Multiple variable phase control circuit

A temperature control circuit for switching an alternating current for a controlled fraction of each half cycle to a water heater comprising:

• a current charging circuit means including a summing capacitor which is charged thereby each half cycle;
• a unijunction transistor having a threshold firing level;
• a timing capacitor associated with said unijunction transistor;
• a feedback circuit means coupled to sense the fraction of the alternating current supplied to the heater each half cycle and discharging said summing capacitor each half cycle in accordance therewith;
• and
• a temperature comparing circuit means for comparing the temperature of the water heater with a desired set temperature and providing an output for further discharging said summing capacitor each half cycle as a function of the temperature difference;
• said current charging circuit means providing for initially charging said timing capacitor each half cycle to a pedestal voltage level dependent on the residual charge on said summing capacitor and thereafter along a ramp whereby the phase angle at which the ramp reaches the threshold firing level of said unijunction transistor can be made to vary in accordance with the residual charge on said summing capacitor to thereby control the fraction of each half cycle that the alternating current is switched to the heater.