I am new at the ohms law and trying to calculate the resistance.
A LED is rated at 20mA and 3.2 v and I want to calculate the resistance.
R= E/I
R = 3.2/0.02 = 160 ohms
Am I doing this right? Thanks for the help!
Am I doing this right? Thanks for the help!
No you are not. What is also required information is the source voltage value being used to drive the led/resistor series network. Say you are driving it from a arduino digital output pin that has a +5vdc output. You take the +5 volt value and subtract the forward voltage drop value for the led, 3.2vdc in your case, that leaves you with 5-3.2= 1.8vdc the the E value to use in the R=E/I. So then 1.8/.02 = 90 ohms. If you were using a +12vdc source to drive the led/resistor string then the results would be (12-3.2)/.02 = 440 ohms.
That make sense?
Lefty
Yes that makes sense :). Yea I am using the arduino pins. I magic number I am looking for is 90 ohms. Thank you!
Bear in mind that 90 ohms is not a standard resistor value (within the E24 resistor range) The next preferred value is 91 ohms so that's the one you should be looking for to stay within your calculation result. However you will find it much easier to get 100ohm value and using that would make negligible difference to your circuit operation.
If you enter E12 or "E24 resistor values" into google you'll find information on standard resistor values.
Am I doing this right?
No you are not. What is also required information is the source voltage value being used to drive the led/resistor series network.
The question now is do you know why what you were doing was incorrect and do you know why you have to use the technique Lefty outlined?
Don
The point is that ohm's law only works for linear components and an LED is not a linear device. That means the current is not directly proportional to the voltage, but proportional to some power law function of the voltage.
That is why you have to apply the law to the resistor which is a linear component.