| 摘要 |
A load L is supplied with constant current by selecting a tap 12a-12g of the secondary 12 of a transformer 7 and selecting a series resistor 22,23 in conjunction with the selection of transformer tap 12-12g. The control unit CU, acting in dependence upon sampled measurements from current and voltage transformers 19,27 may select from the taps 12a-12g by turning ON one of four triacs 15 connected respectively to taps 12a-12d, and one of four triacs 16 connected respectively to taps 12d-12g, and selects one or more of the resistance sections 22 by turning ON transistors 24 individual to those sections 22. Each resistance section 22 may be a single resistor 33 or a plurality of resistors 33 connected in parallel. The resistance of resistance sections 22 may be related to each other in a geometric progression of common ratio two and operation may be initiated by selecting the transformer tapping that gives the lowest voltage and the highest value resistor, whereupon the load resistance of the load may be calculated and the resultant value used when controlling the current. The controlled current is substantially sinusoidal and load L may comprise an airfield lighting system where the lights are in series to ensure consistent brightness and each light is fed by an isolating transformer. |
