EVENTS

2019-08-05

Installation and programming of RGB LED strips

Light-emitting diodes (or LEDs) increasingly often replace standard sources of electric light, such as incandescent light bulbs, halogen lamps or fluorescent tubes. First of all, they are far more energy-efficient, but they also have many other advantages.

In this article you will learn:

LEDs are often used in lighting systems fitted with a wide range of white diodes. However, more and more often colour LEDs are used to illuminate interiors with an eye-catching visual effect. The most advanced solution of this type are RGB diodes, whose colour can be smoothly controlled, but also set to almost any colour in the visible spectrum. What’s more to know about these?

What is a LED?

Light-emitting diodes (LEDs) are semiconductor light sources which emit light when current flows through them. Electrons in the semiconductor recombine with electron holes to release energy in the form of photons. This effect is called electroluminescence.

The colour of the emitted light corresponds to the energy of the photons, which in turn is determined by the energy required for the electrons to cross the band gap of the semiconductor. The band gap is sometimes called energy gap, and is an important aspect of every semiconductor – so the colour of the diode depends on the material used for its construction.

LEDs appeared on the market as commercially available electronic components in 1962. Initially, they emitted low-intensity infrared light. Infrared LEDs are used primarily in remote-control circuits, e.g. in consumer electronics. The first visible-light LEDs were also of low intensity and limited to the red colour. They were manufactured from materials such as gallium phosphide (GaP) and aluminium gallium arsenide (AlGaAs)

Modern LEDs are available across the visible, ultraviolet and infrared wavelengths, with high emission output, which means they generate a lot of light for a low energy cost. These contemporary products are made out of a variety of semiconductive materials, depending on the desired colour range. Red diodes are manufactured using aluminium gallium indium phosphide (AlInGaP), which makes them more efficient than those made of GaP or AlGaAs. The components of blue and green diodes, on the other hand, are manufactured mainly from gallium nitride (GaN) and indium gallium nitride (InGaN). The amount of indium determines the colour – the more indium, the longer the wavelength (e.g. green).

What is the application of RGB diodes?

RGB is an additive colour model in which red, green and blue (as the abbreviated name suggests) lights are combined in different ways to reproduce a wide range of colours. The main application of the RGB colour model is to detect, represent and display images in electronic systems such as TV sets and computers, but it has also been used in analogue photography. Nowadays, however, it is also increasingly used in lighting systems. Before the electronic age, the RGB colour model already had a solid theory based on the human perception of colours.

Mixing red, green, and blue light from LED sources to produce white light requires dedicated electronic circuits to control the blending of the colours and, since different diodes have slightly dissimilar emission patterns, the colour balance may change, depending on the angle of view, even if the RGB sources are in a single package, so RGB diodes are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colours and high energy efficiency.

Multicolour LEDs also offer a new way of creating light of different colours. Most perceivable [i.e. by the human eye] colours can be formed by mixing different amounts of the three primary colours: red, green and blue. This allows for precise and dynamic control over the display of colours. But the problem with using RGB LEDs for accurate colour display in lighting systems is related to the fact that a change in temperature also changes the energy gap of the semiconductor used as a component. Consequently, a change in colour emission of individual diodes (red, green and blue) occurs in the RGB structure. This is not an issue in the case of low power diodes.

How to set the diode’s brightness – pulse-width modulation

The brightness of electroluminescent diode emission is dependent on the current flowing through it. This, however, can be controlled in a vaiety of ways. The two easiest methods are to use a controlled current source or a PWM modulator.

A current source is an electronic circuit that delivers or absorbs electric current which is independent of its voltage. There are two types of current sources: an independent current source delivers constant current, while a dependent current source delivers current that is proportional to some other voltage or the circuit current. Therefore, in order to control LEDs, a dependent source is needed. Most of the actual current sources are made with the use of elements of controlled resistance (e.g. a MOSFET transistor). It is controlled in such a way that the voltage drop on that element also forces the flow of the desired current through the load.

The disadvantage of the solution with the lossy element which forces the flow is its low energy efficiency. The voltage drop at the control element can be quite significant, especially for low currents. Additionally, this way of control, as it needs an analogue input – e.g. control voltage – is difficult to implement in a digital system and requires the implementation of additional elements such as a digital-to-analogue converter.

PWM, or pulse-width modulation, is a method of reducing the average power delivered by an electrical signal by effectively cutting the signal into separate parts when it is switched on and off (without any transition states – as in a rectangular waveform). The average value of the voltage (and current) applied on the load is controlled by quickly switching on and off a specific type of key between the power supply and the load. The longer the key is turned on compared to off periods, the greater the total power supplied to the load.

PWM modulation is particularly suitable for relatively inert loads, such as motors that are not so easily affected by discrete switching. They react more slowly due to inertia. The PWM switching frequency must be high enough not to affect the load. In the case of RGB LEDs, it is not the receiver itself – the light emitting diode – that is inert, but the human eye, which does not perceive the blinking, because it averages the light intensity.

The speed (or frequency) at which the key must switch the load can vary considerably depending on the load and application of the system. In the case of LEDs, the optimum frequency also depends on the specific application. The upper frequency limit is the switching speed of the LED. The switching time of a typical LED is between several hundred and several thousand nanoseconds, which translates into switching frequencies from several hundred kilohertz to several megahertz. On the other hand, the minimum switching frequency is defined by the inertia of the human eye. With a moving object, 200 Hz is used as the minimum switching frequency for the LED control key.

The main advantage of using PWM modulation is that the power losses in the switching devices are very low. When the switch is switched off, the current practically does not flow, and when the key is switched on, the voltage drop on the key is marginal. Power losses, which are the product of voltage drop and current flow, are therefore small in both cases. Additionally, PWM works very well with digital controls, which due to their nature – zero-one control – control the key easily.

What is a LED and RGB LED strip with integrated driver

LED strip is a flexible printed circuit board on which surface-mounted light emitting diodes (SMDs) and other components needed for the operation of the diodes are soldered. It is usually equipped with an adhesive backing.

LED strips were used in the past only in accent lighting, backlighting, task lighting and decorative lighting. Thanks to the increased efficiency of LEDs and the availability of more powerful products, LED strips are now used as high brightness lighting that effectively replaces luminaires with fluorescent or halogen bulbs.

Popular LED strips are also available in a version with multi-coloured LEDs: RGB, RGBW. The latter has an additional, white diode, which emits good quality white light – you’ll learn more about it later in this article. Controlling them with the help of external drivers would be complicated because of the large number of leads needed to control the longer strip. That is why integrated drivers are often used for this type of strips.

How to control LED strips

Most RGB LED strips are constructed using classic RGB LEDs with four leads – a common anode or cathode and a single lead for each of the colours. Cables cannot be connected directly to the power supply, because a driver is necessary for an easy colour change. Although such a solution allows us to control the colour, the user should remember that the whole strip emits the same colour, which can be a limitation in terms of its use. A solution in which integrated drivers, such as Worldsemi WS28xx family of chips, are used on the strip in addition to RBG LEDs has become popular recently.

It is also worth mentioning that classic RGB LED strips are controlled differently than those with drivers. This is mainly due to the fact that in the case of integrated drivers the structure changes – only one line (DATA) is used to control, and not three separate lines for each colour. You can use e.g. control solutions based on Arduino here.

Strips with circuits from this group are usually called programmable or smart, while the driver itself has the form of an integrated circuit, designed to control LEDs. It includes an internal intelligent digital data latch for the input port, its own individual address, as well as a power controller circuit. It also has a precise internal oscillator and a 12V voltage regulator for LEDs. In order to reduce the ripple in the system, individual PWM channels are controlled with a phase shift. This system uses the NZR communication mode.

In the NZR system, the WS28xx family of systems are connected in series. The DIN pin is the data input and the DO is the output. The data is supplied to the DIN pin of the first driver in the chain. Its DO is attached to the DIN of the next one etc. After restarting the chip, the DIN line receives data from the controller. The first chip collects the first 24 bits of data (three times 8 bits for three colours) and then sends them to the internal data latch. The remaining data are sent further by the DO output.

The DO output data is buffered by built-in digital circuits, so the next driver receives a high quality waveform. This increases the range of the chip, as the only limits to strip length are the maximum distance between drivers and the number of available addresses.

When the driver latches the data, the system generates appropriate PWM control signals at the OUTR, OUTG and OUTB outputs, designed to control the red, green and blue diodes on the strip. Thanks to the possibility of addressing the WS28xx family of circuits, it is possible to set the colour and brightness of the RGB diode individually, which greatly expands the range of application. For example, in strips using this system, each diode can emit a different colour and with a different intensity, regardless of the other diodes on the strip.

It is worth mentioning that there are also available comprehensive solutions containing both the RGB LED structures and an integrated addressable driver in one housing, which simplifies the application and reduces the final cos. Such diodes are offered both in a budget version by Worldsemi, and in the version offered by Liteon, with top quality embedded diodes characterised by high repeatability.

Which strip and which controller to choose?

Many different RGB LED strips with integrated drivers are available on the market. These are strips with different power and LED number options, which translates into different brightness levels. Such products range from 30 to 144 LEDs per meter and have a maximum power output of 36W to 86.4W (per 1 meter of strip).

RGB LED strips can be supplied with 5V, 12V or 24V DC. The choice of a specific strip must be dictated by the supply voltage available in the specific system. For example, for a microcontroller system astrip supplied with 5V will work perfectly, and in an industrial system a strip supplied with 24V will be the best choice. In addition, when choosing LED strip for industrial applications, it is worth checking the ingress protection class of the product. If you choose an IP65 rated model, you can count on the reliability of the system, because this class guarantees dustproofness and protection against moisture.

RGB or RGBW – how to choose a suitable LED?

A standard RGB LED strip uses three LEDs (red, green and blue). It can produce a wide range of colours, mixing these three colours and giving an almost white light, but even when all three LEDs are lit to the maximum brightness, the final colour is far from perfect. Therefore,RGB + W LED strips are applied, which use four LEDs: RGB LED and an additional white light-emitting diode.

Although RGB LEDs themselves can produce a colour similar to white, the dedicated white LED in the structure provides a much purer white tone and allows the use of an additional warm or cold white chip. In addition, the white chip provides additional possibilities to mix colours with RGB chips, and in this way you can create an impressive range of unique shades.

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