# Burning out LED's



## Mister Bill (Jan 30, 2014)

I am trying to luminate an LED from my DC track with wheel wipers.
I guess the voltage could be 6 to 12.
I bought a package of Miniatronics Yeloglo White 3 mm LED’s, and after burning out the first one (used the 270resistor), the second one worked fine and is in use (used the 470 resistor).
I tried to build two more using the same procedure and promptly burned out the LED each time.
I was holding the resistor in my bare hand and it might have taken 20 seconds to fiddle with the solder so I got asolid connection. Could I have burned out the resistor with 15-20 seconds ofheat? 
My soldering iron is 18 watts.
On the good one, I cut the long leg ofthe LED. On the ones that burned out, I did not cut the long leg.
Does it matter which end of the resistor is soldered to the long leg? 
Does it matter which LED leg the resistor is soldered to?

Bill


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## mopac (Feb 24, 2011)

Reminds me of the first time I bought an batch of LEDs. I burned them out as fast as I could hook them up. No resistor. I am still not a LED expert but here is what I know.
Resistor can be turned either way.
Does not matter which LED leg you attach to.
There is a + and - side to the legs (it will not burn out LED if you are wrong but it won't light if you are wrong.
I would suggest using a 1000 resistor. Never burned one out with that. If LED is too dim drop down to about 700.
I think a 450 resistor is cutting it too close.


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## JerryH (Nov 18, 2012)

LEDs burn out from too much current. They can do that immediately when power is applied the first time. The resistor limits the current through the LED. Since the voltage varys in on a DC track, you have to use a big enough resistor to reduce the current when the voltage is high. DC voltage could go as high as 18 volts. A LED has a voltage and current rating and you can use those numbers to determine what value resistor to use. http://led.linear1.org/1led.wiz I would use at least a 1000 ohm resistor because of that. The LED will dim as the voltage is lowered. If done with proper prep and heat sinks, it should only take a few seconds to solder a lead. You are likely to fry the LED from too much heat over time rather than the resistor. It doesn't matter which lead the resistor is on.


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## wvgca (Jan 21, 2013)

it doesn't matter which leg of the resistor, or which leg of the led, although i normally put the resistor on the anode [longer leg] as habit ..leds vary as to current draw, and forward maximum voltage, normally between 1.8v and 3.3 volt depending on colour, but most manufacturer's [and web site] recommendations are for 'maximum' brightness, which gives little room for error, and most dc power packs will give more than 12 volts maximum to the track.. I generally use no smaller than a 1k ohm resistor, sometimes up to 4k, I don't consider a model train led that is bright enough to read by to be realistic, if I get a visible circle on my hand six inches ahead of the loco, that's what I prefer...using a 1k or better will give slightly less light than a 470ohm, but leaves room for error on input voltage and will increase light expectancy quite a bit..the voltage drop across the led is relatively constant so that when you increase the applied voltage with the resistor you are changing the current draw ... also if you are driving off dc or dcc track power a diode or bridge rectifier is required as leds are not tolerant of high reverse voltage application..




i


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## traction fan (Oct 5, 2014)

*LEDs on variable dc voltage*

First off, I agree with the prior advice to use a higher value resistor. 1000 ohms sounds like a good place to start.
You might consider using a voltage regulator as well as a resistor. The track voltage varies from 0 to 16 volts or so. If you have access to a voltmeter, check your track voltage at min.-highest speed settings to get a better idea of what voltages your DC power pack puts out.
Part of the problem is the fact that the voltage varies so much. That's where the voltage regulator comes in. If you use say a five volt regulator; it will feed a constant five volts to the LED/ resistor combination, regardless of speed setting.
You may find the 470 ohm resistor now works better than the 1000 ohm one, because the input voltage is lower and steady.
If you're not familiar with voltage regulators; the common type is about the size of a postage stamp, has three legs, and needs a heat sink. The heat sink can be a small piece of aluminum screwed to the projecting metal tab on the regulator. Voltage regulators are available from allelectronics.com.

Good luck;

Traction Fan


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## gunrunnerjohn (Nov 10, 2010)

Simple rule of thumb for LED resistors for maximum brightness. Figure that most red/yellow/green LED's use 2 volts or less, and white or blue ones around 3 volts.

Take the maximum applied voltage you'll have, subtract the voltage drop of the LED, and multiply that number by 50. That's the smallest resistor you should use for the LED.

Using that example, with a white LED and 16 volts maximum track voltage, I come up with 750 ohms.

(16V - 3V) * 50 = 750


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## fcwilt (Sep 27, 2013)

gunrunnerjohn said:


> Simple rule of thumb for LED resistors for maximum brightness.


You know that old saying:

"Give a man a fish and you feed him for a day. Teach him a rule of thumb and he will never understand Ohm's Law."

Or something like that.


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## gunrunnerjohn (Nov 10, 2010)

Maybe he doesn't want to understand Ohms law?


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## Mister Bill (Jan 30, 2014)

Thanks, guys, problem solved.
I was the problem. 
I put my last LED in a clamp and touched the iron to it for about ¾ of a second and got a good bond instead of trying to do it with my fingers.
I was applying too much heat for too long and burning things out.
I now have a nicely lighted tender.

I appreciate all of your advice, but this came as a kit, so I thought that the resistor selection should have been thought out by the manufacturer.



Bill


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## Dano (Aug 26, 2012)

I recently had some passenger cars fitted with LEDs. I think a bridge recifier and resister and capacitor were installed with the lights, no flickering.


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## Lee Willis (Jan 1, 2014)

gunrunnerjohn said:


> Maybe he doesn't want to understand Ohms law?


Ohms law works best when fishing for electric eels, by that way.

For buildings and passenger car lighting, etc. I buy LED's mounted with resistors built in with them so they operate on 12V without any resistors added. I find it just so much easier. I run my building lighting off a small DC supply set to 10 volts (gives a slightly softer light), and I normally cruise my trains at about 10-13 volts, so after one considers the voltage drop through a rectifier, they see 9.5 - 12.5 vdc, which they seem to like. 

When I have to replace/add a light in a loco, I do use standard 2V or so LEDs: often the resist goes inches away where there is room to put it, the area where the light is going being tight.


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## DonR (Oct 18, 2012)

Mister Bill

The easy way to solder an LED or other heat
sensitive part is to first use flux, then
with a dab of solder on the iron tip, touch
the solder to the part to tin it for just a second.

Do the same to the other part or wire.

Put them together and carry a dab of
solder to the connection and touch it
with the iron for just a second. You'll
get a good connection without damage
to the parts.

You might practice using the previously
damaged parts.

Don


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## gunrunnerjohn (Nov 10, 2010)

FWIW, LED's should not be that sensitive to soldering. I've soldered tons of LED's into various places, and I don't have issues losing them to heat. Typically, I cut the leads very short for installation. If I can, I use a pair of forceps to heatsink the LED, but that's not always possible. I've also soldered on lots of surface mount LED's, no heatsinking there.


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## Lee Willis (Jan 1, 2014)

Dano said:


> I recently had some passenger cars fitted with LEDs. I think a bridge recifier and resister and capacitor were installed with the lights, no flickering.


I've taken to putting a string of three tiny 5.5V supercaps in series (for tolerance of 16.5 DCV total), the series of three wired parallel with the LEDs in passenger cars: costs little, yet means the LEDs not only don't flicker, but stay bright through the operation of a direction switch or even a throttle down to toggle the e-unit when running conventional. I've only done three like this so far - I just do it as I get to it.


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## Patrick1544 (Apr 27, 2013)

I use a rectifier and voltage reg . I Don't know about the cap. Where would it go and what will it do for the lighting? What value is used?


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## DonR (Oct 18, 2012)

The cap goes across the rectified DC parallel to the lights (observe polarity and 
max voltage, usually for model railroads it will be 50 v dc.)
It acts as a 'storage battery' with enough charge to keep
lights on without flickering when power pickup loses contact
with the rails.

Don


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## Patrick1544 (Apr 27, 2013)

Thanks Don
What would be the uF rating?


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## gunrunnerjohn (Nov 10, 2010)

My lighting modules use a 330uf cap for the lighting strips in passenger cars, nobody has complained about flickering lights, and I don't see any flickering with any of the ones I've converted. For a single LED, you can probably get away with 100uf or more.


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## DonR (Oct 18, 2012)

John

What uf for the typical 3mm and larger incandescents
that some use in lighted cars?

Don


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## gunrunnerjohn (Nov 10, 2010)

Trying to use capacitors for incandescent bulbs is a whole different kettle of fish, they caps would have to be MUCH larger. I've never even considered it as I'd much rather have the LED lighting.


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## Lee Willis (Jan 1, 2014)

I use a string of three 5.5V .33 Farad (not microfarad) supercapacitors, which gives about 1/10 Farad (because they are in series) as well as the ability to tolerate a full 16 volts. Put in the proper DC orientation, parallel with "12V" LED lights and normally operating at 10-13 volts, these not only provide anti-flicker, but as I said, once full charged (about twenty seconds) mean the lights will operate through even a long direction-switch or throttle down to change the e-unit. In fact they operate for about a minute or more after shut down. They cost a bit more, though . . .


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## fcwilt (Sep 27, 2013)

Supercaps are usually rated in Farads rather then micro-Farads.

The voltage ratings are usually quite low compared to a "normal" capacitor.

Here are some examples:

http://www.ebay.com/bhp/supercap


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## Lee Willis (Jan 1, 2014)

fcwilt said:


> Supercaps are usually rated in Farads rather then micro-Farads.
> 
> The voltage ratings are usually quite low compared to a "normal" capacitor.
> 
> ...


Yeah, those are typical types and prices. It is important to aslo keep in mind that supercaps are definitely "directional" only designed to work in one direction, not as real capacitors, with AC signals. Reversing voltage on them too many times will fail them. 

I buy scads of big ones, mostly on Amazon, souring it for quantity discounts: Cornell Dublier or PowerStor are the brands I prefer. I get only doubles (5.5V limit) in sizes of 1, 1.5, or even 10 Farads, usually coin-type, for use as "electronic flywheels" in 'Streets vehicles: three put across the terminals of the DC motor and it gives a 'Streets car, truck, or bus near total immunity from stutters or stalling due to the marginal electrical contact in will have at very low operating voltages - so fitted they run very smoothly at really low scale speeds. Without them they don't.


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## gunrunnerjohn (Nov 10, 2010)

Some of those prices are a little high, especially for the smaller ones. I like to buy from reputable supply houses, especially when the prices are better. 

Digikey superCAP search, in stock.

You can have them in about two days, and you'll get exactly what you order.


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