I used the following formula to calculate the ohms:
Basically though, if you want 20mA, the resistor should be ([supply voltage] - [led voltage]) / 0.02. Or 50*(supply voltage - led voltage).
So I calculated: 5V - 3.5 (led voltage) / 0.02 = 75 ohms
=> P = V^2/R with this formula I get around 0.33 watts?
Is this not correct?
The formula is correct. Though, since you know both the voltage and the current through the resistor its easier to use
P=UI=1.5V0,02A=0,03W
But as you can see you got the actual calculation wrong. It's 0,03W not 0,3W.
About the 0.25w items on that site: what I don't understand is why there are several items available
starting from 1 W upwards. Is not this one the actual resistance value?best
yves
Since all the values given in the table are typical resistor values they got an error in their database. The values should read 1Ohm, 27kOhm, 560kOhm etc.
But I'm not sure about the prices, looks like you have to order a minimum of 1000 Resistors per value? (that would almost match the prices in Germany 5? if you order >=1000)
Eberhard