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Resistor for LED?

Old 08-09-2002, 12:45 PM
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Post Resistor for LED?

I'm going to be running a few LEDs and need to know what size resistors I need to make them work on a 12v system. Here's the LED specs:

Absolute Maximum Ratings:
Reverse Voltage 5.0v
D.C. Forward Current 20mA

...other info
Forward Voltage @ 20mA(V):
Typ: 3.5 Max: 4.5

This info can be seen at http://www.allelectronics.com/spec/LED-59.pdf

Can anyone help me?
Thanks!
-Eric H


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Old 08-09-2002, 01:23 PM
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OHM's law states that...

V = I x R

V = voltage
I = current
R = resistance

Since the V and I are known you get...

R = 12v / 20ma

R = 600 ohms

When the car is running that voltage figure will go up to 14v or so.

R = 14v / 20ma

R = 700 ohms

You don't want to run the LEDs right at their current limit so use 1k ohm resistors to be safe.

I = 12v / 1k

I = 12ma

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Old 08-09-2002, 01:27 PM
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Thanks man! You've always been a big help in this forum.

-Eric H

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Old 08-10-2002, 06:39 AM
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You're very welcome.

I've been doing this stuff for longer than a lot of guys on this board have been alive. If my experience can be of any help to others then great. I'm glad to help when I can.
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