Resistor value too high?

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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=74690
Printed Date: April 18, 2024 at 12:52 PM

Topic: Resistor value too high?

Posted By: autofan01
Subject: Resistor value too high?
Date Posted: March 18, 2006 at 2:42 PM

Hi!

To keep it short:
When wiring up led's, can you use resistors with a value higher than required? For example, using a 1K instead of 150 ohms? Or a 100 ohms instead of 47 ohms? Of course the wattage would be te same.

The reason I'm asking is because it would be much more convenient to keep only a couple of different values at hand, than to use the exact value as calculated and thus creating the need to keep 20 or more different ones for different scenarios.

Thanks in advance,
Nicolas

Replies:

Posted By: ff-mike
Date Posted: March 20, 2006 at 10:08 AM

You can always use a larger value resistor, but the output is going to go down:
12V 150ohms will do 80mA and .96watts
12V 1 Kohms will do 12mA and .14watts

Posted By: firstrax
Date Posted: March 20, 2006 at 11:08 AM

Assuming a 12 volt source and an LED Vf of 2.1 (typical red) a 150 ohm resistor will drive the LED with 66mA and most likely toast it. A 1k resistor will drive the LED with 9.9mA and be very dim.

Heres the math:

V = battery Voltage

Vf = forward voltage of the diode

If = Current through the diode

Vd = Voltage to be dropped

V - Vf = Vd

Vd / R = If

So a 12 volt battery driving an led with a 2.1 volt forward voltage @ 20mA would require a 495 ohm resistor. To answer your question, do the math for the LEDs you normally use and stock the appropriate resistors. Large changes can have a big impact on the performance of the LED.

Posted By: autofan01
Date Posted: March 20, 2006 at 4:58 PM

Thanks guys for your trouble! I guess you can't change Ohm's law to suit yourself. It's the LAW afterall.
Perhaps my example of using a 1K instead of 150ohm is a bit stupid. What about stocking only like 100, 200, 300, 400 etc. instead of all the inbetweeners? Then the diff wouldn't be that much, right? In most cases the 12V source is not exactly 12V but slightly more, especially in running mode.

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