How do RGB LED lamps work

Technology and connection of RGB LED strips

LED strips are available in every hardware store, often as a package including a power supply unit, controller and remote control. In addition to monochrome ones - cold white, warm white, red, green, blue - there are also RGB LED strips with a variable luminous color. You can get the cheapest strips for less than 3 euros per meter from Chinese retailers via Amazon Marketplace or AliExpress.

When purchasing and connecting RGB LED strips, it is easy to get tangled up in the thicket of different designs. But ultimately there are only a few basic forms that we explain here.

Red, green and blue in one housing

LED triples in the colors red, green and blue can offer the human eye a wide range of colors if they are close enough to each other and light up differently. RGB LEDs combine three individual LED chips in one housing. The square design 5050 with an edge length of 5 mm, soldered onto LED strips with a length of several meters, is widespread.

5050 LEDs do not always contain only RGB triples, but also RGBW quartets with additional white LEDs. There are also RGB (W) LEDs with and without an integrated controller chip. The different types have different numbers of connections. A monochrome LED only needs two: anode and cathode, i.e. plus and minus or supply voltage and ground. With RGB-LEDs there are four connections, with RGBW-LEDs five: All LEDs are connected to the supply voltage with their anodes, the cathodes are brought out individually. This allows the flow of current to be controlled with very simple circuits. RGB (W) LEDs with an integrated controller work fundamentally differently and have only three connections: supply voltage, ground and a line for the serial control signal.

You should not rely on the cable colors of the various LED strips when connecting: In the case of an RGB type with four wires, the positive pole was white and the color channels green-red-blue (the sequence GRB is common); in the case of an RGBW strip, white was used for the white channel, the positive pole was black - which is actually the usual color for a ground line.

Electricity controls brightness

The brightness of an LED depends on how much current flows through it. As a result, the mixed color of an RGB-LED triple changes with the average current intensities of the three LEDs. The simplest cases are pure red, pure green and pure blue - then the other two LEDs are each off. If all LEDs light up at full brightness, the result is theoretically white; in practice, however, most RGB LEDs emit a strong color cast.

Pulse width modulation (PWM) is common to control the average current intensity. In a certain cycle, a transistor switches the current on for a certain time, so that the respective LED lights up for a shorter or longer period per cycle. If the clock frequency is high enough, the eye does not perceive any flickering.

In the case of LED strips with "dumb" RGB (W) LEDs, the controller controls all LEDs together. LED strips with digitally controllable RGB LEDs work fundamentally differently: The individual control enables effects such as running lights or rainbows. A cheap microcontroller like that of an Arduino theoretically addresses up to 1000 RGB triples. In any case, 300 LEDs on a 5 meter strip (60 per meter) are no problem.

The WS2811 controller from the Chinese company Worldsemi (WS), which is built into WS2812B LEDs as a bare chip, is widely used. Strips with these LEDs are also called WS2812B strips, some companies call them rainbow strips or neopixels. There are alternatives to the WS2811, such as the TM1829 once used in the so-called "Aldi LED" strips or the SK6812. Some are compatible with the WS2811, but the SK6812 also knows RGBW operation. The RGB LEDs, which can only be controlled together, are referred to by some as 5050 chips, which is ultimately nonsense: The WS2812B chips are also 5050. And there are narrower strips with 3535 chips that work in the same way.

To control a WS2812B strip, the controller software (or firmware) must know the number of LEDs, otherwise the protocol will not work. Each chip passes the control signal through; if only one chip goes on strike, the strip “behind” can fail. Unlike the “stupid” RGB LED chips, the digital ones do not light up when the supply voltage is connected, but wait for control data.

Supply the LED strips correctly

Strips with jointly controlled RGB LEDs are usually designed for 12 volts, some also for 24 volts. Digitally controllable strips usually require 5 volts and are damaged at 12 volts.

The number of LEDs that hang together on a controller is important. The latter can only cope with the flow of electricity up to a certain limit and the power supply unit must also provide sufficient juice. If the controller or power supply unit is too weak, the strip lights up less brightly in the best case, in the stupid case something breaks. This can be expensive, especially when connecting LED strips to PC mainboards. The latter usually deliver a maximum of 2 to 3 A per strip connection.

There are LED strips with different numbers of RGB LEDs per meter, common ones with 30, 60 and 144. The table shows measured values ​​of three different strips with 60 LEDs per meter each. Halve the values ​​for strips with 30 LEDs / m. Strips with super bright LEDs require more power. Our RGBW sample was a nerd with 30 RGB and 30 W LEDs per meter - there are also some with “real” RGBW LEDs, which then draw around 30 percent more power than RGB types.

Limit the flow of electricity

Power is the product of current and voltage: 5-volt strips draw 2.4 times as much current as 12-volt strips with the same power or number of chips. In other words: For a 5 meter strip with 300 RGB LEDs and a total of around 60 watts of peak power, you need either a power supply with 12 V / 5 A or 5 V / 12 A. The latter is significantly more expensive and the stronger current flow through the long strips lead to waste heat and a voltage drop, which can become visible: The "rear" LEDs then light up darker. To remedy this, feed the supply voltage in the middle of the strip and use cables with a larger core cross-section.

In practice, power consumption and current flow are lower because you want colored light, with the individual colors shining weaker. Anyone who understands the power adapter should avoid the color white or only use it with reduced brightness. Flashing instead of permanent light also reduces the average power consumption.

LEDs heat up a lot at full brightness and break if they overheat. Therefore you should monitor the temperature at least in a trial phase. Because of the risk of overheating, LED strips should not be tried rolled up on the delivery spool. If you want a lot of brightness, you have to ensure heat dissipation, for example by mounting the strips on metal surfaces. The factory-applied adhesive strips often do not last long.

Most LED strips can be shortened, i.e. cut at marked points. How many LED chips belong to a group depends on the type of strip. With the digitally individually controllable RGB LED strips, each LED can be cut off individually. With most analog RGB LEDs, three are attached to common series resistors; you can cut after every third. Our strange RGBW LED strips, on the other hand, could only be separated after every six chips. (ciw)