Light Emitting Diode Facts
How To Use LEDs - LED Facts
LEDs - Light Emitting Diodes - can be quite rugged and durable. They are impervious to shock and vibration. Cold and heat do not affect them. But there are some requirements that need to be met in order to insure long LED life and maximum light output.
First, let's identify the LED positive and negative pins. Virtually every standard LED is marked in the same way as in the images below. The long pin will always be positive and the short pin negative. Also, the negative side will always be marked with a cut out on the side of the LED case. The positive side is also known as the "Anode" and the negative side is known as the "Cathode".
The voltage of an LED depends of its color. As you can see from the chart below, the voltages vary considerably. A white LED runs at about 3.2 volts. But if you were to apply that same 3.2 volts to a red LED, you would have a very intense light and lots of heat for a short time - and then nothing.
LEDs can also very greatly between manufacturers. When working with LEDs, be sure to check the datasheet from the manufacturer for the proper voltage and current requirements for your particular LED.
LED Voltage, Current & Color Chart
In retail LED lights, a current limiting device is almost always used to protect the LED from over current, which will burn it out. A resistor is commonly used to protect LEDs from too much current. For larger lights, normally a voltage and current regulated power supply is built into the light bulb.
You can get around using current limiters by using enough LEDs to match the power supply voltage. For example, if you have a power supply of 12 volts DC and white LEDs that require 3.0 volts, then you can simply string 4 LEDs together in series to safely run them on your power supply.
A Current Limiting Resistor
Series Connected LEDs
If you are working with household 120 volt AC power and want to connect LEDs directly up to it, no problem. Using a white LED with a voltage of 3.0 volts again, then you would need 40 LEDs strung together in series. Take 120 volts and divide by the LEDs 3.0 volts and you get 40.
Life is not always so simple. If you want to hook up an LED in your car, then you can run into trouble. The car battery can vary from between 11.9 volts all the way up to 14 volts DC. This is a wide range of voltage and can cause trouble for your LED. Too low a voltage and the LED will be dim. Too high and you can burn out the LED. In this case, you will need a current limiting resistor. To find the correct resistor value, you will need a little bit of math. Please refer to the following:
R = (Vs - Vled) / Iled
Where "R" is the resistor value we need to find. "Vs" is the voltage source. "Vled" is the voltage of the LED. "Iled is the current of the LED in amps.
To find the value of a resistor for an LED to be used in a car, we must take the highest possible voltage, which is 14 volts. The average LED current is 20 miliamps, or 0.02 amps. Plug in the values:
R = (14 - 3) / 0.02
R = 450 ohms.
So, for an LED that needs 3 volts and 20 mA, you will need a 450 ohm resistor.
Here is another problem. If you are working with household voltage and want to use AC - fine. But if you want to convert the AC voltage to DC, then you will run into a bit more math again. When you convert AC to DC, you usually get a bit higher voltage DC out than AC in. To spare a lot of confusion and calculations, to put it simply, you get roughly about 150 volts DC after converting from AC to DC and using a filter capacitor.
So, if you are using the white LEDs again at 3 volts DC, you will need 50 LEDs connected in series if you do not use a current limiter.
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