Unlike incandescent light bulbs, which light up 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.

While the only definitive way to determine the polarity of the LED is to examine its datasheet, these methods are usually reliable:

Less reliable methods of determining polarity are:

While it is not an officially reliable method, it is almost universally true that the cup that holds the LED die corresponds to the cathode. It is strongly recommended to apply a safe voltage and observe the illumination as a test regardless of what method is used to determine the polarity.

Because the voltage versus current characteristics of the LED are much like any diode (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current. Added to deviations in the process this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.

Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LED's operating range. Thus the power can be considered to be essentially proportional to the current. In order to keep power nearly constant with variations in supply and LED characteristics, the power supply should be a “current source”, that is, it should supply an almost constant current. If high efficiency is not required (e.g., in most indicator applications), an approximation to a current source is made by connecting the LED in series with a current limiting resistor to a regulated voltage source.

Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage of more than a few volts. Since some manufacturers don't follow the indicator standards above, if possible the data sheet should be consulted before hooking up the LED, or the LED may be tested in series with a resistor on a sufficiently low voltage supply to avoid the reverse breakdown. If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallel.

LEDs can be purchased with built in series resistors. These can save PCB space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.

Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.^[28]

Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity, however, more than two electrodes (leads) are required.

LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside which causes the LED to flash with a typical period of one second. This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.

Generally, for newer common standard LEDs in 3 mm or 5 mm packages, the following forward DC potential differences are typically measured. The forward potential difference depending on the LED's chemistry, temperature, and on the current (values here are for approx. 20 mA, a commonly-found maximum value).

Many LEDs are rated at 3 V maximum reverse potential.

LEDs also behave as photocells, and will generate a current depending on the ambient light. They are not efficient as photocells, and will only produce a few microamperes (µA), but will produce a electrical potential—as much as 2 or 3 V depending on the band gap. This is enough to operate an amplifier or a CMOS logic gate. This effect can be used to make an inexpensive light sensor, for example to decide when to turn on the LED illuminator.

[edit] Advantages of using LEDs

• LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.^[29]
• LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
• When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
• LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
• LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
• LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.^[30] Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.^{[citation needed]}
• LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.^[31]
• LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states “less than 100ns.” LEDs used in communications devices can have even faster response times.
• LEDs can be very small and are easily populated onto printed circuit boards.
• LEDs do not contain mercury, unlike compact fluorescent lamps.
• Due to the human eye's visual persistence LEDs can be pulse width or duty cycle modulated in order to save power or achieve an apparent higher brightness for a given power input. The eye will tend to perceive the peak current light level rather than the average current light level when the modulation rate is higher than approximately 1000 hertz and the duty cycle is greater than 15 to 20%^{[citation needed]}. This is also useful when applied to the multiplexing used in 7-segment displays.

LEDs are produced in an array of shapes and sizes. The 5 mm cylindrical package (red, fifth from the left) is the most common, estimated at 80% of world production. The color of the plastic lens is often the same as the actual color of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have clear housings. There are also LEDs in extremely tiny packages, such as those found on blinkies and on cell phone keypads. (not shown).

[edit] Disadvantages of using LEDs

• LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps^{[citation needed]}.
• LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
• LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.^[32]
• The spectrum of some white LEDs differs significantly from a black body radiator, such as the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under LED illumination than sunlight or incandescent sources, due to metamerism.^[33] Color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
• LEDs do not approximate a “point source” of light, so cannot be used in applications needing a highly collimated beam. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.^[34]
• There is increasing concern that blue LEDs and white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.^[35][36]