Blue LEDs
11-06-2002, 07:07 PM
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Blue LEDs
i got some blue 5 volt LEDs from radioshack and i was wondering if anybody knows how to reduce voltage from 12 volts to power these lights. i know i need to wire resistors to them but i'm not sure what to use. any help would be appreciated.
11-06-2002, 07:18 PM
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resistors.
5 Volt LED's huh? whats the item #??
typically LED's work from roughly 1.7-1.9 volts.
For instance, say you want 1.9 volts for the LED and you have a 12 volt source. You'll need a resistor of roughly 505ohms.
to get 5 volts, from a 12 volt source, you'll need a resistor of around 350 ohms.
Typically, a 1/4 watt resistor is good for a single LED... resistors from radio shack come in packs of 5. You can also get 1/2 watt and larger resistors... to power more than one LED on each resistor.
5 Volt LED's huh? whats the item #??
typically LED's work from roughly 1.7-1.9 volts.
For instance, say you want 1.9 volts for the LED and you have a 12 volt source. You'll need a resistor of roughly 505ohms.
to get 5 volts, from a 12 volt source, you'll need a resistor of around 350 ohms.
Typically, a 1/4 watt resistor is good for a single LED... resistors from radio shack come in packs of 5. You can also get 1/2 watt and larger resistors... to power more than one LED on each resistor.
11-06-2002, 08:15 PM
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i have both 5 volt and 3.7 volt leds from radioshack. they are fairly large, and i have seen the ones you are talking about. the package that they came in say 5 volt and 3.7, so i'm assuming that's what they are rated for. but thank you for your help in choosing resistors. how did you figure out the the ohms and watts? thanks again
11-06-2002, 08:32 PM
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the wattage is usually listed on the package of the LED... its a small figure, but from my experience, using more than one LED on a 1/4 watt resistor makes the resistor get hot... meaning its too much for it.
The ohms for the resistor is pretty easy to figure out... heres a formula:
The ohms for the resistor is pretty easy to figure out... heres a formula:
11-06-2002, 08:35 PM
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sweet. thanks for the formula. it's hard to find info like that on the net if you don't know where to look. i've been looking for quite a while but hardly found anything. thank you again and i'll have those blue leds up and running in no time.
11-07-2002, 06:18 PM
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one more question: do i wire the resistor just into the 12v power wire? i don't have to put anything on the negative do i? thanks.
11-07-2002, 10:30 PM
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just make sure you get the polarity correct... as LED's only work one way... the longer lead is the positive (i think, hard to remember) but the package will tell you which is which.
11-08-2002, 04:57 PM
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Originally posted by SumAccordGuy94
resistors.
5 Volt LED's huh? whats the item #??
typically LED's work from roughly 1.7-1.9 volts.
resistors.
5 Volt LED's huh? whats the item #??
typically LED's work from roughly 1.7-1.9 volts.
huh????? most LED's have a forward voltage 3-5vDC . The resitors are used to clamp the current not the the voltage. The bias current is the thing that concerns you, some of the newer LED's can want as much as 100ma, underdriving them won't hurt them though, it will just make it so they aren't that bright.
Your theories are close, but not quite accurate, so here's a little extra mind numbing info. You using an arbitrary .020 without an explanation scared me. The proper formula is (source V - LED Forward V) / Forward Current. In the case of the LED's I used for my dash (LED #74 from allelectronics.com) the max Forward V is 4.5, they have a forward current of 20ma(I have some LEDs that pull 50). So (15volts(rough # for a car electrical system) - 4.5volts) / .020A = 525ohms. I only had 470ohm SMD (Surface Mount Device) Resistors in stock so I hooked a few up and I played with them on the bench and did a 100hour burn-in in a climate oven at 85°C to see how they hung and they did great so that's what I used.
Also it doesn't matter which side the resistor is on.
Here's a quick little how to page I put together to help otehr 6th gen accord owners make their car look pretty
http://www.igd4u.com/ledaccord/
