Led’s keep burning out!

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Garethh
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Led’s keep burning out!

Post by Garethh » Sun May 03, 2020 11:31 am

I spent a bit of time yesterday making a working tail lamp for my brake van using a 3v led, on/off switch and 2 AA batteries. It worked fine with some old batteries I had kicking about but they only lasted a couple of hours before they died. When I replaced with some new ones the led immediately went pop, any ideas? Thought a 3v led on 3v should be fine but electrics aren’t my strong point...

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Re: Led’s keep burning out!

Post by metalmuncher » Sun May 03, 2020 1:32 pm

LEDs really need some form of current limiting to stop them drawing too much current and going pop, the most basic way to do this is a resistor in series with the LED. If you are driving just one LED off 2 AA batteries a resistor of 100 ohms should be a safe value for any LED colour.

Calculation of a more accurate resistance value for a particular application is relatively straightforward, there are online tools to help with this, e.g. http://www.ohmslawcalculator.com/led-re ... calculator. This article has a bit more information on how the calculation works if you are interested: https://www.evilmadscientist.com/2012/r ... -for-leds/

Garethh
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Re: Led’s keep burning out!

Post by Garethh » Sun May 03, 2020 5:33 pm

Thanks for those links, very handy!

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Re: Led’s keep burning out!

Post by Garethh » Thu May 07, 2020 7:08 pm

F05523B6-A62F-4B98-B88F-91CB8A786459.jpeg
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Back in business!

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ge_rik
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Re: Led’s keep burning out!

Post by ge_rik » Fri May 08, 2020 7:11 am

Garethh wrote: Thu May 07, 2020 7:08 pm Back in business!
Glad to hear it.

Something (else) which perturbed me about the 16mm NGM Guide was what the late lamented Brian Dominic has written about using LEDs in models .....
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philipy
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Re: Led’s keep burning out!

Post by philipy » Fri May 08, 2020 8:16 am

You're right Rik, that is a worrrying assertion, in more than one way.
Philip

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Jimmyb
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Re: Led’s keep burning out!

Post by Jimmyb » Fri May 08, 2020 9:06 am

My understanding (eventually) was that LEDs are current based and not voltage based, though in most cases you will need 3 volts, without the requisite resistor the current could easily be too great and dramatically reduced the LED's life. I can only say the person who wrote the article was very lucky.

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SeamusOD
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Re: Led’s keep burning out!

Post by SeamusOD » Sat May 09, 2020 12:32 pm

That is a strange statement and can only assume he was using specific LEDs rated for the voltages he was using, as your bog standard coloured 5mm leds red, green, yellow are rated at 2v, and as jimmyb said are current rated 20-30ma you put 12v on them with no resistor and it will light once !
These "standard" leds have been around for long time and always used with a resistor which are normally 220R for 5v, 330R for 9v and 470R for 12v you can use any voltage you want so long as you keep current correct that is what a resistor does.

There is however 12v leds they have a resistor built in so safe to use on 12v there are others with different voltages 5v being another common one.

There are now in more recent years, other more intense leds that tend to come with a clear case not coloured, these are totally different and much brighter, and come is wide range of "nominal" voltages, but again if its a 3v led you cant put 12v on it without a resistor, try it and see what happens !
Even with these leds if they are rated at 3v and dont state they have a built in resistor like 12v ones mentioned above and you are using the same rated voltage 3v in this case, its still good practice to use a resistor which can be as low as 10R.
Last edited by SeamusOD on Sat May 09, 2020 12:48 pm, edited 1 time in total.

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Re: Led’s keep burning out!

Post by SeamusOD » Sat May 09, 2020 12:48 pm

Forgot to say in your case it went pop because voltage was obviously higher with the new batteries, if only momentarily

Something we discovered in the RC aircraft world we would normally use 4 AA (4.8v) nimh batteries for powering receiver servos etc and some guys like to use the 5 AA nimh packs which are 6v as tend to give bit more power and speed to the servo, but found were having problems burning out some brands of servos and the reason why was when you take a freshly charged 5cell pack off a charger, it can be over 7v if only for a few seconds but enough to cause problems to something with a 6v rating

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Re: Led’s keep burning out!

Post by Boustrophedon » Sun Jun 07, 2020 11:31 pm

To calculate the value of the series resistor for one LED, take the battery voltage, subtract 2, multiply by 50. Gives result in ohms. This assumes 2 volts and 200mA to light the LED. You can do it in your head.

Or you can buy LEDS with built in regulators, look for 12V leds on Ebay. They do need 9V to work though.

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