# LED headlight upgrade in old DC locos. Diode?



## kilowatt62 (Aug 18, 2019)

Greetings, 
Wondering if I should be using a diode in series as protection of sorts for the new LED replacements when DC polarity is reversed for the Locos to back up. If so, any help on diode values appreciated. 
I have some germanium, 1n60, 50ma / 80v diodes laying about. Usable? 

TIA 
KW62


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## DonR (Oct 18, 2012)

Why not just let the headlight go off when
your loco backs up? That's what most HO model
locos do...well, actually, they douse the
headlight and turn on a backup light.

Don


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## kilowatt62 (Aug 18, 2019)

Hello Don, 
Well yes. Light would go off anyway, I suppose. I am just mildly concerned if DC voltage going backwards through a LED is a bad thing. 
Looking for some knowledge on pros & cons of diodes is all. 

Cheers 
Scott, or KW62
“220,221. Whatever works”


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## wvgca (Jan 21, 2013)

a germanium diode would be easy to wire into the DC circuit, and provide some safety against reverse voltage .... although the dropping resistor -should- provide the necessary protection ..


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## gunrunnerjohn (Nov 10, 2010)

A diode will solve the problem of reverse voltage. In truth, the dropping resistor doesn't necessarily provide the reverse voltage protection. I used to use just the resistor in LED upgrades, and I had a number of engines come back with dead LED's. Since I've added a diode to the configuration, I haven't seen a return since.

If you want to LED to be on all the time with track power, simply use a bridge rectifier instead of a diode, that will give you light for either polarity.


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## kilowatt62 (Aug 18, 2019)

“Well I’ll be jiggered.” I obviously didn’t think about a rectifier. Any advice on size/value? Save me an exhaustive search maybe. 

TIA 
KW62


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## rrman987 (Aug 29, 2021)

kilowatt62 said:


> “Well I’ll be jiggered.” I obviously didn’t think about a rectifier. Any advice on size/value? Save me an exhaustive search maybe.
> 
> TIA
> KW62


Almost any in the 1N40xx series will work though you might have to do some looking for smaller size ones if space is premium. Anything 50 volts at 100mA will work.


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## gunrunnerjohn (Nov 10, 2010)

Are you looking for a diode or a bridge rectifier?


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## kilowatt62 (Aug 18, 2019)

GRJ, 
A diode value at first. Then the rectifier point came up so, that as well.


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## gunrunnerjohn (Nov 10, 2010)

I bought 1,000 1N4003 diodes several years ago, any time I need a general purpose diode, that's what I use. As for the bridge rectifier, here's a few 500ma Bridge Rectifiers that would be suitable.


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## T-Man (May 16, 2008)

kilowatt62 said:


> Hello Don,
> Well yes. Light would go off anyway, I suppose. I am just mildly concerned if DC voltage going backwards through a LED is a bad thing.
> Looking for some knowledge on pros & cons of diodes is all.
> 
> ...


To protect the back voltage just use a diode across the leads. Black bar to the + side. That will protect the back voltage.
I struggled with engine LEDs. One problem is voltage spikes on the motor. I used a bridge with 1N4001 diodes and a cap in the center. I have one working in O gage on Ac track. In HO I am not sure if I have one. If the LED blows it will back voltage that does it.. It will not happen right away but eventually.
I took my game one step further and added a voltage regulator because I had the room. That was for passenger car lighting.


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## kilowatt62 (Aug 18, 2019)

GRJ & T-man, 
You guys nailed it for me. Questions answered, as well as some in back of my mind. 
Thank you muchly for ideas, examples, and links. Ya’ll saving me from a lengthy search. 

“Man, I love this forum.” 
KW62


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## T-Man (May 16, 2008)

FYI that LED has worked for years off my AC track. The diode is a smaller switching diode the number has slipped my mind. Like John I bought I bought a 1000, 1N4001 rated at one amp.


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## kilowatt62 (Aug 18, 2019)

I settled on a handful of 1n4001. Still perusing bridge rectifiers. 
More questions to come, I’m sure. 

Thanks guys.


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