Light emitting diodes work like normal diodes and so are only conducting in one direction. Current only flows from the positive terminal to the negative terminal.
Identifying + and -
Round LEDs with legs have a short and a long leg when new. The longer leg is the positive leg (anode), the short leg is the negative (cathode). Other LEDs usually have marks which can be found in their data sheets. If the legs of an LED are already cut often there's still a flattened part on the LED's marking the negative side.
what happens if the polarity is reversed?
In reverse direction usually no current will flow. Too much voltage in reverse direction however will cause a current to flow. The voltage where this happens is called the breakdown voltage as this is where the usually isolating junction inside the LED is breaking down and becomes conductive. The current flowing in reverse direction is called the reverse current and possibly dangerous for an LED (the allowed reverse current is often much lower than the allowed forward current). Also the LED will emit no light when reverse current is flowing. Don't be worried when experimenting with voltages around 5V like Arduino's outputs, but take care when using external power supplies with higher voltages.
Resistor calculation for an LED
In a series circuit an LED "eats" voltage. No matter what current flows through the LED - the voltage it uses remains almost the same. For example a single red LED will always eat 1.4V - no matter if 1mA or 20mA flow through it.
To calculate a resistor value for one LED three things have to be known:
- the supply voltage
- the voltage the LED eats
- the desired current through the LED.
Let's assume we have 5V for supply and an LED which eats 1.4 V. So if we subtract the 1.4 V from the 5 V supply there remain 3.6 V which aren't eaten by the LED. So a resistor has to use the remaining voltage at the desired current. We want our LED to use 15 mA. So the resistor can be calculated: R = U/I which is in this case: R = 3.6V / 0.015 A = 240 ohms.
Connecting an LED to AC voltage
As described above reverse current may destroy an LED - so it is a good idea to let the reverse current flow around the LED with an additional diode or a second LED which is connected in parallel to the LED but in opposite direction.
Unknown voltage of an LED
There are different methods of determining the (forward) voltage of an LED.
Knowing The LED's color
If the wave length is known the minimum voltage to create light can be calculated here or in this LED color chart) - this method is not very reliable due to several reasons. Better measure it using one of the below methods.
Testing with a Resistor
If the wave length is unknown the LED can be connected to an adjustable voltage source via a large resistor and the voltage can be gradually increased until a small current flows through the LED (for example 1mA will be ok for all standard LEDs without destroying them). While running this, the voltage across the LED's legs can be measured. The voltage then can be used for calculations.
If you have no variable voltage source: Use any voltage source between 5V and 10V and a 5000 ohm resistor - so the maximum current is less than 2mA in any case. Then measure the voltage across the LED.
Testing with Multimeter
The diode tester of a Multimeter internally works similar to the above described method. The Multimeter uses a small voltage and a limited current (usually between 0.1 and 1mA). It displays the voltage across the diode. Some multimeters deliver sufficient current to light the LED very dim (use a dark environment).
However this doesn't work for all diode testers I have used before - they may only be able to detect voltages up to 1.5V for example - a blue LED (3V) can't be tested this way. Best try with a white LED if it works with your multimeter. If this works it will be fine with all standard LEDs as well.
Links
- LED Resistor calculator here or there
- LED Array Wizard