Understanding the technical ins and outs of available dimming systems.

» In just about every lighting design, the final detail is the ability to tune the light. If the lighting scheme permits adjustments, such as lenses or color, applying them is a fine art in skilled hands. But dimming can also create the perfect scene. To many, the concept may seem quite simple: to dim the lights, add a “dimmer.” In reality, while a dimmer can be as straightforward as a wall-switch replacement, it can also be as complex as a multi-channel scene controller, a theatrical dimming system, or a programmable facility management system. Add to that the latest generations of dimming ballasts, and modern-day dimming is no longer an easy bet.

An incandescent lamp is dimmed when it operates at a lower voltage than that for which it was designed. As voltage decreases, lamp power and lumens also decrease accordingly. Many peopl· who understand basic electric circuits think incandescent dimmers are rheostats, a type of variabl· resistor. Not true—in fact, the modern incandescent dimmer is actually a solid-state switching device that turns the light on and off 120 times per second. The combined thermal mass and persistence of the filament smoothes out the pulses of power so they are imperceptible. The human eye sees a brighter or dimmer light, depending on the proportion of “on” to “off” time. In other words, although the actual voltage is still 120, the average voltage is reduced. This is called phase-cut dimming. There are two primary types:

• forward phase-cut dimming, in which the lamp is energized only during the last portion of each power-line half cycle. Forward phase-cut dimming is cheap, uses robust electronics, and is suitable for most loads, including regular incandescent lamps and magnetic transformers, as well as neon, cold cathode, and some types of fluorescent dimming ballasts and LED power supplies.

• reverse phase-cut dimming, in which the lamp is energized during the initial portion of each power-line half cycle. Reverse phase-cut dimming is more expensive because it uses more complex electronics, but some loads, especially electronic transformers, operate better and mak? less audible noise when this type of dimmer is used.

In either case, the actual lighting load operates at the power-line frequency (60 hertz in North America and Japan, 50 hertz in most of the rest of the world). Sometimes noise-reduction devices, such as debuzzing coils, are needed to eliminate the hum that these systems generate.

Electronic dimming ballasts, on the other hand, convert the alternating-current power line (50 to 60 hertz) to direct current, and then to a different frequency (usually over 30,000 hertz) to operate the lamp. During the conversion process, it is relatively easy to regulate the power to each lamp, but in order to signal the ballast to dim the lamps to a particular level, several protocols have evolved:

• three-wire analog power dimming These circuits incorporate a special type of forward phase-cut dimmer with three wires (dimmed hot, switched hot, and neutral) to control specific three-wire electronic dimming ballasts. Both Lutron and Lightolier make fluorescent ballasts and controls using three-wire circuits. Three-wire ballasts connect to a wide range of dimmers, such as wall-switch and architectural dimming systems; however, specific dimmers for three-wire ballasts must be used. The three-wire system is the oldest protocol and offers some of the best dimming ballast performance. (See diagrams on page 87 for visual clarification of this and the following three scenarios.)

• two-wire analog power dimming A few ballasts have been cleverly designed to use forward phase-cut two-wire incandescent dimming circuits, principally the Advance Mark X and Lutron Tu-Wire fluorescent ballasts. This system is handy because it employs relatively inexpensive dimmers and is wired using a conventional switched lighting circuit with two wires.

• four-wire 10v analog signal dimming The most common dimming ballasts have two power wires (switched hot and neutral) and two wires that connect to a low-voltage circuit. This is probably the most universal ballast protocol available today for U.S. and European fluorescent lamp ballasts, as well as for some of the latest HID electronic ballasts. For wall- switch dimming, sp9cial dimmers or interfaces must be used, but simple interfaces are available for most architectural and theatrical dimming systems, and for a wide range of daylight sensors. Note that these ballasts cannot turn themselves on and off like DALI ballasts, so in addition to the low-voltage-level control circuit, there must be a line-voltage switch as well.

• dali digital signal dimming European ballast companies recently joined together to develop a digital protocol called the Digital Addressable Lighting Interface (DALI). In a DALI system, the ballast connects only to power and to the DALI bus; on/off switching, as well as light-level control, occurs in the ballast. There are DALI-controlled conventional dimmers and low-voltage electronic transformers as well, essentially eliminating wall-switch dimmers and dimming cabinets as we know them. Control-initiating devices are also digital, allowing the use of everything from pushbuttons and touch switches to daylight sensors and LCD control stations.

• other digital dimming systems Several other digital dimming systems can be found in the marketplace. For example, Universal Ballast offers the AddressPro dimming system, also sold as the Cooper DLS system. In Europe, several companies make products using the Digital Serial Interface (DSI) system. Some products work on both DSI and DALI. Like DALI, these systems include dimming ballasts, conventional dimmers, dimming electronic transformers, and sending devices.

In addition, there are communications systems and protocols to which lighting systems connect. These range from general-purpose communications systems like TCP/IP Ethernet and IEEE 485, to theatrical lighting controls using DMX-512 and building-technology-specific controls like the European Installation Bus (EIB) and BacNET, the U.S. equivalent. (These will be discussed in more detail in Part Two of this series.)

The lighting specifier is fundamentally responsible for ensuring that lighting loads and dimming ballasts are compatible with the intended control system—no easy task considering the many complex options available today.

This point is critical: NONE of the dimming approaches listed above is fully compatible with ANY of the others. If you carelessly specify “dimming ballast” these days, be prepared for an unhappy situation. For instance, the two-wire dimming ballast and respective dimmer is often the least expensive. But I don’t know of a single daylight sensor that can be connected directly to this type of ballast without first being processed by some type of architectural dimming system. Similarly, the generic 10V dimming ballast cannot operate on a standard wall-switch dimmer, but instead requires a specific and harder-to-find dimmer or an interface box. Contractors usually do not spot these problems until installation time, which then involves a rush to get new parts and perhaps some rewiring. But these are the ballasts that are needed to interface to most daylighting photosensors.

As a basic rule, decide at the beginning of the project which dimming approaches will be used. If the design employs control devices from multiple manufacturers, especially automated daylight sensors, it is best to use the 10V analog system, as there is reasonable compatibility among ballasts and control devices. However, if the design involves a basic manual dimming system using wall-switch devices, or if it is based around an architectural dimming system consisting of multiple dimmer cabinets or a digital system of any kind, specify ballasts and dimmers as recommended by the control system manufacturer to ensure compatibility, and do not allow substitutions. Note that to help alleviate this problem, some companies make dimming ballasts that can be connected in two or thre¿ different ways. For example, there is at least one ballast that can be connected to a 0V–10V analog or two-wire dimming circuit, making it compatible with most analog circuits.

Perhaps a bigger question is which type of control system to use for each project. The new digital generation of lighting controls makes the choice of dimming system that much more difficult. Part Two of this series will examine the applicability and performance of these different dimming systems, including some critical issues professional designers must address when implementing them. James Benya

James Benya is a professional lighting designer and principal of Benya Lighting Design in West Linn, Oregon. He is an editor-at-large for A|L.

part two will appear in the Sept/Oct 2006 issue.