The way an LED works
LEDs (Light Emitting Diodes) are taking an increasingly prominent place in lighting technology. The energy-efficient, robust and extremely flexible light sources can be found in powerful LED hall floodlights as well as in ambient lights for residential use. How an LED works and what types there are roughly, you can read in the further here. It should be provided a brief overview – without diving into technical depths.
LEDs are based on the principle of electroluminescence
Technically, they are diodes, which means they are an electronic component. Unlike hot emitters such as incandescent bulbs, in which wires are made to glow, or fluorescent tubes, in which a gas glows. Electroluminescence was discovered as early as 1907 by Henry Joseph Round. The first LEDs came on the market in 1967 (first red, then other colours), which were mostly used as functional light on technical devices and are still used today. 1995 came the first for lighting usable white LEDs on the market.
In electroluminescence, light is produced in one of several layers of semiconducting crystals. This light is monochromatic – the color depends on the material of the semiconductor crystal. Red, yellow, blue and green can be generated directly from the crystals when current flows through them.
White light for lighting technology is not included. This is produced by luminescence conversion using a coating. These phosphor layers – similar to fluorescent tubes – are excited and emit light, which then mixes with the light of the blue LED to form an overall spectrum. Depending on the composition of this layer, the blue LEDs can thus be adjusted to different Kelvin (light colors) and appear white. The light output with rather bluer or cooler light colors (5000K-6000K) usually fall out higher than warmer shades (2700K).
The semiconductor is then mounted on a heat sink, which dissipates heat from the sensitive semiconductor crystals. This ensures the long shelf life of the LED. If the semiconductor is heated for a long time above its specified maximum temperature, the luminosity drops rapidly – until total failure.
Types of Light Emitting Diodes
High-power and low-power
High-power LEDs are light-emitting diodes that operate at more than 20mA. All others are referred to as low-power diodes. High-power LEDs allow more powerful light sources in a smaller space. However, heat dissipation is also a stronger issue here.
The earliest type of LED is the DIP LED (dual-in-line-package). This LED, which can be soldered to the board with 2 wires and comes in a variety of colors, is probably also the best known type. The luminosity of these LEDs decreases quickly. Used they are often as a display in electrical devices.
The SMD LED (surface-mounted device)
The SMD is probably the most common type of LED today from industrial lighting to residential lighting. The semiconductors are soldered into a circuit and form a unit with the heat sink, which can then be glued to a circuit board and soldered by machine. The semiconductor crystal is soldered directly into the heat sink – this solder joint is for direct heat transfer and is one part of the electrical connection. The other part of the electrical connection is taken over by a so-called bond wire, which starts from the crystal. This is very thin in order not to shield any light. In SMD LEDs both low-power and high-power LEDs are used. Extremely bright and flexible lights are therefore possible.
The COB LED (chip-on-board)
With COB LEDs, the semiconductor crystal is bonded directly and packaged onto the aluminum or ceramic board (circuit board) and electrically connected with bond wires. This enables very good heat dissipation in a very small space. Due to the less complex production, a very good price-performance ratio is also possible. The radiation angle is realized by a lens made of epoxy, which is mounted above the semiconductor. Depending on the curvature of the epoxy droplet, the LED radiates wider or narrower. Heat sinks can be smaller with COB modules because a large part of the heat is already absorbed by the COB board.
This technology allows a very dense design of the LEDs and thus a high light output in a small space.