LED lighting and genral information about LEDs
the symbol for an LED is quite straight forward. Basically current can only flow in one direction in a LED from the anode to the cathode. The LED anode kinda looks like the letter A and this is the positive which has a Longer leg than the LED cathode, the negative. The cathode is also identified by a flat edge on the side of the LED as well as looking at it internally through the plastic housing and identifying the triangular piece of metal.
A LED is a semi-conductor made up of two elements one a P-type material which is missing electrons(Holes) and the other a N-type which has extra electrons (an electron has a -ve charge)
current flow is the passing of -ve electrons from the “extra” free electrons of atoms in a conductor towards the “holes” of the other atoms adjacent to it.
following convectional current flow current flows from positive towards negative, electron flow however is in the opposite direction negative towards positive.
If you make the N-type region -ve and the P-type +ve, electrons and holes will move towards the junction and there are plenty of charge carriers there and a current can flow. Like poles will repel each other forcing the junction to close and allowing current to flow. this is known as forward biasing a PN junction.
On the other hand if you reverse bias the PN junction, you place a -ve on the P-type material and a +ve on the N-type material the junction will widen and electrons will not be able to jump the gap and current will not flow except for a slight current flow and this is known as leakage current and is usually 0.7V
When the -ve electrons fill the +ve holes they release energy in the form of light.
The colour of an LED is determined by the semiconductor material, this means it can be packaged in a clear housing and not only in the colours that they come in.
(InGaN) indium gallium nitride – green, blue and white
(AlGaInP) aluminum gallium indium phosphide – red, orange and yellow
the ratios of these compounds determines the colour produced
crazy LED Colours:
originally the manufacturing process to produce white and blue LEDs was very difficult and subsequently the price for these colours where a lot more. In resent years however the better manufacturing techniques means it is now possible to get crazy LEDs in any colour under the sun.
LED wind up torch:
LEDs are very low current devices (typically 20mA) relative to incandescent bulbs (450mA, standard light bulbs) this means you can use other forms of energy very efficiently like the power produced by winding a small dc motor, like the ones found in small toys. Then storing this power in a capacitor to slowly discharge through a resistor in to a LED. The relative cheapness of these components mean its very easy to produce wind up torches and they are very effective and inexpensive.
The best way to dim an LED it through PWM (pulse width modulation) using a very simple circuit like the 555 on this page by varying the duty cycle and adjusting the “on time” the LED will appear to glow brighter or dimmer. PWM is more efficient over using other techniques like resistors.
How to power a LED:
LEDs are not like normal bulbs they have very low resistance and can be thought of as a conductor (piece of wire) when forward biased (following electron flow) if you connect it directly to a battery they will draw the maximum amount of current the power supply can handle. Now typically a LED is designed to run at 20mA (0.02A) and will blow if you run to much current through them, this is why you need to place a resistor in series with the LED to limit the current flow. To work out the current flow follow this equation below this is very simple.
VLED: is the rated voltage of the LED usually between 1.5v- 4v
Vsupply voltage: the voltage you supplying it with
Resistor: the component we trying to work out measured in ohms
and I: the current flow say 20mA
R=(3V – 2V)/0.02
R= 50 ohm
now you probably seen people make LED Throwie’s and you saying they don’t use resistors this is true. and simple really. A 3V button battery has very little current that it can supply and is no where close to burn out a LED.
LEDs are low current devises making them perfect for applications where moisture is involved. One such use is for bathroom lighting. There is no need for mains electricity to directly power these low voltage devices and therefore they are very safe for bathroom areas.
Buy LEDS online: