A circuit for a voltage power optimiser

A controller protection circuit for a voltage power optimiser. The circuit comprising: a first terminal for connecting to a first end of a winding in the voltage power optimiser; a second terminal for connecting to a second end of the winding in the voltage power optimiser; and a thyristor. The thyristor has an anode terminal, a cathode terminal and a gate terminal. The anode terminal is connected to the first terminal. The cathode terminal is connected to the second terminal. The gate terminal is configured to receive a gate control signal. The controller protection circuit also includes a thyristor gate control circuit. The thyristor gate control circuit is configured to: set the gate control signal such that the thyristor is configured to conduct in response to a potential difference between the anode terminal and the cathode terminal of the thyristor; and set the gate control signal such that the thyristor is configured not to conduct in response to a signal received from a voltage controller. The thyristor gate control circuit comprises a normally-on switch having a conduction channel and a control terminal, wherein the conduction channel is connected in series between the anode terminal and the gate terminal of the thyristor; and also a photovoltaic isolator configured to set the gate control signal such that the thyristor is configured not to conduct in response to a signal received from a voltage controller.