How an LED Light Bulb Works

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LED light bulbs are now the preferred form of lighting for domestic lighting as well as many commercial lighting applications.

LED lights use light emitting diodes as their basic component, but they require several other electronic circuits to enable them to operate.

Looking at how an LED light bulb works requires looking at the components within the overall lamp to see what circuits are there and how they work and contribute to the overall operation of the complete light.

Basic LED technology

As might be expected the LED lighting solutions are based around light emitting diodes themselves.

LEDs are a form of semiconductor diode. Diodes allow current to pass though them in one direction. However the light emitting diodes are distinguished from other forms of semiconductor diode in that they emit light when current is passed through them in a forward direction.

The LED circuit symbol for use in the circuit diagrams used for electronic circuit designs consists of the normal diode symbol, but with two arrows coming out of it to depict te light.

Although the effect was first noticed as far back as 1907, it was not until the 1960s when they became commercially available.

Initially the first LEDs were used as small indicator lamps, but the technology developed and they are now used for full lighting applications.

LEDs are low voltage devices, often having only about a volt and a half across them when they are operation, i.e in the forward direction. They also do not like large reverse voltages, so care is needed in the electronic circuit design of any system using LEDs to ensure that they do not experience a reverse voltage. This is particularly true for LEDs used in light bulbs.

LED light circuitry basics

LED lights require more than the basic LED component for their operation. They require other circuits around them to provide the right current and other features for their operation.

The electronic circuit design for the operation of LED lights requires the different circuit functions to come together to enable the safe, reliable, and efficient operation of the overall LED light.

Looking at how an LED light bulb works, it is possible to split the main elements of the design into a number of different blocks. These blocks include both electronic circuit designs as well as a number of physical elements:

• LED array:   Obviously the LED is a major element of any LED light bulb. The design and construction of this element of the overall electronic circuit design is particularly important both in terms of its electrical and physical aspects.

• Driver circuit:   This circuit block provides the correct drive for the LEDs themselves taking the input power, which may be 240V AC or 12V DC, etc and then converting this into the required format for the LEDs used within the light bulb.

• Heatsink:   Even though LEDs are considerably more efficient than other forms of lighting, they still dissipate some heat and this needs to be removed from the lamp otherwise it will cause it to run too hot, or overheat. By keeping the temperature down the reliability and life can be maximised. Accordingly most larger LED lamps will include a heatsink to remove excess heat.

• LED base or connector:   The LED lamp needs a base to enable it to connect to the light fitting. The standard base connections are adopted to that LED light bulbs can directly replace incandescent or filament lamps and CFLs.

• Diffuser:   In order that a more pleasant and even light emanates from the LED light bulb, a diffuser is fitted over the LED array. Not only does this diffuse the light, but it gives protection for the electronics and prevents any possibility of electric shock by preventing direct contact with the electronics.

These different elements of the LED light bulb will be addressed in turn to see how they work.

LED array

The LED light bulb normally has a number of individual LEDs themselves, i.e. the basic electronic components. These are normally surface mount components that are set on a printed circuit board.

An array of LEDs is generally used because this means that the power capability of the individual diodes does not have to be as large enabling large power capability lamps to be made more easily.

Having several LEDs also improves the overall life of the LED light bulb, because not only is the power of each device reduced, but it helps with the removal of heat because a lower amount of heat is generated over a wider area and there is not a single very hot spot for which thermal management would be more difficult.

A further advantage of having an array of diodes is that the light does not come from a single spot and it can be diffused more easily. Using a number of diodes can help with improving the angle over which the light is delivered. The physical aspects can be arranged more easily to ensure a widely angle of light.

LED driver electronics

When looking at how an LED light bulb works, another major element is the driver circuitry. This is probably the main part of the electronic circuit design for the overall LED light.

There are several functions provided by the driver circuitry:

• Power conversion:   The power conversion element of the driver electronics is particularly important. As individual LEDs require very approximately a volt and a half of DC, and the input could be 12 volts DC 240 volts AC or another voltage, then the input must be converted to provide the required output. Typically a switch mode regulator is required to provide a very high level of efficiency. The higher the efficiency of the switch mode regulator, the higher the efficiency of the overall LED light bulb. It also means that less heat needs to be removed and this results in cost savings in the manufacture.

• Drive circuitry:   There will be circuitry on the printed circuit board to drive the LEDs in the manner they require. In some instances a series configuration may be needed to increase the overall drive voltage, whereas in others some may be run in parallel.

• Output protection:   It will be necessary to ensure that any diodes that fail as a short circuit do not cause a catastrophic failure of the whole assembly and the resulting possibility of fire.

• Input protection & filtering:   Input circuitry is required to protect against any line transients that will undoubtedly be experienced. It is also necessary to add filtering to ensure that no harmful emissions can be radiated or travel back along the power line.

In view of the vast numbers of LED lights being manufactured, cost is a prime consideration. The electronic circuit design of the driver aspects of the assembly need to be optimised and the electronic component count reduced to minimise the component cost as well as the manufacturing cost and size of the printed circuit board assembly.

LED light bulbs or lamps are widely used. Looking at how these LED lights work, the different blocks each have a distinct function enabling the overall light bulb to operate effectively and reliably.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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