What voltage do I use for calculations?

Forum 2005-2010 (read only) Hardware Interfacing
1

Ok on the subject of leds and resistiors.

For some reason every kit I get or tutorial I read says use a 220 ohm resistor with leds. I just bought a pack of leds(100 of them, for 1 dollar... an american dollar at that :wink: and I need to get the correct resistors.

Voltage: 3.2V - 3.6V
Reverse current (uA): <=30
Max Power Dissipation: 80mw
Max Continuous Forward Current: 30mA

for my calculations I use
source voltage of 5v
forward voltage of 3.3V
current of 25mA

this gets me close to 75ohms for my resistor....

If I use 9V as source voltage I get close to 220... should I use 9v for calculations? If so why, I thought arduino only put out 5v on the pins. Does this hold true through a shift register?

My confusion here is I don't know if I am using the wrong values for my calculation, or if my leds are just power hungry little buggers...

2

Voltage: 3.2V - 3.6V
Reverse current (uA): <=30
Max Power Dissipation: 80mw
Max Continuous Forward Current: 30mA

for my calculations I use
source voltage of 5v
forward voltage of 3.3V
current of 25mA

this gets me close to 75ohms for my resistor....

Yes you are using the correct values.

The resisitor is value is lower then usual because your LED current and voltage drops are higher than usual. Typically the voltage drop will be under 2 volts and the curent around 10ma so a resistor will be four or five times higher than that.

Not to worry, at a penny each you got a great deal :slight_smile:

BTW, there are lots of LED resistor calculators on the web, for example this one: http://led.linear1.org/1led.wiz

3

ok so typical leds are closer to 3v at 10mA? (assuming an arduino, or 5v source)

The ones I got are 10000 mcd(the white ones) and 5000mcd for the colored ones... are mine just brighter than average hence he higher mA draw?

maybe ill get some 180s and 120s to play with the brightness... I dont want to pwm them for brightness.

4

Yes, a typical LED has a little over 3 volts across the resistor connected to a 5V pin. You have about 1.7 volts.

But that is not so unusual for very high brighness LED such as yours. I wouldn't worry about burning them out. It should only happen if the specs are wrong. And if one does you still have 99 more :wink:

Anyway, its not bad thing using a larger resistor if the brightness is sufficient so put in a larger one if you want.

Have fun :slight_smile: