LEDs - choosing the correct driver for your application

08 February 2013

LEDs are fast replacing other light sources due to their controllability, long life and energy saving potential. However, LEDs need special control gear - often referred to as an LED driver - in order to deliver the correct voltage and current.

There is no such thing though as a universal LED driver, and with the driver having a large impact on the performance of the LED lights, it is essential the correct one is chosen.

LEDs are relatively new to the general lighting market, previously used only for applications that did not require a high lux level, for example emergency lighting and signage. Recent technological advances are leading to LEDs being installed in an ever increasing number of applications. As the most environmentally friendly way of lighting a building, containing no hazardous materials, consuming less energy and lasting longer than any other light source; they are becoming increasingly popular. However the correct LED driver needs to be chosen to ensure best performance is achieved.

LED drivers are the interface between the mains supply and the LED light source. They convert the alternating current from the mains supply, to direct current. LEDs are low voltage light sources and require a constant current to operate at their optimum.

Choosing the correct LED driver for an application is as important as choosing the correct tool for a job; it is determined by a high number of factors and must be chosen correctly in order to ensure safety, reliability and efficiency.

There are many aspects which should be considered when choosing a driver for your application. The type of LEDs being installed and the number should be determined, as well as whether they will be placed individually or in series strings. Size limitations will also need to be considered, as will the main design goals of the installation, such as the efficiency of the lights and the performance. Cost will also play a factor in determining which LED driver is the best for any application and, as part of this, the payback period of the installation.

There are two main types of LED driver – constant current and constant voltage. Constant current drivers, as the name says, supply a constant current to the LEDs whilst constant voltage drivers will need an additional interface circuit to control the LED current.

Constant current drivers tend to be used when one driver is required for each light fitting – whatever the number of LEDs in the fixture the current from the driver will remain the same. A common problem can be overloading the driver, connecting too many LEDs in a series, which means the driver will not deliver the full design current. To ensure this doesn’t occur, all guidelines on the maximum number of LED drivers can cope with should be carefully considered and the maximum specified LED string voltage allowed for.

Constant voltage drivers are best used in applications when the user requires flexibility with the number of luminaires connected to one power supply – as lamps are added, the current will increase to the maximum limit. Note that LEDs should not be connected directly to a constant voltage driver but an interface circuit should be used which will regulate the current into the LED string.

Controls & Dimming
It is not just the type of driver which needs to be chosen. As drivers often come with a multitude of control options, or dimming functions, allowing the intensity of the LED light to be controlled as desired. It is a common mistake that dimming lights will result in a loss of energy; the truth is actually the opposite. Although the driver efficiency will reduce a little as the LEDs are dimmed, overall there will be significant energy saving also, dimming lights to half power can result in the lifespan of LEDs being greatly extended, resulting in savings in maintenance costs.

Dimming LEDs can also mean lights are controlled to a suitable level for the application and enhance ambience whilst creating flexibility in any given space. LEDs can usually be dimmed between 100% and 5% (or even lower, and are controlled either by a dimmer switch, Dali controller or 1-10V controller

There are two main dimming protocols which typically use a phase control dimmer switch - TRIAC (leading edge) and trailing edge. TRIAC is the cheapest and most common method of dimming; however this generates an undesirable amount of Electro-Magnetic Interference, commonly known as EMI. Trailing edge dimming is more expensive than TRIAC but produces much less EMI.

Harvard’s CoolLED Phase Mains Dimmable driver is able to work with Phase-Cut Triac dimmers (leading edge), IGBT dimmers (trailing edge) and sinewave dimmers. The driver can also limit peak current and noise – a common problem for this type of dimming - and has high efficiency and long life.

Harvard also produce a range of programmable dimmable LED drivers, which work with the two most common dimming protocols - DALI and Analogue 1-10V. DALI is widely recognised as the leading intelligent dimming protocol for LEDs, standing for Digital Addressable Lighting Interface. DALI drivers allow users to digitally programme their LED installations using digital signals to send control information to each light. This allows them to set different lighting and ambient levels for displays, thereby maximising their investment. DALI drivers provide innovative dimming capabilities. Harvard’s range of DALI LED drivers provide dimming capabilities across a breadth of output currents. The full range includes 350mA, 500mA, 700mA, 1000mA, 1.2A and 1.4A drivers.

Analogue 1-10V drivers offer a less costly and more basic dimming solution to the DALI digital protocol. The 1-10V analogue drivers can be simply programmed with a simple passive controller or a fixed or variable resistor. These controls use voltage input to manage the intensity of the light, for example, at 10V lights are on 100%, 5V lights are on about 50% and 1V, lights are off. Harvard’s range of Analogue 1-10V drivers delivers dimming capabilities across a wide range of output currents, with the full range including 350mA, 500mA, 700mA, 1000mA, 1.2A and 1.4A drivers.

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