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 LEDLight-emitting diode are already cut often there's still a flattened part on the LEDLight-emitting diode'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 LEDLight-emitting diode is breaking down and becomes conductive. The current flowing in reverse direction is called the reverse current and possibly dangerous for an LEDLight-emitting diode (the allowed reverse current is often much lower than the allowed forward current). Also the LEDLight-emitting diode 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 LEDLight-emitting diode

In a series circuit an LEDLight-emitting diode "eats" voltage. No matter what current flows through the LEDLight-emitting diode - the voltage it uses remains almost the same. For example a single red LEDLight-emitting diode will always eat 1.4V - no matter if 1mA or 20mA flow through it.

To calculate a resistor value for one LEDLight-emitting diode three things have to be known:

  • the supply voltage
  • the voltage the LEDLight-emitting diode eats
  • the desired current through the LEDLight-emitting diode.

Let's assume we have 5V for supply and an LEDLight-emitting diode 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 LEDLight-emitting diode. So a resistor has to use the remaining voltage at the desired current. We want our LEDLight-emitting diode 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 LEDLight-emitting diode to AC voltage

As described above reverse current may destroy an LEDLight-emitting diode - so it is a good idea to let the reverse current flow around the LEDLight-emitting diode with an additional diode or a second LEDLight-emitting diode which is connected in parallel to the LEDLight-emitting diode but in opposite direction.

Unknown voltage of an LEDLight-emitting diode

There are different methods of determining the (forward) voltage of an LEDLight-emitting diode.

Knowing The LEDLight-emitting diode'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 LEDLight-emitting diode 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 LEDLight-emitting diode (for example 1mA will be ok for all standard LEDs without destroying them). While running this, the voltage across the LEDLight-emitting diode'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 LEDLight-emitting diode.

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 LEDLight-emitting diode 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 LEDLight-emitting diode (3V) can't be tested this way. Best try with a white LEDLight-emitting diode if it works with your multimeter. If this works it will be fine with all standard LEDs as well.