Abstract:
A clothes dryer has a system for regulating the inlet air temperature. The system includes a first sensor located in an inlet of the dryer and including a thermistor and a thermostat, a heat source located in a heater box adjacent the first sensor, and a second sensor located in an exhaust of the dryer. The thermistor measures the inlet air temperature of the dryer and cooperates with the controller to prevent the thermostat from reaching its trip temperature and turning off the heat source. Thus, damage due to excessive air temperatures in the dryer is prevented.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to the drying of clothes using a clothes dryer. More particularly, the invention relates to providing a clothes dryer with a combined temperature sensor and electromechanical thermostat for measuring the inlet air temperature, and for controlling the heat source. 
   2. Description of Related Art 
   The drying of clothes via the application of heated air in a conventional clothes dryer is well-known in the prior art. Thermostats and thermistors with electronics are used in such dryers to control heat input, thereby preventing high clothes temperatures that can damage the clothes. Some dryers use both an inlet thermistor and an exhaust thermistor for monitoring air temperature, as well as a bi-metal thermostat for limiting the heat input. This known configuration, however, suffers from a number of shortcomings. 
   Initially, the above-mentioned system of the prior art has a delay between the time the inlet air temperature is sensed by the thermistor and the time the thermostat reacts to an increase in temperature. This delay in response time can result in excessively long drying times due to the thermostat turning the heating element off prematurely. This condition, known as nuisance cycling, lengthens the total amount of drying time necessary to completely dry the contents of the dryer. 
   Another shortcoming of the prior art is a lack of close correlation of the air temperature due to the distance and orientation between the inlet thermistor and the thermostat. This distance and orientation can lead to a difference in the temperature detected by each of the components. 
   Further, the prior art utilizes an inlet thermistor that is separate from the thermostat. Thus, two separate components must be manufactured and mounted to the dryer, thereby adding to the overall cost in both labor and materials. 
   Accordingly, it is desirable to develop a system that more efficiently controls the heat input in a clothes dryer while using the minimum amount of components to reduce overall cost. 
   SUMMARY OF THE INVENTION 
   The present invention meets the shortcomings of the prior art by providing a combined thermistor/thermostat located in the inlet of the heater box of a clothes dryer. The combined device measures the conductive, convective, and/or radiated heat of the heat source of the dryer and regulates the inlet air temperature to the clothes load, thereby providing a more real-time control of the overall dryer temperature and preventing the air temperature from getting too high. The invention disclosed herein combines a thermistor with its fast response time for monitoring inlet air temperature and a bi-metal thermostat wired directly to the heat source. One of the benefits of having a combined device is the close proximity of the two components. This proximity improves the reaction time of the control system to temperature changes, thereby preventing excessive fabric temperatures. The combined sensor of the present invention provides all the above benefits at a cost lower than that of prior art sensors since the thermistor and thermostat are assembled as a single piece instead of two separate components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a system utilizing the combined thermistor/thermostat sensor of the present invention. 
       FIGS. 2A and 2B  are perspective views of the combined thermistor/thermostat sensor of the present invention. 
       FIG. 3  is a control diagram of the system utilizing the combined thermistor/thermostat sensor of the present invention. 
       FIG. 4  is a schematic view of an alternative system utilizing the combined thermistor/thermostat sensor of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to  FIG. 1 , an electric clothes dryer  10  of the present invention is schematically shown, provided generally with a heater box  30 , a drum  60 , a blower  80  and an exhaust  90 . The heater box  30  is provided with an inlet  32  through which inlet airflow  20  passes, and a drum inlet grill  34  through which heated air exits the heater box  30  and enters the drum  60  of the dryer  10 . The air is heated in the heater box  30  by a heating element  36 , preferably a dual element heater. The blower  80  draws the air out of the drum  60 , through a lint screen  70 , and eventually through the exhaust  90  of the dryer, as exhaust airflow  120 . The dryer  10  further includes a thermal cut-off  50  and a thermal fuse  110 . The thermal cut-off  50  ensures a safe condition in the event of a heating element failure. The thermal fuse  110  removes power to the drum motor, thus stopping the airflow and containing any combustible material from being vented outside of the dryer. 
   The clothes dryer  10  is provided with a number of sensors for detecting the temperature of the airflow in the dryer. A combined thermistor/thermostat unit  40  is located in the inlet  32  of the heater box  30  while an exhaust thermistor temperature sensor  100  is located in the exhaust  90  of the dryer  10 . As shown in  FIGS. 2A and 2B , the combined thermistor/thermostat unit  40  includes a thermistor temperature sensor  41  and an electromechanical bi-metal thermostat  42  within a body  43 . The thermistor  41  measures the inlet air temperature of the dryer, and the exhaust thermistor  100  measures the temperature of exhaust airflow  120 . The thermostat  42  opens the heating element circuit when the temperature exceeds a predetermined trip point and closes the heating element circuit when the temperature falls below a predetermined reset point. 
   The thermistor  41  of the combined sensor unit  40  may be a partially encapsulated NTC (negative temperature coefficient) semiconductor molded into a high temperature plastic probe. Alternatively, the thermistor  41  may be a fully encapsulated or metal enclosed device. The thermostat  42  may be of a bi-metal type single pole, single throw switch that opens when the metal is heated to the specified trip point. Thus, the combined unit  40  provides the fast response time of a thermistor along with the safety and reliability of a bi-metal thermostat within one component. 
   Referring to  FIGS. 2A and 2B , the combined sensor unit  40  is depicted in further detail. In addition to the thermistor  41  and the thermostat  42 , the unit is provided with high voltage terminals  44 , which are connected in series with the heating element  36 , and terminals  46  for connection with a controller. The high voltage terminal  44  and the terminal  46  protrude from the body  43 . Further, the unit is provided with mounting means  48  for mounting in the desired location on the heater box  30 . 
   With reference to  FIG. 3 , the combined sensor  40  is connected to both the heating element  36  and a controller  140 . Specifically, the thermostat  42  reacts to the inlet temperature to limit the heat input by the system. In the event that the thermostat&#39;s trip temperature is reached, the thermostat  42  would open the heating element circuit and turn the heating element  36  completely off. 
   Additionally, the thermistor  41  communicates with the controller  140  via a wire harness  130 . The thermistor  41  measures the temperature at the inlet of the heater box  30 , and then provides the temperature signal to the controller  140 . When the thermistor  41  senses that the temperature is becoming too high, the controller  140  operates the heating element  36  at half power until an inlet reset point is reached. Thus, one of the heating elements  36  remains active and continues to heat the airflow. Once the reset temperature is reached, the controller  140  then turns the heating element  36  back to full power. Alternately, the combined thermistor/thermostat  40  could be implemented with a single stage heating element. As a result of this function of the thermistor, the thermostat is prevented from reaching its trip temperature, thus preventing long dry times due to thermostat cycling. 
   With reference to  FIG. 4 , the combined sensor  40 , described above, is shown in a gas dryer  10 ′. The gas dryer  10 ′ is provided generally with a heater box  30 ′, a drum  60 ′, a blower  80 ′ and an exhaust  90 ′. The heater box  30 ′ is provided with an inlet  32 ′ through which inlet airflow  20 ′ passes, and a drum inlet grill  34 ′ through which heated air exits the heater box  30 ′ and enters the drum  60 ′ of the dryer  10 ′. The air is heated in the heater box  30 ′ by burner  38 ′ that is controlled by a bi-level gas valve. The blower  80 ′ draws the air out of the drum  60 ′, through a lint screen  70 ′, and eventually through the exhaust  90 ′ of the dryer, as exhaust airflow  120 ′. The dryer  10 ′ further includes a thermal cut-off  50 ′ and a thermal fuse  110 ′. The thermal cut-off  50 ′ ensures a safe condition in the event of a burner or gas valve failure. The thermal fuse  110 ′ removes power to the drum motor, thus stopping the airflow and containing any combustible material from being vented outside of the dryer. 
   The gas dryer  10 ′ is provided with a number of sensors for detecting the temperature of the airflow in the dryer. A combined thermistor/thermostat unit  40  is located in the inlet  32 ′ of the heater box  30 ′ while an exhaust thermistor temperature sensor  100 ′ is located in the exhaust  90 ′ of the dryer  10 ′. As shown in  FIGS. 2A and 2B , the combined thermistor/thermostat unit  40  includes a thermistor temperature sensor  41  and an electromechanical bi-metal thermostat  42 . The thermistor  41  measures the inlet air temperature of the dryer, and the exhaust thermistor  100 ′ measures the temperature of exhaust airflow  120 ′. The thermostat  42  opens the gas valve when the temperature exceeds a predetermined trip point and closes the gas valve when the temperature falls below a predetermined reset point. 
   The function of the combined sensor  40  in the gas dryer  10 ′ is generally the same as demonstrated above for an electric dryer  10 . Referring again to  FIG. 3 , the thermistor  41  communicates with the controller  140  via a wire harness  130 . The thermistor  41  measures the temperature at the inlet of the heater box  30 ′, and then provides the temperature signal to the controller  140 . When the thermistor  41  senses that the temperature is becoming too high, the controller  140  operates the burner  38 ′ at half power until an inlet reset point is reached. Once the reset temperature is reached, the controller  140  then turns the burner  38 ′ back to full power. As a result of this function of the thermistor, the thermostat is prevented from reaching its trip temperature, thus preventing long dry times due to thermostat cycling. 
   Thus, the present invention provides a more real-time control of the overall dryer temperature, thereby preventing the temperature from getting too high and damaging clothes, and also reducing nuisance cycling in the dryer. Further, dryness accuracy and overall energy efficiency of the dryer are both improved. 
   The combined sensor of the present invention can be manufactured at a cost lower than that of prior art sensors since the thermistor and thermostat are assembled as a single piece instead of two separate components. 
   While certain features and embodiments of the present invention have been described in detail herein, it is to be understood that the invention encompasses all modifications and enhancements within the scope and spirit of the following claims.