Constant Current and Constant Voltage LED Drivers
Constant Current LED Power Supply
What is a Constant Current LED Driver ? Constant current led drivers or power supplies are designed to supply a fixed current mA. Unless you overload a constant current driver, it will always supply this current. All good quality power LEDs will specify an optimal forward current o release the best performance and best lifetime. It is important to select a driver to match the value if best performance and best lifetime is your goal.
With a constant current driver, you can wire several LEDs in series and provided that you do not exceed the constant current driver's maximum power output rating then all the LEDs will be run at the specified forward current.
What changes as you add and remove different numbers of LEDs ? Well, the voltage that is output by the constant current driver will be varied so that there is the correct voltage supplied to (or voltage drop across) each LED. It adjusts the output voltage automatically up and down into a rated limited.
So constant current driver always maintains the defined forward current output but changes the overall circuit voltage automatically dependent on how many LED you to connect to it in series at any one time. Constant current drivers are usually rated at popular LED foroward current specifications such as 350mA, 500mA, 700mA, 1050mA or 1750mA.
Some good examples of quality LED drivers are shown below from Meanwell, Euchips, Easeic and Philips.
The latest technology offers more flexibility for manufacturers of LED light fittings or installers thanks to the multi-level current settings. The power supplies have dip switches (from 3 to 5 in one row) that use internal resistors to adjust the current rating to the right level.
The users must be very careful when they use their own LED driver with an LED arrays (made of various LEDs in series/parallel). There is a high risk to overdrive the chips with high current. As a result, the lifespan will be dramatically reduced. Under driving, the circuit with a lower current will downgrade the performance of the light fitting giving out less lumen.
Higher forward current will make LEDs brighter but if the forward current is not regulated the LED will draw more current than it is rated for and burn out quickly.
A constant current driver is the best way and really only efficient way to drive a collection of high power LEDs as it maintains consistent and safe brightness across all the LEDs in series.
. Because an LED is essentially a diode it will only operate in one direction and that is with a forward voltage. The permitted range of forward voltages for a specific LED may be found in the LEDs product datasheet (graph with Forward Voltage Vs Forward Current)
[caption id="attachment_1674" align="aligncenter" width="478"]
Various brands of contact current (CC) LED driver. Fixed output current or adjustable with dip switches to meet different power levels[/caption]
[caption id="attachment_1678" align="aligncenter" width="532"]
Constant current power supply 40W, 950mA, suitable for large LED fittings, Tubes, Panels and Down Lights[/caption]
Constant Voltage LED Driver
Constant voltage drivers are designed to supply a single direct current (DC) output voltage. The most common constant voltage drivers are available on the market are usually designed to provide output voltage of 12Vdc 24Vdc and recently 36V and 48V for large LED signs. The drivers have a variable amount of current dependent on the load presented to it, up to its maximum rated wattage.
An LED light or strip that is rated for constant voltage usually specifies precisely the level of input voltage it requires to operate correctly. Constant current-voltage driver accepts 100VAC or 240VAV as input and outputs 12Vdc, then it is really just a standard run the mill step down transformer with a standard rectifier smoother circuit bolted on. So as you can see the although constant voltage driver is possible a newish term compared with step down transformer or switching mode power supply.
[caption id="attachment_1675" align="aligncenter" width="516"]
Image 2. Glacial Power led driver with 12V and 24V output.[/caption]
Most LED strip lights use a constant voltage whether it is 12Vdc, 24Vdc or 220VAC. As long as that power supply or constant voltage driver is capable of outputting sufficient wattage than LED srtip light up at the brightness intended, and without significant risk of damage or thermal runaway. There is not really any chance of thermal runaway occurring due to the inherent design of the strip. Examples of typical constant voltage power supply design for use the led strips are available in image 2.
As a general observation LED contact current drivers are only really suitable for low powered single LEDs or strips of individually low powered LED such as are used on most LED strip.