led/resistor issue

Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=104750
Printed Date: May 12, 2025 at 6:09 AM

Topic: led/resistor issue

Posted By: rehtro
Subject: led/resistor issue
Date Posted: May 14, 2008 at 10:18 PM

hello

Hello.

Can anyone tell me what happens when I use a 1/4 watt resistor when I should be using a 1/3 watt to light an led? I thought I did everything correctly with this lighted circuit board but evidently I didn't. The LED lights up fine, but the resistor gets real hot. Turns out that I should have used the 1/3 watt resistor.

OK. Do I need to replace the old 1/4 resistor with a 1/3 watt resistor? What happens if I don't. I left the led illuminated for about 15 minutes and nothing went wrong. What If I throw a couple of cooling fans in there?

I hope I'm not being to vague. I'm trying to be as general as possible.

thanks in advance!

Replies:

Posted By: ryan@tcp
Date Posted: June 04, 2008 at 9:29 AM

If you even have to ask why this is dangerous you probably shouldn't be using a soldering iron without adult supervision. Drive down to your local Radio Shack and buy the correctly rated (1/2W) resistor.

posted_image I can't imagine the through process that one goes through to determine that "throwing a couple cooling fans in there" is a substitute for purchasing the correct six cent resistor.

If you don't have an electronics store near you, Mouser Electronics sells resistors individually. Shipping will be about $5 on a six cent part, but it's better than the $500+ deductible you'll pay your insurance when your car catches fire.

The resistance value will change based on what color LED you are running (from about 560ohms for red to about 270 for blue/white) but regardless you should be using a 1/2watt resistor.

Posted By: KPierson
Date Posted: August 11, 2008 at 1:34 PM

The wattage rating of a resistor is related to the heat dissipation capacity of that resistor.

Resistors resist current flow which creates heat. If your resistor is undersized it will over heat until either it fails or until it catches on fire. A board like you are talking probably wouldn't start a huge fire, but it could happen.

If your 1/4 watt resistor is getting hot then going to a 1/3 watt resistor might not help - like stated above you should go with a 1/2 watt or bigger depending on your application.

What is the resistance of your resistor and what is the voltage you are running it at?

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Kevin Pierson