# LEDs



## norgale (Apr 4, 2011)

Have anAthern SD45-2 and want to put an extra light in the back for the rear number boards. Looks like a grain of wheat bulb in the front so can I add a LED to that or will they not work together? Pete


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## gunrunnerjohn (Nov 10, 2010)

You can use LED's, but you will need a current limiting resistor and maybe a diode. Is this a DCC locomotive? What is the supply to the bulb, AC or DC?


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## norgale (Apr 4, 2011)

It's not DCC. All I know is that I put the engine on the track and then turn on the transformer and it goes. Ya hada ask me all those cornfusing questions huh? I remember buying it back in the 80's if that helps. Pete


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## gunrunnerjohn (Nov 10, 2010)

Well, this is an HO conventional train, so it's DC. The LED will just require a resistor, and of course you'll have to have it connected correctly as far as polarity. Easy, peasy.


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## Bucklaew (Oct 7, 2010)

gunrunnerjohn said:


> Well, this is an HO conventional train, so it's DC. The LED will just require a resistor, and of course you'll have to have it connected correctly as far as polarity. Easy, peasy.


I have some white LEDS that have a 3 VDC drop across them and use a 100 ohm 1/4 watt resister in series with LED, seems to work OK for me.

My source was EBAY in both cases.


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## gunrunnerjohn (Nov 10, 2010)

I guess it would depend on the maximum voltage the train runs at. 100 ohms is a bit low unless you're running at a maximum of 4-5 volts. Most LED's top out at 20ma, and there is no real benefit in brightness to running them more than around 10ma.

With that being said, whatever the maximum operating voltage for your locomotive is, subtract the LED operating voltage, normally in the 2-3 volt range and multiple the remaining voltage by 100, that's the resistor you should be using. If you want to drive to the limit, you can multiply by 50 instead.


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## JohnAP (May 4, 2011)

*White LEDs*

I've been looking for some really white LEDs for my grandson's engine I'm working on. I was thinking something in the 6,000-8,000 k range, 5 mm size.


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## gunrunnerjohn (Nov 10, 2010)

Do you want "warm" white or the standard blueish white for a white LED?


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## JohnAP (May 4, 2011)

*warm led*

I think something on the high end of warm would do best. I'm not overly fond of the blue notes.


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## T-Man (May 16, 2008)

gunrunnerjohn said:


> Well, this is an HO conventional train, so it's DC. The LED will just require a resistor, and of course you'll have to have it connected correctly as far as polarity. Easy, peasy.


What about reverse?

I have had motor spikes too, that burn them out. The small grain bulbs just make it easier.


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## Massey (Apr 16, 2011)

If the engine came out of the 80s you may want to add some 1uf caps to the motor to help reduce voltage spikes. Just like in the boats! This will keep the motor from generating the voltage spikes mentioned above that protect the LEDs from popping. 5mm LEDs in my opinion are too big for HO trains. The ones i like to use are 3mm, smaller, just as bright and you can fit them in more locations. They are sometimes harder to find but they cost about the same as the 5mm.

Massey


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## gunrunnerjohn (Nov 10, 2010)

You can add a diode bridge to handle DC of either polarity, and I agree that the 1uF cap is probably a good idea as well to swamp any spikes. I'm spoiled now that most of my LED's are supplied from the command/control electronics and they've already taken care of filtering.


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## JohnAP (May 4, 2011)

*Being spoiled*

I LIKE being spoiled! God bless my other half and my good fortune, and a Happy Memorial Day Weekend to all!


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## Artieiii (Apr 25, 2011)

Ok Gunrunner,
I am not an electrical guy what is a "diode bridge" and how do I order the correct equipment. What is a 1uf cap? 
I am planning on converting an HO Bachmann cable car to a replica of Mr Rogers neighborhood trolley and want to add running lights.
TIA
-Art


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## gunrunnerjohn (Nov 10, 2010)

Here you go. Basically, it's primary purpose is to convert AC into full wave rectified DC. However, due to the arrangement of the diodes, it'll also convert either polarity of DC into a single polarity DC signal.

Diode bridge


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## Artieiii (Apr 25, 2011)

gunrunnerjohn said:


> Here you go. Basically, it's primary purpose is to convert AC into full wave rectified DC. However, due to the arrangement of the diodes, it'll also convert either polarity of DC into a single polarity DC signal.
> 
> Diode bridge


Gunrunner,
Ok that makes sense to me 4 diodes. I see how that works, very clever. This way the polarity would remain constant despite the train running forward or in reverse. I never purchased diodes, would this be what I am after? How bout that other other thing 1uf cap?
-Art


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## JohnAP (May 4, 2011)

*1 uf cap*

Artieiii, that would be a 1 microfarad capacitor


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## gunrunnerjohn (Nov 10, 2010)

A 1uF cap is just a filter.

What you'll end up with is the bridge rectifier, a resistor, and a 1uF cap. The LED will be connected across the cap, the resistor is in series with the cap and the LED, and of course the bridge is supplying the power to the whole mess from the track pickups.

Note that the cap will be polarity sensitive like the LED, so make sure it's connected correctly.


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## Artieiii (Apr 25, 2011)

gunrunnerjohn said:


> A 1uF cap is just a filter.
> 
> What you'll end up with is the bridge rectifier, a resistor, and a 1uF cap. The LED will be connected across the cap, the resistor is in series with the cap and the LED, and of course the bridge is supplying the power to the whole mess from the track pickups.
> 
> Note that the cap will be polarity sensitive like the LED, so make sure it's connected correctly.


Is this what I am after?
http://www.radioshack.com/product/index.jsp?productId=2062581
And the capacitor?
http://www.radioshack.com/product/index.jsp?productId=2062378 
If this is correct what size resistor needs to be included for this setup?
If it's not too much to ask can you send me a simple wiring diagram so I do it correctly. My plan is to use front and rear lights on the model and possible a few in the roof below the sign. I took physics in college so I can understand simple electronics diagrams.TIA
-Art


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## gunrunnerjohn (Nov 10, 2010)

You don't have to pay for a non-polarized capacitor since you have a DC source. I typically buy my stuff a Digikey or Mouser since they have very low shipping and good prices.

What are the voltages involved here? I'm not an HO guy, so I can't really estimate what voltage the track will have. There are also constant current IC's for less than a buck that will keep the LED current the same for varying voltage, makes the lighting much more realistic. I use the Supertex CL2 for that task.

FWIW, you can do something tricky like have direction sensitive lights for HO easily. That just takes a diode, resistor, and the LED. What happens is you have the diode and LED wired opposite polarity on the track pickups, so when you're going one direction it lights one of them, when you reverse that one goes out and the opposite one lights. Obviously, this only works for locomotives that aren't running DCC, since the track polarity reverses for conventional mode. This adds a bit of panache to the lighting.  If you have marker lights, you can light them as well, perhaps leaving them on all the time, using the bridge I described previously. That roof light is a good example.


There have been plenty of tutorials posted already about wiring LED's, try a search of the forum. 

http://www.modeltrainforum.com/showthread.php?t=2819&highlight=led+wiring

http://www.modeltrainforum.com/showthread.php?t=521&highlight=led+wiring


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## Artieiii (Apr 25, 2011)

*Led's with diode bridge and resistor*

Lookie what I found.....this should do the trick....includes diode bridge and the appropriate reisister. A red one for the back, a white one for the headlight and a few for in the roof.
http://www.modeltrainsoftware.com/bl-212.html
Now THAT's easy peasy gunrunner
-Art


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## gunrunnerjohn (Nov 10, 2010)

Well, you should take note of this comment: *these solid non-flashing LEDs will look great with 7-19 Volts of power input.*

You may have less track voltage at times when you're going slow.


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## Artieiii (Apr 25, 2011)

gunrunnerjohn said:


> Well, you should take note of this comment: *these solid non-flashing LEDs will look great with 7-19 Volts of power input.*
> 
> You may have less track voltage at times when you're going slow.


Damn, I thought I had this figured out. I am running DC power with an MRC Tec 4 controller running a Bachmann Cable car. I am thinking 1/2 throttle should give the lights enough current...Is this a reasonably safe assumption? I guess a smaller resister would let them illuminate at a lower voltage.
-Art


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## T-Man (May 16, 2008)

It is always good to see that a member finds their own solution.

I was stuck with the 80's problem and tried to get LEDs to work for a length of time. So now the secret is out.










Here I have a 5mm white LED with a non polarized capacitor and a 1/4 watt 470 ohm resistor.

Now all I have to do is mount it and test.

I will say the offer for the small T3/4 is enticing since they are not that much fun to solder. My rig is large to show the details. I also used diodes instead of a bridge rectifier.

I like buying the parts and starting from scratch.


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## Artieiii (Apr 25, 2011)

Sweet T-man that's what I was looking for. I did several searches and did not find it. Now I have some reading to do for the holiday.
-Art


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## gunrunnerjohn (Nov 10, 2010)

I'd set the power to the anticipated lowest operating speed and measure the track voltage under load, like with a train running. See what you have to work with.


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## T-Man (May 16, 2008)

That is a good idea if you want the max out of brightness.

Often I use a bread board to mock up the parts for the best result. 
Most of the time, it is just to see if it will work!


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