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LED Dimming

LED dimming

All LEDs can be dimmed, but it depends on the driver that is being used to control them. All LEDs need a driver, which is a piece of electronics. Its main function is to run the LEDs at the correct voltage and current, converting the mains AC supply (typically 230V, 50Hz) to, usually 12 or 24V DC. This is necessary because LEDs don’t operate on mains. LEDs work at low voltages on direct current.

Dimming LED that has a separate driver

If the driver is separate from the lighting fixture or is accessible within it, then the best way to dim the LEDs will be by using a dimming driver. This is how most dimming is achieved in commercial and industrial installations. A dimming driver performs two functions: it is both a driver and a dimmer.

As a driver it converts the mains AC supply (typically 230V, 50Hz) to 12-24V DC. This is necessary because LEDs don’t operate on mains. LEDs work at low voltages on direct current.

As a dimmer it raises and lowers the amount of electrical energy flowing out to the LEDs. There are two mains ways that a driver does this – pulse width modulation (PWM) or amplitude modulation (AM) – and these are discussed below. However, if a driver uses PWM or AM is generally of little interest to an installer or specifier. They are internal to the driver and have little or no effect on the end user and the quality of the dimmed light they will see. The more important question for the specifier and installer is how the driver is to be instructed to make the LEDs brighter or dimmer. There are several ways to control a driver, or instruct it what to do. In each case the driver will require a continuous supply of mains power plus a control signal to tell it what to do. The most common methods of controlling a driver are as follows:

DALI (Digital Addressable Lighting Interface). DALI is the most widespread communications protocol for commercial lighting control in use today. A DALI signal is delivered to the LED driver(s) via two wires that are in addition to the wires delivering the mains supply. Many manufacturers supply LED drivers that will accept a DALI input.

1-10V analogue control. This is an analogue communication protocols. Like DALI, it is delivered to the driver on two wires that are in addition to the mains wires.

Switch Dim, Touch Dim, Push Dim. These all use one extra wire carrying an additional mains supply to a dedicated terminal on the driver. The control device is usually a retractive (non-latching) switch making/breaking the wire carrying the additional live supply; a quick push on the retractive switch will cause the driver to switch the LEDs on/off while a longer push instructs the driver to dim the LEDs up or down.

Sensor Dim, Timer Dim. This function uses a latching switch, typically a motion sensor (PIR or microwave) or a time switch. This connects or disconnects the additional mains supply which indicates to the driver that is should switch the LEDs on, off or dim to a pre-set level.

DMX (Digital Multiplex). This is a communications protocol that originated in theatre lighting but which is also widely used for other colour changing applications such as façade lighting.

Dimming LED that has integrated driver

With most LED lamps the driver is sealed inside. This is also true of many fittings designed for residential use. In both cases there is no means of delivering to the driver any sort of dimming control signal. Therefore, if dimming is required it must be done by fitting an external dimmer to the incoming mains supply to the LED lamp or sealed fixture. This has a significant impact on the design of the driver in the lamp or fitting. An external dimmer increases and decreases the amount of electrical energy flowing into the LED lamp or sealed fitting and it usually does this by switching the supply off and on (for a variable duty cycle) at 50Hz. If the driver is to work under these conditions it has to be designed accordingly. This gives rise to the concept of a “dimmable or non-dimmable” LED lamp or driver.

A dimmable LED lamp is one which is designed to be dimmed by an external (separate) dimmer. However, even a dimmable LED lamp might not be suitable for use with all types of dimmer, so it is important to check what sort of dimmer is recommended for the LED lamp or fixture you wish to dim. The main types of dimmer available are:

Leading edge phase-cutting dimmers (also known as triac dimmers and rising edge dimmers). These work by switching the current off at the zero-crossing point (see diagram) and on again later in the same mains cycle. The amount of energy flowing to the LEDs depends on the duration of the “off” period. The longer the off period the dimmer the LEDs will appear to be.

Trailing edge phase-cutting dimmers. These work by switching the current on at the zero crossing point (see diagram) and off later in the mains cycle. Usually this is done with an electronic component called an isolated gate bipolar transistor (IGBT). The amount of energy flowing to the LEDs depends on the duration of the “off” period. The longer the off period the dimmer the LEDs will appear to be.

To identify the best dimmer to use for a particular LED lamp or sealed LED/driver light fitting check with the manufacturer for list of their recommended dimmers. If there is no such list then follow these steps:

Check the load on the circuit you plan to dim. To do this, add up the wattage of all the lamps/fittings on the circuit and then select a dimmer whose load range (in watts) falls in this value. Be sure to note the minimum load of the dimmer you intend to use.

Select a dimmer that is described as “trailing edge” or which advertises itself as “suitable for LEDs”. In the absence of more specific advice, a trailing edge dimmer is much more likely to work well with a dimmable LED lamp or fixture than a leading edge dimmer.

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