds would not work? may have been bad wireing on my part?

It's a series circuit. That means the resistance can be on either LED lead with equal effectiveness. As a matter of fact if I use two resistors, I often put them on each lead so if something shorts nothing bad can ever happen.

LED's have low internal resistance when conducting. They are like a regular diode with crappy reverse breakdown voltage (that is why AC could shorten life). The resistor limits current to prevent burnout (and control brightness).

The 5mm LEDs are brite and I had to shave them down to fit in the P2k GP18 light housing in the frame. I use electrical tape to cover the roof fans and then put tape across the interior cab sides so you cant see light leaking out. on my C&NW locos I had to paint the inside of the shells Black because the the Yellow parts of the shell was becoming transparent with the brite LEDs.

I'm thinking of removing my incandescent lamps (I'm still in the DC age) and using high intensity LED's. I think I'll use a constant current circuit so whenever track voltage is over a few volts the brightness stays the same. That'll do until I eventually start investing in DCC.

By the way, series connecting LED's reduces wasted power. If you have multiple LED's, the series resistor would be slightly less wattage for the same LED current per LED when they are in series, and it only would take one resistor. Of course the bulbs would have to be in an application that needs them to illuminate the same.

Tom

 

[lmackattack]

good info Tom. Im still learning about LEDs and how they work. Im more of a mechanical person where I need to see things in front of me to grasp why its working one way and not the other. Im a little slower when someone tries to talk me thru things electrical things as you cant see the flow of power untill you shock yourself or short it out!!!

I like the looks of Incandesents for buildings and even in loco headlights. the only problem is I could never get that very brite headlight with them. I hope I can dim the number marker boards on my Diesels a little so that it looks a little more prototipical. untill then....

Trent