Constant Current and Constant Voltage LED Differences

[Erik Sine]

I've been getting calls an emails about the differences between Constant Current LED's and Constant Voltage LED's, an why I stress the fact about CC so much. So, here is a quickie, basic to address those questions with a visual.

I may redo for a better one in the near future but here is one for now

[bonehead]

Excellent video Eric. Now I 'get it'. Thanks!!

[thomaserb]

Thanks for the video - so essentially a constant voltage LED module has an on-board switching regulator ?

And if you just powered the constant voltage LED with your power supply - you would see the current go up when the voltage went down.

[wizzy00]

excellent video Eric

[Erik Sine]

I think I probably stepped on my own hands on this one.

I didn't explain more in depth like I should have and maybe it's not that important for the sake of just keeping it simple but in the video I was just trying to show what the tolerance of LED power supplies can do to both Constant Current & Constant Voltage LED (12v-/+) strings for a sign system and for CC LED's basically nothing, you will always have the string with an on board current driver constant regardless of the voltage, like i.e long runs from string to power supply or other factor, or long strings of LED modules. The regulated current regardless of the voltage will keep each LED module consistent, unlike Constant Voltage LEDs.

Type 1) Non-regulated LED module (Constant Voltage).
- Often uses a simple resistor to adjust power through the LEDs on each module, or sometimes nothing at all
- Requires precise voltage throughout entire system, or a constant-current power supply
- Some vendors do a good job with this type, most get by, some do very poorly

Pros: Cheaper
Cons: Less consistent due to voltage fluctuations, voltage drop or inconsistent forward voltage of LEDs
Often over or under-driving the LEDs, even just slightly. Can cause loading issues for power supplies

Type 2) Current-regulated LED module (Constant Current).
- Uses on-board component to tightly regulate the current through the LEDs
- Operates on standard constant voltage power supplies, or within a voltage range (say 10-15V DC)

Pros: More reliable, more consistent brightness/power, modules not affected by change in voltage
Can work with any power supply within range, or on solar/battery based systems without issue
Cons: Usually cost a little more

There is also a lot to worry about when it comes to CV LED's because the manufacturer is relying on certain factor they cannot control or take into account which may come from the user.

Here is a really good explanation below from Marko which I think is the best about Constant Voltage LED's, which is the most widely sold LED line in the market:

LED modules that take the same voltage onto the board - this is constant voltage POWER SUPPLY.

When that power now meets the module, you can have a constant current driver on board which means the current is REGULATED. This is often referred to as on-board constant current - but it still works with a constant voltage power supply (typically 12VDC).

On an LED module that does not have on-board current regulation, you'll usually find a resistor instead. This is a linear, NON-REGULATED module. These also need a constant voltage power supply - also typically 12VDC.

The difference between the two is that the regulated module will send a specific amperage through the LEDs, regardless of the incoming voltage or whether the LEDs themselves have a tolerance in their own forward voltage. If the driver is set for 50mA per module, that's what the LEDs will get. They will all be exactly the same brightness and they will degrade evenly over their lifetime.

A resistor only design is less reliable because it is dependent on A) the incoming voltage to be accurate B) the LED forward voltage to be accurate and C) that there be no voltage drop in the system. You can't usually control all these variables, so what happens is if a resistor size is chosen for one of the variables, the other two will cause the current going through the LEDs to be wrong. They may be over or under driven and they will likely degrade differently from beginning of a string to the end.

For example - and this is in the simplest, non technical terms:

Risk #1) Say you have 3 white LEDs rated at 3.6V each on a manufacturer spec sheet. 3 of them on a module = 10.8V and so you have to select a resistor to work off that additional 1.2V of energy and allow a specific amperage through the LEDs. This means that if you have exactly 12V from the power supply, the modules will run at theright current. Power supplies usually have a tolerance on their output voltage, though, so you can't fully control this. Further, some power supplies are adjustable, which allows user error into the picture.

Risk #2) The second problem is that pretty much every channel letter module string will suffer DC voltage drop along the chain. The longer the string and the bigger the load, the more likely you'll have voltage drop. LED modules at the beginning of the chain will be the right brightness (assuming exactly 12V), while others will be less and less bright the further you go down the chain. They will be underpowered and degrade more slowly than the ones at the beginning. The sign faces may not have a consistent brightness as well.

Risk #3) This is one that some LED manufacturers don't even realize. The spec sheet might say 3.6V, but LEDs have a tolerance from batch to batch too - and further still - they are almost always lower than the sheet. Most White LEDs from Nichia or Cree or Samsung are actually around 3.1-3.2V each. This is a problem if you're expecting 10.8V but actually using LEDs that add up to only 9.3V total. Using a resistor and being off by 1.5V can mean you're over-driving your LEDs by 50-100%. Yes - double! So while the LEDs will look real bright, they'll die out a whole lot faster. The power supply may not allow this much power through the LEDs - it will hit a limit at some point depending on how it's loaded (or overloaded) - but you'll kill the power supplies this way as well.

Solar Projects - Constant Current LEDs vs Constant Voltage

A further note on good-vs-bad modules... if anyone is doing a solar project where batteries are used, note that the typical 12V battery is actually 13.5-13.8V and that by using a resistor based LED, you'll not only be over-driving the LEDs but also over-loading your battery system. It's critical to go to a regulated current module when doing solar.

The Analogy of Constant Current / Constant Voltage

The best way to understand the difference between a regulated LED module and a linear LED module is as follows:

The LED module is a boat.

Voltage is the depth of the water and Amperage is the speed of the boat. A Constant Current-regulated module is like a boat with a pump that keep the boat at exactly the right buoyancy and speed. It won't go faster or slower based on how it rides in the water, or how fast the water is flowing.

A resistor based module (Constant Voltage) is like a canoe that will ride higher in faster water - which wears the boat down more quickly - or a boat that takes on water and slows down when the water level is low. It is reliant on the water moving at exactly the right speed and the weight of the passengers to be exactly perfect.

[burkeenterprise]

Does this mean that the constant voltage LEDs that are used in a dimming system (as in a building using lutron controls for the lighting) will be brighter at the beginning of the string and dimmer at the ends?

Erik we may have a problem with the casino project we have been talking about!!!!

Can I get by by making more home runs to the power supply and feeding from the middle of each string?

[Erik Sine]

Bob,

The dim rate will depend on the number of modules you have on a string, and based on that if it will be noticeable. Constant Voltage LED's are best with more branched circuits or strings. You can always get in-line splicers and divide a string right in the middle.

That's part of the pro's and con's of CC and CV LED's, the advantage CV has over CC would be for your application.....dimming. Which cannot be done with Constant Current LEDs