Apparatus and methods for controlling fan operation

Claim 1

Apparatus for controlling the operation of electrically driven ventilator means in response to the difference in temperature between first and second locations, comprising an electric switch for controlling operation of said ventilator means, a first temperature responsive diode for sensing the temperature at said first location and a second temperature responsive diode for sensing the temperature at said second location, means for operating said switch in response to temperature differences between said first and second locations to start said ventilator means in operation when the difference in the temperatures between said first and second locations exceeds a small predetermined temperature difference and to stop operation of said ventilator means when the difference in the temperatures between said first and second locations does not exceed said small predetermined temperature difference, said operating means comprising an electrical bridge circuit having first and second input terminals, a direct current electrical source, means connecting said source to said first and second input terminals, said bridge circuit including a pair of parallel branches connected between said first and second input terminals each having an intermediate juncture point and each having a fixed resistor between said juncture point and said first input terminal, said first and second temperature responsive diodes being disposed one in each of said branches between said juncture points and said second input terminal, an integrated circuit differential amplifier serving as a voltage comparator connected between said juncture points and serving to amplify voltage differences caused by temperature differences at said diodes at said first and second locations, a transistor connected to said source and connected to said amplifier to receive the voltage output thereof and being turned on in response to voltage outputs resulting from temperature differences between said first and second locations exceeding said small predetermined temperature difference and being turned off in response to voltage outputs resulting from temperature differences between said first and second locations not exceeding said small predetermined temperature difference, a solenoid in series with said transistor connections to said source adapted to close said switch to turn on said ventilator means when said transistor is turned on and to open said switch to turn off said ventilator means when said transistor is turned off.