Understanding Lamp Dimming Technologies

Understanding Lamp Dimming Technologies

Dimming technology has evolved considerably over the last decade, especially in the arc discharge lamp category. The adjustment of light levels over a linear or continuous range is appropriate for areas like lecture halls, meeting rooms, classrooms and even open-office areas, in addition to restaurants and entertainment locations.

Users can easily reduce energy usage with dimmers.

Dimming technology has evolved considerably over the last decade, especially in the arc discharge lamp category. What began as a device for simply matching lighting levels to various tasks and moods has emerged as a tool for reducing energy usage. The reduction in light output can be accomplished either as a step function or as a continuous function. The adjustment of light levels over a linear or continuous range is appropriate for areas like lecture halls, meeting rooms, classrooms and even open-office areas, in addition to restaurants and entertainment locations.

Dimming is typically quantified using percentages, such as 10% dimming. This percentage can be applied to lamp voltage, lamp power, measured lumen output, or perceived lumen output. Dimming usually refers to the reduction in measured lumen output. Therefore, a 500-lumen light source in a 10% dimmed system would produce 50 lumens.

Perceived lumen output is an important parameter because the human eye overcompensates for diminished light. The opening of the pupil allows more light to enter, causing the lighting level to seem higher than normal. Light in a 25% dimmed system appears to be about 50% of its original level. A 10% dimmed system appears more than three times as bright. And when the light level is reduced to only 1%, it appears to be about 10%. This characteristic should considered when planning a dimming system.

Step dimming.

In the case of incandescent lamps, the three-way A-lamp provides the simplest step function. The step method in fluorescent systems is often referred to as bi-level switching, even though more than two levels may actually be available. For example, in a system with three-lamp fluorescent fixtures, one switch may operate the center lamp in each fixture while another operates the outer lamps. This arrangement provides three levels (one-, two-, or three-lamp operation); the term bi-level refers to the number of switches.

By offering two or three lighting levels, step-dimming ballasts used with occupancy sensors can reduce light output without resorting to on/off cycling of lamp operation, which usually results in an appreciable reduction in lamp life.

In addition, many HID systems currently provide dimming via a stepped, or switched, system. The switched capacitor operation within the core and coil ballast compartment permits a 50% reduction in power to the lamp when an area like a warehouse is unoccupied.

Continuous dimming of incandescent and halogen lamps.

Continuous incandescent and halogen lamp dimming involves controlling the voltage delivered to the lamp. As the voltage is reduced, the lamp current and resulting lamp power are reduced. This can be achieved by using an adjustable transformer to decrease the amplitude of the 60-cycle waveform or by turning off the applied voltage with solid-state controls for a part of the cycle.

In either case a portion of the AC waveform received by the incandescent filament is chopped away. The longer the triac conducts electric power through each half of the voltage waveform, the brighter the light output (Fig. 1 above).

Wall box-mounted incandescent dimmers, rated 600W, 1,000W, 1,500W, and even 2,000W, are useful for a wide variety of applications. Higher power dimming modules can be added to the incandescent dimming function to raise the load capacity to 30kW. Dimming panels with capacities as high as 144kW are also available.

Single-gang switch box dimmers are compact enough to be mounted side-by-side in standard 2-in. deep wall boxes. Side-by-side mounting may force you to remove the side sections of the dimmers, thereby derating the devices. In such instances, consult the manufacturers' capacity derating chart to determine maximum loads for each control in the installation. Some dimmer models have no removable side sections so derating isn't required.

While the use of a simple incandescent dimmer is straightforward, some considerations should be observed. Occasionally, the lamp filaments may generate a buzzing sound near the 50% dimming level caused by vibration. Using rough-service lamps with thicker filaments, smaller lamps, or lower wattage lamps will help eliminate it. However, the most effective and permanent method of reducing the annoying lamp buzz is to install a lamp “debuzzing” coil in series with the lamps. When placed in the circuit, the coil reduces the current inrush during rapid switching cycles.

Radio frequency interference (RFI) may manifest itself as a buzzing noise in audio and radio equipment when solid-state dimmers are used nearby. Although dimmers contain a filter to suppress RFI, sometimes more filtering may be necessary.

Continuous dimming of fluorescent lamps.

Three methods are available for dimming a fluorescent arc discharge light source using electronic ballasts: 0VDC to 10VDC analog, phase control analog, or digital control. The first two methods of dimming are designed for use with controllable analog electronic fluorescent ballasts. These dimmers provide linear lighting control from 100% down to 1% of rated lumen output and are available in single-pole or 3-way designs for 120V and 277V applications.

Analog 0VDC to 10VDC control systems with four conductors are available in single- and two-lamp models, and are the most commonly used dimmers today. One pair of conductors support a low-voltage circuit that provides the dimming control function, and the second pair (line voltage conductors) provide on/off power control. A manual adjustment feature makes it possible to achieve maximum light output from each fixture and establish a reduced light output level as the maximum limit. Dimming ballast designs differ from one manufacturer to the next, so the number of lamps allowed on a lighting branch circuit may depend on a particular ballast type.

Groups of fixtures are connected to the control circuit to receive the same command signal, thus the control can extend beyond a single power circuit layout. Higher amperage applications are controlled with modular systems that incorporate a wall-mounted control unit and remotely mounted power modules capable of handling several hundred T8, T5, or CFL lamps.

Not all ballasts are approved for use with some manufacturers' electronic fluorescent dimmers. Additional ballasts are usually added to a manufacturer's list as they're approved. Specifiers should also note that different ballasts respond differently to photo sensor control signals. Generally, a dimming ballast responds to only a part of the control voltage range of a photo sensor; different ballasts respond to different parts of the control voltage range.

Phase control technology is better suited for an architectural lighting scheme in a smaller space, such as a conference room or an office. A phase control dimming ballast generally uses the same pair of conductors for power and control, providing for a simple, cost-effective retrofit when control from a single location is desired. Some phase control dimming ballast products use a three-conductor line-voltage system.

Ballasts in digital control systems respond to digitally encoded pulse signals instead of a variable analog control voltage. Digital control transmission is free from noise or RFI on the conductors, assuring the reliability of the control signal. This technology also offers a high degree of control flexibility in commercial, institutional, and high-end residential applications.

Digital control offers a number of other useful features. Decentralized microprocessor-controlled dimming panels and control stations provide maximum flexibility and reliability. In most cases, the wiring design is simplified, since the RS-485 network is connected using two sets of twisted-pair conductors in a cable, installed as a simple daisy-chain layout. Virtually no load is too big because the RS-485 network can support as many as 128 control stations and 256 dimming circuits, and it interfaces with RS-232 and DMX-512 protocols. The DMX-512 protocol is a theatrical industry standard now finding application in architectural dimming and control systems.

Dimming HID luminaires.

Electronic ballasts are also entering the HID product area, promising to provide economical dimming applications. Electronic ballasts have been used in low-power (175W or less) HID luminaires, but reliable electronic technology is new to the higher wattage metal-halide (MH) class. The new electronic MH ballasts allow a reduction in energy consumption by continuously dimming the lamp. In some cases, a reduction from 100% to 50% of the rated wattage is possible. High-low control systems save energy, but a linear relationship doesn't exist between wattage and light output. If the light level is at 50% of maximum, the wattage will be about 65% of maximum.

In the past, lamp manufacturers typically established a lower limit to 50% of normal power to the lamp. Below this level the MH lamp could become unstable and be subject to detrimental effects. However, new laboratory work and field testing indicates that the precision of control and the beneficial aspects of lamp operation at high frequencies allow the MH lamp to break through the 50% dimming limitation imposed by conventional ballasts. Because of the way the electronic ballast optimizes lamp ignition, ballast operation, and power control, manufacturers are ready to establish a new lower limit of 35%.

Efforts are underway to design an MH lamp that will be served by an electronic ballast to achieve still greater performance optimization. In fact, one lamp company has designed an MH lamp specifically for high-frequency operation and dimming.

For high-load areas, reduction of HID and fluorescent light output may also be achieved with panel level controllers/voltage regulators that lower the circuit voltage upstream of the ballasts. This method is suitable for areas switched simultaneously, such as retail stores, supermarkets, and large, open-plan offices.

Finding new ways to reduce energy usage can be difficult, but as budget cutbacks continue to make maintaining a facility difficult, it's a necessary evil. Now that dimming systems double as energy misers, lighting specifiers have an option that can offer two functions for the price of one — enough to give any bottom line a little more room to breathe.

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