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Resistor value too high?


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autofan01 
Member - Posts: 20
Member spacespace
Joined: May 04, 2003
Location: South Africa
Posted: March 18, 2006 at 2:42 PM / IP Logged  
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
ff-mike 
Copper - Posts: 199
Copper spacespace
Joined: November 15, 2004
Location: United States
Posted: March 20, 2006 at 10:08 AM / IP Logged  
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
firstrax 
Copper - Posts: 113
Copper spacespace
Joined: January 18, 2006
Posted: March 20, 2006 at 11:08 AM / IP Logged  

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.

autofan01 
Member - Posts: 20
Member spacespace
Joined: May 04, 2003
Location: South Africa
Posted: March 20, 2006 at 4:58 PM / IP Logged  
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.
All your BASS are belong to us!

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