Demand defrost system

Claim 2

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for initiating a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a detection signal and directing said signal along and through a plurality of portions of said heat exchanger along a direct path which passes through the central portion of the heat exchanger between adjacent parallel fins, said path being oriented in a plane generally parallel to said adjacent parallel fins, said detection signal having a characteristic with varies in direct response to build-up of frost on any one of said portions of the planar surfaces of said adjacent parallel fins of the heat exchanger along said path; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said first peripheral location, for sensing said variation in said detection signal characteristic and for producing a control signal in response to said characteristic achieving a selected value; means responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; and means responsive to failure of said detection signal generating means for precluding initiation of said defrost cycle.

Claim 3

The demand defrost system of claim 1 wherein said defrost cycle initiating means sustains said defrost cycle independently of subsequent termination of said control signal.

Claim 4

The demand defrost system of claim 1 including temperature sensing means actuated in response to a selected temperature condition indicating that defrosting is complete for effecting termination of the defrost cycle.

Claim 5

The demand defrost system of claim 3 including defrost cycle termination means for terminating said defrost cycle and initiating the cooling cycle in response to actuation of said temperature sensing means.

Claim 6

The demand defrost system of claim 4 in which the cooling system includes air circulation fan means which blows air across the heat exchanger to the cooling area; said defrost cycle initiating means de-energizing said fan means during the defrost cycle.

Claim 7

The demand defrost system of claim 5 wherein said temperature sensing means is deactuated in response to a selected temperature condition indicating that the heat exchanger is substantially free of liquid moisture for energizing said fan means after the cooling cycle has commenced.

Claim 8

The demand defrost system of claim 1 in which the detection signal is a light signal directed through the heat exchanger.

Claim 9

The demand defrost system of claim 7 in which said sensing means is a light sensitive device producing said control signal in response to a selected reduction in the intensity of light received by the light sensitive device.

Claim 10

The demand defrost system of claim 1 including at least one detection signal inhibitor located in the heat exchanger which varies the characteristic of the transmitted detection signal as a function of frost build-up on the inhibitor.

Claim 11

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, an air circulation fan for passing air directly over the heat exchanger through the fins into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for energizing the heater to initiate a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a light detection signal along a direct path through a plurality of portions of the heat exchanger, said path passing through the central portion of said heat exchanger between adjacent parallel fins, said path also being oriented in a plane generally parallel to said adjacent parallel fins, whereby frost which builds up on any one of said portions of the planar surfaces of said adjacent parallel fins of the heat exchanger blocks the path of the light transmitted therethrough; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said peripheral location, for sensing the variation of transmitted light transmitted through the heat exchanger along said path and for producing a control signal in response to the intensity of light received falling below a selected value, said means for sensing the variation of transmitted light intensity being a light sensitive device having a first conductive state in response to the light received exceeding the selected value and having a second conductive state in response to the light received falling below the selected value; means responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; a switching transistor rendered conductive for effecting energization of first switching control means to produce a control signal in response to the second conductive state of said light sensitive device; and second switching control means energized in response to said control signal for opening a first set of normally closed switching control contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system.

Claim 12

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, an air circulation fan for passing air directly over the heat exchanger through the fins into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for energizing the heater to initiate a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a light detection signal along a direct path through a plurality of portions of the heat exchanger, said path passing through the central portion of said heat exchanger between adjacent parallel fins, said path also being oriented in a plane generally parallel to said adjacent parallel fins, whereby frost which builds up on any one of said portions of the planar surfaces of said adjacent parallel fins of the heat exchanger blocks the path of the light transmitted therethrough; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said peripheral location, for sensing the variation of transmitted light transmitted through the heat exchanger along said path and for producing a control signal in response to the intensity of light received falling below a selected value, said means for sensing the variation of transmitted light intensity being a light sensitive device having a first conductive state in response to the light received exceeding the selected value and having a second conductive state in response to the light received falling below the selected value; means responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; a switching transistor rendered conductive for effecting energization of first switching control means to produce a control signal in response to the second conductive state of said light sensitive device; and second switching control means energized in response to said control signal for opening a first set of normally closed switching control contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system.

Claim 13

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, an air circulation fan for passing air directly over the heat exchanger through the fins into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for energizing the heater to initiate a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a light detection signal along a direct path through a plurality of portions of the heat exchanger, said path passing through the central portion of said heat exchanger between adjacent parallel fins, said path also being oriented in a plane generally parallel to said adjacent parallel fins, whereby frost which builds up on any one of said portions of the planar surfaces of said adjacent parallel fins of the heat exchanger blocks the path of the light transmitted therethrough; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said peripheral location, for sensing the variation of transmitted light transmitted through the heat exchanger along said path and for producing a control signal in response to the intensity of light received falling below a selected value said means for sensing the variation of transmitted light intensity being a light sensitive device having a first conductive state in response to the light received exceeding the selected value and having a second conductive state in response to the light received falling below the selected value, means responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; a voltage comparator for producing an output signal to energize a first switching control means thereby producing a control signal in response to the second conductive state of said light sensitive device; and second switching control means energized in response to said control signal for opening a first set of normally closed switching control contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system.

Claim 14

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, an air circulation fan for passing air directly over the heat exchanger through the fins into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for energizing the heater to initiate a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a light detection signal along a direct path through a plurality of portions of the heat exchanger, said path passing through the central portion of said heat exchanger between adjacent parallel fins, said path also being oriented in a plane generally parallel to said adjacent parallel fins, whereby frost which builds up on any one of said portions of the planar surfaces of said adjacent parallel fins of the heat exchanger blocks the path of the light transmitted therethrough; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said peripheral location, for sensing the variation of transmitted light transmitted through the heat exchanger along said path and for producing a control signal in response to the intensity of light received falling below a selected value said means for sensing the variation of transmitted light intensity being a light sensitive device having a first conductive state in response to the light received exceeding the selected value and having a second conductive state in response to the light received falling below the selected value, means responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; a voltage comparator for producing an output signal to energize a first switching control means thereby producing a control signal in response to the second conductive state of said light sensitive device; and second switching control means energized in response to said control signal for opening a first set of normally closed switching control contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system.

Claim 15

The demand defrost system of claims 10 or 11 wherein said second switching control means, when energized, closes a second set of normally open switching control contacts connected to the heater to initiate a defrost cycle.

Claim 16

The demand defrost system of claim 12 wherein said second switching control means when energized closes a third set of normally open switching control means contacts in series therewith to maintain said second switching control means energized independently of said first switching control means.

Claim 17

The demand defrost system of claims 10 or 11 including a temperature sensitive switch actuated in response to a selected temperature condition indicating that defrosting is complete for effecting termination of the defrost cycle.

Claim 18

The demand defrost system of claim 18 including third switching control means energized in response to actuation of said temperature sensitive switch for opening a set of third switching control contacts connected in series with said second switching control means to de-energize said second switching control means causing said second set of second switching control contacts to de-energize the defrost heater.

Claim 19

The demand defrost system of claim 13 wherein said first set of normally open switching control contacts closes in response to de-energization of said second switching control means to initiate the cooling cycle.

Claim 20

The demand defrost system of claim 14 wherein the temperature sensitive switch is deactivated in response to a selected reduced temperature condition indicative that the heat exchanger is substantially free of liquid moisture for completing a circuit to energize the cooling fan.

Claim 21

The demand defrost system of claim 10 including at least one wire screen located transverse to the light path and in contact with portions of said heat exchanger which blocks the light when frost builds up on the screen.

Claim 22

The demand defrost system of claim 10 wherein the means for generating a light detection signal is an incandescent lightbulb, and wherein the means for sensing the variation of transmitted light includes a photoelectric cell.

Claim 23

The demand defrost system of claim 10 wherein the means for generating a light detection signal is a photoemissive photodiode and wherein the means for sensing the variation of transmitted light includes a photosensitive phototransistor.

Claim 24

In a cooling system with a heat exchanger containing cooling fluid circulating in cooling coils for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the cooling coils and fins attached to the coils and for energizing the heater to initiate a defrost cycle in response to frost build-up comprising: a light emitting device which is directed to transmit a light detection beam through the cooling coil, whereby frost which builds up on the heat exchanger blocks the path of the light detection beam transmitted therethrough; a light sensitive device which senses the decrease in transmitted light intensity after the light beam is transmitted through the heat exchanger and which activates first relay means when the light received falls below a selected value; normally open relay contacts which close to energize second relay means to produce a control signal when said first relay means is energized; a first set of normally closed relay contacts which open to discontinue cooling operation of the circulation fan and heat exchanger when said second relay means is energized; a second set of normally open relay contacts which close to energize the heater to defrost the built-up frost when said second relay means is energized; a third set of normally open relay contacts in series with said second relay means and connected in paralell with said first relay contacts which maintain the second relay means energized independently of said first relay means; temperature sensitive switch means which senses the temperature in the heat exchanger; third relay means in series with said temperature sensitive switch means energized when the temperature sensitive switch means actuates in response to a selected temperature condition indicating that defrost is complete; and a fourth set of normally closed relay contacts, in series with said second relay means, which open when said third relay means is energized to de-energize said second relay means to open said second set of normally open relay contacts and de-energize the heater, and to close th first set of normally open relay contacts to initiate the cooling cycle.

Claim 25

In a cooling system with a heat exchanger containing cooling fluid circulating in cooling coils for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the cooling coils and fins attached to the coils and for energizing the heater to initiate a defrost cycle in response to frost build-up comprising: a light emitting device which is directed to transmit a light detection beam through the cooling coil, whereby frost which builds up on the heat exchanger blocks the path of the light detection beam transmitted therethrough; a light sensitive device which senses the decrease in transmitted light intensity after the light beam is transmitted through the heat exchanger and which activates first relay means when the light received falls below a selected value; normally open relay contacts which close to energize second relay means to produce a control signal when said first relay means is energized; a first set of normally closed relay contacts which open to discontinue cooling operation of the circulation fan and heat exchanger when said second relay means is energized; a second set of normally open relay contacts which close to energize the heater to defrost the built-up frost when said second relay means is energized; a third set of normally open relay contacts in series with said second relay means and connected in paralell with said first relay contacts which maintain the second relay means energized independently of said first relay means; temperature sensitive switch means which senses the temperature in the heat exchanger; third relay means in series with said temperature sensitive switch means energized when the temperature sensitive switch means actuates in response to a selected temperature condition indicating that defrost is complete; and a fourth set of normally closed relay contacts, in series with said second relay means, which open when said third relay means is energized to de-energize said second relay means to open said second set of normally open relay contacts and de-energize the heater, and to close th first set of normally open relay contacts to initiate the cooling cycle.

Claim 26

In a cooling system with a heat exchanger containing cooling fluid for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the heat exchanger and for energizing the heater to initiate a defrost cycle in response thereto comprising: means for generating a light detection signal and for directing said light detection signal through the heat exchanger, whereby frost which builds up on the heat exchanger blocks the path of the light transmitted therethrough; means for sensing the variation of transmitted light transmitted through the heat exchanger comprising a light sensitive device which assumes a first conductive state in response to the intensity of light received exceeding a selected value and having a second conductive state in response to the intensity of light received falling below the selected value and further including a switching transistor rendered conductive for energizing a first switching control means to produce a control signal in response to the second conductive state of said light sensitive device and further including a second switching control means energized in response to said control signal for opening a first set of normally closed switching control means contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system and for initiating a defrost cycle; and a temperature sensitive switch actuated in response to a selected temperature condition indicating that defrosting is complete for effecting termination of the defrost cycle, including a third switching control means energized in response to actuation of said temperature sensitive switch for opening a set of third switching control means contacts connected in series with said second switching control means to de-energize said second switching control means causing a second set of second switching control means contacts to de-energize the defrost heater.

Claim 27

In a cooling system with a heat exchanger containing cooling fluid for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the heat exchanger and for energizing the heater to initiate a defrost cycle in response thereto comprising: means for generating a light detection signal and for directing said light detection signal through the heat exchanger, whereby frost which builds up on the heat exchanger blocks the path of the light transmitted therethrough; means for sensing the variation of transmitted light transmitted through the heat exchanger comprising a light sensitive device which assumes a first conductive state in response to the intensity of light received exceeding a selected value and having a second conductive state in response to the intensity of light received falling below the selected value and further including a switching transistor rendered conductive for energizing a first switching control means to produce a control signal in response to the second conductive state of said light sensitive device and further including a second switching control means energized in response to said control signal for opening a first set of normally closed switching control means contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system and for initiating a defrost cycle; and a temperature sensitive switch actuated in response to a selected temperature condition indicating that defrosting is complete for effecting termination of the defrost cycle, including a third switching control means energized in response to actuation of said temperature sensitive switch for opening a set of third switching control means contacts connected in series with said second switching control means to de-energize said second switching control means causing a second set of second switching control means contacts to de-energize the defrost heater.

Claim 28

The demand defrost system of claim 18 or 21 wherein said first set of normally open switching control means contacts closes in response to de-energization of said second switching control means to initiate the cooling cycle.

Claim 29

In a cooling system with a heat exchanger containing cooling fluid for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the heat exchanger and for energizing the heater to initiate a defrost cycle in response thereto comprising: means for generating a light detection signal along a direct path through a plurality of portions of the heat exchanger, whereby frost which builds up on any one of said portions of the heat exchanger blocks the path of the light transmitted therethrough; a wire screen located transverse to the light path and in contact with portions of said heat exchanger which blocks the light when frost builds up on the screen; means at the end of said path for sensing the variation of transmitted light transmitted through the heat exchanger along said path and for producing a control signal in response to the intensity of light received falling below a selected value; and means response to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle.

Claim 30

In a cooling system with a heat exchanger containing cooling fluid for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the heat exchanger and for energizing the heater to initiate a defrost cycle in response thereto comprising: means for generating a light detection signal along a direct path through a plurality of portions of the heat exchanger, whereby frost which builds up on any one of said portions of the heat exchanger blocks the path of the light transmitted therethrough; a wire screen located transverse to the light path and in contact with portions of said heat exchanger which blocks the light when frost builds up on the screen; means at the end of said path for sensing the variation of transmitted light transmitted through the heat exchanger along said path and for producing a control signal in response to the intensity of light received falling below a selected value; and means response to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle.

Claim 31

In a cooling system with a heat exchanger containing cooling fluid for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the heat exchanger and for energizing the heater to initiate a defrost cycle in response thereto comprising: means for generating a light detection signal and for directing said light detection signal through the heat exchanger, whereby frost which builds up on the heat exchanger blocks the path of the light transmitted therethrough; means for sensing the variation of transmitted light transmitted through the heat exchanger comprising a light sensitive device which assumes a first conductive state in response to the intensity of light received exceeding a selected value and having a second conductive state in response to the intensity of light received falling below the selected value and further including a voltage comparator which produces an output signal to energize a first switching control means thereby producing a control signal in response to the second conductive state of said light sensitive device and further including a second switching control means energized in response to said control signal for opening a first set of normally closed switching control means contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system and for initiating a defrost cycle, a temperature sensitive switch actuated in response to a selected temperature condition indicating that defrosting is complete for effecting termination of the defrost cycle, including a third switching control means energized in response to actuation of said temperature sensitive switch for opening a set of third switching control means contacts connected in series with said second switching control means to de-energize said second switching control means contacts to de-energize the defrost heater.

Claim 32

In a cooling system with a heat exchanger containing cooling fluid for cooling air in the vicinity of the exchanger, an air circulation fan for passing air over the heat exchanger into a cooling area and a heater for defrosting the heat exchanger, a demand defrost system for detecting the existence of frost build-up on the surface of the heat exchanger and for energizing the heater to initiate a defrost cycle in response thereto comprising: means for generating a light detection signal and for directing said light detection signal through the heat exchanger, whereby frost which builds up on the heat exchanger blocks the path of the light transmitted therethrough; means for sensing the variation of transmitted light transmitted through the heat exchanger comprising a light sensitive device which assumes a first conductive state in response to the intensity of light received exceeding a selected value and having a second conductive state in response to the intensity of light received falling below the selected value and further including a voltage comparator which produces an output signal to energize a first switching control means thereby producing a control signal in response to the second conductive state of said light sensitive device and further including a second switching control means energized in response to said control signal for opening a first set of normally closed switching control means contacts connected to the air circulation fan motor and the compressor to discontinue cooling operation of the cooling system and for initiating a defrost cycle, a temperature sensitive switch actuated in response to a selected temperature condition indicating that defrosting is complete for effecting termination of the defrost cycle, including a third switching control means energized in response to actuation of said temperature sensitive switch for opening a set of third switching control means contacts connected in series with said second switching control means to de-energize said second switching control means contacts to de-energize the defrost heater.

Claim 33

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for initiating a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a detection signal having a characteristic with varies in direct response to build-up of frost on any one of a plurality of portions of the planar surfaces of said adjacent parallel fins of the heat exchanger and directing said signal along and through said plurality of portions of said heat exchanger along a direct path which passes through the central portion of the heat exchanger between adjacent parallel fins, said path being oriented in a plane generally parallel to said adjacent parallel fins, said detection signal; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said first peripheral location for sensing said variation in said detection signal characteristic and for producing a control signal in response to said characteristic achieving a selected value; means normally responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; and means operative in response to failure of said detection signal generating means for precluding discontinuance of said cooling operation and initiation of said defrost cycle.

Claim 34

In a cooling system with a heat exchanger having a plurality of fins arranged in generally parallel relationship and having coils containing cooling fluid, for cooling air in the vicinity of the exchanger, a demand defrost system for detecting the existence of frost build-up on the planar surfaces of the heat exchanger fins and for initiating a defrost cycle in response thereto comprising: means at a first peripheral location of said heat exchanger for generating a detection signal having a characteristic with varies in direct response to build-up of frost on any one of a plurality of portions of the planar surfaces of said adjacent parallel fins of the heat exchanger and directing said signal along and through said plurality of portions of said heat exchanger along a direct path which passes through the central portion of the heat exchanger between adjacent parallel fins, said path being oriented in a plane generally parallel to said adjacent parallel fins, said detection signal; means at the end of said path, and at a second peripheral location of said heat exchanger opposed from said first peripheral location for sensing said variation in said detection signal characteristic and for producing a control signal in response to said characteristic achieving a selected value; means normally responsive to the control signal for discontinuing cooling operation of the cooling system and for initiating a defrost cycle; and means operative in response to failure of said detection signal generating means for precluding discontinuance of said cooling operation and initiation of said defrost cycle.

Claim 35

The cooling system of claim 26 wherein said failure responsive means is operative to preclude energization of control signal responsive means in response to failure of said detection signal generating means.

Claim 36

The cooling system of claim 26 including alarm means responsive to failure of said detection signal generating means for producing an alarm.

Claim 37

The cooling system of claim 26 including manually operative means for discontinuing cooling operation of the cooling system and for initiating said defrost cycle independently of the failure of said detection signal override means.

Claim 38

The demand defrost system of claim 26 wherein said defrost cycle initiating means sustains said defrost cycle independently of subsequent termination of said control signal.

Claim 39

The demand defrost system of claim 26 including temperature sensing means actuated in response to a selected temperature condition indicating that defrosting is complete and; defrost cycle termination means operable to terminate said defrost cycle and initiate the cooling cycle in response to actuation of said temperature sensing means.

Claim 40

The demand defrost system of claim 31 including safety means for energizing said defrost cycle termination means upon failure of said temperature sensing means.

Claim 41

The demand defrost system of claim 32 wherein said safety means includes high temperature sensing means actuated in response to a second temperature condition higher than said selected temperature condition, said defrost cycle termination means being operated in response to actuation of said high temperature safety means.

Claim 42

The demand defrost system of claim 32 wherein said safety means includes timing means energized in response to initiation of said defrost cycle and de-energized upon termination of said defrost cycle; said timing means operative after a selected time interval to effect energization of said defrost cycle termination means, said time interval being longer than the time interval when said temperature sensing means would normally be actuated, whereby said timing means is operative upon failure of said temperature sensing means.