LEDs have a very low internal resistance, so when you apply electric power to them, three things can happen:
- No Light: if the voltage is below the forward voltage of an LED, no current will flow, and there is no light emission.
- LED works: if the voltage is above the forward voltage, and the current is not exceeding the amount of power that the LED can convert to light, all is good.
- LED burns (destruction): if the voltage is above the forward voltage, and the current is exceeding a certain threshold, then the LED can no longer convert all of the supplied energy into light. The excess energy is converted to heat, and depending on the amount of excess energy, the LED will burn up and be destroyed immediately or over a course of a longer period of time (minutes to hours)
LED controllers are circuits designed to keep the supplied energy within the specifications of an LED.
A current limiting resistor is a very basic form of LED controller: the resistor takes the excess energy and converts it to heat. This works well for signal LEDs that only take a few mA of current. Resistors do not work well for more powerful LED, though: they would need to be capable of dissipating many watts of power, waste a lot of energy and get very hot.
LEDs can be controlled by constant current (CC) or by constant voltage (CV) power supplies.
Constant Current Controllers
Constant current is the best driver for LEDs. Here is why:
- Light = Current: the light emission of a LED is directly proportional to the current. By supplying a fixed current, you ensure that the LED is always emitting light of the same intensity. This can become especially important with RGB LEDs that mix colors out of red, green, and blue: if the light intensity of one of the colors changes, you will see a noticeable change in color.
- Resistance Varies: Voltage and internal resistance determine the current. A constant current supply continuously monitors the current, and if it changes, the supply changes the voltage to bring back the current to its target value. For LEDs, this automatic adjustment is important because the internal resistance of LEDs can vary based on production differences, and more importantly, the resistance is affected by heat. When LEDs are operated for a longer period of time, they heat up and change resistance. A constant current supply notices the decreasing resistance by sensing a higher current, and automatically reduce the voltage.
In a nutshell, what matters to LED is current, and a constant current supply controls exactly this parameter.
Constant Voltage Controllers
Constant voltage can also be used to drive LED: a fixed voltage is set that is causing the desired current to flow. Remember: voltage is the force that drives a current through resistance.
This would work well if the resistance was constant. However with LEDs, resistance is not constant: it can change i.e. when the LED heats up. That’s why constant voltage can only approximately set the current, and the real current will vary and change based on other factors like the LED temperature and its specific internal resistance.
Saving Money
Constant voltage supplies are technically simpler and cheaper than constant current. The variations in current that you get with constant voltage supplies may not be terribly high, so you might want to tolerate them in exchange for lower component cost.
WHenever exact brightness does not matter (i.e. decorative or ambient lighting), constant voltage supplys may be the most economical solution.
If exact brightness is important (as in RGB LED where slight changes in brightness can affect the emitted mixed color), or if you aim to drive a LED at its absolute maximum specifications, always use a constant current supply.
Powering Drivers, Not LED
Sometimes, LED and LED strips come with built-in electronics and use their own internal drivers (i.e. programmable LED like the WS2812). Cheaper LED strips may also use built-in current-limiting resistors. In all of these cases, you are not driving the LEDs. The LEDs are driven by the internal electronic parts. You are simply supplying power to these.
In such cases, you must use a constant voltage supply and supply exactly the voltage that the internal electronic components require. For example, programmable WS2812 LED typically require 5V and use an internal constant current driver to drive the actual LEDs.
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(content created Sep 08, 2024)