Oh, Please Give Me a Sine

Dimming is a dirty business. Ever since we gave up using resistance dimmers because they were big, heavy, expensive, inefficient, hot and very difficult to remotely control (and in the case of the saltwater variety, also smelly and toxic), we have been playing merry hell with our utility supply.
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Dimming is a dirty business. Ever since we gave up using resistance dimmers because they were big, heavy, expensive, inefficient, hot and very difficult to remotely control (and in the case of the saltwater variety, also smelly and toxic), we have been playing merry hell with our utility supply.

In particular, the introduction of the phase-control dimmer brought with it substantial distortion to the power supply and megawatts of radiated electromagnetic interference--a pair of remarkably antisocial side effects.

At the heart of almost every modern dimming installation, the phase-control process dims by using an electronic switching device such as a thyratron valve, a solid-state thyristor, or more recently, a power transistor to shut off a controlled portion of each power cycle.

Depending on the switching device, that shut-off can remove either the beginning or the end of each power half-cycle, while the switching process itself may be very abrupt or somewhat tapered.

While switching off as gently as possible may reduce the harmonic distortion caused to the supply, the mess made is nevertheless quite substantial.

SWITCHMODE POWER

In the early days of phase control, the dimmer system was often the only substantial source of harmonic currents in an entire studio installation. While avoided where possible (usually by keeping lighting utility feeds well-separated from those for more sensitive equipment), it was tolerated as being a peculiarity of lighting systems that didn't do too much harm.

Since then, the almost universal adoption of switchmode power supplies, which chop up input power and thus produce harmonic distortion, has led to an imminent crisis in our power distribution systems. In some places, the solution appears to be for utility companies to throw money at the problem by what amounts to down-rating substation transformers that have to deal with substantially distorted loads.

When the incumbent transformer malfunctions, they simply replace it with a larger one and pass the cost on to the consumer. It seems that sometimes a megawatt just ain't quite what it used to be.

In the European Union, regulators decided during the early '90s it would be a good idea to reduce the conducted harmonics and conducted voltage fluctuations generated by electronic equipment, and so wrote some new standards (such as IEC/EN61000-3-2) that were scheduled to come in to force toward the end of that decade. When it turned out that no one could actually build compliant equipment, the electronics industries of the EU quietly went about getting the deadline extended until 2001.

That deadline too has passed, with very little activity on the other side of the Atlantic. Nevertheless, the problem shows few signs of going away anytime soon.

On the EMI (electromagnetic interference) front, the story is only a little different. We have ceased to see much evidence of EMI on productions, but only because we go to extraordinary lengths to keep it out. Over the past three or four decades, everyone we've worked with reluctantly came to terms with the reality that electronic dimming produces bucketloads of EMI.

BLEEDING EMI

While we may keep our load cabling in shielded containers for the majority of its journey from dimmer to luminaire, there's always that last few feet between the socket and the fixture that's going to bleed EMI.

(Unless of course, in desperation, your audio department put up the money for shielded cable tails on all your luminaires!)

What everyone else has learned to do is keep signals inside well-shielded cables to avoid receiving our EMI. Whenever confronted by a stressed audio tech, I always 'fess up: "Sure I caused the buzz in that mic channel, and I would be causing the same buzz in every mic channel if the cables also had damaged shielding."

Along with the electrical and electromagnetic side effects of phase-control dimming, there's also the acoustic effect that comes from feeding lamp filaments on a diet of chopped-up power. The sudden discontinuities in the supply cause mechanical vibrations that result in our filaments singing along at 120 Hz. Most of the time, this noise is barely noticed above the cacophony of air conditioning, power supply inductors of various kinds, cooling fans, power cabling, and by no means least of all, the noisy motors in our own robotic equipment.

However, if you need dimming in a space with strict noise requirements such as a studio for recording acoustic performances, then this factor may be important.

All of these unpleasant side effects from our otherwise ideal phase-control dimmers result from sudden current changes that arise from chopping significant chunks from the sine waves of the power supply.

One way to avoid such problems and still achieve a dimming effect is to chop a whole lot of tiny chunks from the supply. The result is still less total power getting to a lamp, but the many changes of current are each quite small. Indeed, with a little help from a simple filter circuit, the output of such a dimmer would be almost indistinguishable from a variable amplitude sine wave, and thus exhibit none of the nasty side effects of phase control.

While the concept of a sine wave output dimmer had been around for many years, it was impractical to build one using the electronic switches available until relatively recently. When the insulated gate bipolar transistor became available to switch at the required voltage, current and speed, many engineers tried their hand at building such a dimmer. The successful designs use pulse width modulation, a process in which a power cycle is divided up into several hundred fixed-sized chunks that are varied in width to achieve an overall change in current flow without changing its smooth sine wave shape.

Driving an insulated gate bipolar transistor that is performing pulse width modulation on large currents at frequencies in the 20-to-50 kHz range, turns out to be a remarkably complex juggling act of currents, times and temperatures. It usually involves many sensors and a processor chip to keep all of the balls in the air. Although many designs have been tried, only a few have made it into production.

There are full-scale commercial sine wave dimmers currently available from several manufacturers, if you have the funds available. The first to market was the Solution Sine from Bytecraft (now State Automation) in Australia, which picked up a number of industry awards at the turn of the century. These were soon followed by the iSine from IES in the Netherlands, now part of ETC, and more recently, the TrueSine from Strand Lighting.

Both the ETC and Strand dimmers are offered as modules in their popular high-density dimming systems.

Oddly enough, most of the marketing emphasis on these dimmers is on their acoustic silence rather than their negligible harmonic supply distortion or their almost total absence of EMI. I would have thought that the key selling point would be that our audio brethren could breathe easier and not have to worry about repairing their mic leads anymore!!