British Kinematography (1952)

June, 1952 STRANGE : DIMMING OF FLUORESCENT LAMPS 187 reduced, the dimmer impedance itself provides the stabilizing feature of the circuit. The separate impedance has the other major advantage that the full circuit voltage is across the lamp prior to the initiation of the discharge, whatever the position of the dimmer control. The dimmer impedance may take a number of forms, including a simple resistor or a saturable reactor. On the face of it, the provision of separate resistors for each lamp circuit may seem prohibitively expensive, but in practice the majority of the components of the step type resistors are of the 1W. or 2W. radio type and the costs of the control boards are only a little in excess of the corresponding incandescent boards. Saturable reactors, on the other hand, if they are to give a wide range of control must be large and expensive. The problem with the fluorescent lamp is a different one from that of controlling an incandescent one. In the latter, the current only falls to about half of the full-up value at blackout, so the impedence required is limited. With the fluorescent lamp, on the other hand, there is roughly linear relationship between light output and current so the impedance range required is very large. The first objective that was set was a range of 400 to 1 . With separate resistors this is a straightforward matter but with saturable reactors the cost rises steeply as the range is increased. Since the first developments of this circuit a number of modifications have been tried out. The chief variation of the basic circuit which has found practical application is what has been described as a cascade circuit. In this two or more lamp circuits are run off a common dimmer by means of a series of matching transformers (Fig. 7). The primaries of these transformers are connected in series with the dimmer impedance and the supply, while the secondaries feed the separate lamp circuits. In the normal arrangement of supplying a number of lamp circuits from a common supply and resistance, the first lamp that strikes reduces the potential available for the other lamps. In the cascade circuit the reverse is true, when one lamp strikes the impedance of its corresponding transformer is reduced and the potential drop across the others is increased. This arrangement is more expensive in actual control gear costs because of the extra transformer but it has considerable advantages in wiring costs, particularly in cases where building limitations come in, or where existing wiring must be used. The Relationship between Cost and Design Before passing on to describe the applications of the circuits, it may be interesting to compare the costs of some different installations. Even the most interesting and elegant solution of a technical problem may remain as an encumbrance to the patent files, or as an illustration in some handbook of the sub iJUJUULZJUULSLJUULBJUL nrrtn VZ ^3 LAMP 3 ■ >jMJL*ffiff*ug. s $=$ LAMP 2 JLAJULAJUULA^ r«AN]ro«(-1C JUUULSLSUULSL. I r**) m nrrjnnrj pnr^ pra^ Fig. 7. A cascade circuit. ject if, in fact, it does not offer either an appreciable saving in cost in installation or running, or some marked advantage in use. The considerable demands made particularly by stage producers, was the first difficulty to be met in the effort to keep the cost down. Completely smooth dimming from full-up to black-out was demanded, and quite rightly the first objective was to see how far this most difficult requirement could be met. Examination of standard dimmers showed that, in fact, some compromise with this optimum condition was generally accepted. For example, dimmers with 100 steps were most commonly used. If the change of brightness in moving from step to step was restricted to about 6 per cent the overall