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Atlas fluorescent auditorium-lighting dimmers

Dimming can be difficult...

Fluorescent lighting dimmer

Fluorescent lighting dimmer

Ordinary filament lamps are dimmer-friendly because the filament is simply a resistive heater that glows white-hot; any device that varies the voltage supplied to the lamp will vary its power dissipation and hence the brightness of its glow. Many techniques can be used to achieve this and subject to minor limitations all can offer smooth control from near zero to full brightness. Variable resistors, transformers and inductors all work well with tungsten filament lamps but have drawbacks of size, cost or difficulty of remote control. For compactness and versatility the phase-angle method is far superior; this works by switching the filament current on and off 100 times per second for a variable fraction of the time. A phase-angle control dimmer designed for tungsten lamps would not be suitable for dimming a conventional fluorescent fixture; special circuitry is needed to achieve a useful dimming range while catering for the rather particular requirements of the tubes.

The characteristics of gas discharge lamps such as fluorescent tubes mandate the use of an external ballast circuit to make them compatible with ordinary mains supplies. Fluorescents also require control gear to operate the electrode heaters and give out a high-voltage starting impulse; these functions are provided by a choke and 'glow starter' (in non-dimmed units) or alternatively a special transformer. Under phase-angle control these components add complication to the dimmer circuit, which needs to be electrically robust by comparison to a tungsten dimmer. In practical installations the range is somewhat limited by the tendency for the tubes to flicker or extinguish at low output, although for architectural lighting the range is usually adequate. Modern dimmable fluorescent installations employ integral high-frequency electronic control gear, each unit of which behaves like an adjustable regulated power supply. This enables flicker-free operation over a wider range of brightness without curtailing tube life or requiring non-standard wiring installations.

A thyratron solution

Mullard thyratrons

Mullard thyratrons

This fluorescent dimmer bank was supplied by Atlas Lighting as part of an auditorium-lighting installation for the St. Albans Civic Centre in Hertfordshire in the 1960s. Five separate electrical circuits of lighting fixtures are dimmed as a group using remote pushbutton controls. The active part of the circuit that handles the load current consists of five pairs of thyratron valves, which are gas-filled high power switching devices closely related to ordinary radio valves. Each Mullard XR1-6400A xenon triode thyratron is rated for 6.4 amps (average) at 1500V inverse peak. The pairs of valves, connected back-to-back, are built into modules equipped with triggering transformers, bias controls, chokes, and heater transformers providing two highly isolated windings each delivering 21A per valve at 2.5V. The power supplies required to bias and trigger the thyratrons are derived from a common control circuit which includes a solenoid-operated clockwork timer arranged to allow the thyratrons to reach working temperature before allowing them to pass load current. The forward drop of each thyratron pair is approximately 16V at full load; with the brightness control set to maximum each dimmer module is short-circuited by a relay to increase efficiency and extend valve life. Thyratrons of this type were used for many other tasks that required the AC mains waveform to be chopped or controlled on a cycle-by-cycle basis, for example spot welder controllers and variable-speed machinery drives. The introduction of smaller, longer-lasting solid state thyristors and triacs eventually made them obsolete for all but the most demanding situations.

Inside fluorescent dimmer

Inside fluorescent dimmer

The dimmer cabinet houses not only the electronic modules but also the remote control mechanism, power factor correction circuits and fuses and circuit breakers to protect the wiring. The user controls consist of wall-mounted pushbutton stations which were originally installed in the projection box and stage areas. Pressing either the 'up' or 'down' button energises a corresponding relay to start a small reversible AC motor in either clockwise or anticlockwise direction. This rotates, via a reduction gearbox, the shaft of a variable resistor that controls the thyratron firing delay circuit. Also on the potentiometer shaft is a cam assembly that actuates microswitches at various points in the dimming range, to control such circuit functions as power-factor correction and thyratron bypassing in the 'full-up' position. The electrical supply configuration is unusual in that two phases of the mains are used; each fixture group is provided with both a phase-angle controlled feed and a constant supply for the tube heater transformers which remains at full voltage even when the dimmer is turned down. A bank of paper-in-oil capacitors mounted in the bottom of the cabinet improves the otherwise poor power-factor at the lower end of the dimming range, under the control of mercury relays selected by the camswitch.

Can it be made to work again?

The unit was obtained from its original location during a refit, although at that time it had been out of use for an unknown duration. It is not clear whether any of the original Atlas lighting fixtures survive to be recovered in the future, so it may be necessary to make up some units of equivalent characteristics if the dimmer is to be demonstrated. Its ultimate durability is also limited by the finite service lifetime of the thyratrons, although a box of spare valves both new and used was found in the switchroom near the dimmer. Here at Electrokinetica we like dimmers with a bit of personality and might in the future be found hiding in the switchroom with the lights off, basking in the cosy glow of the thyratrons, watching the spectacular flashing of the mercury relays accompanied by the click-whirr-click of the motorised resistance...

Motorised potentiometer

Motorised potentiometer

Circuitry inside dimmer

Circuitry inside dimmer

Mercury switches

Mercury switches

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