Abstract:
A defrost heater relay control circuit having reduced power consumption during a de-energized mode of operation is provided. The relay drive circuit utilizes a series connected capacitor to introduce a phase shift in the AC current waveform such that the amount of real power dissipated in the circuit during periods when the defrost heater control relay is de-energized is greatly reduced. This effectively shorts out the relay drive voltage without generating heat due to real power dissipation through the switched circuitry that disables the relay.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to defrost control circuitry for consumer and commercial refrigeration appliances, and more particularly to power reduction circuitry for use with such defrost control systems. 
     BACKGROUND OF THE INVENTION 
     Recognizing that icing of the evaporator heat exchanger in a consumer or commercial refrigeration unit, such as a refrigerator or freezer, many modern appliances provide fixed or adaptive defrost control. Such a defrost control provides heating of the evaporator heat exchanger so as to melt any accumulated frost or ice that may have formed thereon during the refrigeration cooling cycle. Many different methods of controlling the defrost cycle are known in the art, including electromechanical timers and microprocessor control. 
     Typically, such defrosting circuitry employs a heater positioned in proximity to the evaporator heat exchanger within the freezer compartment of the refrigerator or freezer. At controlled intervals while the refrigeration system is not operating during its normal temperature control cycle, the defrost heater is energized. This defrost heater generates enough heat to cause melting of the frost build up or ice on the evaporator heat exchanger, which greatly increases the efficiency of subsequent cooling cycles. 
     While providing a defrost heater greatly enhances the overall efficiency of the refrigeration cycle, the heat generated by the defrost heater will have to be removed in subsequent cooling cycles to maintain the interior temperature of the freezer compartment. A simple rule of thumb is that twice the amount of energy is needed to remove a unit of heat. As such, heat generation within the freezer compartment must be minimized both during the defrost cycle, and when the defrost heater is not energized. 
     Electromechanical defrost timers and modern adaptive defrost controls operate to provide such limited heating only when necessary and only to the extent necessary to accomplish the defrosting of the evaporator heat exchanger. During other periods, the defrost heater is turned off, although the defrost heater control circuitry is still powered. Unfortunately, even when the defrost heater is turn off, this control circuitry still generates a small amount of heat due to the consumption of the standby power by the control circuitry when not in the defrost mode of operation. While small, this heat generated must still be removed during subsequent cooling cycles. As a result, the overall efficiency is decreased and the cost of ownership of the appliance is increased. 
     There exists, therefore, a need in the art for a refrigeration defrost control circuit that reduces the amount of heat generated while in this standby mode of operation when the defrost heater is not energized. The circuitry of the present invention provides such power reduction. 
     These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a new and improved defrost heater control circuit that overcomes one or more problems existing in the art. More particularly, the present invention provides a new and improved defrost control circuit that reduces the heat generated during a standby mode of operation when the defrost heater is not operated. Still more particularly, the present invention provides a new and improved defrost heater control circuit that reduces the amount of real power consumed during a standby mode of operation to therefore reduce the amount of heat generated during such mode. 
     In one embodiment of the present invention the defrost heater control circuit includes a relay to switch power to the defrost heater, and a relay drive circuit. In this embodiment the relay drive circuit utilizes a twenty four volt supply to energize the relay, which connects the heater power supply to the heater element to provide the defrost mode of operation. However, when the relay is turned off, the circuit of the present invention saves real power by effectively shorting out the relay power supply. 
     In a preferred embodiment of the present invention, the defrost control circuit provides a capacitor in series with the line AC voltage to act as a dropping impedance. While this would apparently increase the current to the drive circuit when the relay is off, due to the increased voltage across the capacitor, the result is that the power dissipated across the circuit is primarily reactive, i.e., not real power that would be turned into heat. As a result the amount of heat generated by the circuit when the relay for the defrost heater is held off is substantially reduced. 
     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a simplified single line schematic diagram of one embodiment of a defrost heater control circuit constructed in accordance with the teachings of the present invention; 
         FIG. 2  is a simplified single line schematic diagram of the circuit of  FIG. 1  illustrating current flow during a positive half cycle of the line voltage while the defrost heater relay is energized; 
         FIG. 3  is a simplified single line schematic diagram of the circuit of  FIG. 1  illustrating current flow during a negative half cycle of the line voltage while the defrost heater relay is energized; 
         FIG. 4  is a simplified single line schematic diagram of the circuit of  FIG. 1  illustrating current flow during a positive half cycle of the line voltage while the defrost heater relay is off; 
         FIG. 5  is a simplified single line schematic diagram of the circuit of  FIG. 1  illustrating current flow during a negative half cycle of the line voltage while the defrost heater relay is off. 
     
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates one embodiment of a defrost heater relay drive circuit  10  constructed in accordance with the teachings of the present invention. This circuit  10  controls the on or off state of the defrost heater (not shown) in, for example, a consumer or commercial refrigerator and/or freezer. An advantage provided by the circuit of  FIG. 1  is that the real power dissipation, and therefore heat generation, during periods when the defrost heater is de-energized is greatly reduced compared with other defrost heater relay drive circuits. Since approximately twice the amount of energy is needed to remove a unit of heat from the refrigeration compartment, any reduction in heat generated during periods when the defrost heater is not to be energized greatly enhances the efficiency of the system and reduces the overall cost of operation and lifetime costs of ownership of the appliance. 
     As illustrated in  FIG. 1 , the power for the circuit is taken from the line voltage via power terminal  12 . A transient limiting resistor  14  couples the terminal  12  to a node between a parallel combination of capacitor  16  and resistor  18 . This parallel combination is coupled to the anode of diode  20  and the cathode of diode  48 . The cathode of diode  20  is connected to a parallel combination of Zener diode  22  and capacitor  24 . In the illustrated embodiment, this combination forms the twenty four volt supply to operate the defrost heater drive relay  32 . 
     This parallel combination is then coupled to another parallel combination of Zener diode  26  and capacitor  28 , which are then coupled to ground  30 . This second parallel combination provides the five volt supply for use by the controller  38  and other control circuitry. The anode of diode  48  is also coupled to this ground connection  30 . One terminal of the coil of the defrost heater drive relay  32  is coupled between the two Zener diodes  22 ,  26 . The other terminal of the coil of the relay  32  is coupled to a node that connects the cathode of the Zener diode  22 , diode  20 , and positive terminal of capacitor  24 . 
     This node also connects to the emitter of transistor  34 . The collector of transistor  34  is coupled through the resistor network  40 ,  42 ,  44 ,  46  to the node between Zener diodes  22 ,  26 . The base of transistor  34  is coupled through a resistor to the collector of transistor  36 . The emitter of this transistor  36  is coupled to ground, while the base is coupled through a resistor to a controller  38 . As will be discussed more fully below, this controller  38  operates to energize or de-energize the relay  32  through the control circuit  10 . 
     In the discussion that follows, operation of the circuit  10  will be described for both the condition when the relay  32  is energized to provide power to the defrost heater to initiate and maintain a defrost cycle, and the condition when the relay  32  is de-energized to stop the defrost cycle. Since power is derived from the AC line voltage, the discussion will cover two situations for each condition. The first operation that will be discussed will be during the positive half cycle of the line voltage, followed by a discussion of the operation of the circuit during the negative half cycle of the line voltage. In each of these conditions, reference will be made to an additional line superimposed on the schematic illustrating the primary current flow during the various conditions. 
       FIG. 2  illustrates the primary current flow  50  while the relay is energized during the positive half cycle of the AC line voltage coupled to terminal  12 . This current flow  50  will flow through the transient limiting resistor  14  and primarily through the series connected capacitor  16 , due to the relative impedance to the AC line voltage between capacitor  16  and resistor  18 . This current will flow through the diode  20  and will charge capacitor  24  to the clamped voltage dictated by Zener diode  22 . In one embodiment of the present invention this voltage is clamped at twenty four volts. 
     The current through diode  20  will also flow through the coil of relay  32  to energize this relay  32  to start the defrost cycle. The current will then flow to the L 1  terminal. This primary current flow path exists when the controller  38  has a low output to the transistor  36 . This low output maintains transistor  36  in an off state. As a result, the voltage at the base of transistor  34  is positive, which keeps transistor  34  also in an off state. In this state, no current can flow through transistor  34 . 
     During the negative half cycle as illustrated in  FIG. 3 , the controller maintains the same output to transistor  36  which also maintains transistor  34  in the off state. During this negative half cycle current flows in the opposite direction from the L 1  terminal though the capacitor  28 . The voltage developed across capacitor  28  will be clamped by the Zener diode  26 . In one embodiment of the present invention this voltage is clamped to approximately five volts for use by the controller  38 . The current then flows through diode  48 , capacitor  16  and transient suppression resistor  14  to terminal  12  as illustrated by line  52 . The relay  32  is kept energized by the discharge of capacitor  24  through the coil of relay  32  during this negative half cycle. 
     As the AC line voltage again transitions to a positive half cycle, current flow as illustrated by line  50  of  FIG. 2  will again occur. As a result of this cyclical operation, the relay  32  will continue to be energized during the defrost cycle to provide power to the defrost heater. 
     Once the controller  38  has determined that the defrost cycle is to be ended, the controller  38  provides a positive output to transistor  36  as illustrated in  FIG. 4 . This positive output turns on transistor  36 , which then pulls the base of transistor  34  low. As a result, transistor  34  turns on to allow current flow therethrough. While in this condition, during the positive half cycle of the AC line voltage connected to terminal  12 , the primary current flow will be as illustrated by line  54 . 
     Specifically, current will flow from the terminal  12  through the resistor  14  and capacitor  16 , through diode  20  transistor  34  and the resistor network  40 - 46 . The current will continue to flow to the L 1  terminal. The result of this operation is that the voltage supplied to the coil of relay  32  is pulled down through transistor  34  such that it is below the drop out voltage of the relay  32 . As a result, the relay  32  becomes de-energized and the voltage to the defrost heater is turned off. 
     During this state the current flows through the resistor network  40 - 46 . If this current flow were in phase with the line voltage, the power dissipation across this resistor network would be real, and would result in the generation of heat. However, as discussed above, the generation of heat during periods other than the defrost cycle would decrease the efficiency of the system based on the increased load on the refrigeration system to remove this heat from the freezer compartment. However, since the current flows through capacitor  16 , a phase shift in the current occurs such that the real power dissipated across the resistor network is reduced. In other words, the inclusion of capacitor  16  results in the power dissipated being reactive power, not real power that would otherwise be turned into heat. This reduction in heat generation provides a significant advantage over prior defrost drive circuits that consumed standby power and generated heat when not in the defrost mode. 
     When the AC line voltage is in its negative half cycle, the current flows as illustrated by line  56  in  FIG. 5 . This primary current flow goes from the L 1  terminal through capacitor  28  (and Zener diode  26  once the voltage reaches the clamping voltage), through diode  48 , capacitor  16  and transient suppression resistor  14  to the AC line terminal  12 . During this negative half cycle, the relay  32  is also not energized, which maintains the defrost heater in an off condition. 
     As will now be apparent to those skilled in the art in view of the foregoing disclosure, inclusion of the series capacitor effectuates a phase shift of the current waveform relative to the voltage waveform such that the amount of real power dissipated in the form of heat is greatly reduced or eliminated. As a result, this drive circuit minimizes the heat generated during the standby mode of operation for the defrost heater so as to not increase the load on the refrigeration system. As a result, the overall efficiency of the entire system is increased, which results in a reduced cost of operation and lifetime cost of ownership. 
     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.