Linear Fluorescent Dimming Ballasts: Technology, Methods, Protocols

By Craig DiLouie, Lighting Controls Association

Published August 2004

Dimming linear fluorescent lamps can provide a number of significant benefits to owners of commercial lighting systems:

• Flexibility, enabling the lighting system to adapt to multiple activities and changing space needs.
• Cost savings, derived from direct energy savings as well as load reduction during peak demand periods, which can be accelerated by using dimming ballasts in a system that can also include occupancy sensors, daylight sensors and time-clocks.
• Higher worker comfort, satisfaction and performance, achieved by allowing occupants to choose their own light levels.
• Increased lamp life for applications where lamps can be dimmed instead of frequently switched.

Dimmable fluorescent systems combine the long life and energy efficiency of fluorescent lamps with the controllability and full-range dimming capabilities of incandescent systems. In this white paper, we will discuss how linear fluorescent lamps are dimmed, then compare popular methods for dimming with pros and cons of each.

Dimming ballasts

Linear fluorescent lamps produce light when an arc of electric current is established across the lamp from one cathode to the other, causing the gas to emit energy that is converted into visible light by the phosphor coating the inside of the glass bulb. Fluorescent lamps require a ballast to operate, an electrical device that provides the proper starting voltage to initiate the arc and then regulates the current flowing through the lamp.

The ballast can be configured so that it 1) receives a signal from a control device and subsequently 2) changes the current flowing through the lamp, thereby achieving a gradual controlled reduction in lamp output. The characteristics of the control signal affect the duration and extent of the change in current and subsequent lamp output.

Dimming ballasts are available for operation of linear and compact fluorescent lamps. In this article, we will focus on linear fluorescent lamps.

Most commercially available dimming ballasts for operation of these lamps are electronic rapid-start or programmed-start ballasts, and all linear lamps operated by these ballasts feature bi-pin bases typical of rapid-start lamps.

Rapid-start ballasts preheat the cathodes with a small voltage, which reduces the amount of voltage needed to start the lamp. After preheating the cathodes, the ballast provides the high voltage required to initiate the arc.

Programmed-start ballasts are rapid-start ballasts that preheat the electrodes more accurately to minimize damage to the electrodes during the startup process (according to a program) and therefore can optimize lamp life. While supplying the preheat voltage, the ballast minimizes the lamp voltage, thereby reducing glow current during this phase with its associated degrading effect on lamp life. As a result, programmed-start ballasts can provide up to 100,000 starts, ideal for applications where the lamps are frequently switched, such as space with occupancy sensors.

Dimming methods: analog vs. digital

Several methods can be used to achieve the dimming effect. Because the dimming ballast must be able to communicate with connected controllers, the method becomes the basis for a protocol, or common operating parameters adopted by all manufacturers of dimming ballasts and controllers that use that method. This assures interchangeability between the ballast made by a particular manufacturer and various controllers made by controls manufacturers. Check the ballast manufacturer for compatibility between its ballasts and various controls.

The primary methods are:

• Analog: The analog electronic dimming ballast includes components that perform these functions: electromagnetic interference filtering, rectification, power factor correction and ballast output to power the lamp. There are several analog methods, including 0-10VDC, two-wire phase-control, three-wire phase-control and wireless infrared, with 0-10VDC being most popularly used.
• Digital: The digital electronic dimming ballast includes components that perform these functions: electromagnetic interference filtering, rectification, power factor correction, a micro-controller and ballast output to power the lamp. The micro-controller functions as a storage, receiver and sender of digital information. The micro-controller can store the ballast address, receive control signals and send status information.

Analog dimming systems are established and common, while digital dimming systems are relatively new to the industry. Both provide the essential function of controlling the lamp output based on input from a control device. Both enable the construction of networks of controls and ballasts wired to local and central points where control signals can originate, either manually or based on a program.

Analog is the standard dimming method, typically presents a lower cost, and is compatible with a wide range of common control devices. The dimming ballasts can be on a low-voltage or line-voltage control circuit. Analog ballasts and controls are compatible as long as they are configured to the same method—e.g., 0-10VDC, etc.

Digital provides a higher degree of granularity of control capability, such as ability to individually address and group the ballasts, gain feedback information from ballasts, manage a variety of zones and scenes, and provide a lighting system that can easily accommodate changes over time.

Protocols

Dimming ballasts must be configured to understand and act upon the control signal coming from a control device over either low- or line-voltage wires. To ensure compatibility, protocols have been developed around the various dimming methods.

It should go without saying that items designed to operate on different protocols are not compatible and should not be operated together. Doing so will result in the dimming system failing its purpose, as well as potentially damaging the equipment.

Analog: There is currently no standard for the operation of analog dimming ballasts. While there is a 0-10VDC control ANSI standard for the entertainment industry, it does not apply to dimming ballasts. As a result, equipment may work well together as a system but dimming performance may not be consistent among different ballast types and ballasts made by different manufacturers. A 5V signal for one ballast might result in a 50% dimming level but 30% on another, for example.

Digital: For digital ballasts, the Digital Addressable Lighting Interface (DALI) protocol, part of Europe’s IEC Standard 60929, provides a standard. DALI offers the possibility of true interchangeability between ballast manufacturers and defines light output for all levels of dimming signals, ensuring consistent dimming performance across all dimming ballasts regardless of type or manufacturer. This ensures that different ballast types can mingle in the same control area and simplifies commissioning.

Methods/Interfaces

The dimming method is an important consideration, since it often defines the range of possible change in the lamp output and also the wiring configuration, which in turn affects capability as well as cost. As with everything in lighting, there are tradeoffs.

Digital: Digital ballasts are recommended to use a Class 1-rated 5-conductor cable that uses one hot (live), one neutral, one ground and two polarity-insensitive control wires, all routed together in the same conduit. It is also possible to install the ballasts and controls as a Class 2 installation, in which case the control wires must be routed through separate conduit as the power wires. Check with the ballast and controls manufacturers whether their products are rated for Class 1 installation.

Manufacturers of DALI-based digital ballasts include Advance Transformer Co., Lutron Electronics, OSRAM SYLVANIA, Tridonic USA and Universal Lighting Technologies.

The other digital protocol is proprietary, developed by Energy Savings Inc. (ESI), which was purchased by Universal, whose digital ballasts are now marketed under the AddressPro brand.

Analog (0-10VDC): Dimming is accomplished by controlling the amplitude of the current flowing through the lamp via reduction in the lamp power. As lamp power decreases, lamp voltage increases proportionally to maintain heating of the lamp cathodes and prevent the lamp from being extinguished.

0-10VDC ballasts use four wires, with two line-voltage leads (hot and neutral) to power the ballast and two low-voltage control leads to change the light level. Depending on wire insulation and control switch ratings, the control wires may either be routed in the same raceway (Class 1) or in a separate raceway (Class 2). In general, the system may be installed as Class 1 if the control wires carry the same voltage rating as the power wires and the control device is rated for Class 1.

This wiring scheme adds labor and material costs to the installed system cost, but enables the dimming ballast to be linked to other ballasts and control devices in a larger system, which in turn can be linked to local occupant controls and central control.

Typically, 0-10VDC ballasts have violet and gray control wires. The gray wire is internally connected to provide a ground reference.

When the voltage level is near or above 10VDC, the ballast responds with full light output. As the voltage decreases, the ballast reduces light output. The ballast can also be connected to a switch or relay to enact bi-level control, providing full light output when the switch opens and reducing it to a specified minimum when the switch closes.

Note that some manufacturers provide command regions in the 0-10VDC range; a signal less than 0.3V might signal the ballast to shut down, for example. Be sure that the specified controllers are compatible with any such added feature for the chosen ballast.

Manufacturers of 0-10VDC dimming ballasts include Advance (Mark VII), Lutron (TVE), OSRAM SYLVANIA (Quicktronic Helios and PHO-DIM), Universal (Ballastar, SuperDim) and GE.

Analog (Two-Wire Phase-Control): Also called AC dimming, phase chop dimming or two-wire dimming, phase-control dimming entails “reading” the AC power supply signal’s “starting point” or zero crossing point, then turning on the current after a preset waiting time. This “cuts out” part of the cycle and results in dimming. The extent of the waiting time, usually 0-8.3 milliseconds or one-half the waveform, is related to the dimming level.

Phase-control ballasts use the same two line-voltage leads for both power and ballast control. The ballast receives the dimming signal through the dimmed hot wire connected to the power line.

Because the standard wiring configuration is utilized, phase-control dimming ballasts represent a lower-cost dimming solution, typically found in architectural dimming applications such as conference rooms, boardrooms and individual offices. It is also ideally suited to retrofits, stand-alone applications and cost-sensitive projects. In addition, the control signals are less sensitive to interference than low-voltage analog signals.

Currently, Advance is the only manufacturer that offers a full line of phase-control dimming ballasts (Mark X Powerline). Lutron makes available a limited offering (Tu-Wire). OSRAM SYLVANIA and Universal Lighting Technologies have discussed developing such ballasts and offering them in the near future.

Analog (Three-Wire Phase Control): The three-wire phase control configuration is based on the original magnetic dimming ballast conventions from the 1960s. This control method uses a third wire (in addition to hot and neutral) to carry the (typically) phase control signal to the ballast. All three wires are rated Class 1 and can be run within the same conduit. Lutron manufactures three-wire phase-control dimming ballasts (Hi-lume, Compact SE and ECO-10).

Wireless Infrared Control: Some manufacturers also have wireless infrared control available. This method uses an IR transmitter to perform the control function and does not require any additional wires. The dimmer is included either in the ballast or as an additional device in the light fixture. This is a good retrofit solution, and allows for occupant fixture control. These types of ballasts are available from Lutron (ECO-10).

The best solution (when choosing analog ballasts)

A major difference between the three main analog dimming ballasts is the equipment required to control them. They are all “hard-wired” to the control circuit or zone, and one control device can control one zone. All of the ballasts wired to the same purple and gray wires (0-10VDC) and wired to phase-cut dimmed leg for two- and three-wire control will be controlled together. For building-wide control, these control wires must be connected to some type of dimmer which is then connected to the other dimmers and some type of building-wide network, presumable with some type of central control. There are two main system topologies for this system: Centralized: All dimming control wires for an area are pulled back to a dimmer cabinet or cabinets mounted in the electrical closets, and then these dimmer cabinets are connected together and to a central controller via a network. Distributed: The dimming control wires are connected to a device that is mounted nearby, such as on the wall or in the plenum, and then these control devices are all connected together and to a central controller via a network. Either topology can be used to achieve building-wide control.

0-10VDC Two-Wire Phase-Control Three-Wire Phase-Control Wireless Infrared

0-10VDC: 0-10VDC ballasts have the advantage of needing only small low-voltage components in the control device, so they are easiest to use in a “distributed” system. 0-10VDC control allows the on/off control to be separated from the dimming control, allowing a combination of centralized switching and distributed dimming equipment to be used.

Two-Wire Phase-Control: Two-wire phase-control ballasts have the advantage of not needing any additional wiring between the control device and the ballast, which makes them very attractive for new centralized dimming applications as well as retrofits. They also don’t require a separate switched power leg, so the hardware required to dim these ballasts is exactly the same as the hardware required to dim incandescent loads. This means that most (if not all) dimmer manufacturers include a way to adjust the dimming curve of their dimmers to allow the control of two-wire ballasts from their dimmer cabinets.

Three-Wire Phase-Control: Three-wire phase-control ballasts draw very little current on the dimmed leg, which means that they can be dimmed without causing much heat to be generated at the dimmer. This allows devices that are intended only for this type of load to be smaller and also appropriate for use in a distributed system.

While considering all of the factors, the best solution for any given application, of course, depends on the application need. For example, is the primary goal energy savings, visual need or some other application need? What kind of dimming performance is required—100% to 1%, 5% or 10%? The choice of dimming ballast often comes down to specifier preference, dimming system compatibility, total installed cost (including wiring), and availability for the fixtures being used.

Finding the best solution (all ballasts)

The major dimming methods are compared side by side in Table 1, which is viewable by clicking here.

Dimming issues

Important issues related to dimming include perceived brightness, perception of light level reduction, power quality and energy consumption.

Perceived Brightness: As lamps are dimmed, light level decreases but the human eye may perceive a higher light level than is actually recorded by a light meter. This yields the “square law” curve, the theoretical relationship between measured light level and perceived brightness:

Perceived Light (%) = 100 x square root (Measured Light (%)/100)

Consider this example (courtesy Lutron): At full brightness, the measured light level is 60fc. At the lowest dimmed level, 10% perceived light is desired:

• 1% measured light (0.6fcd) is perceived as 10% (desired result)
• 5% measured light (3fcd) is perceived as 22% (2x brighter than desired)
• 10% measured light (6fcd) is perceived as 32% (3x brighter than desired)

Perception of Light Level Reduction: A dimming issue for some applications is at what point in the change in light level will occupants notice the change.

The Lighting Research Center studied the relative threshold for detection of gradual reduction in light levels. Four sessions were conducted. Sessions A and B were conducted in a room with more than twice the light level of Sessions C and D.

The results are shown below:

Detection of slow light level reduction. Courtesy: Lighting Research Center.

The A, B curve shows:

• More than 90% of the population would not notice a 10% reduction in lumens
• About 75% would not notice a 15% reduction in lumens
• About 55% would not notice a 20% reduction in lumens

The Lighting Research Center concluded that since the subjects in the experiment were aware that the light level was about to change, which does not match real world conditions, the experiment results can be considered a maximum.

Power Quality: Total harmonic distortion (THD) has been reported to increase on 0-10VDC dimming ballasts as lamp output decreased (Specifier Reports: Dimming Electronic Ballasts, Lighting Research Center, October 1999).

Max. THD of less than 3% at full light output, for example, increased to a max. THD less than 25% at minimum light output. The increase in THD in turn decreased power factor—to a pronounced degree in some ballasts.

The Lighting Research Center concluded that since THD is a percentage of the fundamental current, a high THD at low fundamental current levels associated with low light output levels may not be a concern, as the actual distorted current is small.

Phase-control ballasts also experience THD, but the extent is unknown; in the 1999 Specifier Reports, Advance reported that their ballasts experienced less than 10% max. THD at full light output, but claimed that current THD and power factor at minimum light output depends on the control device used as well as the ballast.

Energy Consumption: Dimming ballasted lighting system may require higher wattage to operate than fixed light output systems, and do not experience an even lumens-to-wattage reduction. As an illustration, consider a fixed light output ballast powering two F32T8 lamps (see Table 2); the lighting system draws 65W of power.

A 0-10VDC ballast requires higher wattage to operate, and at 3% lamp output consumes 19% of the full input wattage. A phase-control ballast also requires higher wattage to operate, and at 5% lamp output consumes 22% of the full input wattage.

Note also that shorter lamps are less energy-efficient than longer lamps in dimming applications; each lamp has two electrodes that require the same amount of heating, but represent a larger percentage of the power consumption for the smaller wattage (shorter-length) lamp.

Table 2. Comparison of two 120V fixed light output (2) T8 lamp electronic ballasts from Advance Transformer with a 120V (2) T8 lamp 0-10VDC dimming ballast and a 120V (2) T8 lamp phase-control dimming ballast.

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