LED

From Syncleus Wiki

The LED (Light Emitting Diode) converts a supplied current into light. They are available with output frequencies from the infrared to the ultraviolet and are widely used for status indicators, displays, and fiber optic communications.

Characteristics

Current rating

LEDs can only handle so much current before they will overheat and suffer reduced lifespan. The specific value will be given in the device's datasheet, but 20mA is a decent rule of thumb for small indicators. Other parts (superbrights, surface-mount, arrays) will vary enormously and you need to see the datasheet.

Forward drop

LEDs are still diodes, and will have a forward voltage drop V_f. For LEDs this is largely dependent on the frequency of the emitted light:

$E = hc/\lambda = q_e V_f\rightarrow V_f = \frac{hc}{\lambda q_e} = 1.241\times10^{-6} / \lambda$

Then some minimum voltage drops include:

Color	980nm IR	640nm Red	580nm Yellow	520nm Green	470nm Blue	370nm UV
V_f	1.27V	1.93V	2.13V	2.39V	2.64V	3.35V

Things aren't perfect in the real world; non-laser LEDs don't emit monochromatic light and will have voltage drops that vary depending on current and from LED to LED - the range will be given in the datasheet. A typical red LED might drop $1.9V \pm .2V$ . Some kinds of LED will drop much more voltage than this table suggests.

Reverse voltage limit

LEDs have much lower reverse breakdown limits than other diodes, sometimes as little as 5V. It will be in the datasheet, specified as the reverse voltage at which some μA of current flow.

Viewing angle

Most leds put nearly all of their light into a cone with a certain angular size. It should be in the datasheet, usually defined as the angle where intensity falls to half of the peak.

Operational Life

The process of running very slowly damages LEDs, reducing their light output. There is no point at which they will just stop glowing but their efficiency will fall. This can be as low as 5 to 10 thousand hours for blue LEDs and as long as 100 thousand or more for low-power high-efficiency red ones.

Types

Indicator - small, low-power individual lamps used as on/off output by computers and circuitry.
Power - LEDs designed for much higher currents to output light for the purpose of illumination. Very large power LEDs or arrays may need heatsinks.
IR signalling - Today most common in TV remotes, but also widely available for hobbyist purposes.
Surface mount - Almost any type of LED has comparable smd parts.
Array - multiple LEDs mounted in arrays, often 5x7, 5x8 and 8x8, for use as displays

Powering LEDs

LEDs are current mode devices, meaning that they function based on supplied current rather than voltage. This also makes powering them accurately and efficiently more difficult than simply applying a voltage through a resistor.

However, the simple resistor is a starting point. This is the simplest way to power a LED without blowing it up. Here current flows up, through the LED, through the resistor, and back to complete the loop. The sum of voltages around a closed loop must be zero (Kirchoff's Laws), so

5 - V l e d - I R = 0

. Assume it's a typical red lamp, so

V l e d = 2

. Then $IR = 3 \rightarrow I = 3/R = 3/300 = 10mA$ .

But, how much power is being used? In a purely resistive system, $P = V I = I 2 R$ . So the LED is using 2V x .01A = 2 milliwatts, while the resistor is turning 300x.01x.01 = 3 milliwatts into heat. Only 40% of the power used goes to the LED - pretty crummy, especially as the LED itself isn't all that efficient at turning electric into photons. This will only get worse if the voltage is higher, as then nearly all the voltage (and thus power) must be wasted by the resistor.

The battery and resistor replaced by a current source. An ideal current source will establish whatever potential it takes to make a certain amount of current flow, so the one shown here will output exactly

V l e d

. As a result, all power consumed goes to the LED - a 150% increase in efficiency compared to the voltage source/resistor!

Because LEDs offer nearly zero resistance once the supplied voltage exceeds their forward drop, they do not share current well. Trying to apply power to multiple LEDs in parallel will result in the one with the lowest voltage drop handling almost all of the current (unless 2 or more happen to have almost exactly the same forward drop), while the others are ghostly dim at best. If they are different colors, only the one closest to red will illuminate.

Driving LEDs is an extremely common application, and a great many integrated circuits are available from many manufacturers and retailers just for this purpose.

LED