d c supply 12 this causes a firing pulse to be supplied to thyristors 36a and 36b at time t 19 which is the termination of the normal operative period as time t 19 is subsequent to time t 6 this has little effect on the operation of thyristor bridge 24 while control signal source 134 has been shown as an independently controllable element in fig8 it will be appreciated that the signal source will typically include reference and feedback regulating circuitry coupled to the energy source utilization circuitry which adjusts the firing angle of the thyristors in bridge 24 to achieve desired operating conditions in the energy source utilization circuitry for example the firing angle of the thyristors may be regulated in accordance with the output voltage of d c power supply 12 to provide a level of voltage 50 below which the circuitry will not take power from the power supply the current i in the energy source utilization circuitry may be used to regulate thyristor firing angles so as to control the rise in voltage which takes place as the output of power supply 12 increases from zero to maximum current and to establish a protective current limit if necessary it will be appreciated that.