Today's Lesson

Some (Belated) LED Driving Definitions

It's been brought to my attention that I have been using a few terms throughout the Light Lessons that I have neglected to explain or define. An oversight on my part, but one that will now be rectified.

Constant Voltage

Well this one's not too complex. A constant voltage LED fitting requires a set voltage to operate. These fittings will contain some internal driver electronics that will regulate the current through the LED. Generally a constant voltage LED light will operate on a range of input voltages (as is the case with our range of LED strip lighting).

A constant voltage driver will supply a set voltage with no current control other than a maximum limit.

Constant Current

A constant current LED fitting is one that requires a set current to operate correctly. Generally these products contain no internal regulation electronics and an external current supply connects directly to the LEDs within the fitting. One such product is the ICON LED Downlight.

A constant current driver will supply a set current over a specific output voltage range (outside of this range the output current could be higher or lower). It is important that the LED fitting and driver that you use match in both the current specification and the output voltage range.

Difference between a dimming protocol, signal and drive

In a dimmable LED light setup you can have up to 3 separate signals. To try to reduce confusion, we will define them as follows.

Protocol

A protocol is something like DMX, DALI or C-Bus. A protocol is essentially the "language" between control systems and the lights themselves in networked installations. In order to then dim a LED fitting you require some "module" to convert from the protocol to a dimming signal or drive (this module could also be built into the fitting, such as in the Excalibur RGB flood light which features a DMX input).

Signal

A dimming signal cannot directly drive (or power) a LED fitting. It can be in various forms but in most cases it would either be 0-10V (an analogue dimming signal) or PWM (a digital dimming signal). The dimming signal would then either feed into the external driver (this is the case with the ICON LED downlight) or directly into the LED fittings (an example of this is the PowerLED strip)

Drive

The dimming drive signal "piggybacks" the dimming signal onto the power for a LED fitting. If the drive is constant voltage, then it will have a PWM output.

If the drive is constant current, then output may work by reducing the output current to correspond with the set level (from the protocol or signal) or it may be PWM.

PWM

PWM stands for Pulse Width Modulation. How it works (at least in terms of LED lighting) is it changes the ratio of how long the LED is on compared to how long it is off (this ratio is known as the duty cycle) while maintaining a constant frequency (remembering that a frequency is just a signal that repeats within a set time). In LED lighting this frequency is generally somewhere in the vicinity of 100Hz, this avoids visible flicker. You can get a fair grasp of how the duty of the PWM signal alters brightness from Figure 1.

Figure 1. How changes in teh duty cycle of PWM signal changes LED apparent brightness

The human eye averages this on and off cycle and creates an "apparent" brightness (this is the same basic theory behind how television screens work).

Active High/Low

On some LED drivers and fittings, you may see some references to active high or active low PWM. A PWM signal has two lines; an active high signal turns the positive line on and off, whereas an active low PWM signal switches the negative (or ground) line on and off.

Why PWM is used to Dim LEDs

PWM is the method suggested to dim LEDs by pretty much all LED manufacturers (I'm talking about the guys who make the small LED components, not the people who put them into lights). There are two main reasons for this.

• First, if you dim a LED by reducing its drive current, you can get a colour shift. By using PWM, the drive current is always the same, and therefore this cause for colour shift is eliminated.
• Secondly, if you dim by reducing the drive current, not all the LEDs will turn off at the same point, slight differences in the manufacturing process makes for small differences in the minimum current LEDs need to produce light.