| 摘要 |
A controller ( 10 ) for a heating cable. The heating cable includes at least first ( 1 ) and second ( 5 ) conductors and a separation layer ( 4 ) interposed between the conductors ( 1, 5 ). The conductors ( 1, 5 ) and the separation layer ( 4 ) extend along the length of the cable and the electrical resistance provided by the separation layer ( 4 ) between adjacent portions of the conductors ( 1, 5 ) has a negative temperature coefficient. The controller ( 10 ) includes a first switch ( 13 ), arranged for connecting the first and second conductors ( 1, 5 ) electrically in series at one end of the cable such that if the first and second conductors ( 1, 5 ) are connected at the other end of the cable to respective poles of a power supply (L, N) currents flow in opposite directions through adjacent portions of the conductors ( 1, 5 ). The controller ( 10 ) further includes a second switch ( 16 ), arranged for connection in series between the first conductor ( 1 ) and a pole (L) of the power supply at the other end of the cable. The controller ( 10 ) further includes a first resistor ( 15 ) connected in parallel with the second switch ( 16 ) and voltage measurement means ( 18 ), arranged to measure the voltage across the first resistor ( 15 ), the voltage across the first resist or ( 15 ) when both switches ( 13, 16 ) are open being responsive to changes in the electrical resistance of the separation layer ( 4 ). The controller ( 10 ) is arranged to control the supply of power to the cable as a function of the measured voltage across the first resistor ( 15 ) when both switches ( 13, 16 ) are open.
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