Fail-safe relay drive system for cooking apparatus

A fail-safe control system for operating a power switching device to control a heating element in a cooking appliance, said control system comprising:

• logic circuit means having an output port and logic means for switching said output port between first and second logic states, wherein said logic means is adapted to switch said output port from a first state to a second state for a predetermined first period of time and thereafter repetitively switch said output port from said first to said second state for a predetermined second period of time that is less than said first period of time;
• drive circuit means connecting said output port with a power switching device for applying power to said power switching device in response to said output port being in said second logic state and for removing power from said power switching device after power has been applied thereto longer than a third predetermined period of time that is less than said first period of time;
• and
• means for maintaining said drive circuit means in a state applying power to said power switching device during said first period of time in order to ensure that such power is applied to said power switching means for a sufficient period of time to operate the latter.

A fail-safe control system for operating a power relay to energize an electrical load in a cooking apparatus, said control system comprising:

• first and second driving means connected in series with said power relay and a source of power, each of said driving means being switchable between an on state and an off state for electrically connecting said power relay with said source of power only when both of said driving means are in said on state;
• logic circuit means having a port adapted to produce first and second signal levels;
• a first branch circuit connecting said port with said first driving means, said first branch circuit including a first comparison means for producing a first biasing signal to turn on said first driving means when a signal is presented to an input of said first comparison means which has a given relationship to a first predetermined level, and a capacitor connected between said port and said input of said first comparison means;
• a second branch circuit connecting said port with said second driving means, said second branch circuit including a second comparison means for producing a second biasing signal to turn on said second driving means when a signal is presented to an input of said second comparison means which has a given relationship to a second predetermined level, and means for connecting said port with said input of said second comparison means;
• and
• said logic circuit means including means for producing said first signal level for a predetermined first period of time that is sufficient to establish a given charge condition on said capacitor and for producing said second signal level for a predetermined second period of time after said first period of time.

A fail-safe control system for energizing a control winding of a power relay to thereby actuate a heating element in a cooking apparatus, said control system comprising:

• a microcomputer programmed to produce a signal on a selected output port;
• first and second transistors having emitter-collector junctions connected in series with each other, with a control winding of a power relay, and with a source of power;
• each of said transistor emitter-collector junctions being switchable between a conducting state and a non-conducting state in response to a bias signal applied to a base of said transistor;
• a first branch circuit connecting said port with said base of said first transistor, said first branch circuit including a first operational amplifier having first and second inputs and an output, said first branch circuit further including a capacitor connected between said first input and said port, a first reference voltage connected with said second input and means for connecting said output to said base of said first transistor, said first operational amplifier including means for producing a first biasing signal at said output when said first input has a given voltage relationship to said second input;
• a second branch circuit connecting said port with said base of said second transistor, said second branch circuit including a second operational amplifier having first and second inputs and an output, said second branch circuit further including means for connecting said first input of said second operational amplifier with said port, a second reference voltage connected with said second input of said second operational amplifier, and means for connecting said output of said second operational amplifier with said base of said second transistor;
• and
• said second operational amplifier including means for producing a second biasing signal at said output thereof when said first input thereof has a given relationship to said second input thereof.