To be able to deliver it’s designed light output, consistent with reasonable lifetime, an LED requires a controlled source of DC power.

An LED is essentially a semiconductor diode and, as such has an anode and a cathode (positive and negative connections). The power supply for a LED must deliver ripple free DC (direct current), not AC (alternating current). This can be from a battery (eg, as in a torch), or via a LED DRIVER, which itself is commonly supplied from AC mains power.

The vast majority of drivers used to power LED lighting in domestic, commercial and industrial situations have mains power input, and a controlled constant voltage or constant current DC output which must comply with the specifications as set out by the LED manufacturer. This output must be stable in regard to time, ambient temperature and load, or the performance and life of the LED will suffer. Fortunately, this is easily achievable with modern electronics.

PROTECTION

Like any electronic device, an LED driver is designed to work up until it’s maximum operating temperature is reached. At or just before this point (case temperature typically 90-100C), internal sensors invoke a protection mechanism and shut the output down so that the driver does not self destruct. The driver may re-start when the temperature drops below a pre-determined figure, or upon re-start. Care must particularly be taken when mounting a driver in a sealed enclosure where lack of free air circulation can de-rate the output considerably.

In the event that the output current is exceeded or the output is short-circuited, the driver may current limit or shut down. Auto recovery will occur when the fault is removed.

Different manufacturers use different methods of input protection.Typically, mains input LED drivers offer an input voltage range of 100 – 305V AC, 47 – 63Hz (or 130 – 430V DC). This enables these models to be used in countries which have different mains supply voltages and frequencies Some drivers feature input over-voltage protection, which shuts the driver down should the input voltage rise too high. Similarly, some manufacturers offer models with a wide input voltage, eg 180 – 550V AC, which can also be used across two phases.

“Inrush current” is a topic needing brief mention, particularly where several SMP supplies are fed from a common circuit breaker. A typical driver will likely have an inrush current rating of 50 – 70A, so it doesn’t need many drivers on the one circuit to trip a typical switchboard circuit breaker ( if all drivers are turned on simultaneously, of course)

FANS

It is common practice to cool electronic equipment by using an internal fan, and LED drivers are no exception.

Fans are not popular with many end users who resent the even low noise intruding into the peace and quiet of their relaxed home environment. Likewise, fans are typically incompatible in an outdoor weather environment..

R F I

Electronic power supplies need to meet international standards for spurious RF emissions. However, the occasional installation can lead to a perfect storm, particularly when miniature lamps with their own built in SMPS are used. As well as being directly radiated through the air, the AC input and DC output wiring can act as an antenna, often interfering with TV and radio reception. Strategically placed toroidal chokes often solve the problem.

DIMMING

Lights are dimmed for a variety of reasons. Dimming reduces power consumption, generally increases the lifetime of the lights themselves, and can add to the ambience of the room.

The lamp must firstly be dimmable ( not all are ), and the driver and light must be compatible. Lamps incorporating their own SMPS may be particularly challenging.

If the installation is to use existing wiring, this may have some bearing on the dimming method used.

Without going into significant detail, the popular methods are

1. PWM, where the current is rapidly switched ( 1000Hz) on and off, and the ON time of the duty cycle determines the average power to the lamp
2. 0-10V ( or 1-10V)
3. Resistance
4. Push-on switch
5. Triac ( phase cut, leading edge, trailing edge)
6. DMX
7. DALI

Satisfactory dimmability can sometimes be difficult to achieve, for reasons not readily visible in the data sheets. Very often the size of the driver must be closely matched to the load, or it will not dim fully to off, or to full power. If existing lighting is being upgraded without replacing concealed wiring, the best option may be triac dimming. Such dimmers offering acceptable performance are only just appearing on the market, and they are still relatively expensive. Probably cheaper than a re-wire ! 

At this point, we need a basic appreciation of the concepts of constant current and constant voltage. Our web article “Constant current vs constant voltage” will provide a more in-depth discussion.

CONSTANT CURRENT POWER SUPPLY

Commonly used CC drivers provide a fixed (current limited) output of typically 350mA or 700mA. Some manufacturers design their fittings to be as low as 200mA, and sometimes as high as 1500mA. “Constant current” means that the output of the driver is limited to this maximum figure. Often there may be a number of LEDs in a fitting, and they would be connected in a series (or series parallel) arrangement so as to provide the light output required. In an installation, a number of such fittings would be connected in series. The series connection ensures that the same current flows through each fitting.

The laws of semiconductor physics provide that, when driven to it’s rated power, the LED or LED array will have a design voltage drop across it. For example, 3.5W, 350mA LED will have a voltage drop across it of 10V. 

To be able to drive say, 5 such fittings (in series), the driver would be required to produce an output voltage of at least 50V. If it’s rated output is significantly less than 50V, the LEDs won’t light up. If it’s more, that’s OK, as the current is limited to 350mA. The output of the driver will adjust so that not more than 10V appears across each fitting. Everything is good!

So, in this application, we require a constant current driver rated at 350mA, minimum of about 20 watts. A typical power supply used in this application  would be able to operate between 15 and 30 watts, with a usable voltage output in the range 30 – 60V. Within this range, the driver will supply a maximum of 350mA, and the output voltage will adjust itself accordingly.

Constant current drivers are available in outputs up to around 250 watts, but these are rare and are used in high power applications such as high bays, floodlights and street lights. It is important to realize that constant current drivers of this size may have an output up to 300 or more volts! More typical sizes are in the range 20 to 60 watts.

It is not uncommon for a given constant current driver to be able to offer a choice of output currents, eg 350, 500, 600, 700, 900mA, selected by a dipswitch underneath the cover.

Many constant current drivers are dimmable, using such techniques as 0-10V (1-10V), PWM, DALI, PUSH, resistance or DMX controllable.

As the LED warms up, it’s voltage / current characteristics will change. Likewise, as the driver warms up, it’s output will drift somewhat. Regardless of temperature, the output current must not exceed 350mA, or the purpose of the exercise is defeated.

CONSTANT VOLTAGE POWER SUPPLY

Common examples of a constant voltage drive include the familiar laptop power adaptor, or mains adaptors used to power monitors or small TV sets.

The concept of constant voltage is a little trickier to grasp than that of constant current, and constant current holds some traps for the unwary. In everyday use, domestic and commercial, constant voltage configuration is much more common than constant current.

While a constant current driver keeps the output current constant, a constant voltage driver keeps it’s output voltage constant (up to it’s rated current output).

Power supplies for laptops, computer monitors, or small TV sets are examples of constant voltage driver.

Like it’s constant current counterpart, a typical constant voltage driver has a mains voltage input and an output of 12 or 24V DC. It could be rated from 0.5A up to 200A (for a 12V model, this would correspond to 6 watts up to 1200 watts).

Because they are typically higher in power and physically larger than constant current, larger constant voltage models may have an internal fan for cooling, and most manufacturers offer IP67 waterproof models, for example for use in illuminated signs.

Some constant voltage drivers allow for the output voltage to be adjusted slightly, typically 10% either side of nominal. Like for constant current above, variants may offer dimming via PWM, DALI, 0-10V, resistance or control with DMX.

Over voltage / current / temperature protection as discussed above is generally standard.