Polarity
Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage above this threshold. The cause of damage is overcurrent resulting from the diode breakdown, not the voltage itself. LEDs driven directly from an AC supply of more than the reverse breakdown voltage may be protected by placing a diode (or another LED) in inverse parallel.
The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet. However, these methods may also be used:
| sign: | + | – |
|---|---|---|
| terminal: | anode (A) | cathode (K) |
| leads: | long | short |
| exterior: | round | flat |
| wiring: | red | black |
| marking:* | none | stripe |
| pin:* | 1 | 2 |
| PCB:* | round | square |
(*)Less reliable methods of determining polarity
Read more about this topic: LED Circuit
Famous quotes containing the word polarity:
“The same polarity of the male and female principle exists in nature; not only, as is obvious in animals and plants, but in the polarity of the two fundamental functions, that of receiving and penetrating. It is the polarity of earth and rain, of the river and the ocean, of night and day, of darkness and light, of matter and spirit.”
—Erich Fromm (1900–1980)