People are often intimidated by LEDs but there is really no need.  This article has been written to dispel some of the myths, help understand how to use LEDs and provide a calculator to help with resistance values.

Lets start with "what is an LED?". An LED is a Light Emitting Diode. It is a device that, when connected correctly, attempts to regulate the voltage across it, in doing so it has to give off energy which it does in the form of light. If connected the other way around it is just a standard diode. (A diode is a device that allows current to flow in one direction only). If the current supply to the LED is too high then the energy to give off is too high, it heats up and burns out (fails). To stop the failure we need to limit the current available and we do that by using a resistor or and led driver device. LED drivers are used in small lighting systems and MFU's But not in the advanced lighting systems available. So for a small light system or an MFU, so long as you do not mix colours then you do not need resistors.

Ah! another point, LEDs come with different working voltages the original domed LEDs need 4 volts for blue and white and pink LEDs to work and the reds, yellows, oranges and greens only need 2 volts. It gets a bit more complicated because as technology improved SMD (Surface Mount Device)  or chip LEDs were produced, originally the had the same voltage characteristics as the original LEDs, but newer ones work on 3volts.

Here are some pictures of LEDs

First Standard LEDs, from the left are 2mm, 3mm,5mm,8mm and 10mm.

 
 

They do come in more sizes, different shapes and with coloured lenses. They also come in different output ratings such as standard, super bright and ultrabright. For us it does not really matter which except to say that standard can handle lower currents than super bright and Ultrabright can handle the most. Do not worry about that right now, we will get to it later.

Then SMD LEDs

 
In the same way as the domed LEDs SMDs come in different sizes and ratings. The number is an indication of the size.

sizes are as follows:

  • 0402 = 1.0mm x 0.5mm
  • 0603 = 1.6mm x 0.8mm
  • 0805 = 2mm x 1.25mm
  • 1206 = 3.2mm x 1.6mm

Small or large SMD or discrete LEDs are all connected the same way.

On the discrete LEDs the long leg is always positive ( the anode) and the short leg the negative ( the cathode). If you have chopped the legs then its not so bad as the internals all follow the same construction so you can figure out which is which using the picture above.

SMD LEDs vary sometimes they have a cut out at one end, sometimes one solder surface is larger and sometimes they have a sign printed on them. Confusing but they were designed to be soldered direct to a circuit board by machine, so in general they come in sealed strips with feed holes on one side and they are all the same way round. These can be tricky to use, particularly the small ones which need a small soldering iron to work on them. Do not despair as you can also buy them with pre-soldered leads. The leads can be varnished copper wire or tiny multicore plasti coated wire. Sometimes the wires are colour coded but more often than not they are the same colour but the longer wire is usually the positive and the short the negative.

Using LEDs

Using LEDs is pretty simple so long as you remember a couple of things

  • use an appropriate resistor in series with the LED
  • do not mix led types in parallel (same ends all joined together) as some may not work
  • best practice is one resistor per led but you can use multiple LEDs to one resistor. Multiple LEDs with one resistor can cause a hint of flickering.
  • calculate the minimum resistance required and use a value above that.

Once you have the correct resistor in series with your led it cannot blow up. It does not matter which end of the LED that the resistor is attached but use the same one to avoid problems later. We always put the resistor between the battery + and the LED. If it is connected backwards it will not light up, connected forwards it will light up, BUT no resistor it will blow up unless its on an led driver.

Calculating the resistor value

There is an equation that works out the current flowing through a resistance for a given voltage.

 

V=IR so to find the resistance we divide V by I or   R=V/I

 

 V is the voltage across the resistor, I is the current and R is the resistance. V is also usually the supply or battery voltage. We need a certain voltage across the LED so that complicates it a bit because the voltage  across the resistor is now the supply voltage mines the LED voltage.  So the equation becomes:

R-(V-Vled)/I

 

You can use that or you can use our LED calculator. You will need the following information:

  • led working voltage
  • led maximum current draw
  • Battery voltage

Do not forget that on a rechargeable battery the specified voltage is the mean voltage and the fresh charged and charged voltages can be higher So a freshly charged 7.2 volt NIMH pack can be as high as 10.5 volts but will settle to 8.4 volts fairly quickly. A fully charged 2s lipo pack will be 8.4 volts and a 3s 12.6v.