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Resistor question

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thisisliam:
Hi everyone

I have a 12v DC, 650mA power supply that I'm trying to power a small LED with. A little research tells me I need a 680r resistor... Would appreciate some input from someone a little more versed with this. I have attached the spec chart for the blue LED I'm needing to use.

Thanks for the help!

ataradov:
Yes, 680 Ohm resistor will work, and will result in ~13 mA of forward current. It is fine for the LED, but if it is just an indication LED, then it might be too bright.

thisisliam:

--- Quote from: ataradov on May 24, 2019, 06:52:29 pm ---Yes, 680 Ohm resistor will work, and will result in ~13 mA of forward current. It is fine for the LED, but if it is just an indication LED, then it might be too bright.

--- End quote ---

@ataradov - thanks for the quick reply! It is just an indicator LED - would you suggest a different resistor so it isn't so bright?

Zero999:
Yes, a 680R resistor seems sensible and will give 12mA to 15mA, depending on the forward voltage of the LED.

As mentioned above, it might be too bright, unless you have a yellow LED, which tend to have a poorer efficiency, compared to the other colours.

mariush:
Voltage power supply  - Forward Voltage of LED (or sum of forward voltages)  = Current x Resistance.

So if you have a single RED led with a forward voltage of approximately 2v and you want 10mA (0.01A) of current with a 12v power supply, then :

12v  - 2v = 0.01 x R ====>  R = (12-2) / 0.01  = 10 x 100 = 1000 ohm

Another example, if you have 3 blue leds in series, which have a forward voltage of 3v each, and you want 20mA to go through them :

12v (power supply voltage) - 3 leds x 3v (forward voltage of each led)  = 0.02 A (20mA current)  x R   ====>  R = (12 - 9) / 0.02 = 150 ohm


Also, power dissipated in a resistor can be calculated with formula P = IxIxR where I is current and R is resistance.

So for example, with the 150 ohm resistor above, you have P = 0.02A x 0.02A x 150 = 0.06 watts, which means you can use a resistor rated for 0.125 watts.
You typically want to stay below 75% of the resistor's power rating, for safety and reliability reasons and keep the resistor within acceptable temperatures.  If the power dissipated was close to 0.1w, it's better to use a resistor rated for 0.25w



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