any of said resistors alone the coupling principle of fig3 indicates that for higher input voltages the switch 302 should remain open so that only resistor 109 is used for maximum primary current limiting for optimal performance at lower input voltages the resistance of resistor 109 is too high so as a response to a lower input voltage the switch 302 is closed to take the lower combined resistance of resistors 109 and 301 into use for maximum primary current limiting fig4 illustrates an alternative embodiment to that in fig3 in the circuit arrangement of fig4 the resistance used for primary current sensing is a series connection of the traditional current sensing resistor 109 and another resistor 401 which latter is additionally connected in parallel with a shunting switch 402 a control signal for driving the switch 402 is taken directly or derived indirectly from the input voltage as is represented schematically by arrow 403 since the combined resistance of two serially connected resistors is always larger than the resistance of any of said resistors alone the coupling principle of fig4 indicates that for higher input voltages the switch 402 should remain open so that the combined resistance of resistors 109 and 401 is used for maximum primary current limiting for optimal performance at lower input voltages said combined resistance is too high so as a response to a lower input voltage the switch 302 is closed to short circuit resistor 401 leaving only the resistance.