a constant of value 0 77 the power converter apparatus 40 shown in fig2 derives its power from a photovoltaic array 41 made up of photovoltaic cells 42 the positive terminal 43 and the negative terminal 44 of the photovoltaic array 41 are connected to the switching power converter 45 and the latter is in turn connected to a load 46 an oscillator 47 is connected to an inhibiting control function on the switching power converter 45 and inhibits the operation of the switching power converter 45 for fifty milliseconds every ten seconds a diode 51 has its anode connected to the positive terminal 43 of the photovoltaic array and its cathode connected to a capacitor 52 the other terminal of the capacitor 52 is connected to the negative terminal 44 of the photovoltaic array 41 an upper resistor 53 and a lower resistor 54 form a resistive divider across the capacitor 52 and the junction between the resistors 53 and 54 is connected to one input of a comparator 57 the other input to the comparator 57 is connected to the common connection point of a resistive divider comprising a positive rail resistor 55 and a negative rail resistor 56 the relative values of the resistors 53 and 54 and 55 and 56 are arranged such that the voltages.