resistor selection

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Forum Name: Miscellaneous - Off Topic
Forum Discription: Topics that just don't fit anywhere else.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=116656
Printed Date: April 29, 2024 at 4:19 AM

Topic: resistor selection

Posted By: necessaryevil
Subject: resistor selection
Date Posted: October 02, 2009 at 12:36 AM

i want to install a led in my laptop charger, on the board the pin holes are there as well as the resistor holes so i need to figrure out what size resistor i need. heres the specs with my tester it shows 2.75a@19.16v case says 3.5a@18.5v. please help

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time is not wasted when you are wasted all the time

Replies:

Posted By: i am an idiot
Date Posted: October 02, 2009 at 5:37 AM

At that voltage a 2K or greater resistor will be more than enough to protect your LED. Anything between 2K and 10K should work just fine for you.

Posted By: KPierson
Date Posted: October 02, 2009 at 5:38 AM

The optimal and min values of the resistor are 100% dependent on the LED you decide to go with. You need to know the max forward current to determine the min value you can go with. However, often times, the min value will produce too much light, so you'll want to add more resistance to dim it down a bit.

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Kevin Pierson

Posted By: Mad Scientists
Date Posted: October 04, 2009 at 1:57 PM

Wouldn't Vf of the LED also be needed to calculate?

Jim

Posted By: KPierson
Date Posted: October 04, 2009 at 4:33 PM

I've never once used the Vf to calculate a resistor and I've never had a problem.

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Kevin Pierson

Posted By: i am an idiot
Date Posted: October 04, 2009 at 6:30 PM

After trying to dim a few different LED setups by increasing the resistance I quickly realized that the resistor value is not as critical as most people think. You can do the math and get your resistor value right where you need it if you like. I noticed that if the math called for a 470 ohm resistor, I figured that a 1K resistor would be half as bright. I do not remember how large of a resistor I had to use to actually get the lamp to dim a bit, but it was large. I think it was like 6.8K to get a slight difference. I have never seen an LED that would not work on 12 volts with a 470 ohm resistor. Just to make sure I do not burn up an LED for anybody asking for help, I always suggest a 1K resistor. Since this guy has a 19 volt supply, I doubled the resistance. The 2K or the 10K will work just fine. There may not even be a noticeable difference in the brightness of the 2. But if you guys insist on doing the math, go ahead and whip out the calculator.

Posted By: necessaryevil
Date Posted: October 31, 2009 at 4:05 PM

after a few trips down to the shack i found that the 4.5k was just the right thing for me thanks a ton guys

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time is not wasted when you are wasted all the time