A current output apparatus adapted for electrical connection to a load to output an electrical current thereto, comprising: a current output terminal for connecting to a first terminal of the load to pass an electrical current thereto; a current input terminal for connecting to a second terminal of the load to receive said electrical current returned from the load; a first transformer winding and a second transformer winding, both defining a transformer with a transformer drive winding of an external power supply; a voltage regulator (20, 23, 24) that is configured to receive an input current from a terminal of a full-wave bridge rectifier unit having a positive voltage polarity and is operable to supply to the current output terminal an electrical voltage regulated to differ from an electrical voltage applied to the current input terminal by an amount sufficient to cause said electrical current to flow from the current output terminal to the current input terminal via the load, the voltage regulator (20, 23, 24) comprising an output port; wherein the voltage regulator is operable and arranged to supply to the current output terminal an electrical voltage of positive polarity or of negative polarity for supplying said electrical current to a load presenting to the current output terminal a voltage of positive polarity or of negative polarity, respectively, characterised by further including a synchronous rectifier arranged to rectify an AC voltage signal supplied by the first transformer winding coupled thereto, and to supply the rectified voltage to the voltage regulator wherein the synchronous rectifier comprises field-effect transistors, FETs, of P-type and N-type, and a control unit arranged to synchronously control the conductivity of the transistors to supply to the voltage regulator a DC electrical voltage of negative polarity, wherein the control unit includes the second transformer winding coupled to the gate terminals of said FETs of the synchronous rectifier, wherein the polarity of voltage at the second transformer winding is controllable for applying a positive-polarity voltage to the gate terminals of FETs of the synchronous rectifier coupled to an end of the first transformer winding when that end has a negative-polarity voltage such that a negative-polarity voltage of the first transformer winding is applied to the output port of said voltage regulator (20, 23, 24). a current output terminal for connecting to a first terminal of the load to pass an electrical current thereto; a current input terminal for connecting to a second terminal of the load to receive said electrical current returned from the load; a first transformer winding and a second transformer winding, both defining a transformer with a transformer drive winding of an external power supply; a voltage regulator (20, 23, 24) that is configured to receive an input current from a terminal of a full-wave bridge rectifier unit having a positive voltage polarity and is operable to supply to the current output terminal an electrical voltage regulated to differ from an electrical voltage applied to the current input terminal by an amount sufficient to cause said electrical current to flow from the current output terminal to the current input terminal via the load, the voltage regulator (20, 23, 24) comprising an output port; wherein the voltage regulator is operable and arranged to supply to the current output terminal an electrical voltage of positive polarity or of negative polarity for supplying said electrical current to a load presenting to the current output terminal a voltage of positive polarity or of negative polarity, respectively, characterised by further including a synchronous rectifier arranged to rectify an AC voltage signal supplied by the first transformer winding coupled thereto, and to supply the rectified voltage to the voltage regulator wherein the synchronous rectifier comprises field-effect transistors, FETs, of P-type and N-type, and a control unit arranged to synchronously control the conductivity of the transistors to supply to the voltage regulator a DC electrical voltage of negative polarity, wherein the control unit includes the second transformer winding coupled to the gate terminals of said FETs of the synchronous rectifier, wherein the polarity of voltage at the second transformer winding is controllable for applying a positive-polarity voltage to the gate terminals of FETs of the synchronous rectifier coupled to an end of the first transformer winding when that end has a negative-polarity voltage such that a negative-polarity voltage of the first transformer winding is applied to the output port of said voltage regulator (20, 23, 24).