Dimming: In Forward And Reverse

Dimmers have quietly become the least spectacular and least controversial devices in the lighting inventory.
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Dimmers have quietly become the least spectacular and least controversial devices in the lighting inventory. The contemporary dimmer is generally so flexible and reliable that it gets very little attention. Since the moving light came on the scene, many of our industry's finest engineering and marketing brains have shifted their focus away from dimmers. No longer is there a headlong rush to incorporate DVD players and cordless mouses into dimmer racks; instead efforts are aimed toward adding tailfins, oil pressure gauges and sunroofs to the all-singing, all-dancing moving lights.

The overwhelming majority of today's solid-state dimmers operate on the principle of Phase Control, a technology that has been around in various forms as long as television itself. However, in its earliest incarnation, phase control with Thyratron valves was neither reliable enough nor sufficiently affordable to make it popular. Phase control technology came into its own in the late 1950s with the appearance of the Silicon Controlled Rectifier (SCR). This was later supplemented by such solid-state devices as the Triac, the Metal Oxide Silicon Field Effect Transistor (MOSFET) and most recently, the Insulated Gate Bipolar Transistor (IGBT).

The basis of phase control dimming is very simple. Rather than attempting to restrict the amplitude of the current flowing through a lamp, as other forms of dimming had done, phase control works by switching off parts of each cycle of the alternating current supply. Although hacking chunks out of the supply in this way should theoretically cause the lamp to flicker at 120 Hz (once for each positive or negative excursion of the 60 Hz supply) in practice, the thermal inertia of the filament averages out the brightness at a lower level, with no perceptible flicker.

(click thumbnail)Fig. 1: Phase Control

(click thumbnail)Fig. 2: Reverse Phase Control
Achieving accurate and repeatable dimming requires that predictable chunks be excised from each mains cycle. This is accomplished by detecting the start (zero-crossing point) of each half cycle, then waiting for a predetermined period before switching on the current. It is the timed switching of the current in phase with the alternating current supply that gives Phase Control its name (see Fig. 1).

The SCR is a solid-state switching device that only conducts current in one direction (like a rectifier), although it doesn't conduct at all until a trigger voltage is applied to its gate electrode. However, once it starts conducting, it goes directly from fully -- off to fully -- on and is very difficult to turn off. This shortcoming is not as limiting as it may at first appear, as the SCR stops conducting when the current reverses halfway through the power cycle. As an SCR can only handle one half of an alternating current supply, a pair of them -- wired in inverse parallel -- is required to handle the full supply. The Triac is a later development that, for most purposes, can be considered the equivalent of an inverse pair of SCRs, in a single device.

As the first solid-state phase control dimmers employed SCRs and Triacs for switching, the electrical characteristics of these devices required that the unwanted power be cut from the start (or leading edge) of each half power cycle (thus, the name Forward Phase Control). The very rapid switch-on time of SCRs and Triacs means that they produce very little waste heat.

Unfortunately, that rapid current increase does produce both harmonic distortions in the supply mains, as well as Electromagnetic Interference (EMI), radiated from the load circuits. To reduce these problems, a substantial inductor is incorporated into the circuit to slow the rate of current increase. Such an inductor is generally both large and heavy, limiting the possibilities for both portability and miniaturization of these phase control dimmers. Despite these shortcomings, the majority of dimmers built over the last three or four decades successfully employ SCRs and Triacs for forward phase control.

POWER DOWN

Power MOSFETs and IGBTs are essentially very high-powered transistors. Unlike Triacs and SCRs, they cannot only be switched on and off with relative ease, they can also be switched on and off s-l-o-w-l-y, to reduce harmonic distortion and EMI. These devices could potentially be used as phase control switches to make a dimmer that didn't need an inductor. When they eventually became available in formats capable of switching enough power to dim a lamp filament, other forms of phase control became possible.

Reverse Phase Control dimmers (see Fig. 2) now available employ MOSFETs or IGBTs to remove power from the end (or trailing edge) of the power cycle. There is some debate as to whether a reverse phase control dimmer produces less EMI or mains distortion than the forward phase control variety, but conclusive proof has yet to emerge. One dimmer system to appear in recent times dynamically switches between forward and reverse phase control. The manufacturer claims that this technique reduces the harmonic distortion in the supply mains.

Using IGBTs and MOSFETs to achieve slow turn-on times has enabled the production of phase control dimmers with acceptable EMI and harmonic distortion, without the need for big, heavy inductors. However, the heat dissipated in the switching devices of these dimmers is fierce, necessitating very active thermal control measures, such as large heatsinks and often noisy fans. Although it may be possible to make a more compact dimmer without a big inductor, the need for heat dissipation prevents the dimmers from shrinking very much in size.

ALTERNATIVES

One innovative approach to the heat dissipation problem has been to build the dimmers into a long metal duct that attaches to a lighting pipe, thus providing a series of dimmers: each immediately adjacent to the fitting it is controlling. This configuration brings with it a range of additional considerations, including maintenance access, acoustic noise, as well as power and control infrastructure.

The most recent step in electronic dimming, has been to use the switching capabilities of the IGBT to remove a large number of tiny chunks throughout the power cycle, rather than one big chunk at either the beginning or end of the cycle. This approach can produce a power cycle that looks remarkably like the sine wave shape of the original power, only lower in amplitude. Referred to as Sine Wave dimmers by the Dutch and Australian companies who have independently developed them, these devices produce negligible EMI and mains distortion. To achieve this feat, the dimmers incorporate very sophisticated monitoring and control electronics and complex cooling systems, making them substantially more expensive than any other form of dimmer.

Despite all the interest being lavished on the newest, sexiest forms of moving lights, it turns out interesting things are still happening in the not-so boring world of dimming.