A review of LED function will help us understand when LED intensity will change. An LED will not flow current or produce light when the voltage across it is below its forward voltage. As an LED’s voltage approaches its rated forward voltage the LED current will start to flow current and the LED will produce light. As the voltage is increased above the point where current starts to flow the current quickly increases. As the LED’s voltage is forced beyond its rated forward voltage the current will continue to increase beyond its ratings to the point of LED failure. This is why LED fixtures supplied by a voltage source have some additional device to limit LED current.
One common current limiting device is a resistor. The voltage supplied to the fixture in excess of the LED forward voltage will be applied across the resistor. The amount of current the resistor will allow to flow is proportional to the amount of voltage across it. In this way, the brightness of the LED can be varied by varying the supply voltage. This can be an asset in applications where you want to vary LED brightness by varying supply voltage. It can be a detriment in applications where you want consistent light output among fixtures that might have slightly different supply voltages or the supply voltage varies over the length of the fixture.
Another way to limit current is with a current regulator. Like a resistor limit, the voltage in excess of the LED forward voltage is applied across the regulator. Unlike a resistor, a regulator flows a particular amount of current regardless of the voltage supplied. This is good for applications where you want consistent brightness with some voltage variation. Dimming by varying the supply voltage doesn’t work though.
Power dissipation is another issue to consider when comparing resistor limited fixtures to current regulated fixtures. Since a current regulated fixture’s current is constant as the voltage varies the power dissipated in the LEDs is also constant. Only the heat generated by the regulator increases with increases with the supply voltage. With a resistor limited fixture the current increases as the supply voltage increases above its intended operating point. This increases the heat generated in the LEDs and the resistor. To mitigate this effect a larger value resistor can be used but this increases the heat generated by the resistor under all operating conditions decreasing the efficacy of the fixture. Usually a resistor limited fixture will have lower efficacy than a current regulated fixture.
Current regulated fixtures can’t be dimmed by a DC voltage source but can still be dimmed by using an AC voltage source. This is because the supply voltage is not always above the LED forward voltage. With a 60Hz supply the LEDs will flicker on and off at 120Hz as this is how often an AC supply crosses through zero volts. When the voltage is supplied through a phase-cut dimmer the amount of time the supply is at zero volts increases and the time the LED is off increases and the brightness decreases.
Resistor limited fixtures can be dimmed by varying the supply voltage either DC or AC. Current regulated fixtures can not be dimmed by varying a DC supply but can be by phase-cut dimming an AC supply. The compromise for using a resistor limited fixture is that there may be less consistency between fixtures and lower efficacy compared to a current regulated supply.