Reactive load drive and surge protection

Claim 1

An electrical drive circuit arrangement to control the energisation of a reactive load with a high alternating voltage including a control stage to generate an alternating control signal and apply it to an output of the control stage, an electrically isolating control signal transfer means having an input connected to the output of the control stage to transfer the control signal to an output of the transfer means while isolating the output from the control stage, an output stage including power control semiconductor devices and having a control input connected to the output of the transfer means, an energisation input for a high voltage electrical supply in excess of 300 volts d.c. and an output for the connection of a said reactive load to which in operation the output stage applies said high voltage alternating energisation in dependence on the control signal, the high voltage being prevented from reaching the control stage via the input of the output stage by the isolating transfer means, said control stage further including means to provide circuit condition signals and a noise-resistant guard means responsive to said signals to permit generation of the control signal only when said condition signals have selected values, the transfer means further including means to restrict the signal to the input to the output stage to the control signal permitted by the guard means, and the output stage including means to generate a protection signal on the occurrence of a current surge through said devices in the high voltage energisation of a connected reactive load, the protection signal being constrained to a potential low enough for direct application to the control stage without the need for isolating transfer to protect against high voltage in the control stage and the control state including means responsive to surge protection signal to act on the control signal and remove any energisation of the load by the output stage to protect the stage.