LED Drivers: Constant Current vs. Constant Voltage - LEDSupply Blog

‘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?

Link to BLUE DIAMOND

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.

Sorry, I totally changes the focus of my quesiton, or really added a question by editing my post completely, but you had already responded. I should have just left it and added question. That was dumb.

Thanks for clarifying about nominal current.

Just to be clear about efficiency. As I am about to buy a driver and trying to decide between 2 different ones.

So, 2 scenarios:
1: total wattage is 400w, current is mA per strip for a total of 10 strips. Voltage is 40v.
2: total wattage is 400w, current is 750mA per strip for a total of 10 strips. Voltage is 53.3v.

Scenario 2 would be more efficient? In other words efficiency comes from lower drive current even if the voltage increases?

Your scenarios don’t make sense, if you are using the same LED strip for both scenarios. The higher the current passing through a strip, the higher the voltage drop across them. Here is how I would try to explain how efficacy works:

Say you run mA through 10 LED strips at a Vf of 40V, and it yields a total of 70,000 Lumens. This would give you an efficacy of 70,000 Lumens / 400W, or 175 Lumens/W. If you ran 500mA through the same LED strips in the same arrangement, the Vf would probably be in the neighborhood of 38.5V, and the output would be roughly 36,000 Lumens (possibly slightly more). Under this scenario, your input power is 38.5V x mA = 192.5W, and your efficacy is 36,000 Lumens / 192.5W = 187 Lumens/W.

So, your efficacy increases from 175 Lumens/W to 187 Lumens/W, but you are getting barely over 1/2 half as much total light. If you put 20 LED strips in parallel and ran them all at 500mA, you would get about 72,000 Lumens at 385W (20 strips x 0.500A x 38.5V). This would give you more light with less power, but you would use 2x the number of LED strips to accomplish it.

Does that make sense?

Yes, it makes sense. And it opened up a greater understanding. I should be focusing on lumen output instead of wattage. I was not remembering that wattage does not simply translate to amount of light.

I’m using these 2ft srtrips: https://www.bridgelux.com/sites/default/files/resource_media/DS132%20Bridgelux%20EB%20Series%20Gen3%20Data%20Sheet%%20Rev%20A.pdf
Which have a typical forward voltage of 19.1Vf and a nominal current of 700mA. And a typical flux @ 25c of lumens. And efficiency: 19.1*.700 = 13.37w = /13.37 = 186 lm/w.

Now, if I want to find the lumens and wattage of an array of these strips driven by a particular driver how do I do that?

10 of these strips using this driver: meanwell HLG-240H-48 https://www.meanwell.com/Upload/PDF/HLG-240H/HLG-240H-SPEC.PDF

This driver has total current of 5A, and a voltage range of 24-48v. I would like to wire these 10 by wiring 2 in series 5 times, then taking those 5 sets of 2 and parallel them together.

For more High Efficiency Constant Current LED Strips(fi,nl,cs)information, please contact us. We will provide professional answers.

I can find the current per each of the 5 sets by dividing the driver current by 5. which is 5A/5 = 1A. Using the datasheets “current vs forward voltage” graph I can see that at 1 amp the voltage is about 19.6v. If I double the voltage: 19.6v * 2 = 39.2v. Each series pair uses 39.2v * 1A = 39.2w, multiply this by 5 = 39.2 * 5 = 196w. And the number of lumens is simply 10 * = . So efficiency is: /196 = 127 lm/w.

Clearly my calculations are incorrect. What am I doing wrong?

What you are missing is the increased lumen output from the increased current. That is given in the Figure 4 graph.

The lumen output will be about 140% of what it is at the test current (700mA), so roughly lumens per strip. Ten of these will give about 34,860 lumens for an efficacy of 34,860lm / 196w = 178lm/w.

In a real application, this won’t quite be true, because light output and Vf both drop slightly at elevated temperatures for a given current (which will be the case here due to self heating). The Vf drop adds slightly to efficiency, but the loss in light output more than makes up for that, so overall efficacy is reduced by about 3.5% percent if the case temperature rises to 60°C. This comes from Figures 7 and 8 on page 7 of the datasheet.

Thank you for breaking that down for me, very valuable.

It all makes sense now, but I want to be clear about one part. When calculating the total wattage that my strips will use, I take the driver’s max current and divide that by how many strips I have, then take that current and consult the data sheet graph “current vs forward voltage” to get the voltage. Then multiply that voltage by the total driver current. Is this the case regardless of how I wire the array, no matter how complicated a combination of serial and parallel wiring?

For example, if I wanted to wire 24 strips in series of 8, then parallel those sets of 8 in 3 times. I still just divide the driver max current by 24, find the voltage for that and multiply that voltage by the max current to find the wattage?

The arrangement makes a difference. If you have only three strings of LED “strips” in parallel, then each string will take 1/3 of the current pushed out by the constant current LED driver, regardless of how many “strips” are in the string (assuming you have the same number of strips in each string).

If you wanted each strip to run at 1A, then you need an LED driver which will output 3A and it would have to produce nearly 160V to guarantee that they turn on, since each strip will require 19.6 volts and there are 8 of them in series. In this case, since each string is 1A, the power used by each strip is 1A x 19.6V, or 19.6W, and the total power is 24 x 19.6W = 470.4W. You could also figure it out by looking at the total voltage of each string and multiplying that by the current through that string, so 156.8V x 1A = 156.8W, and since there are three strips, the total is 470.4W – same answer.

On the other hand, if you arranged them in 6 parallel strings of 4 strips per string, your LED driver would have to be able to output 6A to give each string 1A (6A / 6 strings = 1A per string). Since you only have 4 strips in each string in this arrangement, the voltage required would be nearly 80V (19.6V x 4 in series = nearly 80V). The total power of all of the strips will be exactly the same doing it this way. Each strip will have 1A passing through it and it will take about 19.6V, for a total of 470.4W.

You can get the same light output a number of different ways. You can put all of them in parallel, but then you would need a supply which output 24A at 20V (not available), you could put them all in series and use a 1A, 480V supply (not available, and dangerously high voltage), or any number of options in between these two extremes (2 x 12, 3 x 8, 4 x 6, 6 x 4, 8 x 3, 12 x 2). As long as you can find a driver that can output the appropriate current and voltage for the arrange you choose, you will get the same light output and will use about the same power.

So, what, exactly is your goal with these questions? Are you trying to get the most light out of 24 of those strips? Are you trying to figure out how to best arrange them for a particular power supply, or what?

@tilopa108,

The “most efficiency” is not the same as the “most light”. Which of these do you want?

If you are assuming that an LED power supply will always push the full stated power into the LEDs, that is not the case. They will vary their voltage until the load draws their specified current level, unless the maximum voltage of the supply is reached before it reaches its specified current, at which point most supplies will maintain the maximum voltage at a reduced current.

The highest efficiency (or more precisely, efficacy, in the case of light output) comes from running each LED strip at low current, but you will get less light out of each strip that way. The most light comes from running each strip at high current, but your efficacy will be reduced this way.

So, if you run half the max current through each LED strip you will get just barely over half the lumens compared to running at max current, but it will take less than half the power, so it is more efficient.

If your goal is to get the most light out of a given number of LED strips (which is not going to be the most efficient because you will have to run them at high current to get the most light), state how many strips you want to use, and we’ll find the best LED driver for that purpose.

Yes, my figure of 118% was from just looking at the graph, and, of course, I’m just eyeballing it so it is not very accurate. But I see the way you did it is more accurate. It just looks like the region from 100% to 120% is a steeper curve, but maybe that is just an optical illusion.

My one concern is the wiring size, and potential voltage drop. I was going to use awg 18 solid core, but now that looks like it would not handle the 12A coming from the driver. Could I use a higher gauge (like 16) from the driver to the first wire nut (using wago 5 connector), which will connect the first 3 strips? Not sure how it works, 12A coming out of the driver, and each strip it hits reducing the current in the wire by 800mA? Maybe it is best to just use awg 16 for all of it?

Also, is there any concern about voltage drop with this low voltage high current situation, if I’m only going about 4 feet max wire distance from driver?

Lastly, with the hlg-240h-20b the b version lets me dim. So, if I dim it down it reduces current and thereby increases efficiency? Obviously it decreases wattage and lumens, but this is a nice feature because the other driver I was considering had a better efficiency but max power of 200w, so this gives me the option to have that same driver in one, so to speak.

Thanks again. Got my digi-key order in my cart, ready to pull the trigger before end of day.

Sticking with the 15 strips, you could also wire 3 parallel strings of 5 in series. This would reduce your wire losses to an insignificant level.

Something like the HLG-240H-CB or the HLG-240H-CA could work for this, as they output anywhere from 59V to 119V at 2.1A. This would drop your max current per strip to 700mA, and thus drop your total luminus flux back to 37,350 lumens, but it would be significantly more efficient, due to a slight efficacy improvement plus almost no wire losses (~0.3w in 10ft of 18AWG).

Both of these supplies are dimmable, the “B” version via PWM, 1-10V external dimmer, or resistance, and the “A” version via an integrated potentiometer.

For a little more money, you could also go with the HLG-320H-CA. This one outputs 2.8A maximum, but you can reduce that current via the integrated potentiometer, allowing for a broad range of output current.

If you want to learn more, please visit our website Custom LED Light Strip.