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How to find out what resistor to use with led's and laser diodes?
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Ohm's law is what you need to work out ANY voltage/ current/ resistance problems.
Ohm's basic law is E = IR.
E is voltage in volts;
I is current, in amperes;
R is resistance, in ohms.
To work out a resistance for a LED, you need first, the LED's specifications. You need the operating voltage, and you need the recommended current.
Let's say these were 2.5 volts, and 20 mA.
Now, you need the supply voltage you have.
Let's say that's 9 volts.
You subtract the LED's voltage from the supply voltage, = 6.5 volts here.
This represents the voltage that has to be DROPPED in the resistor.
Now Ohm's Law. You need a transposition of the basic law here, as R is the unknown.
R= E/I will do it.
E is 6.5 volts, I is 20 mA (.020 amperes).
6.5/.020 = 325 ohms. This is the value of resistor required for this example. The nearest standard value is 330 ohms, so that is adequate.
Check wattage! Watt's law is similar to Ohm's; W = EI.
E & I are the same as in Ohms, W is the symbol for watts.
For the example, 6.5 x .020 = 0.13W.
1/2 watt resistors are more than adequate for LED's. 1/4 watt could be used here, but 1/8 watt would be just too small, and could burn out.
Laser diodes use just the same process to work out a resistance.
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