bonzoronnie
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 posted on 27/2/08 at 09:19 AM |
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LED resistor question
I am about to embark on fitting some ultra bright LED's in the boot of my sons car to highlight his amp & sub.
Yes, I admit it, He is a CHAV 
The question is. How many LED's can be used for each of the rsistors I have.
12v dc supply (car)
Voltage limits of LED's 3.2v - 3.6v
Resistor value 510 ohm .25 w
Will i need a resistor for each LED or will it be possible to run several LED's from a single resistor. !!??
Ronnie 
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Mr Whippy
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| posted on 27/2/08 at 09:25 AM |
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would this not be a better thing to fit?
http://www.hawaiian-tropical-flowers.com/dasdanhulgir.html
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02GF74
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| posted on 27/2/08 at 09:47 AM |
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you need to know the current you want in each LED and from the spec sheet the volt drop across them,
I am assuming you are just using 5 mm LEDs so typically you would drive them at 20 mA and volt drop across them is 3.5 V.
As you have a 12 V (in reallity more like 14 V) you can fit 3 LEDs in series.
so you have 1.5 V to drop across your resistor. As you are drawing 20 mA, the resistor value is 76 ohm.
The books say you should not connect LEDs in parallel from one resistor as one may draw more current than another so the brightness will vary or may
not even turn on at all and current may be hogged by one LEDs so it blows mening there are now even less LEDs to share the current so another may blow
and so on until are are gone.
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bonzoronnie
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| posted on 27/2/08 at 09:59 AM |
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Mr whippy
I'd love too. Imagine the look on his face.
_________________________
02GF74
Thanks for the input 
Yes they are 5mm LED's
Full spec on the pack is.
Model: JL5-W20-25-B10000
Iv 10,000
VF 3.2 3.6
Wd 465-475
Ronnie
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RazMan
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| posted on 27/2/08 at 10:28 AM |
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It would be much easier to fit one or more of the LED strips available on eBay. I fitted over 100 LEDs this way in my car for interior lighting - no
chavvyness I assure you
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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MikeRJ
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| posted on 27/2/08 at 10:48 AM |
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As 02GF74 says you should not parallel LEDs, their forward voltage varies significantly so you will get one LED hogging most of the current.
You can put a few LED's in series and use a lower value resistor as suggested. However, the downside of this is that relatively small changes
in the cars battery voltage will cause significant current changes in the LEDs due to this low resistance. I would stick to one or two LED's
per resistor, but if using more than one LED in series you will need lower value resistors e.g.
R = Vbat - Vled / Iled
Vbat = car battery voltage (nominal 13.8v)
Vled = forward voltage drop of all LEDS i a series chain i.e. number of LEDs * 3.5
Iled = Required current through LEDs, usualy 20mA (0.02 A) for a reasonable life.
For 2 LEDs
R = ( 13.8 - (2*3.5) ) / 0.02 = 340, a 330R resistor will be the closest in the E24 series. The resistor will be dissipating about 140mW, so a 1/4
watt resistor will be adequate.
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ChrisW
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| posted on 27/2/08 at 12:19 PM |
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Why not just use 12v LED's that have the resistors built in already?
If not, resistors are dirt cheap, and you won't need particularly big (in terms of power dissipation) for LEDs. Just use one per LED and have
no worries??
Chris
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02GF74
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| posted on 27/2/08 at 12:35 PM |
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quote:
Originally posted by MikeRJ
You can put a few LED's in series and use a lower value resistor as suggested. However, the downside of this is that relatively small changes
in the cars battery voltage will cause significant current changes in the LEDs due to this low resistance.
good point but there is more than one way to skin a cat ; all depends on how you want to chieve this. You could use a 12 V regulator to ensure the
voltage is stable but is ti worth the expense/conplexity?
one/2/3 resistor per led is a poisble - take you pcik on which one you want,
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trikerneil
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| posted on 27/2/08 at 12:40 PM |
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have alook at the LED series/parallel array wizard HERE
Neil
ACE Cafe - Just say No.
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britishtrident
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| posted on 27/2/08 at 01:26 PM |
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quote:
Originally posted by ChrisW
Why not just use 12v LED's that have the resistors built in already?
If not, resistors are dirt cheap, and you won't need particularly big (in terms of power dissipation) for LEDs. Just use one per LED and have
no worries??
Chris
Available from Maplin.
I used these for use with the reversing sensor on my tintop --come compelete with holders and bevels and 1 metre or so of pre wired tails.
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bonzoronnie
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 posted on 27/2/08 at 07:17 PM |
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Thanks for all of your help guys.
At least I have a clearer picture of what is required.
Yes it will be far easier to use 12V LED's But i just happen to have several hundred 3.5v LED's kicking about the garage. Plenty of
resistors too.
Just looking to save a bit of time on the solder & shrink tube malarkey.
One again, thanks to all.
Ronnie
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ChrisW
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| posted on 27/2/08 at 07:35 PM |
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Also try www.ultraleds.co.uk. Not the cheapest, but certainly convenient for one-offs without having to have them sent from the far East. Good range
of products too.
Chris
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bonzoronnie
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| posted on 27/2/08 at 07:56 PM |
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Cheers for that link Chris.
A lot of handy stuff there.
Ronnie
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