Dim Me Up! A lighting seer consults his crystal ball about the next generation of dimmers. By Andy Ciddor Issue: June/July 2001 Phase control They don’t move, they don’t have many impressive buttons, wheels or video monitors, and they generally don’t really look very cool or stylish. But dimmers are integral components of any lighting system. Thanks to the ascendancy of the thyristor dimmer, as well as the abundant technological advances made in this area, dimmers are becoming hot commodities on the market. Understanding the electronics of the next generation of dimmers and its history is crucial to making purchasing decisions in the future. The last great leap forward in dimmer technology corresponded to the move from parallel analog control signals, to a single digitally multiplexed (DMX512) signal. In the analog system, each dimmer had its own control wire, which fed the control signal directly from its faders on the lighting console (i.e. 512 dimmers = 513 wires). In the digitally multiplexed system, a binary code for the level of each dimmer is sent sequentially down a single control line (i.e. 512 dimmers = 3 wires). For a dimmer rack to decode the signals for its particular dimmers, it requires at the very least some simple data processing. Of course, if you have to put a processor in your dimmer rack, it’s very tempting to have it perform a few extra tasks as well. The 1990s were the era of dimmer racks replete with test functions, dozens of adjustable parameters and LCD panels, which display the fault status of the rack and the weather forecast for the next city on your tour. Under the hood, the method of dimming the lamps has changed very little. Phase Control, the principle underlying the dimmers we use today, has been around for more than sixty years. During that time it has been implemented with a range of electronic switching devices including the Thyratron valve, the Silicon Controlled Rectifier (SCR), the Triac and, most recently, the Insulated Gate Bipolar Transistor (IGBT). Phase control dimming is accomplished by turning on the electronic switch for only a selected portion of each cycle of the supply current. Until the advent of the IGBT, none of the switches was robust enough to turn off the current once it had started to flow, and therefore dimming was achieved by waiting for the required point in each power cycle before switching on. Reverse Phase It is this sudden switching of the current, part way through the power cycle, which is the problem with phase control dimmers. The rapid change in current produced by switching the line, generates substantial radio frequency electromagnetic interference (EMI) and harmonic distortions in the power line. The EMI is radiated out from every dimmer, cable and luminaire in the rig. This can create interference with the lighting and sound control systems, radio microphones, in-ear monitoring systems, cell phones and even the listening assistance system. Harmonic distortions in the power line cause vibration and acoustic noise in dimmers, rattling lamp filaments, interference in audio, video and computer-based equipment, overheated neutral conductors; they may also damage service transformers. The most recent technology used for switching in phase control dimmers is a member of the transistor family, rather than a thyristor like the SCR and Triac. The difference is that an IGBT can do a great deal more than simply switch the current on at the required time in the power cycle: It can turn the current either on or off, and do so slowly. In fact, you can make an IGBT turn on as slowly as the combination of a thyristor and a big, heavy choke inductor, and this is what several dimmer manufacturers have done for some time now. However, the IGBT suffers from some limitations when used this way, as the current handling capacity of the present generation of IGBTs, limits the output power of the dimmers to around 1.5kw at 110v. More importantly, as with any device that attempts to limit the flow of current in a circuit, it must dissipate a proportion of the power that would otherwise have gone to the load. Because IGBT’s are switched on more slowly than SCRs or Triacs, they are obliged to handle considerably more heat than their thyristor cousins, and therefore require either substantially more natural ventilation, or forced air cooling with fans and their associated noise problems. The IPS dimmer from Rosco/ET cleverly solved both the heating and load limitation problems, by packaging a group of one kilowatt dimmers in a section of electrical raceway, which is hung from a lighting pipe in the rig. This not only spreads the heat load out along the pipe for better dissipation, but it also reduces the impact of the load limitation because each luminaire has its own local dimmer. After a decade in production, this format has so far failed to capture the imagination of a significant number of master electricians, theater consultants or electrical engineers. Meanwhile, in the European world of 220/240 volt power, where the same IGBTs will handle loads of twice the power, racks of “transistor dimmers” have been available for a year or two. There they have met with some acceptance, due to their lighter weight and lower interference, despite their higher prices. Another approach that has been tried with the IGBT is to perform Reverse Phase Control dimming, where the current is switched off partway through each supply cycle. Although there was some initial enthusiasm for this concept, it has not been widely adopted, possibly because it appears to offer few (if any) advantages over forward phase control. The future of the IGBT and of electronic dimming has already begun to take a new course, although the U.S. may well be one of the last places to feel its effect. The impact of the EMI produced by phase control dimming is becoming intolerable in a world of cell phones, personal digital assistants, and wireless data networks. It is already in the process of being regulated in the European community, although its effect is probably greater in this country. Even more importantly, the mess caused in the power grid by harmonic distortion from dimmers, and a host of other electronic devices, is approaching a critical point for both consumers and utilities, and must inevitably be limited. The European Union (EU) regulations regarding Electromagnetic Compatibility (EMC), Conducted Harmonics and Conducted Voltage Fluctuations have already been framed and enacted, but have not yet come into force. US regulations, which must inevitably come, are likely to be similar. The performance levels set in these regulations simply cannot be met by Phase Control dimmers. A different approach to dimming will be required: one where no big chunks are cut from the power cycle—exactly like the resistance dimmer of yesteryear. Several groups around the world are developing dimmers that use IGBTs to cut thousands of small chunks out of each power cycle, leaving something that looks remarkably like a sine wave power cycle, only smaller. One such dimmer—the Bytesize VST from Bytecraft in Australia, was on display at the Las Vegas LDI show in 2000. The VST has already been installed in The Studio at the Sydney Opera House. The complete lack of filament noise from lights being dimmed, facilitates the high quality recording and broadcast of live music performances. Right now these dimmers are expensive, but so were thyratron and SCR dimmers when they each made their appearance. The new direction for the dimmer is to be even more unobtrusive than at present. They will sit quietly dimming our shows without any sign of their presence in the electrical, acoustic or electromagnetic environment. At last. sd Andy Ciddor has been involved in lighting for over three decades; as a practitioner, teacher and technical writer. He can be connected via e-mail at aciddor@lightinglinks.net