speed8
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 posted on 23/4/06 at 10:40 PM |
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LED resistors
My schoolboy physics is getting forgotten these days.
I'm using small led's as my warning lights for indicators, neutral, that kind of thing. I keep blowing them so figure that I probably need
a resistor in there. How do I figure out the resistor size to use with them?
Cheers
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Bob C
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| posted on 23/4/06 at 10:55 PM |
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lets say you want 20mA (whatever makes 'em bright enough) and you have 14V supply (with engine running). So that goes down to 13V when you
consider the LED's forward voltage. 13V/20mA is 650ohm. You can't get 650ohm resistors, but 680ohm is close enough.
Having said all that - you'll probably find it's too bright, or not bright enough. Best thing is to get hold of a 2kohm potentiometer
(variable resistor). Set it to mid range & use it as the load resistor (wire to one end and the wiper terminal). Wind it up & down till the
brightness is right. Now take it out of circuit and measure it's resistance. Buy a fixed resistor close to that resistance for that lamp.
If you have a mix of green orange yellow & red LED's the brightness will tend to vary - also there are standard and "high
efficiency" types. Using the potentiometer you can attempt to even things up!
After being lit for a year, most LEDs reduce to about 70% original brightness. Nothing to worry about, just pointing out that these things do actually
age.
Bob
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speed8
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 posted on 23/4/06 at 10:56 PM |
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cheers. i'll go pick some up in the morning
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Chippy
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| posted on 23/4/06 at 11:00 PM |
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Why go to all that bother, buy some 12v ones, all colours from Radio Spares, or Maplins. They can be had in standard, or extra bright, cheap as chips.
They even do natty little plastic holders for them. atb Chippy
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speed8
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| posted on 23/4/06 at 11:18 PM |
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Good point. The ones I'm blowing are from Maplins but at 20p each I'll try some 12V ones.
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02GF74
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| posted on 24/4/06 at 09:53 AM |
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as Bob C said but depending on the LED and current trhough it, forward voltage drop can be anywhere between 1.5 and 5 V. We can use 2.5 as a rough
number.
(14 - 2.5) / current of 20 mA give a resistor value of about 560.
I am not sure what the max voltage the maplin 12 V ones can take; I have them in my Land Rover and not blown one yet.
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trextr7monkey
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| posted on 24/4/06 at 03:41 PM |
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try
www.rapideducation.co.uk we buy in large bags for fractions of a penny each
If youare not looking for anything flash could easily sort you a pile out
ATB Mike
http://www.flickr.com/photos/14016102@N00/ (cut and paste this dodgey link)
Our most recent pics are here:
http://s129.photobucket.com/albums/p211/trextr7monkey/
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MikeR
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| posted on 24/4/06 at 05:29 PM |
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can anyone buy from them or is it only educational establishments?
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trextr7monkey
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| posted on 25/4/06 at 09:53 PM |
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As far as I am aware anyone can use them - they used to be called Rapid electronics with a massive catalogue, schools geta slimmed down version with a
selectionof commonly used items.
As name suggests the service is quick but they can get sticky about taking stuff back if you cock up the order forms!!
They have a decent website which allows you to hunt for things you might want, hard copy of catalogue provides examples and answers FAQs like this
one.
http://www.flickr.com/photos/14016102@N00/ (cut and paste this dodgey link)
Our most recent pics are here:
http://s129.photobucket.com/albums/p211/trextr7monkey/
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trextr7monkey
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| posted on 25/4/06 at 09:55 PM |
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PS not sure if they have a minimum postage rate- check first!!
u2u me if there's anything you think we might be able to sortyou out with
http://www.flickr.com/photos/14016102@N00/ (cut and paste this dodgey link)
Our most recent pics are here:
http://s129.photobucket.com/albums/p211/trextr7monkey/
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