LED Drivers

LED technology is the most recent and biggest change in the lighting industry in recent times. A lighting appliance is called a Luminaire
(often referred to as a light “Fitting”) and the part that creates light is called the Light Source.
Luminaires that have LEDs as their light Source are more efficient and cost effective than older technologies.

What is an LED?

LED is an abbreviation for Light Emitting Diode.

A Diode is an electronic component which only allows electricity to flow through it freely in one direction. An LED is a type of Diode which emits light when the electricity flows through it.

Many LEDs have a yellow "phosphor" coating, which converts the LED output into white light.

LEDs work well with batteries because the electricity flows one way, from positive to negative (as we measure it). This type of flow is called Direct Current or DC. The problem arises when we want to power LEDs from the mains supply, which changes direction 50 or 60 times every second, which is called Alternating Current or AC.

If we try to power the LEDs straight from and AC supply, the supply voltage would normally be too high for the LEDs and they would flicker 50 or 60 times every second as the current tries to flow in the opposite direction, being blocked by the LED's "one-way-street" effect.

The answer is to use a Power Supply, called an LED Driver, that converts mains AC into lower voltage DC, making it suitable for the LED(s)

Power Supplies

A Power Supply Unit or PSU is any electronic circuit that provides power to the other electronics within an appliance.
This can be either from a battery or from a mains electricity supply.

The PSU may need to convert the original source of electrical energy into something safe and appropriate for the equipment that it is driving. The most important things to know when choosing a PSU are the input and output specifications.

Firstly, it is necessary to cover some basics about electricity in order to understand the specifications of Power Supplies.

Electricity - The Basic Principles

Electricity (whether AC or DC) has 2 major properties – Voltage and Current
(text colours blue for voltage and red for current will be used to help differentiate them throughout this article)

Voltage is sometimes known as Potential or Potential Difference (P.D.) and is measured in Volts (V)
This is the amount of energy stored as an electrical charge within a conductor. If there is more voltage in one side of an electrical circuit compared to the other, this will cause electricity to flow from one side to the other.

Current is the actual flow of the electricity through the circuit and is measured in Amperes or Amps (A) The current can be considered as the rate of delivery of the energy contained within the voltage and it is critical that the wiring and components have enough capacity or strength to withstand this electrical flow.

Wiring and circuitry have an innate Resistance, which is measured in Ohms (?). The resistance works against the current and is the remaining factor in the relationship between voltage & current This relationship is expressed as a very simple mathematical equation called Ohm’s Law

V = IR which stands for Voltage = Current × Resistance and is measured as Volts = Amps × Ohms (V = A × ?)

The LEDs in a luminaire have an internal resistance and may be combined with other components which also have inherent resistance. This resistance combines to determine how much current will flow through each LED.
Another effect caused by the resistance is that some of the electrical energy is dissipated as heat.

The last basic principle is that Power is the amount of overall energy used by the appliance.
This is the combined product of voltage and current and is measured in Watts (W)
The calculation for this is P = IV which stands for Power = Current × Voltage

Any LED source needs the correct Power Supply or Driver to operate. It is imperative to match the specifications of the driver to those required by the LEDs. The above electrical measurements are the most critical ones to observe.

Constant Voltage LED Drivers

Constant Voltage LED Drivers convert the electricity to a fixed voltage and can deliver up to a maximum current.
So long as the LEDs demand less than this maximum current, the Constant Voltage Driver can drive them.

LED light sources that use Constant Voltage drivers have resistors or other current-limiting components onboard. The most important parameter to match is the fixed voltage, which must be the same for the driver and LED source. The next thing to ensure is that the maximum current of the driver is greater than or equal to that of the LEDs.

An example of a Constant Voltage LED driver application is shown below.
This LED driver would be suitable for any 12V LED appliance up to 1A (current) or 12W (power)
In this case, it doesn’t matter if the LED circuit has a current demand much lower than 1A, so long as it is not higher.
It can drive a 12V 400mA cluster of LEDs without any problem.
It can drive two 12V 400mA clusters of LEDs wired in parallel because the total current draw is 800mA
It cannot drive three 12V 400mA clusters of LEDs wired in parallel because the total current draw is 1200mA

Voltage vs Current

In general, the light output of an LED luminaire is relative to its Power consumption.
If a luminaire is rated at 10W, then you might expect a 20W version of the same type to produce approximately twice the amount of light output.
The maximum output Power of the driver is the Voltage multiplied by the maximum Current (Pmax = V × Imax)

An example would be the aforementioned LED driver rated at 12Vdc 1000mA. This driver converts 230Vac mains electricity into 12Vdc and can deliver Current up to 1000mA, which is the same as 1 Amp (1A), so the maximum Power would be 12V × 1A = 12W

However, if the same driver is designed to operate at 24Vdc, it is able to deliver twice the power at the same current: 24V × 1A = 24W
This means that it can deliver twice the power, which can drive double the number of LEDs, provided that they are wired to operate from 24Vdc.
The fact that the current remains at 1A means that there is no need to upgrade any wiring or connectors either.

Higher voltage is especially useful for long runs of LED TAPES, which carry the electrical current along the thin, flexible Printed Circuit Board (PCB) that the LEDs are soldered onto.

Each LED has a standard Forward Voltage in the direction it allows current to flow and the LEDs on a tape can be wired in different ways so that they can share the supply voltage across different numbers of LEDs, enabling them to be powered from different supply voltages .

Maintaining a low current means that there is less heat generated in the wafer-thin conductor and running the same current at higher voltages can deliver higher power to drive longer runs of LED tape than is possible with lower voltages.
Less heat generated per Watt of power and fewer drivers per LED cluster results in a more efficient lighting scheme.

Below is a graphic comparison of 12Vdc and 24Vdc LED tapes, where each LED is receiving 4 Volts.

Constant Current LED Drivers

Constant Current LED Drivers convert the electricity to a range of voltages at a fixed current.
This type of driver is used to drive LEDs in lighting products that are designed to provide energy-efficient lighting.
These luminaires usually have no current-limiting components onboard (such as resistors).
There is also an optimal level of current required to give the maximum light output without over-heating the LEDs.

Heat is a problem for LEDs because it causes them to demand even more current.
This effect is accumulative and if it is not controlled, the lighting appliance would work inefficiently, and the LEDs would eventually burn out.

To avoid this, the driver has circuitry to keep the electrical current constant and instead vary the voltage to manage the demand from the LEDs. Usually, one Constant Current driver is specified to drive each single luminaire.

The specifications of a Constant Current driver must be accurately matched to the requirements of the LED(s).

The first criterion is that the output current of the driver must match the current rating of the LEDs. The current should match exactly but if this is not possible, it is safest to match as closely as possible below the current rating of the LEDs.

The next criterion is that the voltage range of the driver must fall within the rated voltage range of the LEDs. If the driver delivers a lower voltage than the minimum required by the LEDs, they will not light and if it delivers a higher voltage than the maximum for the LEDs, the operation will be unstable and the light may flicker or fail completely.

Note: the voltage required to make the luminaire light up is the sum of the forward voltages of groups of LEDs that are wired in series.
This can be calculated if the forward voltage of each LED is known and the wiring layout is known.

Generally, it is best to keep within the voltage limits as stated in the luminaire specifications.

Special Types of LED Drivers

For both Constant Voltage and Constant Current LED drivers, there may be various special adaptations.
Some of these special adaptations are described briefly below.

Programmable Drivers have a switchable range of voltages and/or current settings, designed as generic replacements for different LED luminaires.
In the case of Constant Voltage programmable drivers, these will have several different voltage settings and can still deliver any current up to their maximum handling capacity.
In the case of Constant Current programmable drivers, these will have several current settings and a selection of voltage ranges to match the luminaire as closely as possible.

Remote Controlled Drivers can be switched remotely and are used as part of a smart home lighting scheme or commercial building control.
The control may be wired or wireless or digitally operated over a local network.

Emergency Packs are control devices attached to an LED driver or may be integrated into the LED driver and is connected to a battery pack. The emergency pack detects when there is a mains power failure and switches the LED luminaire to be powered from the battery pack, usually at a lower level, to make escape routes visible in case of an emergency.

Dimmable Drivers enable LED luminaires to operate at a range of brightnesses below the maximum output for energy-saving or mood lighting purposes. There are many different dimming technologies applied to LED drivers and this subject, along with the others above, is covered in more detail in our separate web support article titled Dimming and Lighting Control.