It's had a good run, but its days are numbered. The reign of conventional SCR dimmers will soon be coming to an end, and the hottest candidate for the throne is the sine wave dimmer.
SCR dimming has been around since the early 1960s and came into full force a decade later as the technology became better understood and the components, more affordable. Over the years, the trusty SCR dimmer has become smarter, more compact, slightly more efficient, and conversant in digital languages. Some are even clever enough to talk back to you, but the truth is, the basic operating principle has remained the same for 40 years. Properly designed SCR dimmers can provide beautifully smooth intensity control, and they are also very robust and relatively inexpensive.
SO WHY CHANGE?
The problem with SCR dimming is that it handles intensity control in a very crude, but effective, way by slicing the waveform in half and using lots of copper and iron (the choke) to increase the rise time and the inertia of the incandescent filament to smooth out the result. When the SCR is turned on, the current goes from zero to full in a couple of microseconds, causing a sharp current spike that results in a wide spectrum of radio frequency interference. It also causes resonance in the lamp filament, a phenomenon known as “lamp sing.”
The resonating effects of the filament may also reduce lamp life, as the filament is stressed each half cycle when operated at a dimmed level. Certain types of filament geometry and reflector design can also amplify lamp sing. The popular PAR lamp is particularly susceptible to the effects of dimming because the filament construction is one of the noisiest. The parabolic reflector not only effectively directs the light from the filament, but also the sound!
SCR dimmers have a filter choke in series with the load to increase the switch-on “rise time” to between 200 and 800µS for a modern high-specification dimmer. But even with good quality filtering, it is still possible to hear a lamp filament resonate, particularly as the intensity is changed. Noise is also present at the dimmers themselves, caused by vibrations from winding of chokes. Often, these vibrations are not sufficiently damped and transfer to the dimmer chassis.
O, BRAVE NEW WORLD
If SCR dimming were restricted to performing arts and television, all this noise might not be such a big deal, but dimming is everywhere today, not only in offices and shops, but also in homes.
The world is very different now compared to when the SCR dimmer was introduced. Electromagnetic noise emissions have come into focus due to the rapidly increasing use of computers and wireless communications. Take the modern office building. It often contains multiple communications networks, not only to handle data among computers, but also to enable centralized facility management. Add to this the increasing use of wireless local area networks and cell phones, and it is clear that the airwaves are getting crowded. We are starting to view noisy SCR-based dimmers with increasing suspicion.
Or take any modern theatre. You are likely to find more computers and processing power there (from processors in lighting controls and automated lights, audio, scenery control, office networks, PDAs, etc.) than in an entire city 25 years ago.
We must also address the growing importance of environmental issues, such as energy efficiency. SCR dimmers are actually quite efficient, and very little gets lost in the dimming circuit itself. But when operated at anything less than full output, the SCR dimmer presents a distinctly non-linear load, creating what is known as “triplen harmonics.” This means that the phase currents in a three-phase system do not cancel out as intended, but, rather, add up. In the worst possible case, the neutral current can be up to 73% higher than any one-phase current. The harmonics also produce audible noise and overheating in the distribution wiring and feeder transformers and can lead to penalties from utility companies. In the European Union, legislation is pending that would require dimming systems to produce lower harmonic levels than are currently possible with SCR dimming.
Harmonic currents in an electrical system cause interference with other equipment and also represent non-linear power consumption. This translates to added capital and running costs. Capital costs exist because the electrical system for a traditional dimming installation has to accommodate a reactive power component (harmonic currents). Additional running costs are represented by charges for reactive power (VAr) that is drawn from the main supply but not used.
The usual rule of thumb is to overrate cables, wiring infrastructure, transformers, generators, and such by approximately 40%. That means more copper wiring and bigger, more expensive transformers.
Finally, with its distorted waveform, SCR dimming is simply unsuitable for many loads, including most electronic transformers and electronic ballasts for fluorescent and metal-halide sources. In some cases, the load will perform badly; in other cases, the load and the dimmer may sustain permanent damage.
Many people mistakenly consider an SCR dimmer “at full” to be the equivalent of a non-dimmed outlet. Even if the dimmer is held at full at all times, the choke, in series with the SCRs, creates a non-linear condition that may be unhealthy for electronic ballasts. And if someone accidentally adjusts the dimmer below full, the results could be disastrous. In short, providing power for $100,000 of automated lighting from your SCR dimmer rack is simply a bad idea.
MEET THE IGBT
Experts in the field have known about these drawbacks to SCR dimming for quite some time, but there have not been any feasible alternatives until a certain component came of age. The IGBT is a semiconductor that is fast replacing both regular transistors and the trusted SCR in many power control applications. IGBT stands for Insulated Gate Bipolar Transistor. Invented in the late 1970s by Frank Wheatley at RCA and currently in its fourth or fifth generation of development, IGBTs are the preferred component for power control applications. They are significantly more efficient and easier to control than most other power semiconductors. IGBTs are commonly available with ratings up to 1200 amps and about 1700V, making them suitable for use in just about any dimming application imaginable.
REVERSE PHASE CONTROL
The first attempts to use the IGBT in lighting dimmers employed a principle known as reverse phase control (RPC). Here, the IGBT effectively replaces both the SCR and the choke. RPC dimming applies the voltage to the load at zero volts and turns it off once the desired voltage has been reached.
RPC dimmers can be very quiet and compact. They work well for filament loads or electronic low-voltage transformers but are unusable with inductive loads. Neon, ballasted loads, fans, and small electric motors will generate destructive inductive kickback energy when dimmed by an RPC dimmer. RPC dimmers either should not be used with these loads or should switch to forward-phase control (FPC) to dim such loads. In both cases, FPC or RPC, the dimmers are producing triplen harmonics.
SINE WAVE DIMMING
Taking advantage of the new generation of fast and robust IGBTs and inexpensive microprocessor control has made it probable that the successor to the SCR is sine wave dimming.
This is a relatively new technology, but there are already thousands of channels in daily operation. One of the largest installations of sine wave dimming is the Amsterdam Opera, also known as Het Muziektheater. This 1,600-seat auditorium recently installed over 800 channels of sine wave dimming, supplied by IES of Holland. Other major European installations of sine wave dimming include several stages of the National Theatre complex in London and the Residenz Theatre in Munich.
HOW IT WORKS
A sine wave dimmer can best be described as an electronic autotransformer. The output is a sine wave with continuously variable amplitude. The deformation (distortion) of the sine wave is very small, typically 1-2%.
Sine wave dimming is really a misnomer, as this kind of dimmer will output whatever waveform supplies it. Feed this dimmer with a square wave, and the output will be a square wave, too. “Variable amplitude by means of pulse width modulation” is a better description, but that hardly qualifies as a marketing buzzword hence, “sine wave dimming.”
In a sine wave dimmer, the incoming power is chopped up into many slices of varying width to create an output that is essentially a true sine wave with variable amplitude. This sounds simple but requires very precise control of the IGBTs and constant monitoring of currents, voltages and temperatures to ensure reliable operation.
In a sine wave dimmer, the IGBTs are being used as fast switches, and the losses are much smaller than in FPC or RPC dimming. Sine wave dimmers are almost as efficient as SCR dimmers, around 98% at 230VAC and 96% at 120VAC.
The switching of the IGBTs is done at a high frequency, typically 30-50kHz, and while this creates electrical noise, the filters needed to eliminate this noise are a lot smaller than the bulky filter chokes needed for effective filtering of a comparable SCR dimmer. Sine wave dimming practically eliminates the main problems of FPC and RPC dimming: noise and harmonics.
The advanced technology of sine wave dimming makes it possible to offer many new and useful functions and features on dimmers. For example, sine wave dimmers can offer instantaneous overload cut off and the ability to handle indefinite shorts on the load with a return to full function as soon as the short is removed. Circuit breakers are fitted in order to comply with electrical regulations but should only trip in case of serious malfunctions.
The most frequently discussed advantage of sine wave dimming is the complete lack of filament noise, or “lamp sing.” Audible noise from lamp filaments and cables is generally uncontrollable and often distracting, depending on the specific characteristics of the lighting installation, cable lengths, wire gauge, etc. It may seem improbable, but even traditionally noisy filaments are absolutely silent when driven by a sine wave dimmer. A few rare exceptions exist, but those lamps would resonate when plugged straight into the wall, too!
Not only is the noise from the filaments eliminated, but the dimmers themselves are also quiet. Only the cooling fans are audible. This allows greater freedom of dimmer placement and enables distributed dimming to the extent of adding dimmers, if required. It may be argued that the best SCR dimmers are “good enough,” but wouldn't we prefer total silence to a little bit of noise? Newly designed concert halls are specified with very low noise floors, and still, the best SCR dimmers may not be quiet enough.
With SCR dimming, the effectiveness of harmonics and interference filtering can vary greatly with the quality of the power supplied. By nature of its design, sine wave dimming easily accommodates poor power conditions and fluctuating generator supplies.
ONE SOCKET FEEDS THEM ALL
If you want to plug a low-voltage load into an SCR dimmer outlet, you will need a transformer. Transformers cost money, take up space and, like dimming filter chokes, create an audible noise when subjected to phase-control dimming. With sine wave dimming, you can select your maximum output voltage, allowing direct connection of any low voltage load: 12V, 24V, 28V, 80V, etc.
If you want to plug in an automated light or any other device that needs a true non-dimmed feed, you will need a separate non-dim distribution panel or a non-dim module for your dimmer system. With sine wave dimming, any channel can be a non-dim with the added bonus of automatic voltage compensation, without any negative effects. The output is always a true, undistorted sine wave. And even if the channel is accidentally dimmed, the supply is still a true sine wave and should not permanently harm the equipment.
If the plot from the LD calls for a 4kW HMI backlight, but the ballast is too big and bulky to fit next to the fixture, should you rent or buy a very long head-to-ballast cable? With sine wave dimming, the dimmer can often replace your ballast (provided that the current capacity is sufficient). Want to dim cold cathode or neon? Sine wave dimming handles that, too.
THE MONEY MATTERS
True, sine wave dimmers are more expensive than corresponding SCR dimmers — a lot more expensive, in some cases. This is partly due to the complexity of sine wave technology but also to the relatively small number of dimmer channels produced and, perhaps, to a lack of serious competition in the marketplace. Right now, the market is dominated by a small number of specialist manufacturers like IES, Helvar-Electrosonic, and Dynalite.
Strand recently introduced the SST Dimmer Modules, and other major players, including ETC, will most certainly start to offer sine wave dimming in the next year or two. Once that happens, the prices are bound to drop.
Looking beyond the initial capital outlay of buying a sine wave dimming system, there are several advantages that can offset the higher cost. Sine wave dimming does not require overrated supplies, wiring, and power transformers, and it yields lower operational costs thanks to the lack of reactive power components (harmonic currents). Charges for reactive power (VAr) that is drawn from the supply but not used are also omitted from operational costs.
Unfortunately, installation and operational budgets are often separate, so the total cost of ownership and operation is not always obvious using conventional methods of comparison.
At installations where sine wave dimming has replaced SCR dimming, there is some indication that the lamp replacement is less frequent than before. This makes sense, as the completely smooth waveform of sine wave dimming causes much less stress than the sharp edges of the SCR dimmer.
As mentioned earlier, sine wave dimming saves on electricity and electrical infrastructure, decreasing environmental stress. In some cases, such as the Het Muziektheater in Amsterdam, the energy-saving aspect has been recognized by the Dutch government through an energy-efficiency grant.
In closing, I will stick my neck out to say that the days of SCR dimmers are numbered. Like the dinosaurs, they are unable to adapt to the changing environment. Sine wave dimmer technology not only eliminates the problems of SCR dimming, but it also offers new and useful functions. Existing installations of conventional SCR dimming will certainly remain for a long time, but any new installation should definitely consider sine wave dimming. The advantages are just too great to ignore.
Mats Karlsson works in sales for BellaLite. (www.bellalite.se.)
SELECTED MANUFACTURERS OF SINE WAVE DIMMING PRODUCTS
IES (International Electronic Services)