Control Protocols

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Different control protocols are used to communicate between the components of a lighting control system. Until recently, most of these involved analog signals, but digital control is becoming more popular and offers the potential for more consistent performance across products and, in certain systems, provides the flexibility to reconfigure control zones without changing the wiring configuration. In the case of dimming, the control protocol is used for communication between the wallbox dimmer and the dimming module, or the ballast in the case of discharge sources such as fluorescent or HID lamps.

Available protocols include:

  • Analog 0-10V signal
  • Two-wire
  • Three-wire
  • DALI (Digitally Addressable Lighting Interface)
  • Proprietary Digital Systems
  • Wireless Control

0-10V Analog Control

0-10V analog control has historically been the most popular means of communicating the desired dimming level to fluorescent ballasts. This system has been applied to both wall box dimming control and photosensors that adjust electric lighting systems in response to the available daylight. The ballast has connections for two wires that deliver a control signal in addition to the standard power line and neutral. Unfortunately, no standardized voltage to light output relationship exists to which all such ballasts and systems must conform, so ballasts from different manufacturers do not begin dimming at the same voltage (which is typically around 7 to 8V), nor do they provide the same light output at a given voltage. This is not important for wallbox dimming but can be important for photosensor-based control.

Two-wire Control

Two-wire control applies either pulse width modulation or phase control, which involve modifying a square and sine voltage wave, respectively. Pulse width modulation involves varying the width of a square wave that delivers voltage to a source, and is commonly applied with solid state lighting (LEDs). Phase control can be arranged to cut out either the leading edge or the trailing edge of the voltage sine wave on a circuit. For low-voltage halogen luminaires, leading edge dimmers are typically applied with magnetic transformers and trailing edge dimming with electronic transformers. Leading edge phase control commonly cuts out voltage from the zero crossing point onward to reduce the RMS voltage delivered to a source. This type of dimming is also applied to two-wire fluorescent dimming ballasts. Two-wire dimming is relatively easy to apply in retrofit applications since it can make use of existing system wiring.

Three-wire Control

Three-wire fluorescent dimming is a proprietary control system that provides phase control dimming via a third wire, which is often referred to as the dimmed hot wire. Since the voltage powering the lamps is not carried on this wire, the dimming hardware can be much smaller. Control of fluorescent ballasts to below 1% of full light output is possible. This means of control is more suited to new construction since three wires must be run from the dimming control device to a ballast.


DALI is a standardized digital control protocol that permits manufacturers of different system components, such as ballasts, wallbox dimmers, photosensors, occupancy sensors, relays, etc. to create products that can function interchangeably on a digital control loop. Some of the basic features of a DALI control system are listed below. A DALI control bus can have up to 64 different connected devices (switches, dimmers, sensors, ballasts, etc.). Each device on the bus has a digital address and consumes some of the power provided on the control loop, which is delivered through a power supply and is quantified in mA of current. The mA provided by the power supply on a DALI loop should not exceed the combined mA draw of all the devices on the loop. Distributed intelligence is provided by a microprocessor located within each connected device (such as a ballast, switch, scene control, etc.), permitting the devices to both listen and respond to commands and to return data when queried. One of the unique features of a DALI control system is that control zones need not conform to how the equipment is wired. Ballasts on different electrical circuits can be assigned to the same control zone since each is addressed individually through the DALI network. It is also possible to arrange equipment so that it is switched together by wiring it together on a DALI relay, but this is less common and reduces the overall flexibility of the system. A DALI network can also recognize faulty lamps or ballasts and report these to a central command/control center for prompt maintenance. The commissioning process to setup a DALI network involves assigning each controlled component, such as a ballast or relay, to one or more control zones or scenes. The ease with which this task can be performed (and then modified at a later date) is determined by the software used to operate and manage the DALI network. A DALI bus can have up to 16 different control zones and up to 16 different scenes. A control zone can be thought of as a group of lighting equipment that is controlled together, either from a switch or through a single command that is distributed across one or more busses. A scene is a fixed arrangement of different lighting equipment which may be applied at different on/off or dimmed settings and is activated by a single command to the DALI busses.

Proprietary Digital Systems

Proprietary digital control systems are also available to permit different equipment such as occupancy sensors, photosensors and switches or dimmers to talk to relays and dimming hardware. One advantage of these systems is that they can be easily adapted to retrofit situations, adding additional control capability to meet current building and energy codes. The control wiring may be Cat5 cable which does not need to be run in conduit and is therefore easy to install.

Other proprietary systems operate similar to a DALI control system. Some apply the DALI protocol with additional features that are not formally included in the DALI protocol.

Wireless Control

Wireless systems are also now available. Wireless devices include occupancy sensors, photosensors, and wall station control. In these systems, control signals are passed between devices by low power radio waves instead of through control wires. Many of these remote devices (occupancy and photosensors) are powered by batteries, which can last up to 8 or 10 years, or may be powered by an integrated photovoltaic element. Some wall devices are powered by the kinetic energy of flipping a switch and therefore don’t require a battery. By avoiding the need to run wires to many of the control system’s components, wireless systems can save time and installation costs, and are particularly well-suited to retrofit applications where minimal disturbance to the existing facility and its occupants is necessary.

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Lead Author(s): Rick Mistrick

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