# LED size for light conversion



## kix662003 (Mar 8, 2013)

I know there are old posts with several thoughts on which size LED is best for converting locomotive head lights to LED. I don't remember anything about a 2mm, but these look like they will work if they fit HO and S gauge. Anyone used these instead of the smaller ones?


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## gunrunnerjohn (Nov 10, 2010)

The only issue with the 2mm is the big lump at the rear. If that isn't a problem in your situation, they will work fine. For O-scale, I normally use 3mm LEDs.

If you need really small stuff, you can use the SMT LED's with some fine wires soldered to them, they really fit into some small places!


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## kix662003 (Mar 8, 2013)

gunrunnerjohn said:


> The only issue with the 2mm is the big lump at the rear. If that isn't a problem in your situation, they will work fine. For O-scale, I normally use 3mm LEDs.
> 
> If you need really small stuff, you can use the SMT LED's with some fine wires soldered to them, they really fit into some small places!


Thanks, John. I like that these have the optical tubes already attached to the LEDs, but wonder if they're long enough? I've read that the tube is required when making the conversion. At this point, I'm only thinking about converting the front lamp on the engines. Only one of my four HO engines has a working head light.


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## gunrunnerjohn (Nov 10, 2010)

Note that you don't necessarily have to have the LED right up against the front of the headlight lens, I've had some that I had them back a bit and they worked out fine.

If you want a steady light for plain DC operation, you'll need a bridge rectifier, a current limiting resistor, and the LED for the headlight.


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## kix662003 (Mar 8, 2013)

gunrunnerjohn said:


> Note that you don't necessarily have to have the LED right up against the front of the headlight lens, I've had some that I had them back a bit and they worked out fine.
> 
> If you want a steady light for plain DC operation, you'll need a bridge rectifier, a current limiting resistor, and the LED for the headlight.


The specs are here. All my HO equipment is plain DC. Any particular rectifier numbers come to mind base on the specs? Thanks for the help!


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## gunrunnerjohn (Nov 10, 2010)

You can use any really small bridge rectifier, since the LED's at full current only consume 20ma. Just take whatever the maximum voltage you'll ever put on the track, subtract three volts, and multiply the results by 50, that's the value of resistor you need for current limiting, 1/4w is plenty of power rating. Here's a suitable bridge for 38 cents if you buy 10, and Digikey shipping for this would only be a couple of bucks. You can buy the resistors there as well.

http://www.digikey.com/product-detail/en/DF02M/DF02MDI-ND/149483


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## kix662003 (Mar 8, 2013)

gunrunnerjohn said:


> You can use any really small bridge rectifier, since the LED's at full current only consume 20ma. Just take whatever the maximum voltage you'll ever put on the track, subtract three volts, and multiply the results by 50, that's the value of resistor you need for current limiting, 1/4w is plenty of power rating. Here's a suitable bridge for 38 cents if you buy 10, and Digikey shipping for this would only be a couple of bucks. You can buy the resistors there as well.
> 
> http://www.digikey.com/product-detail/en/DF02M/DF02MDI-ND/149483


That's inexpensive enough. I assume that I'll need one bridge rectifier for each engine that I convert, so getting 10 isn't a problem with running 4 HO engines. Thanks again for the help.


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## gunrunnerjohn (Nov 10, 2010)

Yep, a single bridge, the resistor of the appropriate value, and whatever LED you plan on using. You can also get clever with diesels front/rear lights or steamer backup lights and use a single diode to the track voltage for directional lighting.


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## ktcards (Sep 22, 2012)

gunrunnerjohn said:


> Yep, a single bridge, the resistor of the appropriate value, and whatever LED you plan on using. You can also get clever with diesels front/rear lights or steamer backup lights and use a single diode to the track voltage for directional lighting.


I always throw in a capacitor to eliminate the flicker when the train passes over a switch or a bad spot in the track.

K


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## gunrunnerjohn (Nov 10, 2010)

I usually do as well for stuff like passenger cars. For locomotives, all my LED conversions are for command stuff, and the drivers don't take well to large capacitive loads, so I depend on the existing electronics for flicker resistance.


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## kix662003 (Mar 8, 2013)

Since my locos are all from the late 70s to late 80 and are all analog DC, I guess I need to pick up a few caps too. The LEDs are leaving Hong Kong today and will take about 2 weeks to get here depending on Customs. Does Digikey sell a Capacitor that will work well with the LED and Rectifier Bridge?

GRJ - Was looking at this layout on YouTube and can only imagine what yours will look like with a helix... Not hard to see what he does when he's not in the cabinet shop.


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## gunrunnerjohn (Nov 10, 2010)

Digikey has plenty of capacitors that will work for filtering your DC for steady lights. I'd probably try some of these for 14 cents each: http://www.digikey.com/product-detail/en/ESK107M025AE3EA/399-6650-1-ND/3082782


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## kix662003 (Mar 8, 2013)

Thanks again for all the input and guidance!


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## gunrunnerjohn (Nov 10, 2010)

Let us know how you make out.


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