Dimming LEDs from Centralized Control AC Power Supply
LED dimming can be achieved through PWM (pulse width modulation). By adjusting the duty cycle of a "non-visible" pulse train, the average light output will appear to vary when viewed by the human eye. During each pulse (when the LEDs turn on) the current remains constant, even while at a dimmer setting. This method is preferred over a current control circuit for several reasons. Mainly, the PWM routine is very easy to control though an on-board microcontroller, as where a varying the current would require more complex circuity. Additionally, the PWM dimming method will provide a consistent output between opposing LEDs, while operating at any level. A linear or current control device would possibly result in some LEDs appearing brighter than others, which is undesirable in most applications.
You will need to have two seperate feeds routed from the LEDs back to your centralized control AC power supply. The first will branch off to feed current to the LEDs. It is a good idea to include a local constant current source for each LED circuit. This means your main AC power supply does not even have to provide an constant current output, since this function is provided at the board level by an array of constant current sources in proximity to each LED string. Also be sure to select a power supply with a DC output, not AC. You may also require a capacitor bank in proximity to the LEDs. This will supply current directly to the LEDs during their initial rise.
The second function required is the PWM controller. This may be incorporated into the centralized AC power supply, but does not have to be. The PWM unit could also be a separate unit altogether. The purpose is to output a pulse train via a signal line, to each external LED group. If transmitting over long distances, it is important to consider the possibility of noise on the data line. This may be a result of an induced signal from an outside source, or ringing. The proper filtering components can help to reduce noise. The receiving end of the data line will need to interface with each constant current source. If you select a current source IC with an enable pin, this task becomes very simple. An alternative option would be to use a separate switch to activate each LED group, such as an N-channel MOSFET. Using the field effect transistor as a low side driver will allow you to switch each LED bank on by connecting the PWM data line directly to the gate of the transistor.