It may have something to do with the high levels of eucalyptus oil in the air or the summertime hole in the ozone layer overhead, but something has caused an outbreak of sine wave dimmer technology in Australia.

Phase control has been our best approximation to an ideal dimmer for more than 30 years, though chopping chunks out of mains cycles is an inherently messy business. The ease of remote operation, relative robustness, and high efficiency of thyristor (SCR and Triac) dimmers have made them almost universally acceptable for lighting control applications ranging from architectural and domestic to studio and theatre. Ironically it is the acceptability of phase control dimming which will lead to its eventual demise.

Phase control devices achieve their dimming effect by switching on (1) the current to the load partway through each mains cycle; the delay in switch-on time is related to how much of a dimming effect is achieved. It is this switching that is the downside to phase control, as the switching of load currents causes a range of unwanted side effects including: radio frequency interference being emitted from the load wiring; the generation of harmonics of the load current, which appear as distortions in the mains supply; and acoustic and mechanical vibrations throughout the electrical installation.

For a couple of decades, while the problems associated with large numbers of phase control dimmers were isolated to theatres, studios, nightclubs, and other exotic venues, the entertainment industry was left to itself to deal with the problems. This was generally achieved through the use of shielded audio and video cabling, isolating transformers on audio systems, and all of the other remedies that are now part of daily life. Then came energy management and the smart building, when architects and engineers started putting dimmers at every distribution panel in every structure larger than a garage--that's when the trouble really started.

While it may be difficult trying to convince a sound technician to check the integrity of the braid on the lead of a buzzing microphone, it's going to be a lot more difficult to convince every occupant of a high-rise office or apartment building to do the same. Mains harmonic disturbances from a theatre may annoy the local power utility, but whole buildings with the same problems are becoming a major irritant. In Europe an ISO standard has now been set for mains harmonic distortion that from January 2001 will effectively eliminate the installation of phase control dimming as we know it.

While the research and development teams of every dimmer designer have been constantly working to improve the electronic dimmer, it has somehow transpired that three companies in Australia--Jands Electronics, Bytecraft, and Dynalite--have announced sine wave output dimmers, using the next wave of electronic technology.

Dimmers with a sine wave output are hardly a new phenomenon. The resistance dimmer in its many forms, whether a pair of electrodes in a container of salt water, a rheostat-like slider, or a mechanically operated "grand master" system, always produced a nice clean sine wave output which varied only in amplitude. It was only the fact that these dimmers were heavy, expensive, load-dependent, produced vast amounts of waste heat, and required some form of direct mechanical operation that caused them to be bypassed for other technologies. Resistance dimmers were never responsible for buzz in PA systems, nor did they produce harmonic distortion in the mains, but those of us who have had the privilege of running productions on them are hardly ever heard to lament their passing.

Attempts have been made in the past to produce a sine wave output dimmer by using various semiconductor devices as the variable resistance, but each of these has run up against the difficulty of the available semiconductor technology not being able to handle the currents, voltages, and heat dissipation required for a robust, reliable production dimmer. The new wave of sine wave output dimmers has taken a different approach, using the switch-mode technology widely used in devices as diverse as the ballasts of many discharge luminaires and the power supplies of virtually all computers and lighting consoles.

All three companies set about developing their sine wave output dimmers for similar reasons. Each of them depend on their overseas markets rather than local sales for their existence, and each of them is anxious to have a dimmer that meets new performance and regulatory requirements.

Dynalite may have started out making disco chasers and production dimmers, but made the move into architectural dimming and control several years ago, to the extent that it now represents the vast majority of its business. Dynalite's IALD Award-winning architectural network control system DyNet and its dimmer and switching interface units are now sold through branch offices in China, Singapore, and the UK, besides its Australasian dealer network.

Since March 1998 Dynalite has been shipping the SVC (Sinewave Voltage Converter) dimmer, a two-channel, 7A architectural unit being promoted as a device capable of dimming highly reactive loads such as discharge and fluorescent lamps, without causing any additional power factor or harmonic distortions. Installations include the control of metal-halide sources in the Superdome for the Sydney 2000 Olympics and Sydney's Central Station bus interchange, while in St. Mary's Cathedral, SVCs are being used to control high-pressure sodium vapor lamps. A second generation of the SVC, currently in late beta-testing, features a constant output mode that will maintain a preset light output through an additional boost regulation stage.

Bytecraft, originally a lighting electronics service company, has been successful in Asian and European markets with its Bytesize IDS (Intelligent Dimming System) production dimmers for about a decade, even selling them into Japan, a feat which has been likened to exporting oil to Saudi Arabia. Its new dimmer, the 12x2.5kW Bytesize VST (Variable Sinewave Technology), is a move to retain its technological leadership in Europe as phase control dimming goes the way of the saturable reactor and the autotransformer (variac). The engineering prototypes of the VST won an award at PLASA 98 while the first production units have recently been installed as part of what will be a 192-channel system in the new studio at the Sydney Opera House.

Jands Electronics, best known for the Echelon and Jands Hog desks, recently updated all of its dimming products in line with international electromagnetic interference and safety standards. This past January, Jands announced the development of a prototype of its SWDim sine wave dimmer. Although no further development of the SWDim has been announced and the dimmer has not yet been publicly demonstrated, reliable sources say field tests of the prototype have been very encouraging.

The principle behind all of these sine wave dimmers is very simple: The incoming mains is switched on and off between 600 and 1,000 times per mains cycle (30-50kHz) with the on time (width) of each pulse being proportional to the required output power from the load, a method known as Pulse Width Modulation (PWM). The very finely chopped output is then filtered back into a continuous waveform through an inductor similar to the choke on a phase control dimmer, but very much smaller, as the pulse frequency is higher. The shape of the filtered output waveform is almost identical to the input waveform, only its amplitude is different, precisely what happens in a resistance dimmer. Bytecraft's Ted Fregon was at pains to point out that a good PWM dimmer isn't really a sine wave dimmer at all; it only puts out a modified version of whatever is put in. If you were to feed a square wave into the dimmer, that is precisely what would come out, at whatever amplitude the dimmer was set to (a real-world example of Garbage In, Garbage Out).

The semiconductor used for the high-speed, high-current, and high-voltage switching is the Insulated Gate Bipolar Transistor (IGBT), a device which has been around for a number of years, slowly becoming available for controlling higher currents and voltages. Indeed, Rosco/Entertainment Technology has been using them for some time in their IPS modified reverse phase control dimmers. As today's IGBTs are right at the limit of their capabilities switching present load currents, it is extremely important that they are prevented from overheating, a task that has become a major preoccupation for dimmer designers.

If PWM is a tried-and-true technique and IGBTs have been around for several years, the question must be asked as to why sine wave dimmers have taken so long to develop. This dimmer (in common with many of today's systems, including a number of advanced aircraft) is inherently unstable--that is, if you leave the system to run its present course for any significant period of time it will fail. What is required is the constant monitoring of parameters such as load resistance, load current, line voltage, IGBT temperature, airflow over heat sinks, and the control signal level. Each of these measurements is compared with a set of ideal parameters and used to calculate a new set of control signals to drive such things as the width of the output pulses or the speed of the cooling fans.

Only with a large number of sensors to gather data, a fast processor, and some clever software is it possible to maintain the complex dynamic equilibrium necessary for a sine wave dimmer to function, and this is where Dynalite and Bytecraft have put their efforts. Jands on the other hand claims that at its present stage of development, the prototype SWDim is able to maintain stability by monitoring and processing only output current levels.

Once you have a dimmer that is built around a bunch of sensors and a fast processor it becomes possible to offer functions on the dimmers which until now had only been dreamed of. Bytecraft VST and Jands SWDim production dimmers have no fuses on the channel outputs, yet both offer instantaneous overload cutoff and the ability to handle indefinite dead shorts on the load with a return to full function as soon as the short is removed. Soft lamp start is available to protect both the lamp from inrush current and the IGBT from being damaged. Constant voltage or constant current (VST only) output can be specified, allowing for low-voltage loads to be run directly from the dimmer without the flicker usually found when phase control dimmers are used in this way.

Above all, these dimmers produce insignificant radio frequency electromagnetic interference, negligible mains distortion, and no rattle or singing from lamp filaments (the reason they were installed in the Sydney Opera House). Unfortunately these products, at approximately twice the cost of their phase control forebears, do produce a substantial hole in your bank balance, but if you need a dimmer with these characteristics--and sooner or later we all will--then it's down under to the Land of Oz (2) to get your sine wave dimmers.

1. In the case of the newer reverse phase control dimmers, the current is switched off, which theoretically should produce less significant effects.

2. A local and somewhat charming reference to a country whose inhabitants pronounce its name as Oztralia and believe that they live in a kind of magic land. Some folk there even refer to Sydney as Emerald City.

Andy Ciddor of The Kilowatt Company has been a practitioner, educator, and writer in the field of production technology for 30 years. He can be reached at aciddor@kilowatt.com.au.