[06E90Rostik]

I looked up an Ohm Law on wikipedia but could figure out how to use the equation.

Say if I have 12V curent and 100mA LED, what resistor do I need to get to get the max lumens out of LED and not to blow it?

And also if anybody knows what is the tolerance means on the reisistors

Thanks in advance

[tintivilus]

Originally Posted by 06E90Rostik

I looked up an Ohm Law on wikipedia but could figure out how to use the equation.

Say if I have 12V curent and 100mA LED, what resistor do I need to get to get the max lumens out of LED and not to blow it?

And also if anybody knows what is the tolerance means on the reisistors

Thanks in advance

V=IR doesn't help too much if you're not sure what your numbers are.

The key bit of data is that unlike a light bulb, the voltage dropped across an LED is constant regardless of the current. It varies a bit depending on the composition of the LED, but 1.7V is a good rule of thumb.

The equation's V is the voltage across the resistor: 12 - 1.7 = 10.3

Your I is the current you want flowing through the LED, or your 100mA = 0.1A

Therefore your R is going to be in the neighborhood of 100 Ohms.

Tolerance is just how accurate the resistor's rating is: a 100 ohm 10% resistor is guaranteed to be between 90 and 110 ohms. The lower the tolerance, the more precise the manufacturing process, the more expensive the part.

The other spec on a resistor is the power rating, usually in Watts. (Watts = Volts * Amps) This application calls for at least a (10V * 0.1A = 1 Watt) resistor. You want to be conservative on this (bigger = better) but you don't need to go overboard. Resistors with higher wattage ratings usually handle heat better as well.

Have fun tinkering

[06E90Rostik]

Thanks a lot tintivilus that helps so much!

[06E90Rostik]

BTW yesterday when I posted the question, I was browsing wikipedia for the resistor articles. I came across this nifty resistor configurator, not sure how it helps but still

http://www.dannyg.com/examples/res2/resistor.htm

[lkevinl]

Originally Posted by 06E90Rostik

BTW yesterday when I posted the question, I was browsing wikipedia for the resistor articles. I came across this nifty resistor configurator, not sure how it helps but still

http://www.dannyg.com/examples/res2/resistor.htm

That is pretty nifty. It helps you identify the correct resistor by sight.
The first colored bar represents the 10's place, the second colored bar represents the 1's place and the third colored bar represents 10 to the 'x' power which is multiplied by the number from the first two bars. The fourth bar is the tolerance.

[SoYank]

There's another aspect of Ohm's Law dealing with parallel resistors. Given a 10 Ohm and a 20 Ohm resistor. Put them together, side by side, and connect the leads. You don't get a 30 Ohm resistance. The formula for parallel resistors is:

1/((1/R1)+(1/R2)+(1/Rn))

So, with the resistors given, this comes to 1/.3 = 3.333 Ohms.

More on the tolerance. The fourth band on the resistor will be either gold or silver. Gold indicates a tolerance of ±5% of the value defined by the first three bands. Silver indicates a tolerance of ±10%. No band (only three on the resistor) indicates a tolerance of ±20%.

[06E90Rostik]

Originally Posted by lkevinl

That is pretty nifty. It helps you identify the correct resistor by sight.
The first colored bar represents the 10's place, the second colored bar represents the 1's place and the third colored bar represents 10 to the 'x' power which is multiplied by the number from the first two bars. The fourth bar is the tolerance.

Originally Posted by SoYank

There's another aspect of Ohm's Law dealing with parallel resistors. Given a 10 Ohm and a 20 Ohm resistor. Put them together, side by side, and connect the leads. You don't get a 30 Ohm resistance. The formula for parallel resistors is:

1/((1/R1)+(1/R2)+(1/Rn))

So, with the resistors given, this comes to 1/.3 = 3.333 Ohms.

More on the tolerance. The fourth band on the resistor will be either gold or silver. Gold indicates a tolerance of ±5% of the value defined by the first three bands. Silver indicates a tolerance of ±10%. No band (only three on the resistor) indicates a tolerance of ±20%.

Bookmarking this thread. So much helpful info. Thanks guys!