Temperature control systems

Claim 1

A temperature control system comprising, in combination: I. a source of alternating current; II. an electrically operated switch including a pair of normally closed electric circuit controlling contacts and a control winding having a first end connecting with a terminal of said current source; III. a silicon controlled rectifier having its anode connecting with the second end of said control winding and its cathode connecting with another terminal of said current source; IV. an electrical resistance heating element having a linear positive temperature coefficient of electrical resistance; V. a transformer having first and second primary windings of a relatively large and relatively small number of turns, respectively, and a second winding, said primary windings being oppositely wound so as to induce magnetic fluxes of opposite phase and the ends of said secondary winding connecting across the cathode and control electrode of said rectifier such that only when the current in said first primary winding is greater than that in said second primary winding the voltage at the gate of said rectifier will be in phase with that at the anode of such rectifier; VI. first electrical circuit means including one contact member of said normally closed pair of contacts of said electrically operated switch connecting with said first terminal of said source of current and the second contact member of said pair of contacts connecting in multiple with a first end of said heating element and through an adjustable electrical resistance unit with a first end of said first primary winding; VII. second electrical circuit means connecting the second end of said heating element with a first end of said second primary winding; and VIII. third electrical circuit means connecting the second ends of said primary windings with each other and with another terminal of said current source.