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What type of LED driver do I need? With multiple options, finding an LED driver may be more difficult than you think. There are many factors to consider when choosing the most suitable factor for you. An important choice is to choose a constant current LED driver or a constant voltage LED driver. Under normal circumstances, LED drivers are considered constant current devices, so why do manufacturers also provide constant voltage drivers for LEDs? How can we tell the difference between the two?
Constant Current LED Drivers vs. Constant Voltage LED Drivers
Constant current and Constant voltage drivers are both viable options for a power supply for LED light sources, what differs is the way in which they deliver the power. LED drivers are the driving force that provides and regulates the necessary power to make sure the LEDs operate in a safe and consistent manner.
What is a Constant Current LED Driver?
The constant current LED driver is designed for the specified output voltage range and fixed output current (mA). LEDs rated for operation on constant current drivers require a specified current supply, usually in milliamperes (mA) or amperes (A). These drivers change the voltage along the electronic circuit to keep the current constant throughout the LED system. Mean Well's AP constant current driver is a good example, as shown below:
Higher current ratings do make the LED brighter, but if not regulated, the LED will draw more current than it is rated for. Thermal Runaway refers to excess current beyond the LEDs maximum drive current which results in drastically lower LED life-spans and premature burn outs due to increased temperature. A constant current driver is the best way to drive high power LEDs as it maintains a consistent brightness across all LEDs in-series.
What is a Constant Voltage LED Driver?
Constant voltage drivers are designed for a single direct current (DC) output voltage. Most common constant voltage drivers (or Power Supplies) are 12VDC or 24VDC. An LED light that is rated for constant voltage usually specifies the amount of input voltage it needs to operate correctly.
A constant voltage power supply receives standard line voltage (120-277VAC). This is the type of power that is typically output from your wall outlets around the home. Constant Voltage Drivers switch this alternating current voltage (VAC) to a low direct current voltage (VDC). The driver will always maintain a constant voltage no matter what kind of current load is put on it. An example of a constant voltage power supply is below in the Mean Well LPV-60-12.
The LPV-60-12 will maintain a constant 12VDC if the current stays below the 5-amp maximum shown in the table. Most often, constant voltage drivers are implemented in under-cabinet lights and other LED flex strip applications but it is not limited to those categories.
So how do I know what type of LED driver I need?
The case for constant current drivers:
If you take a look at high powered LEDs, one unique characteristic is the exponential relationship between the applied forward voltage to the LED and the current flowing through it. You can see this clearly from the electrical characteristics of the Cree XP-G2 below in Figure 1. When the LED is turned on, even the smallest 5% change in voltage (2.74V to 2.87V) can create a 100% increase in current driven to the XP-G2 as you can see at the red marks current went from 350mA to 700mA.
Now higher current does make the LED brighter, but it also will eventually over-drive the LED. See Figure 2 for Cree’s specifications of the maximum forward current and the de-rating curves in different ambient temperature conditions. In the example above we would still be alright driving the XP-G2 LED at 700mA, however, if you didn’t have a current limiting device, the LED would draw more current as it’s electrical characteristics changed due to temperature increase. This would eventually push the current way above the limit…especially in hotter environments. The excess forward current would result in extra heat within the system, cut down on the LEDs lifespan, and eventually ruin the LED. We call this thermal runaway which is explained in more detail here. This is the reason the preferred method of powering high powered LEDs is with a constant current LED driver. With a constant current source, even as the voltage changes with temperature the driver keeps the current steady while not over driving the LED and preventing thermal runaway.
When do I use a constant voltage LED driver?
The above example uses high-power LEDs and is small in scale, because we only talked about using one LED. For real-world lighting, it is neither convenient nor economical to build everything manually from a single diode, and LEDs are usually used together in series and/or parallel circuits to produce the desired results. Fortunately for lighting designers, manufacturers have introduced many LED products to the market, which have assembled multiple LEDs together, such as LED string lights, LED light bars, LED light bars, and so on.
The most common LED light bar design is a group of LEDs connected in series, and a current-limiting resistor is connected in series.
When LEDs or LED arrays are constructed like this, they usually state the voltage to operate. Therefore, if you see that your strip requires 12VDC, don't worry about the constant current driver, you only need a 12VDC constant voltage source, because the manufacturer's built-in onboard circuit has already adjusted the current.
Advantages of using constant current LED driver
Therefore, when you build your own lamps or use our high-power LEDs, it is best to use a constant current driver.
Advantages of using constant voltage LED driver
Use constant voltage LED drivers only when using LEDs or arrays that have been designated to use a specific voltage.
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