It is now more than four years since a group of industry experts gathered at PLASA 96 to prophesise about the future of dimming technology. The discussion was prompted by the anticipated changes in European regulations regarding electrical disturbances and noise. These changes, which were expected to almost ‘outlaw’ traditional triac and thyristor dimming systems by 2001, were intended to protect electrical supplies in the face of an ever increasing collection of noise-inducing office and domestic electronic equipment such as photocopiers, computers, motors and of course, dimmers.
   Dimming technology has developed significantly since the introduction of SCRs, even if the basic principle of dimming a light has remained the same. Original pulse-fired dimmers gave way to hard-fired dimmers; then closed-loop dimmers with stabilisation feedback, different response curves for TV and theatre applications, high rise-time chokes to meet TV noise limits, dimmers with current monitoring, voltage stabilisation, fault reporting. Each of these developments were ever more sophisticated, complex, more costly. The limits of the original analogue design criteria become overstretched.
   Digital dimmers offered a sophisticated software-based answer to the expanding list of specification clauses. The list of digital benefits seemed endless, until somebody mentioned the historic issue of noise. If anything, problems associated with electrical noise worsened as the demand for more compact and cheaper dimmers had forced manufacturers into replacing large and bulky (but nevertheless effective) copper and iron chokes for wedding-ring toroidal styles. For all the advances, the noise from filaments, cables, dimmers and trunking was getting worse.
   Growing concerns over electromagnetic compatibility and acoustic noise of filaments led dimmer designers to look for an alternative to the traditional and very successful methods.
   Alternative dimmer technologies had been tried by many manufacturers in the past 20 years or so. MOSFET transistors, GTO thyristors, high frequency switch-mode dimmers, sine-wave reconstructing dimmers, DC dimmers. However, nothing could match the cost, compactness, simplicity and efficacy of the digital triac dimmer. Manufacturers hoped the problem would go away and that changes to European standards would disappear through industry efforts to re-designate higher-power dimmers outside the scope of the legislation. However much the situation was re-defined and legislated for, the central problem still existed; when a power switching device switches rapidly during a mains half-cycle the resultant electrical disturbance causes interference, acoustic noise from the filaments and harmonic currents.
   IES was one dimmer manufacturer aware both of the problems and changes in legislation which were on the horizon, and decided to take positive action. Based in Veenendaal in The Netherlands, IES had been experimenting with alternative dimming technologies since the mid 1980s, producing a line of MOSFET dimmers and experimenting with GTOs. The initial reason was not to find the holy grail of dimming silence, but to move dimming technology along to accommodate the demands of an enthusiastic European rental market. We began by researching possible power control techniques for a dimmer design which was extremely rugged, could accommodate a wide range of loads and power supplies, and which was less sensitive to fault conditions. The cost of MOSFET dimmers, and their tendency to premature failure due to thermal run-away led us to adapt our existing IGBT tech-nology used by our industrial electronics division for the control motors and battery chargers.
   The IGBT (Insulated Gate Bipolar Transistor) is a high-power transistor which can be controlled like any other transistor; switched on and off or controlled in a linear mode. For high power control, efficiency is a key factor, and so a switch-mode is the optimum solution. Like a triac dimmer, the IGBT switches power to the lamp each half-cycle, but it can also switch current off as well as switching it on. This ‘reverse-phase’ technique means that the huge current surge each half cycle (which rise to peak currents of 20 times the nominal - that’s 84A for a 1kW lamp) is eliminated, and any harmonics created go to balance out the harmonic currents experienced by other dimmers in the system. The technology is extremely versatile, and for critical applications where lowest levels of interference and noise is specified, dimmers may be configured in a lower-density arrangement which accommodates the higher heat dissipation required to meet the noise limits. A sine wave version is also available, but the high levels of electrical interference, increased weight and cost make this impractical for most European applications.
   Since 1995, IES have produced a range of dimmers using IGBT devices in conjunction with their own custom hardware and software technology which is called ‘itec’. IES’s itec technology monitors voltage, current and temperature at a rate of 40kHz and dynamically adjusts the operation of the IGBT to ensure that the current waveform is optimised for performance and protection of the device and the load circuit. In a normal overload situation using a triac dimmer (such as a filament failure or a wiring fault), a high current builds up within microseconds and after a delay, during which time a thermal or magnetic device responds to the overload, power is mechanically disconnected. With an itec dimmer, an overcurrent is measured within 2µS and the dimmer switched off in another 5µS. This is much faster than any thermal or magnetic protection, and therefore also protects cables and other components in circuit. As the fault could be either temporary (a filament briefly shorting out as it collapses) or permanent (a wiring error) the itec dimmer checks the circuit repeatedly over a period of 10 seconds. If the fault has cleared, the dimmer soft-starts and brings back power to the circuit. An added benefit of this feature is when multiple paralleled loads are connected to one dimmer, as is now the case with 575W lamps where four could be used in parallel. If one filament fails with a momentary short circuit, power is not mechanically disconnected and the other three lamps are not affected. However, if the fault continues, the dimmer is disabled until remedial action is taken.
   For some time, IGBT dimmers were considered unreliable, slow to respond to a snap change, and could not work successfully in a chase situation. It is interesting that the main drive for this technology, and the early adopter of it, has been the rock and roll industry, so there is obviously a solution. The reason for the assumption that IGBT dimmers are slow dates back to MOSFET time. In order to protect the device from the effects of inductive load cables and high impedence (‘soft’) main supplies, the dimmers went through a gradual switch-on process from cold during which time the dimmer was able to measure the rate of change of current and take action if the device was at risk of being damaged by a short circuit or overcurrent demand. The technique is still used, but the speed of the measurement is now running at 40kHz, plus the latest devices used are capable of withstanding high overload currents means that the speed of a cold-start has improved to the point where it is equivalent to a triac dimmer. A continuous chase or flash effect is noticeably faster using IGBT than a digital triac dimmer.
   The reliability issue is only proved by experience. In the first months of the initial IES installations in Europe, higher levels of device failures than expected were recorded, and much effort was put into selecting devices and fine-tuning the software to achieve the performance of the dimmers and the added features suggested by users. There had been a much wider variation in device specification tolerance than had been expected. Also mains supply voltage and impedence varied widely throughout Europe which, added to the different characteristics of installed load cables, gave rise to early concerns about IGBT mortality rates. However, detailed research on a number of sites identified the criteria and this provided the route to a universal solution.
   The latest generation of itec modules were first installed in the UK in mid-1998, and those used in distributed dimming applications (coincidentally with low impedence mains) have worked faultlessly ever since. The initial problems experienced with installed versions of itec modules were cleared within months, and by mid-1999 the design of the ‘generation five’ IGBT circuit and power ratings was finalised. The largest installation in Europe so far is at the Cottesloe Theatre at the Royal National Theatre in London where 600 channels of IES Executive IGBT dimmers were installed in September 1998 to provide performance lighting, houselighting and working light dimming. Since then, IES dimmers have become established throughout Europe including installations in national theatres, opera houses and broadcast TV studios. The IES range comprises installed dimming systems (based on a core Executive 30kW dimmer pack with customer-specified 19-inch cabinets), Power-Modules and PowerBars for distributed dimming applications. A quantity of PowerModule IGBT distributed dimmers was installed in July 1998 at the BBC’s 24-hour multi-media TV complex in London, and installations have followed in several other central and regional BBC and independent studios since then.
   For many sectors of the industry, distributed dimming offers significant benefits over the traditional installations, particularly in greenfield sites where the costs of installing dedicated power systems and load cables can swap the budget for dimming. With a distributed dimming installation, mains power and DMX outlets are all that is needed throughout the venue, and in most cases this is a simple and economic electrical contracting job. These power and data points can then be used for a wide variety of distributed dimmers, automated lights, scanners, scrollers and other effects. IES’s proprietary DimSTAT network software communicates to all IES devices in the system to allow dimmer characteristics to be adjusted remotely, status information and fault reports to be displayed on a PC, and even the uploading of new operating software. The growing adoption of IES dimmers in distributed dimming projects has come about because of the sophistication of the remote set-up and the self-correcting electronic short-circuit protection.
   Many applications still require installed versions of IGBT dimmers, and the flexibility of IES’s Executive dimmer pack has proved itself time and again. In addition to all the features of a distributed version, the Executive offers self-contained processing for up to twelve 2.5kW dimmers, dual DMX lines (each dimmer can be set to any DMX address from either line with a range of arbitration options), and internal preset memories. DimSTAT software is networkable, and even includes libraries of set-up characteristics for different lighting loads.
   IES’s installed dimming system is so flexible, clients are invited to specify the size, style and layout of each cabinet, its cable entry points, plus the integration of auxiliary power switching. IES also produces custom matching cabinets for main switchgear.
   But in the end, all things come down to cost and reliability. Reliability has been proven with over two years of successful performance in a wide range of venues. IGBT dimmers are not the cheapest on the market compared with triac dimming, but when the total dimming installation is assessed they are affordable, and the features and benefits available make them good value for money. But don’t ask me. Ask my customers!