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An LED is a special semiconductor which emits light when current is passed through it. There are many different physical styles. The emitted color spectrum is usually very narrow. It can generally be specified as a specific wavelength in the electromagnetic spectrum. The emitted color selection is somewhat limited. The most commonly available colors are red, green, amber, yellow, blue and white. The red, green, yellow and amber have a working voltage of approximately 1.8 volts. You can refer to the data sheet for each LED to find the exact value. The actual working voltage is determined by the breakdown voltage of the particular semiconductor material.
Most LEDs are designed for very low power consumption and low output (light output). Even though the LEDs sold as super-bright LEDs may seem bright if you look directly at them, they are still low power LEDs. The 'super-bright' LEDs are typically in a standard LED package and require no heatsink to operate at the recommended current. LEDs that are really bright can be seen for miles. These are typically used in flashlights. One example is the Cree XM-L LED (datasheet HERE). The following image shows the XM-L LED in a ThruNight Catapult V2. The LED is soldered to a heatspreader. This increases the effective surface area of the LED to promote greater thermal transfer. The heatspreader is flat against a large piece of aluminum that serves as a heatsink. Between the aluminum, there is a layer of thermal/heatsink compound. When the reflector assembly is screwed on, it presses the heatspreader against the heatsink. All of this is required for a high power LED to operate at full power.
For more information or to see what high power LEDs can do, search for Catapult XM-L beamshot with Google or on Youtube. For more information on high-end flashlights, visit the Flashlight page of this site.
LED Drive Current:
Working voltage (Vf)=1.8 volts Desired current flow=15ma (.015 amps) Power supply voltage=12 volts
12-1.8=10.2 10.2/.015=680 ohms
A 680 ohm resistor will limit the current to a safe level although I would probably use a 1000 ohm in a vehicle because the charging system voltage would be higher than 12 volts. Any resistor between 680 and 4700 ohms would probably work fine. Choose a resistor with a power rating greater than or equal to the power dissipation given by the calculator above.
The diagram below shows the parts of an LED.
You must also know that an led has polarity. This means that the positive and negative terminals must be connected correctly for it to operate properly. As you see in the diagram below, if the polarity is reversed, the LED will NOT light. If the reverse voltage is beyond what the LED was designed to handle, it may be damaged. Click on the 2 buttons in the demo below to see the LED operation with either polarity.
The image below shows a few different styles.