I am tring to hook up some LED's (5mm) 3.7 volt typ. into my computer case. I have 5.15 volts coming out of the power supply.
What size resistor would I need to drop the voltage to 3.3 volts? I am not to good with figuring these things out could someone help???
Thanks in advance ... Carl
It depends upon how bright you want the LED to be.
A general guide to powering general LED's is to supply them with 20ma. Some "Superbright" LED's require more than that...maybe 30ma upwards.
The best option is to look at the datasheet for the LED you have and use the recommended value in the formula.
Say you have an LED which is required to be driven at 20ma. The formula is:
R = (Vs - Vl)/I
R is the resistance required,
Vs is the Voltage - Source,
Vl is the Voltage - LED,
I is the current required.
so,
Vs = 5.15V
Vl = 3.7V
I = 0.02
5.15 - 3.7 = 1.45
1.45 / 0.02 = 72.5Ω
The nearest higher value standard resistor to this, would be 82Ω, (Grey, Red, Black), which would actually supply the LED with 17.7ma.
Now, if you wanted to have the LED a little dimmer, you could supply it with less current using a higher value resistor. Let's use 10ma:
5.15 - 3.7 = 1.45
1.45 / 0.01 = 145Ω
The nearest higher value standard resistor to this, would be 150Ω, (Brown, Green, Brown), which would actually supply the LED with 9.7ma.
Does this help?