Hi all,
I just started with Arduino Starter Kit. One of the first example is to connect an LED and a resistor in 5V circut. Reading Project 01 in Arduino Project Book on page 30, there's an explanation why 220-ohm resistor was used. It reads 23mA "is just about the maximum you can safely use with these LEDs". Then I checked datasheet at http://arduino.cc/documents/datasheets/LEDR-L-7113ID-12V(Ver1287713938.7).pdf and (for my understanding) it looks like the maximum current at 5V is 2.3mA not 23mA. I am probably using the datasheet incorrectly but my reasoning is following:
Electrical / Optical Characteristics at TA=25°C table (characteristics, page2) reads forward current for 12V typical value is 8.5 and maximum value is 11.5
On page 3 of characteristics there's a Forward Current/Forward Voltage chart, indeed for 12V you can read 8.5mA, however for 5V it is ~2.3mA but not 23mA, see the image attached.
So according to how I read the datasheet (forward current at 2.3mA for 5V) the resistor should be 5V/0.0023A=2174ohm so this could be 2200ohm at the same time according to project book it should be 5V/0.023A=217ohm so ~220ohm. Again why book say forward current for 5V is 23mA while datasheet say it is 2.3mA?
Hi,
Your LED apparently isn't a standard LED, a standard LED doesn't have an internal resistor. (page 1). Maximum forward voltage of a normal single LED is also about 4 volts and far less (~ 2.4 v) for a red one.
On page 2 of the sheet it says 8.5 - 11.5 mA forward current and 14v max forward voltage.
My best guess is that you don't need an external resistor to limit current, unless you want to feed it more as 14 volts.
As long as the voltage applied is higher as the forward voltage (~2.4 volt for a red led), the LED will probably give light. But since yours is already limited to 12-14 volt, current will indeed be much lower when applying 5 volts only. Adding another resistor at 5v will only result in even lower current/less light.
Most leds will indeed need current to be limited to 20-25 mA max as explained in the book. Your calculations are not exactly right though.
You need to subtract the forward voltage of the led from the supply-voltage first, before dividing.
A red led at 5 volt and 20mA will result in the next calculation, (5v-2.4v)/0.02A = 130 Ohms
A blue led (~3.4v forward voltage) connected to a 5v supply will need an (5v-3.4v)/0,02A = 80 ohm resistor to limit current to 20mA.
It's not always possible to find the right resistor when calculating btw, if you can't find a 130 Ohm resistor, it's better to use a 150 Ohm one instead of a 100 Ohm resistor. You're always safe when current is lower as the maximum your led can handle.
The datasheet says: "12V internal resistor." so the LED package already contains the series resistor to limit the current to a safe level when 12V is applied. The calculations for what resistor to use assumes a bare LED with no internal resistor. Does the starter kit not come with any plain LEDs?
