What Are LEDS?
LED stands for Light Emitting Diode. Diodes only allow current to pass in one direction and the diodes... you guessed it... emit light! The light is emitted by the movement of electrons in a semiconductor material.
Unlike ordinary bulbs, LED's don't have a filament so the don't burn out quickly, and they don't get especially hot. This means they can last a long time. And the big bonus is in effectiancy. Normal filimants need to be heats to red hot and that produces the light. This heat is wasted energy. In LEDS heat is a small bi-product rather than what is needed to generate the light. Here are some of he pros and cons of LED Lighting.
What do they look like?
LED's will come with a couple of important features to note.
- Forward voltage (Vf), the voltage drop from the positive side to the negative side (Eg 3.2 volts). If you measure the voltage at one side, (Eg 12 volts), the voltage on the other side will be that much less (Eg 12-3.2 = 8.6 Volts). This is normally be expressed as a range, with anything lower producing little to no light and anything above damaging or reducing the life span of the LED. Normally LED's are have a forward voltage of 1 to 4 volts depending on brightness and colour.
- Forward Current (If), the current needed to power LED. Normally they are in the 20-25 mA range.
These two are the most important features to note but there are also...
- The light output, in brightness (mcd) and viewing angle (degrees)
- Size mm
- Wavelength (Colour)
How Can we wire them?
- It all runs of the basic equation Voltage = Resistance * Current.
The first step is to remove our LED voltage from the Supply voltage to work out the
left over or voltage to be dropped by a resistor. Lets look at two examples.
1) 5 Volt Supply and 1, 3Vf 25mA LED, 5V - ( 1 * 3V) = 5V - 3V = 2V
2) 13.4 Volt Supply (from our car battery) and 3, 3.2Vf 22mA LED's. 13.4V - (3 * 3.2V) = 13.4V - 9.6V = 3.8V
- Next we work out the value of the dropping resister by dividing the dropped voltage
by the current required by the LED's. Quite often you won't be able to find exact
values, so just get the next highest.
1) 2V / 0.025A = 80 Ohms
2) 3.8V / 0.022A = 172 Ohms
- The last thing is to work out how many Watts will be used to make sure our resister can handle it.
1) 2V * 0.025A = 0.05 Watts, So well need a least a 1/4 Watt, 80 Ohm resister
2) 3.8V * 0.022A = 0.0836 Watts, So we'll need a least a 1/4 Watt, 172 Ohm resister.
- Some things to note.
If you wire LEDs in series (like our second example), make sure they do not not exceed
more than 80 percent of the supply voltage. Our second example uses 9.6 Volts out of 13.4 or
71.6%. If you use more it will reduce the stability and predictable of current consumption.
Do not put LEDs in parallel with each other. It will work, but it's not reliable. LEDs
become more conductive as they warm up, which may lead to more current being used in
one LED to another, damaging them. LEDs in parallel need their own individual