Heater control with high-limit thermal safety shutdown

A method of providing a high-limit thermal safety shutdown for a heater that is powered by AC power via an AC power circuit, comprising:

• positioning a high-limit switch in the AC power circuit that is capable of turning the AC power to the heater on and off;
• and
• controlling the high-limit switch with a high-limit control circuit that is powered by low voltage DC power from a DC power source and that is programmed to (i) compare a temperature sensed by a temperature sensor that is positioned in close enough proximity to the heater to sense heat produced by the heater to a preset high temperature limit parameter, (ii) cause the high-limit switch to turn off the AC power to the heater if the temperature sensed by the temperature sensor exceeds the preset high temperature limit, and (iii) cause the high-limit switch to turn on the AC power to the heater if the low voltage DC power to the high-limit control circuit has been stopped and then started again and if the temperature sensed by the temperature sensor does not exceed the preset temperature limit.

A method of providing a high-limit thermal safety shutdown for a heater that is powered by AC power via an AC power circuit, comprising:

• positioning a high-limit switch in the AC power circuit that is capable of turning the AC power to the heater on and off, said high-limit switch including a normally open relay switch that is powered by a coil to close and thereby to turn the AC power on and that opens when the coil is not powered;
• and
• positioning a PTC thermistor in the heater, directing electric current to flow through the PTC thermistor in series with a coil power circuit so that the coil is powered from current that flows through the PTC thermistor, providing a drain circuit that closes when current stops flowing through the PTC thermistor and that remains closed to drain current away from the coil when current starts flowing again through the PTC resistor.

High-limit thermal safety shutdown apparatus for a heater that is powered by AC power via an AC power circuit, comprising:

• a high-limit switch in the AC power circuit that is capable of turning the AC power to the heater on and off;
• and
• a high-limit control circuit that is powered by low voltage DC power from a DC power source and that is programmed to (i) compare a temperature sensed by a temperature sensor that is positioned in close enough proximity to the heater to sense heat produced by the heater to a preset high temperature limit parameter, (ii) cause the high-limit switch to turn off the AC power to the heater if the temperature sensed by the temperature sensor exceeds the preset high temperature limit, and (iii) cause the high-limit switch to turn on the AC power to the heater if the low voltage DC power to the high-limit control circuit has been stopped and then started again and if the temperature sensed by the temperature sensor does not exceed the preset temperature limit.

High-limit thermal safety shutdown apparatus for a heater that is powered by AC power via an AC power circuit, comprising:

• a high-limit switch in the AC power circuit that is capable of turning the AC power to the heater on and off, said high-limit switch including a normally open relay switch that is powered by a coil to close and thereby to turn the AC power on and that opens when the coil is not powered;
• a PTC thermistor positioned in the heater and connected electrically in series with the coil by a PCT thermistor circuit;
• a closable and openable drain circuit connected to the PTC thermistor circuit electrically in parallel with the coil in a manner that drains current away from the coil in a closed mode, said drain circuit including a drain circuit control that closes the drain circuit automatically in response to a no current condition in the PTC thermistor circuit and that opens the drain circuit in response to a manual input.