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Old 03-17-2019, 10:31 AM   #1
Join Date: Feb 2018
Location: Connecticut
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LED headlight wiring -- basic questions

Some VERY basic questions regarding LED headlight wiring.

In a few days, I'm going to make my first attempt at changing an HO loco headlight from incandescent to LED.

For my first try, I intend to rewire an old [Lifelike] Proto2000 SD9 with a new SoundTraxx [non-sound] decoder:

I'll be using these "pre-fab" LED lights that I picked up on ebay:

They have resistors pre-installed and are ready to attach to a 12v power output.

A few questions I need clarified.

I understand that LED's have a "positive" and "negative" side, is this correct?

If so, will I damage an LED if I mistakenly connect it "backwards" to the decoder?
(of course, I want to know this BEFORE I make the mistake!)

Once those issues are sorted out, more info is needed.

When connecting an LED to the decoder board, the "switched" side of the LED (i.e., the wire that is actually "controlled") is the NEGATIVE side, is this correct?

And... so it follows... the NON-switched, "common" side is actually the POSITIVE side, is this also correct?

That should do it, unless there are any other "first time tips" you might wish to pass along...

Last edited by J.Albert1949; 03-17-2019 at 11:12 AM..
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Old 03-17-2019, 12:54 PM   #2
Join Date: Apr 2015
Location: somerset, nj
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i don't believe you risk damaging the LEDs if they are wired up with polarity reversed (they are a diode that blocks the flow of current in one direction). I wired white LEDs in passenger cars with simply a resistor across the DCC rails. I'm sure some would insist you need a diode with a higher reverse voltage rating in series as well.

the following diagram and linked page should explain how to wire the LEDs to the decoder. The blue wire is positive. The decoder LED connections actual ground the LED allowing it to draw current from the positive source. Don't forget the resistor.

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Old 03-17-2019, 03:06 PM   #3
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Location: Jacksonville, Fl
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Usually, the long wire on an LED connects to the
positive side but your LEDs have connected red and
black wires. Was there no indication that came with
them to let you know which is the positive lead?

Those appear to be awfully heavy wires for the
tiny LED amp draw. Will they fit in your loco?

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Old 03-17-2019, 08:52 PM   #4
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Location: Michigan
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I cannot say about damaging, as I can't remember when I first began with led. I want to say it just did not light up. But what I do remember is, if your looking at the diode(the actual bulb), the smaller or shorter side is the anode (positive), and larger size is the cathode (negative).

With you having resistors, your pretty much all set. Soundtraxx, just follow the diagram honestly and you will be good. One thing when I was looking at your post, I was thinking, I hope you ordered the "yellow or dayglo" type, as the others are blue in color. Just a little tip...

Hope your install goes well. Just take it slow.. Even if it takes you two days to complete...

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Old 03-18-2019, 07:53 PM   #5
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Just use a nine volt battery to check polarity. The LED also has a flat side but I can never remember which one.
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Old 03-18-2019, 08:30 PM   #6
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the flat side is the negative on a led, the longer leg [if it still exists] is the positive ..


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Old 03-19-2019, 12:29 PM   #7
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Good idea on using a battery.
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