Converting on-off furnace controllers to proportional operation

The majority of galvanometric on-off furnace controllers can be converted to proportional operation, with a consequent improvement of control characteristic, by injecting a low-frequency modulating signal into the thermocouple circuit. This note describes a simple battery-powered oscillator designed to be added to existing controllers; one lead from the thermocouple is broken and the free ends are connected to the output terminals of the oscillator. An extension to the basic modulator enables the circuit to compensate changes in the voltage of the mains supply to the furnace.