Do LED strips need constant current or constant voltage?

&#;What type of LED driver do I need?&#; Searching for LED drivers can be more difficult than you think with the variety of options out there. There are plenty of factors to look at when choosing the one that works best for you, we have a thorough run-through of this in our guide on LED drivers here. One important choice is that of choosing a constant current LED driver versus a constant voltage LED driver. Now, it&#;s known that LED drivers are considered constant current devices, so why do manufacturers offer constant voltage drivers for LEDs as well? How can we tell the difference between these two?

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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. Understanding the difference between the two types can:

1. Aid in properly powering LEDs
2. Avoid serious damages to your LED investment

What is a Constant Current LED Driver?

Constant current LED drivers are designed for a designated range of output voltages and a fixed output current (mA). LEDs that are rated to operate on a constant current driver require a designated supply of current usually specified in milliamps (mA) or amps (A). These drivers vary the voltage along an electronic circuit which allows current to remain constant throughout the LED system. Mean Well&#;s AP Constant Current Driver is a good example 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 is with high powered LEDs and on a smaller scale as we only talked of using one LED. With lighting in the real world, it isn&#;t convenient or economical to build everything by hand from a single diode, LEDs are usually used together in series and/or parallel circuits to create the desired outcome. Fortunately for lighting designers, manufacturers have introduced many LED products to the market that have multiple LEDs already assembled together like LED rope light, LED strips, LED bars, etc.

The most common LED strips are designed with a group of LEDs in series with a current-limiting resistor in line with them. The manufacturers make sure the resistors are of the right value and in the right position so that the LEDs on the strips will be less prone to the variation of the voltage source as we talked of with the XP-G2. Since their current is already being regulated, all they need is a constant voltage to power the LED(s).

When LEDs or an array of LEDs are constructed like this they will typically state a voltage to be run at. So if you see that your strip takes 12VDC, don&#;t worry about a constant current driver, all you will need is a 12VDC constant voltage source as the current is already being regulated by on board circuitry that the manufacturer has built in.

Advantage of using a constant current LED driver

So when you&#;re building your own fixture or working with our high powered LEDs, it is of your best interest to use constant current drivers because:

1. They avoid violating the maximum current specified for the LEDs, therefore avoiding burnout/thermal runaway.
2. They are easier for designers to control applications, and help create a light with more consistent brightness.

Advantage of using a constant voltage LED driver

You use a constant voltage LED driver only when using an LED or array that has been specified to take a certain voltage. This is helpful as:

1. Constant voltage is a much more familiar technology for the design and installation engineers.
2. The cost of these systems can be lower, especially in larger scale applications.

Feel free to take a look over at our guide to LED strips that has a lot of devices that can be run from constant voltage. Also, if you need help selecting a constant current LED driver, head over to our helpful post on how to choose the right one.

It&#;s probably safe to say that LEDs (light emitting diodes) have become the standard source for all lighting needs. This is because they are our most efficient lighting option, but did you know they can be made even more efficient?

Firstly, it's important to note that all LED fixtures use direct current, or DC power, rather than alternating current (AC) power. Secondly, the drivers used for LED lighting can provide DC power that either has a constant voltage or constant current. This article will compare the benefits and disadvantages of using either constant voltage or current drivers with LED lighting. To do so, first this article will delve into the relationship between current and voltage with respect to the formula for power, and then what this means for the difference between constant current and constant voltage drivers.

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The Relationship Between Current And Voltage

Current and voltage characteristics of diodes have an interdependent relationship; if voltage increases, the current increases exponentially, like in the image below. Conversely, if current increases, voltage only increases incrementally (not nearly as dramatically).

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BLUE DIAMOND Product Page

Image Source: Voltage/Current Relationship of a Diode

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Have a look at the formula for power below to get a better understanding of the general relationship between voltage, current, and power. 

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Formula for Power

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Types of LED Drivers

To differentiate between the two types of drivers, constant voltage drivers maintain a consistent voltage, where the current can fluctuate depending on the required power. On the other hand, constant current drivers provide a consistent current, and the voltage fluctuates depending on the power required. Aside from LED drivers being either constant current, or constant voltage, they can also be integrated or external. External LED drivers are usually constant voltage (typically 12V, 24V, or 48V), and are located separately from the fixture (or remotely). One benefit to this is that heat dissipated from the driver will not affect the LED fixture. Integrated LED drivers, on the other hand, are connected to the light fixture (or bulb), and are usually constant current. Being integrated drivers, they exist in very close proximity to the actual light fixture, so any heat dissipated from them could negatively affect the performance and lifetime of the actual LED's. Damage is especially likely if the drivers are inefficient, and therefore emit more wasted energy in the form of heat.

For more information on LEDs, you can read our blog post: How DC Lighting Is Taking Over The Lighting Industry

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The Disadvantages of Using Constant Voltage Drivers with LED lighting

Although external LED drivers, in theory, cause less wear to LED fixtures (because the heat that they emit won't affect the LED's), they are usually constant voltage, and this can be harmful to the LED's, without the proper protection in place. To explain this further, using constant voltage drivers with LED fixtures means that a single, unchanging voltage is sent to the fixture. If the full power sent to the LED fixture needs to change (to dim or brighten a light, for example), the current will be what changes to affect the total power sent to the fixture. But, even if just the slightest adjustment to the voltage is made, current can increase or decrease exponentially. This means that using a constant voltage driver with an LED light can easily supply way too much current to an LED, which can significantly shorten the lifespan of an LED fixture. Fixtures with constant voltage LED drivers typically have lower efficiency and efficacy (lumens per watt), because extra components are required to protect the LED's from accidentally consuming too much current. Overcurrent protection (OCP) devices for LED fixtures with remote drivers are typically resistors. If you have an LED strip lying around, you can actually see the resistors in-between the LED's on the strip that are used to protect them from burning out.

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These maintain current at a relatively safe level for LED lighting to operate without burning out prematurely. OCP typically makes use of resistors to self-regulate current when it exceeds the limit of the LED, so that LEDs can use constant voltage drivers. unfortunately, when the resistor limits the current to protect the LED, it burns off the extra current as heat, ultimately wasting power to protect the LED's. Despite the lower efficiency being a disadvantage, constant voltage drivers are less costly than constant current drivers, so they are often used in many applications, like LED strip lights, because the manufacturer would not typically spring for more expensive drivers in these cases. 

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Advantages of Constant Current Drivers for LED Lighting

Constant current drivers provide LED lights with the correct current in a controlled fashion. This type of driver maintains a constant current by fluctuating the voltage sent to the LED fixture. As mentioned before, when considering the voltage-current characteristics of a diode, current changes dramatically for small changes in voltage. For this reason, voltage levels are easier to control with precision, and can thus be adjusted more accurately because they don&#;t change as dramatically as current would in a constant voltage driver. This precision is what makes constant current drivers a better option for LED lighting applications.

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Because LEDs do not receive extra current when a constant current driver is used, using these drivers in LED lighting applications reduces wear on LED light fixtures, thus increasing their lifespan. This is why constant current drivers are primarily used in applications where efficiency and power need to be optimized, such as in high power lighting applications (outdoor, industrial, and commercial). These drivers are also very reliable when it comes to maintaining a constant brightness because they can more easily maintain a consistent light output. As a result of all these benefits, thermal runaway is also minimized. One major disadvantage to constant current drivers is that they can be much more costly. However, they are still much more efficient to use with LED lighting than constant voltage drivers, because a constant current LED fixture doesn't need resistors to protect the diodes from overcurrent faults. So springing for constant current drivers for LED lighting applications usually pays for itself.

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Further Comparing Constant Voltage and Constant Current Drivers

Based on what we&#;ve now learned about both constant current and constant voltage drivers, it is safe to say that constant current is the better option for LED lights. Constant current drivers can provide consistent brightness levels, whereas constant voltage drivers can typically cause flickering because of where they lack when it comes to controlling current fluctuations with precision. This lack also significantly shortens the lifetime of LED lighting that uses constant voltage drivers. LED lighting systems that use constant voltage drivers are also typically more inefficient because of the need for current regulation. On the other hand, constant current drivers don&#;t need any additional components, making their systems more efficient in LED lighting applications. Although, constant current drivers are typically more expensive than constant voltage drivers, their increased efficiency more than makes up for the extra cost.

Here&#;s a recap on some of the main differences between constant voltage and constant current drivers in LED lighting applications:

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Constant Voltage

Constant Current

• Fluctuating current and stable voltage

• Fluctuating voltage and constant current

• Current changes exponentially in a way that's hard to control in order to keep voltage consistent (when power needs to change)

• Voltage moderately fluctuates, and is easy to control, which creates a more stable current when power needs change

• Needs a current limiter

• Regulator built-in

• Less costly

• More costly

• Small scale uses

• Large scale uses

• Short lifespan

• Long lifespan

• Compicated

• Simple

• Excessive current

• Right amount of current

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For more information, please visit High Efficiency Constant Current LED Strips.