Fluorescent dimming is a new frontier in lighting made popular by the industry's increasing interest in extending the technology's capabilities beyond the traditional architectural and conference room applications. The strength of dimming lies in its ability to save energy and enhance productivity by providing occupants with personal dimming control. According to one ballast manufacturer, fluorescent dimming systems can provide as much as 65% in energy savings when compared with standard T8 fixed output systems. And according to studies conducted by the Light Right Consortium, the Lighting Research Center, and other lighting organizations, occupants report greater job satisfaction and demonstrate improved performance when given the ability to dim the local lighting to their own preferences.

A basic fluorescent dimming system includes a dimming ballast and a control device. These items, in turn, may need to communicate with other equipment and systems, such as occupancy sensors and building management systems. To ensure interoperability between these disparate parts, a protocol is necessary.

When choosing a fluorescent dimming ballast, engineers are often confronted with a choice of protocols. Ballasts and dimmers must be properly matched with the same protocol, or they won't work together and could even become damaged during the attempt. The first choice is between digital and analog technology.

Digital vs. analog. Digital technology, which is based on protocols like the digital addressable lighting interface (DALI) or BACnet, makes it possible to address and control each digital ballast separately on a low-voltage network. The digital option is essentially in an early adopter phase, and as such hasn't developed very deep market penetration yet. It's suitable for applications that require a high degree of individual ballast addressability and control or a high level of flexibility to easily provide for future lighting changes. Digital dimming ballasts are offered by at least three well-known manufacturers.

Despite the current buzz over DALI's capabilities, analog is still overwhelmingly used, so engineers seeking the benefits of dimming need to understand analog-based options. Analog technologies used for dimming electronic fluorescent ballasts include 0-10VDC, phase-control, and proprietary protocols. The 0-10VDC and phase-control protocols are open protocols, and therefore allow interoperability between components from different manufacturers. On the other hand, each item that makes up proprietary control system is only compatible with other items made by the same manufacturer.

0-10VDC. Standard 0-10VDC controls are readily available from a number of manufacturers, and all major ballast manufacturers make 0-10VDC dimming ballasts. The dimmer sends a dimming or brightening control signal through a pair of 0-10VDC control wires to the lamp via the dimming ballast. Dimming is then accomplished by restricting the amplitude of the current flowing through the lamp. Lamp voltage increases with decreasing lamp power during dimming.

Phase-control. While phase-control dimmers are readily available from the majority of controls manufacturers, only one manufacturer currently offers a full line of phase-control dimming ballasts. A second manufacturer offers a limited lineup, and two other manufacturers are eyeing the technology with interest. With phase-control dimming, parts of the AC power supply are “cut out.” The half-cycle's starting point or zero crossing point is detected, then after a preset time period the current is switched on, effectively “cutting out” a part of the cycle. Despite this method, the lamps don't flicker.

So which method is best? As is the case with all things in lighting, it depends on the application, its requirements, and what you're looking to do. Each protocol offers distinct advantages for different needs. The answer may even be neither and that a proprietary protocol is desirable.

Open vs. proprietary. If you choose open analog protocols, you can be fairly confident that the items you plan to buy will be interoperable even if they come from different manufacturers. Most major ballast manufacturers have adopted at least one of these protocols, and most control manufacturers make products that are compatible with both. The biggest benefit of open protocols, however, is that the owner can take advantage of the competition to gain the best service and pricing. In contrast, proprietary protocols may be advantageous should the owner want to limit the number of suppliers on an installation. The down side is that the owner is tied to one manufacturer and must pay the pricing set by that manufacturer for all components now and in the future. In addition, proprietary technologies typically incorporate unique wiring configurations that can add labor costs to the installation.

0-10VDC vs. phase-control. If you decide against a proprietary system, the next step is choosing between 0-10VDC and phase-control. The biggest question about which is preferable is whether the application is architectural or energy management in nature. Given that the biggest difference between 0-10VDC and phase-control is in the wiring required to power the ballast and change the light level, the key question to ask is, What are we trying to accomplish in the controlled environment? If the goal is simply to dim the lights, as is the case in architectural applications, then the simplest and most cost-effective approach is to use phase-control ballasts that will use standard wiring and minimize labor costs. If the goal is to combine a building management system with photosensors and dimmers, as is the case with energy management applications, then 0-10VDC would be the simplest and least expensive route to take. These different parameters result in a number of unique advantages and disadvantages for each technology. In short:

  • 0-10VDC ballasts use four wires: two line-voltage leads for on/off power control, and two low-voltage leads for ballast control (Fig. 1). This technology makes it possible to integrate multiple control sources like photocells, manual dimmers, and building management systems, but bear in mind that the installation costs associated with the additional wiring can be substantial.

  • Phase-control ballasts use two wires for on/off power control and ballast control (Fig. 2). The technology is used almost exclusively for architectural dimming applications like conference rooms, boardrooms, or individual offices. The cost of installation is minimized for both new installation as well as retrofit because it's possible to use the standard wiring configuration.

Digital ballasts, incidentally, use a special wiring configuration that typically requires only a single set of shielded, twisted-pair, low-voltage wires. Again, these three open protocol technologies — DALI (digital), 0-10VDC (analog), and phase-control (analog) — aren't compatible with each other.

Based on this difference, it's possible to generate a side-by-side comparison of the two analog technologies (See Table).

Neither analog dimming method is inherently better than the other. Each has its own advantages and disadvantages. The best approach depends on the owner's needs and the application requirements. Either way, the user can have confidence that the components in the dimming system will be interoperable — if they're all designated for the same protocol — and meet performance expectations.

Glaser is president of HUNT Dimming, Fort Collins, Colo., and the Lighting Controls Association, Rosslyn, Va.