resistors for 12v apps 12v or more?
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=132142
Printed Date: May 08, 2025 at 12:37 AM
Topic: resistors for 12v apps 12v or more?
Posted By: duffmanrha
Subject: resistors for 12v apps 12v or more?
Date Posted: September 05, 2012 at 9:02 PM
I'm wiring in some indicator LEDs for something and since I need to use a resistor, I am calculating that out. I know how to find out what resistor I need, and going to next production size, etc. but I'm not clear on what voltage rating I should use. A car is obviously a 12v system, BUT when the car is running, the voltage can be 13-14v - in this case my measured voltage at idle is ~13.5v. Should I be using this value to calculate for my resistor, or should I really use 12v instead? If I get a resistor for 13.5v and the voltage drops, won't the LED go out?
Replies:
Posted By: i am an idiot
Date Posted: September 05, 2012 at 9:19 PM
It is not really that critical. Place a 1K resistor in series with the LED, and call it a day.
Posted By: DYohn
Date Posted: September 06, 2012 at 11:26 AM
Resisters are not voltage rated, they are power rated. ------------- Support the12volt.com
Posted By: KPierson
Date Posted: September 07, 2012 at 4:17 PM
If you are really worried about a slight amount of dimming caused by normal voltage fluctuation then a DC-DC power supply could be used. If you use a 7809 voltage regulator connected directly to the battery you will, assuming you always have at least 10vdc, always have a solid 9vdc power supply to power the LEDs.
Generally speaking I always use the worst case scenario for calculating resistor values, but that is more important for proper wattage calculations then anything else.
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Kevin Pierson
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