Heater control

A control system for controlling parallel heater elements comprising:

• a plurality of electrical heater elements;
• a plurality of temperature sensors associated with a plurality of the heater elements, the temperature sensors effective to generate temperature signals;
• at least one temperature controller effective to generate a control signal based on at least one temperature signal;
• an alternating current power supply connected to the electrical heater elements through a power transmission system;
• a zero detection circuit effective to generate a signal when the voltage of the alternating current power supply passes through ground potential;
• a firing controller that will block electrical power to a heater element, starting when a zero detection signal is detected an ending when another zero detection signal is detected, for a fraction of the alternating current cycles in a time period, the fraction being a function of the control signal;
• a remote modem in the proximity of the heater elements effective to convert the temperature signals to signals transmitted through the power transmission system and to detect a control signal transmitted through the power transmission system;
• and
• a master modem effective to convert the control signal to a signal transmitted through the power transmission system and to detect temperature signals transmitted through the power transmission system.