In the early 1960s, there was a radical shift in theatrical dimming with the introduction of SCR dimming. It took some time to get this new technology to a stable level, but it quickly replaced many earlier dimming technologies. Once again, we find ourselves in the early stages of “the future of dimming” in the form of sine wave dimming. First seen in Europe, it is now beginning to appear in the North American market. As with the advent of SCR dimming, there will be a learning curve, and with sine wave dimming, we are all just at the beginning of our educational process.

Sine wave dimming has become an alternative technology to the long-running SCR dimming technology that has been dominant for the past 40 years. There are a lot of advantages that sine wave has over the venerable SCR, and with the developments of the technology from both Electronic Theatre Controls and Strand Lighting, you may be enjoying the benefits sooner, rather than later. It all depends on your application, but the two main advantages of sine wave dimming are absolute quiet and the ability to dim almost any load. Depending upon your requirements, sine wave dimming may just be the answer that you are looking for in control.

Sine wave dimming got its start in Europe about 1998 with Dutch company IES. After some problematic installations, the company worked through — and learned from — those mistakes and, in the process, moved the technology forward. Concurrently, Bytecraft in Australia developed a sine wave product and, while not as well known, did have some installations with its product.

Electronic Theatre Controls (ETC), who had been working on its own sine wave dimmer, acquired IES last June in order to gain its expertise and field experience. The company has introduced its Sensor®+ SineWave dimmer for the 120V markets and is retaining the IES products for the 230V markets. The Sensor+ SineWave SW-24 dimmer rack from ETC has 48 2.4kW sine wave dimmers.

Strand Lighting had been developing its own sine wave dimmer, the SST module, and incorporated it into its SLD dimming racks with 48 2.4kW dimmers. Recently, the company has introduced its C21 series dimming system, which uses Strand's TrueSine sine wave dimming technology, with a higher dimmer density per rack of 96 2.4kW sine wave dimmers. This system will now supersede the SLD system.


The advantages of sine wave dimming appear to outweigh the drawbacks, but you should be aware of what makes a good sine wave dimmer as well as what features and benefits are most important to your application.

Peter Rogers, vice president of marketing for Strand Lighting explains, “The number one benefit is that the loads are silent. If you are dealing with a concert hall environment — and they are building them quieter and quieter now — then silence is really what you are after.”

When it comes to advantages, Steve Terry, ETC vice president of research and development, also believes silence is crucial. “The leading drivers are going to be acoustic quietness and allowing any load capability,” Terry says. “SCR dimmers tend to be picky about the kinds of load that you can plug into them and have them dim properly.”

Rogers says it is important to understand the load abilities of the sine wave dimmer. “People think a sine wave dimmer can dim anything,” he says. “The thing that catches them the most is fluorescent ballasts. We try to get people to think of a sine wave dimmer as an electronic autotransformer; whatever you can dim with that, you can dim with one of these dimmers. You can dim a motor with an autotransformer, and you can dim it with a sine wave dimmer. You can't dim a fluorescent load with an autotransformer because it is expecting a chopped waveform, and then the sine wave dimmer will be no different. That said, it dims neon and cold cathode beautifully. The transformer heating on those devices is much lower because they are not seeing a chopped wave form.”

Of course, there is a long list of other advantages of sine wave dimming. Both Rogers and Terry agree on the benefits of lower installation costs. “Because there are no reflected harmonics in an all-sine wave system, your electrical installation costs are going to be lower because you don't need K-rated transformers, and you don't need oversized neutrals,” says Rogers.

It is especially important for consultants and end users to look at this area in order to specify what they really need. Terry explains, “There are no harmonics being drawn off the power line. Sine wave dimmers draw current in a sinusoidal waveform, which means that we eliminate neutral over-currents in the feeders to the dimmer rack.”

This point leads to another consideration: the aging of the power grids. Sine wave installations yield fewer costs in improvements to the power feeds. “In big cities, this becomes an even bigger deal because in an older city like New York or Chicago — or, as we discovered in Toronto — the electrical infrastructure is aging, and it is close to maxed out,” Rogers points out. “When you start having to build in all the additional load created by regular dimmers, all that harmonic content actually translates into power consumed. When you have sine wave dimming, there is none of that. The power factor is unity.”

Other major advantages include immunity to bad power, very precise curve and fade resolution, and no load sensitivity. “They are not load sensitive like a regular SCR dimmer,” says Rogers. “You can have a 1mW LED, and it will dim perfectly all day long. The other interesting application we see is for a voltage regulating non-dim. Think about the application in some of these big theatres where we feed the racks elevated mains — 130V/140V — so we reduce voltage drops in the wiring. People have been putting non-dims into these systems and finding that the load on the catwalk just outside the dimmer room has turned into toast because the relay is opening, and there is 140V there. They can't put a regular dimmer in the line because the wave form isn't going to be good for the computer or the audio amplifier they are trying to power. With a sine wave dimmer, we can adjust the voltage and have it switch on at the regulated output.”


Just so you don't think that sine wave dimmers are all sweetness and light, there are a few drawbacks, but like most technologies, given time and development, many of these will go away. At this point, sine wave dimmers do cost more than SCR dimmers. With increased volume, cost will decrease. Costs aside, there are a few other drawbacks to consider.

“One drawback is that it just has a lot more parts in it,” says Terry. “It is a much more complicated approach than SCR dimming, which has a very low failure rate. With sine wave dimming, you have many more parts, many more interconnects. Just by statistics, you are going to expect some lower reliability. We don't know how low because we don't have enough system-hours accrued on any of these product lines, but we can predict that it is going to be somewhat lower than SCRs.”

Rogers sees the response time as an issue for applications that are seeking immediate response. “I think that someone doing rock and roll would tell you that a sine wave is maybe half a beat slower to respond to control, and the reason for that is all the manufacturers have to protect the IGBTs [Insulated Gate Bipolar Transistors, the solid state power device technology that makes newer dimming technologies possible],” he says. “The dimmer turns the lamp on gradually to heat the filament so the IGBTs aren't getting the cold inrush, and while this happens really quickly, it is still a half beat behind a regular dimmer. The response to control is slightly slower, although in a normal theatrical application you would probably never see it.”

In that vein, cooling is very important to protect the IGBTs and the components in a sine wave dimmer. The dimmer racks need to have proper air flow and cooling to protect the dimmers. “They run as hot as a high-rise time regular dimmer, so you've got some air conditioning load issues that are going to be higher than a 350 microsecond choke dimmer,” says Rogers. “It is not a big deal, but it is something you need to pay attention to.”


Besides looking at the advantages and drawbacks, end users should also educate themselves on what to look for in a sine wave dimmer. Terry suggests some criteria: “first, curve consistency on different loads,” he says. “Getting tight insensitivity to load in terms of the curve is a significant performance parameter that users need to be concerned about. Second, the ability to deal with any load, inductive, resistive, or capacitive. When one buys a sine wave dimmer, one expects to be able to plug in anything into it and have it dim, if the load is designed to dim, and certainly not have it fail. Third, cold filament inrush performance — how fast do they respond, and do they respond consistently from dimmer to dimmer with widely varying loads on the output of the dimmer? Next item is short circuit and overload performance. There are many ways to protect a sine wave dimmer in terms of overloads and short circuits, and the behavior and performance of those circuits are important. You don't want dimmers nuisance tripping on cold filaments; you want enough intelligence in the dimmer that under no condition can you hurt the dimmer or the branch circuit wiring because you don't shut down in the presence of an overload or a short circuit. Finally, is the sine wave dimmer really a sine wave? It is very easy to create a sine wave dimmer that doesn't deliver a pure representation of the sine wave at its output over widely varying loads and levels. If you buy a sine wave, you really want it to be a sine wave.”


The dimmer modules and racks will evolve, and manufacturers are already looking at alternative packaging designs for the technology. Strand has already taken note of the demand for rolling rack versions. “The interesting group that is perking up to this whole sine wave thing is motion picture studios,” says Rogers. “They are buying sine wave rolling racks as fast as we can make them. They are using it when the lights, the audio, and the camera are all close to the subject.”

Both companies are looking down the road at distributed dimmers. “One of the other benefits to sine wave dimming is that it lends itself extremely well to distributed applications,” says Terry. “That is convection cooled sine wave dimming that makes no noise from the dimmer unit itself and can be located next to the lamp or right on the electric pipe. It is a very interesting architecture that is being picked up a lot in the European opera houses — Amsterdam, Copenhagen, and others — where the over stage electrics are almost entirely serviced by distributed sine wave dimmers, and the rest of the facility is serviced by rackmount dimmers. We believe strongly in that hybrid model of distributed and centralized dimming.”

As the first installations in North America are completed, we will get our first look at sine wave dimming in action. Many of the initial applications are concert halls, house lights — particularly under balcony lights — as well as silencing the hum of music stand lights. The installations will continue to get bigger, and our knowledge will increase. Who knows what the future holds, but as sine wave dimming technology moves forward in North America, the dimmers may be quiet, but the conversation certainly won't be.