How Do Meanwell HLG LED Power Supplies Work?

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If you're working on an LED project, you'll undoubtedly require a power supply. Meanwell HLG series products are frequently recommended as a reliable product that feature constant voltage and constant current with useful built in dimming functions.

For the first time buyer, a quick look at their spec sheet and wiring diagrams will likely leave you confused. We'll explain the product and power features below.

Meanwell HLG Overview

Meanwell HLG LED power supplies are categories first and foremost by their output wattage that ranges between 40W and 600W. Within each wattage class, there are different DC voltage levels that range from 12V DC to 54V DC.

You can decipher these specs just by looking at their part numbers - for example, HLG-60H-24 refers to their 60 Watt, 24V power supply.

Constant Voltage and Constant Current?

Meanwell HLG products advertise "dual mode constant voltage and constant
current output" - which can cause some confusion as this product feature differs from other constant current or constant voltage products you may be familiar with, and their "hybrid" nature can make understanding its operation a bit more challenging.

A helpful thing to remember is that Meanwell HLG power supplies are first and foremost constant voltage power supplies, and operates in constant current mode under certain circumstances only. What do we mean by this?

Let us use Waveform Lighting's 24V LED strip product, paired with Meanwell's HLG-60H-24 as an example case. First, we note that the LED strip's spec sheet indicates a current draw of 225 mA per foot, which is the most important number to know when calculating the power requirements.

What happens when we connect 1 foot of the LED strip to the Meanwell HLG 24V power supply? The power supply will act like a simple, constant voltage power supply, and provide 225 mA of output at 24V DC. At 2 ft, it will provide double the current at 450 mA, at the same voltage level of 24V DC. In other words, its voltage remains constant.

When is constant current mode activated?

Continuing with our example, let's see what happens when we extend the length of the LED strip. According to the HLG-60H-24 spec sheet, the rated current is 2.5A. So, at 225 mA power draw per foot for the LED strip, a 10-ft LED strip segment's power draw (2.25A = 225 mA x 10) will still remain below the rated power supply current of 2.5A, and operate in constant voltage mode (i.e. 2.25A output at 24V DC).

What happens when we increase the the LED strip length to 12 ft? Our calculations would indicate that its current draw (2.7A = 225 mA  x 12) will now exceed the power supply's rated current. This would typically be a big no-no for regular constant voltage power supplies. A higher quality product may go into overcurrent protection mode and cease operation, and a lower quality one may simply burn out and be permanently damaged, and in some cases become a fire or electrical shock hazard.

Meanwell's HLG series, however, will continue to operate, but will enter constant current mode, with output current now being fixed at its rated current of 2.5A. But how could it continue operating 12 feet of LED strip that has a current draw of more than 2.5A?

The answer is that the HLG power supply, in constant current mode, will simply adjust the output voltage downwards, while keeping its output current fixed at 2.5A.

Remember that LEDs, and by extension LED strips, are diodes and resistors with a positive relationship between current and voltage. In other words, increasing the current will increase its voltage, and vice versa.

Hold on - you might be wondering - isn't the LED strip rated for 24V DC input specifically? Yes, it is, but operating it at a lower voltage level is completely permissible and safe.

In other words, the Meanwell HLG power supplies have a built-in overcurrent protection, which, when activated, is constant current operation mode. If, in our example case, the length of LED strip is continually increased such that its power draw also increases, the power supply will need to continue to lower its voltage so that the LED strip only draws 2.5A.

HLG power supplies also have a minimum voltage output for each specification. For the HLG-60H-24, the minimum output voltage is 14.4V DC - it is incapable of providing power below this voltage level.

The chart below shows the possible operation modes of Meanwell HLG-60H-24 when used with Waveform Lighting's 24V LED strip. The green region represents drive modes permissible based on the Meanwell specification. Each of the diagonal lines represents the various current-voltage combinations depending on the length of LED strip used.

HLG Type A

There is a final suffix to the Meanwell HLG power supply part number nomenclature, which designates its dimming function. For example, HLG-60H-24-A refers to Type A dimming, while HLG-60H-24-B refers to Type B dimming. No suffix letter indicates a non-dimming model - the following options do not apply.

Type A dimming allows output current and voltage to be "adjustable through built-in potentiometer." First, let's look at this in detail.

On the top side of Type A HLG power supplies, you will see two holes, marked Io ADJ and Vo ADJ. Inside of the hole is a trim potentiometer ("trimpot"), similar to the one shown blue. Use a screwdriver to turn the trimpot position clockwise or anti-clockwise to adjust the voltage and current levels.

But when we say "adjust voltage and current levels" - what exactly are we adjusting?

Recall that the HLG-60H-24 has a rated voltage of 24.0V, and a rated current of 2.5A. The trimpot allows for a limited range of adjustment of these parameters. Specifically, voltage can be adjusted to between 22V and 27V, while current can be adjusted to between 1.5A and 2.5A. (Reference spec sheets row 6 and 7). In other words, you can adjust the power supply to become a constant voltage 22V power supply that acts as a constant current power supply once 1.5A in current draw is reached.

Do note that while useful, Type A HLG's potentiometer method does not allow for a wide range of dimming, nor is it accessible to users.

HLG Type B

HLG Type B allows for dynamic and wider dimming control. How is this achieved?

Unlike Type A power supplies, HLG Type B includes a second pair of input wires, separate from the AC power source. This second pair of wires is used as an input signal to adjust the output brightness.

What type of dimming signal input is used? Meanwell HLG provides for three different options:

1) 1-10V Signal Input (Analog)

Commonly used in wall-dimmers and controls systems. (Not compatible with TRIAC dimmers). The input signal varies between 1V and 10V, and the resulting output DC power corresponds to the input signal. Note that the 1V-10V is a signal only - it is independent of the output DC power and not used to power the LEDs.

This is the most practical way to achieve dimming in a permanent home or commercial installation, as it connects to established wall-dimmers and allows for a reliable and aesthetically pleasing setup.

2) 10V PWM Input (Digital)

Instead of an analog 1-10V signal, a digital pulse-width modulated signal at 10V is used to dictate the output DC power. The duty cycle of the input dictates the output DC power.

This PWM signal is frequently confused with PWM dimming output - these are two different concepts. Here, we are referring to the input signal being PWM, and does not in itself indicate whether the LEDs themselves are pulsed using PWM (see below).

This approach is useful for users building their own microcontroller circuits, including Arduino and Raspberry Pi projects which have the digital PWM output capability.

3) Additive Resistance Input (Analog)

Similar to 1-10V above, this method uses a potentiometer or other form of variable resistance to adjust the dimming signal input. Unlike 1-10V dimming, where the voltage differential is provided externally, additive resistance uses the 10V signal provided by the DIM+ wire.

This is likely the most straightforward method requiring just a potentiometer to achieve the desired dimming outcome. It is most suited for DIY projects or simple testing of LED systems, but will require a bit of electrical wiring for a permanent installation.

Dimming Output

In response to the various dimming input signals, the HLG power supply will respond correspondingly with a different level of PWM DC power output. It is important to clarify that this PWM output is different from the PWM input signal discussed in method (2) above.

For more on how PWM works, read our article here.

This is a frequently misunderstood feature of HLG power supplies due to their marketing as "constant current" drivers - when dimmed, these power supplies use PWM to achieve the desired dimming effect. For some applications, this PWM output may introduce undesired flickering associated with PWM.

Finally, note that the DC power output ranges from 10% to 100% - so the HLG power supplies may be insufficient if you are trying to achieve a full 0%-100% dimming range.

Bottom Line

Meanwell HLG power supplies are excellent products with great performance and reliability (MTBF > 1 million hours) and unique dimming plus constant voltage + constant current functionality. The range of features can make the product somewhat challenging to understand, but by learning how the constant voltage and constant current transition occurs, as well as the dimming input signals, you will also be able to harness the power and flexibility of these products.

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