Patent Publication Number: US-8528352-B2

Title: Defrost timer for refrigerator and refrigerator

Description:
TECHNICAL FIELD 
     The present invention relates to a defrost timer used for a refrigerator and a refrigerator having such a defrost timer. 
     BACKGROUND OF THE INVENTION 
     While refrigerators are running, their coolers, also called evaporators of the refrigerators, are frosted and often get ice condensed on the coolers. Such frost and ice must be removed periodically for a smooth operation of the refrigerator. For this purpose, many refrigerators have defrost timers. The defrost timer turns off periodically a compressor of the refrigerator so that it allows the temperature of the evaporator to be high enough to melt and dry the ice and frost formed on the evaporator. 
     Since refrigerators are ubiquitous appliances, there is a high demand to lower the cost of refrigerators, their replacement parts and maintenance fees. At the same time, durable refrigerators are desired because it is burdensome and costly to fix or replace the refrigerators. Nowadays, some consumers want a refrigerator that lasts for two decades. 
     One bottleneck to realize durable refrigerators is their defrost timer. A typical defrost timer is made of mechanical parts such as a motor, gears, cams and levers, which are designed to count time and turn off the evaporator at a preset time. Mechanical defrost timers are cost-effective due to the facts that they do not contain numerous components and each component is not an expensive part. However, their mechanical components make it difficult to produce a durable defrost timer that lasts over a decade. Because the mechanical components keep receiving forces and moving all the time, they are easily worn out. In addition, the mechanical defrost timer makes ticking or clicking noise, which can be heard in a quit room. Some people mind such noise in late night. Therefore, quiet defrost timers are required to make the refrigerator more quiet. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is a defrost timer for refrigerator, including a circuit board, a first terminal, a second terminal, a third terminal, a fourth terminal, a switching unit, a first AC line, a second AC line, a third AC line and a fourth AC line. The first terminal is located on the circuit board and is coupled to one position of an alternative current source. The second terminal is located on the circuit board and is coupled to a heater of the refrigerator. The third terminal is located on the circuit board and is coupled to other position of the alternative current source. The fourth terminal is located on the circuit board and is coupled to a compressor of the refrigerator. The switching unit is electively coupled between the first terminal and the fourth terminal. The first AC line is provided on the circuit board and is coupling the first terminal and the switching unit. The second AC line is provided on the circuit board and is coupling the second terminal and the switching unit. The third AC line is provided on the circuit board and is coupling the third terminal and the switching unit. The fourth AC line is provided on the circuit board and is coupling the fourth terminal and the switching unit. The distance between the third AC line and the fourth AC line is at least 5 mm. 
     Another aspect of the present invention is a defrost timer for refrigerator, including a switching unit, a timer unit and a DC supply unit. The switching unit is electively coupled to a compressor of the refrigerator. The timer unit controls the switching unit according to a counted time. The DC supply unit supplies direct current to the timer unit. It is preferable that the switching unit contains a photocoupler and an AC relay, both parts of which are coupled to each other in series and both parts of which are coupled to an AC line in parallel. It is preferable that the timer unit contains a timer for counting a time, a CPU for controlling the switching unit according to the time counted by the timer, and a flash memory for storing data outputted from the CPU. It is preferable that the DC supply unit contains a capacitor coupled in series to an AC source, a bridge diode coupled between the AC source, and a zener diode coupled in parallel to the bridge diode. 
     Another aspect of the present invention is a defrost timer for refrigerator, including a switching unit, a timer, a CPU and a flash memory. The switching unit is selectively coupled to a compressor of the refrigerator. The timer counts a time. The CPU controls the switching unit according to the time counted by the timer. The flash memory stores data outputted from the CPU. The CPU is configured to write periodically a value reflecting the time counted by the timer into the flash memory and control the switching unit and turn off the compressor when the value reaches a predetermined threshold. 
     Another aspect of the present invention is a defrost timer for refrigerator, including a switching unit, a CPU and a flash memory. The switching unit is selectively coupled to a compressor of the refrigerator or a heater of the refrigerator. The CPU controls the switching unit. The flash memory stores data outputted from the CPU. The CPU is configured to write periodically a value reflecting a running time of the compressor into the flash memory and control the switching unit and turn off the compressor or the heater when the value reaches a predetermined threshold. 
     Another aspect of the present invention is a refrigerator having a compressor, a condenser, an evaporator to compress, condense and evaporate a coolant, and a defrost timer coupled in series to the compressor. The defrost timer is the defrost timer as described above. 
     Another aspect of the present invention is a refrigerator having a compressor, a condenser, an evaporator, a thermostat, an optional heater and a defrost timer. The compressor, the condenser and the evaporator compresses, condenses and evaporates a coolant. The thermostat is coupled in series to the compressor. The thermostat selectively couples an alternative current source to the compressor. The heater warms the evaporator. The defrost timer is coupled in series to the compressor and the thermostat. The defrost timer is the defrost timer as described above. Current to the defrost timer is arranged to be off while the current to the compressor is off by the thermostat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a block diagram of an embodiment of a defrost timer showing an approximate occupied area of different units. 
         FIG. 2  depicts a circuit diagram of an embodiment of a defrost timer illustrating a terminal unit, a switching unit and a DC supply unit. 
         FIG. 3  depicts a circuit diagram of an embodiment of a defrost timer illustrating a timer unit. 
         FIG. 4  illustrates a schematic plan view of a circuit board of a defrost timer. 
         FIG. 5  depicts a block diagram displaying various components of a controller shown in  FIG. 3 . 
         FIG. 6  illustrates a flowchart describing an overview of operational steps for a defrost timer. 
         FIG. 7  illustrates a flowchart describing in detail the operational steps outlined in  FIG. 6 . 
         FIG. 8  illustrates a flowchart describing in detail the operational steps outlined in  FIG. 6 . 
         FIG. 9  depicts a schematic memory map of the memory unit shown in  FIG. 5 . 
         FIG. 10  represents a perspective view from upper front of an example of a refrigerator having a defrost timer pertaining to the present invention. 
         FIG. 11  represents a perspective view of the refrigerator shown in  FIG. 10 , viewed from behind the refrigerator. 
         FIG. 12  depicts a schematic circuit diagram of an embodiment of connections between various electric parts of the refrigerator shown in  FIG. 10 . 
         FIG. 13  depicts a schematic circuit diagram of an alternative embodiment of connections between various electric parts of the refrigerator shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Embodiments of the present invention relates in general to a defrost timer and refrigerators containing the defrost timer. It is specifically relates to a new generation of electrical defrost timers where specific examples are described in detail. 
     §1. Defrost Timer  100   
       FIGS. 1-9  represent the preferred exemplary embodiments of the present invention, describing a defrost timer  100 . The defrost timer  100  is a control device that turns on and off a compressor of the refrigerator. More specifically, the defrost timer  100  switches off the compressor at a certain time for a predetermined period. Thereby, frost and ice on an evaporator of the refrigerator are removed. Because the defrost timer  100  has features described further below, the defrost timer  100  is quiet and durable. In addition, the defrost timer  100  is cost-effective, thereby inexpensive to produce. 
     §1.1 Overview of the Defrost Timer  100   
     Referring first to  FIG. 1 , a block diagram of the defrost timer  100  is shown. The defrost timer  100  may include a terminal unit  210 , a switching unit  300 , a DC supply unit  400 , and a timer unit  500 . These units are provided on a quadrilateral printed circuit board (PCB)  200 , where electrical connections of the defrost timer  100  are physically supported.  FIG. 1  also shows approximate positions and areas, occupied by different units of the defrost timer  100 , on the circuit board  200 . 
     The terminal unit  210  provides electrical connections to an alternating current (AC) source, to a compressor and, if necessary, a heater of the refrigerator. The compressor and the heater of the refrigerator are positioned outside of the defrost timer  100 . The switching unit  300  turns on and off the electrical connection between the compressor and the AC power source. The timer unit  500  controls the switching unit  300  by counting a time internally. The DC supply unit  400  provides a direct current (DC) to the timer unit  500 . 
     §1.2 Circuit Design of the Defrost Timer  100   
       FIGS. 2-3  illustrate a circuit diagram for each unit of the defrost timer  100 . The terminal unit  210 , the switching unit  300  and the DC supply unit  400  are shown in  FIG. 2 . The timer unit  500  is shown in  FIG. 3 . 
     §1.2.1 Terminal Unit  210  and Switching Unit  300   
     As shown in  FIG. 2 , the terminal unit  210  may include an active terminal TAB 1 , a heater terminal TAB 2 , a neutral terminal TAB 3 , and a compressor terminal TAB 4 . The active terminal TAB 1  and the neutral terminal TAB 3  are configured to connect respectively to an active terminal and neutral terminal of an AC outlet, which correspond to terminals of the alternating current (AC) source. In this embodiment, the heater terminal TAB 2  and the compressor terminal TAB 4  are configured to couple, respectively, to the heater and compressor of the refrigerator. In another embodiment, the heater terminal TAB 2  may not be connected to anything for refrigerators without a heater for defrosting. Additionally,  FIG. 2  show metal lines that connect terminals TAB 1 -TAB 4  from the terminal unit  210  to the switching unit  300  and the DC supply unit  400 . 
     The switching unit  300  may include an AC relay RY 1 , a photocoupler TR 1  and a resistor R 3 . In the switching unit  300 , a primary switching line  301  is provided between the active terminal TAB 1  and the neutral terminal TAB 3 . In other word, the primary switching line  301  is connected to the active terminal TAB 1  and the neutral terminal TAB 3  in parallel. On the primary switching line  301 , a coil part of the AC relay RY 1  and a light reception part of the photocoupler TR 1  are provided in series. In the switching unit  300 , a secondary switching line  302  is provided between a DC source, which will be described further below, and the timer unit  500 . On the secondary switching line  302 , a light emission part of the photocoupler TR 1  and the resistor R 3  are provided in series. 
     The active terminal TAB 1 , the compressor terminal TAB 4  and the heater terminal TAB 2  are connected to a switching part of the AC relay RY 1 . In the AC relay RY 1 , the active terminal TAB 1  is configured to connect to either of the compressor terminal TAB 4  or the heater terminal TAB 2 . Thereby, the AC relay RY 1  selectively couples the AC current source from the active terminal TAB 1  to the compressor terminal TAB 4  and the heater terminal TAB 2 . When the primary switching line  301  is off, the AC relay RY 1  is configured such that the active terminal TAB 1  and the compressor terminal TAB 4  are connected. 
     For convenience, a line that connects the active terminal TAB 1  and the switching part of the AC relay RY 1  is called a first AC line  211 . A line that connects the heater terminal TAB 2  and the switching part of the AC relay RY 1  is called a second AC line  212 . A line that connects the neutral terminal TAB 3  and the coil part of the AC relay RY 1  is called a third AC line  213 . A line that connects the compressor terminal TAB 4  and the switching part of the AC relay RY 1  is called a fourth AC line  214 . 
     §1.2.2 DC Supply Unit  400   
     The defrost timer  100  includes the DC supply unit  400  to provide a DC current to the timer unit  500  and the switching unit  300 . The DC supply unit  400  may include a varistor VR 1 , a capacitor C 1 , a resistor R 1 , a resistor R 2 , bridge diodes D 1 -D 4 , a zener diode D 5 , and an electrolytic capacitor C 2 . 
     The varistor VR 1  is coupled to the active terminal TAB 1  and the neutral terminal TAB 3  in parallel. The varistor VR 1  functions as a protective bypass. 
     The capacitor C 1 , the resistor R 1  and the resistor R 2  are coupled to the active terminal TAB 1  in series. The resistor R 1  is connected to the capacitor C 1  in parallel and the resistor R 2  is connected to the capacitor C 1  in series. In the DC supply unit  400 , the capacitor C 1 , the resistor R 1  and the resistor R 2  constitute a step down unit  401 , which lowers AC voltage inputted from the active terminal TAB 1  and the neutral terminal TAB 3 . 
     As shown in  FIG. 2 , one AC pin of the bridge diodes D 1 -D 4  is coupled to the active terminal TAB 1  through the step down unit  401 . The other AC pin of the bridge diodes D 1 -D 4  is connected to the neutral terminal TAB 3 . The plus pin of the bridge diodes D 1 -D 4  is coupled to a voltage source (V+). The minus pin of the bridge diodes D 1 -D 4  is coupled to a ground. The bridge diodes D 1 -D 4  functions as a rectifier. 
     The zener diode D 5  and the electrolytic capacitor C 2  are coupled in parallel between the voltage source (V+) and the ground. In the DC supply unit  400 , the zener diode D 5  and the electrolytic capacitor C 2  constitute a constant voltage unit  402 , which provides a stable and constant voltage. 
     §1.2.3 Timer Unit  500   
       FIG. 3  illustrates a circuit diagram of the timer unit  500 . The timer unit  500  may include a controller U 1 , a time input unit  510 , a time display unit  520 , a mode selection unit  530 , an acceleration mode activation unit  540 , and auxiliary connections  550 . In this embodiment, the controller U 1  is composed of, for example, a complementary metal oxide semiconductor (CMOS) integrated circuit (IC). Additionally, the controller U 1  features three pins: P 1 , P 3  and P 4  for sending and receiving input and output signals. The controller U 1  is coupled to all other units of the timer unit  500 , including: the time input unit  510 , the time display unit  520 , the mode selection unit  530 , the acceleration mode activation unit  540 , and the auxiliary connections  550 . The secondary switching line  302  from the switching unit  300  is also coupled to the controller U 1 . The controller U 1  will be explained further below in more details. 
     The time input unit  510  allows a user to input a current time into the timer unit  500 . The time input unit  510  may include a resistor R 9 , a resistor R 17 , a capacitor C 4 , a resistor R 10  and a tactile switch S 1 . The tactile switch S 1  is coupled to pin P 3 _ 4  of the controller U 1 . 
     The time display unit  520  displays a time inputted from the time input unit  510 . The time display unit  520  features a light-emitting diode (LED) D 6  and a resistor R 8 . The LED D 6  is coupled to pin P 1 _ 7  of the controller U 1 . 
     As will be described further below, the defrost timer  100  has two modes of operations: 1) clock mode and 2) integration mode. The mode selection unit  530  sets the operational mode for the defrost timer  100 . The mode selection unit  530  may include a resistor R 13 , a resistor R 14  and a resistor R 15 . In the defrost timer  100 , the resistor R 15  is designed to be easily replaceable such that the operational mode of the defrost timer  100  is determined by the resistance value of the resistor R 15 . The mode selection unit  530  is coupled to pin P 1 _ 8  of the controller U 1 . 
     The acceleration mode activation unit  540  enables an accelerated cycle of operation performed by the controller U 1 . This allows manufacturers and repairers be able to quickly verify whether the defrost timer  100  and the refrigerator including the defrost timer  100  function properly. The acceleration mode activation unit  540  may include a resistor R 11 , a resistor R 18 , a capacitor C 5 , a resistor R 12  and a jumper switch S 2 . The jumper switch S 2  is coupled to pin P 1 _ 5  of the controller U 1 . 
     The auxiliary connections  550  provide and receive power, reference voltage, and data input/output to and from the controller U 1 . The auxiliary connections  550  contain a connector CN 2 , a resistor R 4 , a resistor R 5 , a resistor R 6 , a resistor R 7 , a capacitor C 3 , a resistor R 16 , and a capacitor C 6 . The controller U 1  receives a DC voltage from the DC supply unit  400  at pin Vcc. The controller U 1  receives a 0 V supply from the DC supply unit  400  via capacitor C 3  at pin Vss. In addition, the controller U 1  receives a reference voltage from the DC supply unit  400  at pin VREF. The controller U 1  may transmit data from pin TXD 1  to one of the connector CN 2  pins, which can further be coupled to an electrical device outside of the defrost timer  100 . The controller U 1  may also receive data at its RXD 1  pin from one of the connector CN 2  pins. The controller U 1  may receive a reset signal, which resets any process performed by the controller U 1 , at its RESET pin from one of the connector CN 2  pins. The pin MODE of the controller U 1  is also coupled to one of the connector CN 2  pins. 
     Referring back to the  FIG. 2 , the secondary switching line  302  which is connected to the light emission part of the photocoupler TR 1 , is coupled to pin P 1 _ 1 /AN 9  of the controller U 1 . Thereby, the controller U 1  is able to switch on and off the photocoupler TR 1 , which in turn switches on and off the AC relay RY 1 . 
     §1.3 Electrical Action of the Defrost Timer  100   
     In this section, with reference to the  FIGS. 2-3 , the electrical action of the defrost timer  100  is explained. 
     As explained above, an AC signal such as AC (120V) from the alternating current (AC) source is provided to the defrost timer  100  through the active terminal TAB 1  and the neutral terminal TAB 3 . When the voltage of the AC signal is accidentally too high, the AC signal runs through the varistor VR 1  to protect the timer unit  500 . Then, the AC voltage is lowered to an acceptable level, for example 5V, through the step down unit  401 . The AC signal is then rectified by the bridge diodes D 1 -D 4 . The current formed by rectifying the AC signal is flattened by the constant voltage unit  402 . Therefore, a constant DC signal is provided to the timer unit  500  and the switching unit  300 . 
     The timer unit  500  controls the switching unit  300  using the DC signal provided by the DC supply unit  400 . Detailed operation of the timer unit  500  will be explained in a later section. In short, the controller U 1  selectively turns on and off the current of the secondary switching line  302  according to an internal program. When the controller U 1  turns off the secondary switching line  302 , the primary switching line  301  is also off. In this embodiment, the first AC line  211  is selectively coupled to the fourth AC line  214  in the AC relay RY 1 . Thus, the provided AC signal runs from the active terminal TAB 1  to the compressor terminal TAB 4 , which is coupled to the compressor of the refrigerator. On the other hand, when the controller U 1  turns on the secondary switching line  302 , the primary switching line  301  becomes on. In this embodiment, the first AC line  211  is selectively coupled to the second AC line  212  in the AC relay RY 1 . Thus, the provided AC signal runs from the active terminal TAB 1  to the heater terminal TAB 2 , which is coupled to the heater of the refrigerator. While the primary switching line  301  is on, only a small AC current flows into the switching line  301  due to the high impedance of coil part of the AC relay RY 1  (for example 1-10KΩ). 
     §1.4 Advantage of the Defrost Timer  100  on Circuit Design 
     The defrost timer  100  doesn&#39;t have a mechanical component that keeps moving. Therefore, it doesn&#39;t make a noticeable noise while running. Furthermore, it is durable and can last for over a decade. 
     In the defrost timer  100 , AC signals are flowing into the coil part of the AC relay RY 1 . This means, the defrost timer  100  doesn&#39;t drive the relay by a DC current. In addition, the controller U 1  is composed of a CMOS IC, which needs only a small amount of electric power. Therefore, the DC power needed in the defrost timer  100  is very small. This enables the capacitor C 1 , the resister R 1  and the resister R 2  to lower the AC voltage for generating DC voltage. In addition, this allows manufacturers to not use a transformer in the defrost timer  100  for lowering the AC voltage. Transformers are expensive electric parts that are large and heavy in terms of size and weight. Since the defrost timer  100  doesn&#39;t contain any transformer, the production cost of the defrost timer  100  is low. In addition, the defrost timer  100  is small and light, therefore easy to transport and handle. Furthermore, since the zener diode D 5  is used to generate a constant DC voltage, and the zener diodes are relatively cheap compared to other elements with similar function, the production cost of the defrost timer  100  is lower. 
     §1.5 Part and Line Arrangement of the Defrost Timer  100   
     Referring next to  FIG. 4 , a schematic plan view of the defrost timer  100  with different parts and lines arranged on the circuit board  200  are shown. For the sake of simplicity, drawings of some parts and lines are omitted. In this figure, AC lines are only shown with hatching. However, AC lines on the bottom side of the circuit board  200  are omitted. As shown in this figure and outlined in  FIG. 1 , an area provided for the terminal unit  210  and the switching unit  300  occupies at least one third of the entire area of the circuit board  200 . Furthermore, an area provided for the terminal unit  210 , the switching unit  300 , and the DC supply unit  400  occupies at least a half of the entire area of the circuit board  200 . Such arrangement makes it easy to design the circuit board  200  to effectively prevent the discharge of AC between different parts and lines. 
     On the circuit board  200 , slits  221 - 228  and screw halls  231 - 232  are formed. The screw halls  231 - 232  are used to install the defrost timer  100  in or on the refrigerator using some screws. 
     As shown in  FIG. 4 , the first AC line  211 , the second AC line  212 , the third AC line  213  and the fourth AC line  214  are formed on the circuit board  200 . In this embodiment, the distance between the third AC line  213  and the fourth AC line  214  (DIS 1 ) is set to be at least 5 mm. This arrangement effectively prevents the discharge of AC signals between the third AC line  213  and the fourth AC line  214  even though a high voltage such as 120 V is provided between the two lines. For the same reason, it is preferable that the distance between the third AC line  213  and the first AC line  211  and the distance between the third AC line  213  and the second AC line  212  are set to be also at least 5 mm. It is also preferable that the distance between the neutral terminal TAB 3  and the compressor terminal TAB 4  is set to be at least 5 mm. Furthermore, it is also preferable that the distance between the neutral terminal TAB 3  and the active terminal TAB 1  and the distance between the neutral terminal TAB 3  and the heater terminal TAB 2  are also set to be at least 5 mm. Although not limited, the maximum distance between these lines and terminals may be set to 5 cm. 
     In this embodiment, the AC relay RY 1  is provided approximately at the center of the circuit board  200 . Such arrangement makes it easier to handle the defrost timer  100 . Since the AC relay RY 1  is relatively larger and heavier than the other parts used in the defrost timer  100 , the gravity point of the defrost timer  100  becomes closer to the center of the circuit board  200  if the AC relay RY 1  is located near the center of the circuit board  200 . This makes the defrost timer  100  less prone to accidentally flip over while it is being put, for example, on top of the refrigerator while a worker is trying to install the defrost timer  100  into the refrigerator. One indicator of the ‘approximately center’ is that at least a part of the AC relay RY 1  is between one quarter and three quarter in width of the circuit board  200  and between one quarter and three quarter in length of the circuit board  200 . Furthermore, it is preferable that the AC relay RY 1  is placed so that at least a part of the AC relay RY 1  is placed between one third and two third in width of the circuit board  200  and between one third and two third in length of the circuit board  200 . 
     The photocoupler TR 1  is located adjacent to the AC relay RY 1 . This arrangement enables the AC lines on the circuit board  200  not to be excessively long. This makes it easy to design the circuit board  200  to effectively prevent the discharge of AC signals between the lines. One indicator of the ‘adjacent to’ is that the distance between the photocoupler TR 1  and the AC relay RY 1  is at most 2.5 cm. Furthermore, it is preferable that the distance between the photocoupler TR 1  and the AC relay RY 1  is within 1 cm. Although not limited, the minimum distance between the photocoupler TR 1  and the AC relay RY 1  can be set to 1 mm. 
     As shown in  FIG. 4 , the photocoupler TR 1  contains five pins. Among these pins, the AC signal is inputted into the pins TR 1 _ 1  and TR 1 _ 2 . The pin TR 1 _ 1  is coupled to the third AC line  213 . The pin TR 1 _ 2  is connected to a line that couples the photocoupler TR 1  and the AC relay RY 1  in series (AC line  215 ). In this embodiment, a first slit  221  between the pins TR 1 _ 1  and TR 1 _ 2 , is provided on the circuit board  200 . This effectively prevents the discharge of AC signals between the pins TR 1 _ 1  and TR 1 _ 2 . In this respect, it is preferable that the width of the slit  221  is at least 0.5 mm. Existence of the slit  221  allows the distance between the third AC line  213  and the AC line  215  (DIS 2 ) to be set close to each other. Therefore, in this embodiment, the distance DIS 2  is smaller than the distance DIS 1 . This enables an smaller area in which the switching unit  300  occupies the circuit board  200  and thus it allows the defrost timer  100  to be more compact. It is still preferable that the distance DIS 2  is at least 1 mm. This effectively prevents the discharge of AC signals between the third AC line  213  and the AC line  215 . Although not limited, the maximum width of the slit  221  and the maximum distance (DIS 2 ) may be set to 1 cm. 
     The second slit  222  is provided between two pins RY 1 _ 1  and RY 1 _ 2  of the AC relay RY 1  and between the pin RY 1 _ 2  and one pin of the photocoupler TR 1 , which is on the secondary switching line  302 . The second slit  222  effectively prevents the discharge of AC signals between pins RY 1 _ 1  and RY 1 _ 2  and between the primary switching line  301  and the secondary switching line  302 . The third slit  223  is provided between the AC relay RY 1  and the timer unit  500 , which effectively prevents the discharge of AC signals between them. The fourth slit  224 , the fifth slit  225 , the sixth slit  226 , the seventh slit  227  and the eighth slit  228  are also provided on the circuit board  200 . They also prevent the discharge of AC signals between some parts and metal lines. However, it is preferable that slits are not provided between the terminals TAB 1 -TAB 4 . This provides a mechanical strength to the circuit board  200 . Therefore, the circuit board  200  is resistant to the breakage even when a plug is plugged in and out again and again to the terminals TAB 1 -TAB 4 . 
     On the circuit board  200 , the LED D 6  is provided adjacent to the tactile switch S 1 . This configuration brings an advantage which will be described later. One indicator of the ‘adjacent to’ is that the distance between the LED D 6  and the tactile switch S 1  is at most 2.5 cm. 
     §1.6 Miscellaneous Remarks 
     In the above exemplary embodiments, the distance between the first AC line  213  and the second AC line  214  (DIS 1 ) was at least 5 mm. In other embodiments this distance may be smaller than 5 mm. Furthermore, in an alternative embodiment the distance DIS 1  may even be smaller than the distance DIS 2 . In yet another embodiment, the distance DIS 2  may be smaller than 1 mm while width of the slit  221  is smaller than 0.5 mm. 
     In addition, in the above exemplary embodiments, the AC relay RY 1  is located approximately at the center of the circuit board  200 . Other embodiments may place the AC relay RY 1  near the edge of the circuit board  200 . Moreover in the above exemplary embodiments, the photocoupler TR 1  is adjacent to the AC relay TR 1 . However, in an alternative embodiment the photocoupler TR 1  may be placed apart from the AC relay TR 1 . 
     In the above exemplary embodiments, the capacitor C 1 , the resistor R 1  and the resistor R 2 , the step down unit  401 , are coupled in series to the active terminal TAB 1 . The step down unit  401  may also be coupled in series to the neutral terminal TAB 3  in other embodiments. 
     In addition, the step down unit  401  of the exemplary embodiment is constituted of the capacitor C 1 , the resistor R 1  and the resistor R 2 . In alternative embodiments, the step down unit  401  may be constituted of only a capacitor or a resistor. In yet other embodiments, the step down unit  401  may be constituted of other parts such as a coil or a transformer. 
     Furthermore, the constant voltage unit  402  of the exemplary embodiment includes the zener diode D 5  and the electrolytic capacitor C 2 . In other embodiments, the constant voltage unit  402  may include other part such as a 3-terminals regulator. 
     Moreover, in the above exemplary embodiments, the AC relay RY 1  is driven by an AC signal. The AC relay RY 1  may be replaced by a DC relay in other embodiments such that the DC relay is driven by a DC signal. Furthermore, in the above embodiments, the AC relay RY 1  and the photocoupler TR 1  are coupled to switch on and off the current into the compressor of the refrigerator. Alternative embodiments may use different elements, other than the relay and the photocoupler, for switching on and off the current into the compressor as a part of the switching unit  300 . 
     In the above embodiment, all the electrical parts of the terminal unit  210 , the switching unit  300 , the DC supply unit  400  and the timer unit  500  are placed on the circuit board  200 . In yet other alternative embodiments some essential parts of the defrost timer  100  may be placed on a different circuit board or in some other part of the refrigerator. 
     In the above embodiment, an AC signal 120V is supplied to the defrost timer  100 . Other AC signals such as AC 100V, 220V or any other AC voltage may also be supplied to the defrost timer  100 . Some embodiments may even supply a DC voltage to the defrost timer  100 . 
     Additionally, the defrost timer  100  may include partially some mechanical components as well. 
     §1.7 Architecture of the Controller U 1   
     As described before, the controller U 1  of the timer unit  500  switches on and off the connection to the compressor and the heater of the refrigerator. With reference to  FIG. 5 , a block diagram of input/output connections between various components of the controller U 1  and input/output connections between inside and outside of the controller U 1  are shown. As shown in this figure, the controller U 1  may include a central processing unit (CPU)  610 , a timer  620 , a memory  630 , an analog/digital (A/D) converter  640 , and input/output (I/O) ports  650 . The memory  630  has a random access memory (RAM)  631 , flash memories  632 , and a program read only memory (PROM)  633 . The flash memories  632  have two sets of flash memories; a first flash memory  632   a  and a second flash memory  632   b.    
     The timer  620  counts a time and transmits the counted time to the CPU  610 . The CPU  610 , which runs programs stored in the PROM  633 , outputs and stores temporary data in the RAM  631 . The CPU  610  outputs and stores data, which are designed to retain even during a power failure, into the first flash memory  632   a  or the second flash memory  632   b . According to the programs stored in the PROM  633  and the time counted by the timer  620 , the CPU  610  outputs signals to the I/O ports  650  to control the switching unit  300  or to control the time display unit  520 . Some signals from outside of the controller U 1  are transmitted to the CPU  610  through the I/O ports  650  or through the I/O ports  650  and the A/D converter  640 . Examples of such signals may include inputs from the time input unit  510  and the acceleration mode activation unit  540  and signals from the mode selection unit  530  and the auxiliary connections  550 . 
     §1.8 Operation of the Controller U 1   
     As described before, the CPU  610  runs programs stored in the PROM  633 . The defrost timer  100  has two modes of operations: 1) a first mode of operation which is suitable for a refrigerator without a heater to defrost its evaporator, and 2) a second mode of operation which is suitable for a refrigerator with a heater to defrost its evaporator. When the defrost timer  100  is set to operate in the first mode, the CPU  610  runs a first program which will be described further below and is written in the PROM  633 . When the defrost timer  100  is set to operate in the second mode, the CPU  610  runs a second program which will be described later and is also written in the PROM  633 . As explained previously, either of these two modes are selected by replacing the resistor R 15  in the mode selection unit  530 . According to the resistance value of the resistor R 15 , two kinds of signals, for example a high signal or a low signal, may enter the pin P 1 _ 0 /AN 8  of the controller U 1  from the mode selection unit  530 . In the case where a high signal is inputted into the controller U 1  from the mode selection unit  530 , the CPU  610  runs the first program. On the other hand, in the case where a low signal is inputted into the controller U 1  from the mode selection unit  530 , the CPU  610  runs the second program. 
     §1.8.1 Overview of the First Program 
       FIGS. 6-8  illustrate flowcharts describing operational steps of the defrost timer  100  that are mainly run by the CPU  610 . In this specification, 24-hour system, which begins at 0:00 and ends at 24:00 hours, is used to indicate the time. In what follows, the overview of operational steps for the first mode of operation will be explained using  FIG. 6 . The first mode of operation is designed such that the defrost timer  100  initiates its defrosting period at a certain time every day. For example, by running the first program, the defrost timer  100  turns on the compressor of its refrigerator from 0:00 to 1:00 o&#39;clock and from 4:00 to 24:00 o&#39;clock. Then, the defrost timer  100  turns off the compressor from 1:00 to 4:00 o&#39;clock, during which the defrosting mode is performed. As shown in  FIG. 6 , the first program is composed of four steps: 1) an initialization step S 100 , 2) a compressor-on step S 200 , 3) a defrost-on step S 300 , and 4) a time input step S 400 . Each of these steps is further divided into more detailed steps, which will be explained further below. 
     The initialization step S 100  is executed when an AC power begins to be supplied into the defrost timer  100 , for example after a power failure. In this step, the CPU  610  retrieves data, related to the time counted by the timer  620 , from the flash memories  632  before the power is turned off. 
     In the compressor-on step S 200 , the CPU  610  maintains the compressor in the On position. During this step, the CPU  610  writes periodically, e.g. every five minutes, data related to the time counted by the timer  620  into the flash memories  632 . 
     When the time counted reaches a predetermined time, for example 1:00 o&#39;clock, the CPU  610  begins the defrost-on step S 300 . In this step, the CPU  610  turns off the compressor of its refrigerator for a certain period of time, for example three hours. Thereby, defrosting an evaporator of the refrigerator is achieved. Once the time counted reaches a certain time, for example 4:00 o&#39;clock, the CPU  610  finishes the defrosting step and initiates again the compressor-on step S 200 . During the defrost-on step S 300 , the CPU  610  also writes periodically data related to the time counted by the timer  620  into the flash memories  632 . 
     At any time during the compressor-on step S 200  and the defrost-on step S 300 , the CPU  610  accepts an input about current time from a user. When the user pushes the tactile switch S 1  (see  FIG. 4 ), the CPU  610  initiates an interruption process, which is represented by the time input step S 400 . During this step, the CPU  610  receives and updates information related to the current time and uses this information for further time counting. 
     §1.8.2 The First Program 
     In this section, the first program will be explained in more detail, using  FIGS. 7-8 . In the first program, it is assumed that the AC power entered into the defrost timer  100  is off until the initialization step S 100  is executed. 
     &lt;&lt;Initialization Step S 100 &gt;&gt; 
     As shown in  FIG. 7   a , the initialization step S 100  is composed of two steps: Steps  101 - 102 . 
     &lt;Step  101 &gt;: Once the power is on, the CPU  610  seeks the maximum value of the time counted by the timer  620  from the flash memories  632  before the power is turned off. The CPU  610  stores this maximum value as a value ‘t’ in the RAM  631 . For example, as shown in  FIG. 9 , the CPU  610  seeks the maximum value stored in the flash memories  632 , and retrieves the value ‘525600’ from the flash memories  632  and stores this value as the value ‘t’ in the RAM  631 . The value ‘t’ represents the time in minutes counted by the timer  620  until the power entered into the defrost timer  100  is turned off for the last time. 
     &lt;Step  102 &gt;: The CPU  610  also obtains an address ‘p’ where the maximum value was stored in the flash memories  632 . The CPU  610  stores also the value ‘p’ in the RAM  631 . In the embodiment of  FIG. 9 , the maximum value is stored in the first flash memory  632   a  at the address ‘2400h’. Thus, the CPU  610  obtains the value ‘2400h’ and stores this value as the address ‘p’. 
     &lt;&lt;Compressor-on Step S 200 &gt;&gt; 
     As shown in  FIG. 7   b , the compressor-on step S 200  is composed of eleven steps, Steps  201 - 211 . In brief summary, the CPU  610  first turns on the compressor of the refrigerator (Step  201 ). While the counted time ‘t’ is below a preset time ‘T 1 ’ or above a preset time ‘T 2 ’, the CPU  610  keeps performing the following processes: Steps  202 - 211 . According to the time counted by the timer  620 , the CPU  610  writes a value ‘t’ at an address ‘p’ in the flash memories  632  with an interval ΔT (Steps  203 ,  204  &amp;  208 ). At this time, the CPU  610  tries to writes the value ‘t’ at a different address where the value ‘t’ is written for last time in the same flash memory  632   a  or  632   b  (Steps  205 - 208 ). If a writing error occurs, the CPU  610  writes the value t in a different flash memory  632   a  or  632   b  (Steps  209  &amp;  210 ). When the value ‘t’ reaches the preset value ‘T 1 ’, the process proceeds to the defrost-on step S 300 . 
     &lt;Step  201 &gt;: The CPU  610  turns on the line to the compressor. This is done by turning off the secondary switching line  302 , leading the first AC line  211  and the fourth AC line  214  to be connected and the first AC line  211  and the second AC line  212  to be disconnected at the AC relay TR 1 . 
     &lt;Step  202 &gt;: The CPU  610  determines whether a remainder of the value ‘t’ divided by a cycle length ‘C’ is below the preset time ‘T 1 ’ or above the preset time ‘T 2 ’. In the case where the above mentioned condition is satisfied, the process proceeds to Step  203 . In the case where the above mentioned condition is not satisfied, the process terminates the compressor-on step S 200 . The cycle length ‘C’ in this embodiment is the length of a day in minutes, which is 24×60=1440. The remainder of the value T divided by the cycle length ‘C’ gives a value of time in minutes on the day in which the defrost timer  100  is running. The preset time ‘T 1 ’, in this embodiment, is a predetermined threshold value corresponding to a time when the defrosting process is supposed to initiate. In the same way, the preset time ‘T 2 ’ is a value corresponding to a time when the defrosting process is supposed to terminate. As an example, in the case where the defrosting process starts at 1:00 o&#39;clock and finishes at 4:00 o&#39;clock, the preset time ‘T 1 ’ is 1×60=60 while the preset time ‘T 2 ’ is 4×60=240. 
     &lt;Step  203 &gt;: The CPU  610  waits for a certain period of time ‘ΔT’, for example 5 minutes, which refers to a time counted by the timer  620 . 
     &lt;Step  204 &gt;: The CPU  610  increases the value T by the value ‘ΔT’. For example, the CPU  610  increases the value ‘t’ to ‘525605’ if the value t is set to ‘525600’. 
     &lt;Step  205 &gt;: The CPU  610  increases the address p by a record size (or a cell size of the flash memory  632 ) ‘ΔP’. For example, in the case where the record size ‘ΔP’ is 6 bytes, the CPU  610  increases the address ‘p’ to ‘2406h’ if the address ‘p’ is set to ‘2400h’. 
     &lt;Step  206 &gt;: The CPU  610  determines whether the address ‘p’ is the same or above a limit address of flash memory ‘Pmax’. In the case where the above condition is satisfied, the process proceeds to Step  207 . If not, the process jumps to Step  208 . 
     &lt;Step  207 &gt;: The CPU  610  sets a start address of flash memory ‘P 0 ’ as the address ‘p’. 
     &lt;Step  208 &gt;: The CPU  610  writes the value ‘t’ in the flash memory  632  at the address ‘p’. For example, if the value ‘t’ is ‘5256005’ and the address ‘p’ is ‘2406h’, the CPU  610  writes the value ‘5256005’ at the address ‘2406h’ in the flash memory  632 . 
     &lt;Step  209 &gt;: The CPU  610  determines whether a writing error occurred in the previous steps. If the writing error occurred, the process jumps to Step  210 . If the writing error did not occur, the process proceeds to Step  211 . 
     &lt;Step  210 &gt;: The CPU  610  sets the corresponding address of the address ‘p’ in the other flash memory  632  as a new address ‘p’. For example, if a writing error occurs at an address ‘2406h’ in the first flash memory  632   a , the CPU  610  sets the address corresponding to ‘2406h’ in the second flash memory  632   b  as the address ‘p’. Then, the process jumps back to Step  205 . 
     &lt;Step  211 &gt;: The process goes back to Step  202 . 
     &lt;&lt;Defrost-on Step S 300 &gt;&gt; 
     As shown in  FIG. 8   a , the defrost-on step S 300  is composed of eleven steps: Steps  301 - 311 . Briefly, the CPU  610  first turns off the compressor of the refrigerator (Step  301 ). While the counted time ‘t’ is the same or above the preset time ‘T 1 ’ and the same or below the preset time ‘T 2 ’ (Step  302 ), the CPU  610  keeps performing the same process as described in the compressor-on step S 200  (Steps  303 - 311 ). 
     &lt;Step  301 &gt;: The CPU  610  turns off the line to the compressor. This is performed by turning on the secondary switching line  302 , leading the first AC line  211  and the second AC line  212  to be connected and the first AC line  211  and the fourth AC line  214  to be disconnected at the AC relay TR 1 . 
     &lt;Step  302 &gt;: The CPU  610  determines whether a remainder of the value ‘t’ divided by the cycle length ‘C’ is the same or above the preset time ‘T 1 ’ and the same or below the preset time ‘T 2 ’. If yes, the process proceeds to Step  303 . If not, the process terminates the defrost-on step S 300 . 
     &lt;Steps  303 - 311 &gt;: Since these steps are the same as steps: Steps  203 - 211 , explained above, their explanations are omitted. 
     &lt;&lt;Time Input Step S 400 &gt;&gt; 
     This step is initiated as an interruption process when the tactile switch S 1  is pressed for a certain period of time, for example three seconds, during the Step  203  or the Step  303 . The signal from the tactile switch S 1  enters the pin P 1 _ 0 /AN 8  of the controller U 1  through the time input unit  510 . As shown in  FIG. 8   b , the time input step S 400  is composed of four steps: Steps  401 - 404 . 
     &lt;Step  401 &gt;: First, the CPU  610  turns on the LED D 6  for a short period of time, for example 10 seconds, by outputting a signal from the pin P 1 _ 7  of the controller U 1 . This notifies a user that the user may presently input the current time. 
     &lt;Step  402 &gt;: While the LED D 6  is on, the user inputs the current time by pressing the tactile switch  51  according to a switch pushing times parameter, defined in Table 1. For example, when the current time is 16:37, the user pushes the tactile switch S 1  8 times. Thereby, the CPU  610  receives an input related to the current time. If the number of times the tactile switch S 1  is pressed is not proper, the CPU  610  doesn&#39;t perform the following steps and terminates the time input step S 400 , thereby turning off the LED D 6 . 
     &lt;Step  403 &gt;: according to the current time inputted from the tactile switch S 1 , the CPU  610  updates the value ‘t’ in the flash memories  632  so that the value ‘t’ reflects the inputted time, which is larger than the previous value of ‘t’. For example, the value T may be updated by the following equation: t=((t/1440)+1)×1440+Tin×60 where ‘Tin’ is the inputted time, which refers to the setting time parameter, defined according to the Table 1. In this equation, the remainder of the division is rounded off. 
     &lt;Step  404 &gt;: The CPU  610  blinks the LED D 6  based on the number of times the tactile switch S 1  is pressed at the Step  402 . This blinking process enables the user to confirm whether his/her intended time is inputted properly. 
     When the time input step S 400  is finished, the process goes back to the original step, where the CPU  610  was executing before initiating the time input step S 400 . For example, if the CPU  610  was performing the Step  203  before the time input step S 400 , the process goes back to the Step  203 . 
     If the time the user inputted does not correspond to the time the user intended to input, the user may push the tactile switch S 1  for three seconds again. In this case, the time input step S 400  begins initiating and the user may input the intended current time again. 
     §1.8.3 Advantage of the First Program 
     As described above, in the first program, the CPU  610  receives an input related to the current time through the time input unit  510 , and the CPU  610  adjusts the value ‘t’ based on this input. This program enables the CPU  610  to initiate the defrosting cycle at a certain time more accurately. In fact, although the controller U 1  counts the time internally, this counted time may be shifted by hours due to a power failure. Thus, the defrost timer  100  pertaining to the present invention allows the CPU  610  to adjust the initiating time of its defrosting cycle accordingly. This feature enables the CPU  610  to perform the defrosting cycle at a specific time frame, for example from 1:00 o&#39;clock to 4:00 o&#39;clock every day, with an increased accuracy. 
     In addition, the CPU  610  writes the value ‘t’, which reflects the time counted by the timer  620  into the flash memories  632 . This value is retained even while the power to the defrost timer  100  is failed. Therefore, the time counting resumes from the time when the power failure is occurred. If the power failure is short, the time counted is not behind much. Therefore, the CPU  610  may still perform its defrosting cycle at a pretty accurate time. 
     Besides, the CPU  610  writes periodically the value ‘t’ into the flash memories  632 . This may increase the longevity of the flash memories  632 . The flash memories  632  may write data only a limited number of times. By writing data into the flash memories  632  with a certain interval time ‘ΔT’, the defrost timer  100  may reduce the number of times where data is written into the flash memories  632 . This elongates the longevity of the flash memories  632 . 
     In this respect, in this embodiment, the interval time ‘ΔT’ is set to 5 minutes. It should be noted that the interval time ‘ΔT’ is not limited to this length. However, it is preferable that the interval time ‘ΔT’ is set to at least 2 minutes. According to the inventor&#39;s calculation, this length enables to keep rewriting in the flash memories  632  for a long period of time such as over a decade. Although not limited, the maximum interval time ‘ΔT’ may be also set to 2 hours. 
     Furthermore, in the above Example, the CPU  610  writes the value T at an address ‘p’ of the flash memories  632 , which is different from the address ‘p’ where the value ‘t’ was written for the last time. This avoids writing data repeatedly in the same cell of the flash memories  632 . This feature further enhances the longevity of the flash memories  632 . In this embodiment, the CPU  610  writes the value ‘t’ into the flash memories  632  sequentially. In other embodiments, the CPU  610  may write in to the flash memories  632  randomly, in other word at a random address of the flash memories  632 . 
     It is preferable that the controller U 1  has at least 1 kilobyte of total size for the flash memories  632 . According to the inventor&#39;s calculation, this size enables to keep rewriting the flash memories  632  for a long period of time such as over a decade. Although not limited, the maximum size may be set to 1 megabyte. 
     In this embodiment, the controller U 1  includes two flash memories: the first flash memory  632   a  and the second flash memory  632   b . This feature may prevent the breakage of the defrost timer  100  efficiently. Even in the case one of the flash memories  632  breaks, the defrost timer  100  may still keep running without any problem. Although in this embodiment, the defrost timer  100  includes two flash memories, other embodiments may include defrost timers  100  with more than two flash memories. On the contrary, in an alternative embodiment, the defrost timer  100  may use only one or not any flash memory. 
     Additionally, in this embodiment, the CPU  610  writes the value T into a same flash memories  632  as long as the same flash memory  632  allows the CPU  610  to rewrite its value. This feature may bring an easier coding scheme for a programmer. In addition, the debug of the program may also be easier. 
     As shown in Table 1, the number of times the tactile switch S 1  is pressed by the user is always smaller than a numeral, e.g. the setting time parameter, represented by time in hour. For example, when the current time is ‘16:37’, the setting time parameter is set to ‘16:00’ and the numeral represented by time in hour is ‘16’. In this case, the number of times the tactile switch S 1  may be pressed is set to ‘8’, which is smaller than ‘16’. This makes it easier for the user to input the time as the maximum number of times to push the tactile switch S 1  by the user is only 12 times. In other embodiment, if the numeral of the time is ‘0’ represented in hour, the user may not have to push the tactile switch S 1 , meaning the number of times the tactile switch  51  is pressed is ‘0’. 
     §1.8.4 Overview of the Second Program 
     In what follows, the second program will be explained using the same  FIGS. 6-8 . It has to be noted that only matters different from those explained in the first program will be explained in this section. First, an overview of the operational step for the second mode will be explained, using  FIG. 6 . The second operational mode is designed such that the defrost timer  100  initiates its defrosting cycle after the compressor is on for a certain period of time. For example, by running the second program, the defrost timer  100  defrosts the evaporator of the refrigerator for 20 minutes after the compressor was on for about 10 hours. Then, the defrost timer  100  switches on the compressor again for another 10 hours. The second program is mainly composed of three steps, an initialization step S 100 , a compressor-on step S 200  and a defrost-on step S 300 . Contrary to the first program, the time input step S 400  is inactivated in this program. In other word, the second program does not allow a user to input his/her intended input time. 
     Similar to the first program, the initialization step S 100  is executed when an AC power begins to be supplied to the defrost timer  100 . However, the second program is designed to be used in a refrigerator that turns off the power to the defrost timer  100  while its compressor is off. In the initialization step S 100 , the CPU  610  retrieves a data from the flash memories  632  related to the time counted by the timer  620  before the power to the defrost timer  100  is turned off. 
     In the compressor-on step S 200 , the CPU  610  keeps the switch in a position where the line to the compressor is on. During this step, the CPU  610  writes periodically data related to the time counted by the timer  620  into the flash memories  632  as long as the power to the defrost timer  100  is on. 
     When the time counted reaches a predetermined threshold, for example 10 hours, the CPU  610  begins the defrost-on step S 300 . In this step, the CPU  610  turns off the compressor of the refrigerator and turns on the heater for its defrosting cycle for a specific period of time, for example 20 minutes. Thereby, defrosting of the evaporator is achieved. After the specific period of time, the CPU  610  finishes this step and initiates the compressor-on step S 200  again. During this step, the CPU  610  also writes periodically data related to the time counted by the timer  620  into the flash memories  632 . One remarkable feature of the second mode is that the defrost timer  100  is designed to be installed in a refrigerator so that the defrost timer  100  is connected to a switch, such as a thermostat, which turns on and off the compressor, in series. Thus, when this switch is off, the power supply to the defrost timer  100  is off as well as the power supply to the compressor is off. This means that the defrost timer  100  doesn&#39;t count any time while the compressor is off by other devices, e.g. the thermostat. In other word, the defrost timer  100  is configured so that the CPU  610  writes a value, which reflects the running time of the compressor into the flash memories  632 , by taking into account the operation of other devices of the refrigerator, such as the thermostat, which controls the compressor. 
     §1.8.5 The Second Program 
     In this section, the second program will be explained in more detail, using  FIGS. 7-8 . As explained previously, only different matters from the first program will be explained. 
     &lt;&lt;Initialization Step S 100 &gt;&gt; 
     The initialization step S 100  is the same explained in the first program. 
     &lt;&lt;Compressor-on Step S 200 &gt;&gt; 
     &lt;Step  201 &gt;: This step is the same as the in the first program. 
     &lt;Step  202 &gt;: In the second program, the values of the cycle length ‘C’, the preset time ‘T 1 ’, and the preset time ‘T 2 ’ are different from those set in the first program. The cycle length ‘C’ is the sum of the compressor-on time and the defrost-on time. For example, when the compressor is on for about 10 hours and the defrosting is on for 20 minutes, the cycle length ‘C’ is 10×60+20=620. The preset time ‘T 1 ’ is defined as a value corresponding to the period of time when the compressor is on. In the case where the compressor is on for 10 hours, the preset time ‘T 1 ’ is 10×60=600. The preset time ‘T 2 ’ is defined as a value corresponding to the sum of period of time when the compressor and the defrosting cycle are on. In the case where the compressor is on for about 10 hours and defrosting is on for 20 minutes, the preset time ‘T 2 ’ is 10×60+20=620. 
     &lt;Step  203 - 211 &gt;: These steps are similar to those explained in the first program. 
     &lt;&lt;Defrost-on Step S 300 &gt;&gt; 
     &lt;Step  301 &gt;: The CPU  610  turns off the line to the compressor while it turns on the line to the heater. This is performed by turning on the secondary switching line  302 , leading the first AC line  211  and the second AC line  212  to be connected and the first AC line  211  and the fourth AC line  214  to be disconnected at the AC relay TR 1 . 
     &lt;Steps  302 - 311 &gt;: These steps are similar to those explained in the first program. 
     §1.8.6 Advantage of the Second Program 
     As described above, in the second program, the CPU  610  is configured to write the value ‘t’, which reflects a running time of the compressor, into the flash memories  632 . Since the amount of frost and ice accumulated on the compressor correlates with the running time of the compressor, this configuration enables the defrost timer  100  to initiate its defrosting cycle before the amount of frost and ice becomes large. This leads to an efficient operation of the refrigerator. 
     Since other advantages of the second program will become more in the context of the refrigerator, such advantages will be explained in the following sections. 
     §1.8.7 Acceleration Mode 
     Referring back to  FIG. 4 , when the jumper switch S 2  is closed, a signal from the acceleration mode activation unit  540  enters the pin P 1 _ 5  of the controller U 1 . In this case, cycles corresponding to the compressor-on step S 200  and the defrost-on step S 300  are performed with a shorter period of time. For example, in the first program, the cycle length ‘C’ becomes 24 minutes. In addition, the period during which the defrosting cycle is performed becomes 3 minutes by setting, respectively, the preset time ‘T 1 ’ and ‘T 2 ’ to the following values: 1 and 4. In the second program, the cycle length ‘C’ becomes 12 minutes. In addition, the period during which the defrosting cycle is performed, becomes 2 minutes by setting the preset time ‘T 1 ’ and ‘T 2 ’ to 10 and 12 values respectively. In each case, the interval ‘ΔT’ is set to 1 minutes. This enables manufacturers and repairers of refrigerators to verify easily whether the defrost timers  100  are working properly within their refrigerators. 
     §2 Refrigerator 
     In this section, refrigerators with the defrost timer  100  pertaining to the present invention will be described using  FIGS. 10-13 . 
     §2.1 Overview of the Refrigerator 
       FIG. 10  shows a perspective view from the upper front of a refrigerator  700 . 
       FIG. 11  shows a view of the refrigerator  700  seen from behind. As shown in these figures, the refrigerator  700  may include a compartment system  710 , a heat-exchange system  720 , an electric system  730  and accessory parts  750 . 
     The compartment system  710  has a housing  711 , and a door  712 . The door  712  is attached to the housing  711  so that it can be opened and closed. When the door  712  is closed, inside of the refrigerator  700  is insulated from outside. This inside insulated compartment is called refrigeration room  713 . 
     The heat-exchange system  720  includes a compressor  721 , a condenser  722 , an accumulator  723 , and an evaporator  724 , each of which is connected to each other by a pipe. The heat-exchange system  720  may also have a coolant, which circulates internally. As shown in  FIG. 11 , the compressor  721 , the condenser  722  and the accumulator  723  are placed outside of the compartment system  710 , more specifically behind the housing  711 . The evaporator  725  is placed inside of the compartment system  710 . 
     In the refrigerator  700 , the heated coolant is compressed by the compressor  721 . This compressed coolant emits heat and is condensed in the condenser  722 . The boiling point of the coolant is lowered by the function of the accumulator  723  in which its internal pressure is first elevated and then lowered. The condensed coolant evaporates in the evaporator  724 . When the coolant evaporates, it takes heat around the evaporator  724 . Thereby, the refrigeration room  713  is refrigerated. The heated coolant goes back to the compressor  721 . 
     The accessory parts  750  contains a defrost timer cover  751 , which includes a hole  752 . As shown in  FIG. 11 , the defrost timer  100  is attached to the behind side of the refrigerator  700  by screws using the screw holes  231  and  232  (see  FIG. 4 ). Then, the defrost timer  100  is covered by the defrost timer cover  751 . The hole  752  is located above the tactile switch S 1 , or at the same position if seen perpendicularly from a sight facing to the defrost timer  100 . Therefore, the user may press the tactile switch S 1  through the hole  752 . As described previously, the LED D 6  is located near the tactile switch S 1 . Thus, the user may easily see and recognize the light from the LED D 6  through the hole  752 . Although not written in the figure, an instruction of how to adjust a time of the defrost timer  100  is given on the defrost timer cover  751  with a table similar to the Table 1. Therefore, the user may easily adjust the time of the defrost timer  100 . 
     In one embodiment, the refrigerator  700  may include an electric system  730 , hereinafter referred to as a first electric system  730   a . In another embodiment, the refrigerator  700  may include another type of the electric system  730 , hereinafter referred to as a second electric system  730   b . In what follows, the first and second electric system  730   a  and  730   b  will be explained, respectively. 
     §2.2 First Electric System  730   a    
       FIG. 12  illustrates a schematic circuit diagram of the first electric system  730   a . The refrigerator  700  having the first electric system  730   a  is often called a mechanical refrigerator or direct-cool refrigerator. This type of refrigerator is commonly used, for example, in hotel rooms. As shown in this figure, the first electric system  730   a  may include an AC plug  731 , the defrost timer  100 , a thermostat  732 , an overload protector  733 , the compressor  721  and a positive temperature coefficient (PTC) thermistor  734 . Since the refrigerator  700  with the first electric system  730   a  doesn&#39;t contain a heater to defrost the evaporator  724 , the first program is preferably used to defrost the evaporator  724 . 
     In this embodiment, the AC plug  731  may include three terminals, an active terminal, a ground terminal, and a neutral terminal. The active terminal of the AC plug  731  is coupled to the active terminal TAB 1  of the defrost timer  100 . The neutral terminal TAB 3  is coupled to the neutral terminal of the AC plug  731 . In other word, the active terminal TAB 1  and the neutral terminal TAB 3  of the defrost timer  100  are connected to the AC plug  731  in parallel. Thus, the power is always provided to the DC supply unit  400  of the defrost timer  100 . For convenience, the line which connects the compressor terminal TAB 4  to the neutral terminal of the AC plug  731  is called a compressor line  739 . The thermostat  732 , the overload protector  733 , the compressor  721  and the PTC thermistor  734  are provided on the compressor line  739 . More specifically, the thermostat  732 , the overload protector  733 , and the compressor  721  are coupled in series to the compressor terminal TAB 4  and the neutral terminal of the AC plug  731 . The PTC thermistor  734  is coupled to a part of the compressor  721  in parallel. In the first electric system  730   a , the heater terminal TAB 2  is open, in other word is not connected to anything. 
     In the first electric system  730   a , the thermostat  732  and the compressor  721  are coupled to the ground terminal of the AC plug  731 . 
     The thermostat  732  is provided in the refrigeration room  713  to monitor the temperature of the refrigeration room  713 . The PTC thermistor  734  is provided near the condenser  722  to monitor the temperature of the condenser  722 . 
     While the defrost timer  100  selectively couples the active terminal TAB 1  to the compressor terminal TAB 4  at the AC relay RY 1 , in other words, when the defrost timer  100  executes the step of compressor-on step S 200 , an AC signal flows into the compressor line  739 . In this way, the AC signal flows from the active terminal of the AC plug  731  through the active terminal TAB 1 , the AC relay RY 1 , the compressor terminal TAB 4 , the thermostat  732 , the overload protector  733 , the compressor  721 , and the PTC thermistor  734  to the neutral terminal of the AC plug  731 . Thereby, the compressor  721  runs and circulates the coolant in the heat-exchange system  720 . Therefore, the refrigeration room  713  is being cooled down. When the temperature in the refrigeration room  713  is lower than a certain temperature, the thermostat  732  becomes off. In this case, the AC signal doesn&#39;t flow into the compressor line  739 . Thus, the compressor  721  is turned off. When the temperature of the condenser  722  goes higher, the resistance of the PTC thermistor  734  becomes higher as well. This leads to decrease the current flow into the compressor  721 , leading to a suppressed performance of the compressor  721 . When the current to the compressor  721  is too high, the overload protector  733  shuts off the current to the compressor  721 . 
     While the defrost timer  100  selectively couples the active terminal TAB 1  to the heater terminal TAB 2  at the AC relay RY 1 , in other words, when the defrost timer  100  executes the step of defrost-on step S 300 , the AC signal doesn&#39;t flow into the compressor line  739 , which means the compressor  721  is being turned off. Thereby, the refrigerator  700  performs its defrosting cycles, where the frost and ice on the evaporator  724  will be melted down and removed from the evaporator  724 . 
     §2.3 Second Electric System  730   b    
       FIG. 13  illustrates a schematic circuit diagram of the second electric system  730   b . The refrigerator  700  having the second electric system  730   b  is often called a fan-type refrigerator. This type of refrigerator is common in more expensive refrigerators, which have more functions. It should be noted that matters with similar explanation as those in the first electric system  730   a  will be omitted in this section. As shown in  FIG. 13 , the second electric system  730   b  may include an AC plug  731 , a thermostat  732 , the defrost timer  100 , an overload protector  733 , the compressor  721 , a PTC thermistor  734 , a running capacitor  741 , a fan  742 , a defrost thermostat  743 , a heater  744 , a thermal fuse  745 , a lamp switch  746 , and a lamp  747 . Since the refrigerator  700  with the second electric system  730   b  includes the heater  744  for defrosting the evaporator  724 , the second program is preferably used to defrost the evaporator  724 . 
     In the second electric system  730   b , the thermostat  732  is provided between the active terminal of the AC plug  731  and the active terminal TAB 1  of the defrost timer  100 . In other word, the thermostat  732  is coupled in series to the active terminal TAB 1  of the defrost timer  100 . Thus, the thermostat  732  may be used to turn on and off the power into the defrost timer  100  as well as the compressor  721 . The overload protector  733 , the compressor  721 , the PTC thermistor  734 , and the running capacitor  741  are provided on the compressor line  739 . The running capacitor  741  is also coupled in parallel to the PTC thermistor  734 . This prevents excessive current from flowing into the PTC thermistor  734  when the resistance of the PTC thermistor  734  is low. 
     In the second electric system  730   b , the heater terminal TAB 2  is coupled to the heater  744 . For convenience, the line which connects the heater terminal TAB 2  to the neutral terminal of the AC plug  731  is called a heater line  749 . On the heater line  749 , the defrost thermostat  743 , the heater  744  and the thermal fuse  745  are provided in series. The heater  744  is placed in adjacent to the evaporator  724  to defrost the evaporator  724 . The defrost thermostat  743  and the thermal fuse  745  are placed adjacent to the heater  744  position to monitor the temperature of the heater  744 . 
     The fan  742  is coupled in parallel to the compressor terminal TAB 4  and the neutral terminal of the AC plug  731 . Thus, the fan  742  may be turned on and off by the thermostat  732  and the defrost timer  100 . The fan  742  is placed near the evaporator  724 . 
     The lamp switch  746  and the lamp  747  are connected to each other in series. They are coupled to the active terminal of the AC plug  731  and the neutral terminal of the AC plug  731  in parallel. The lamp switch  746  is placed such that it may be switched on and off according to opening and closing of the door  712 . The lamp  747  is placed in the refrigeration room  713 . Thus, in the refrigerator  700  with the second electric system  730   b , the refrigeration room  713  becomes bright when the door  712  is opened because the lamp switch  746  and the lamp  747  are turned on. 
     In the second electric system  730   b , the thermostat  732 , the compressor  721  and the fan  742  are connected to the ground terminal of the AC plug  731 . 
     While the defrost timer  100  connects the active terminal TAB 1  and the compressor terminal TAB 4 , the AC signal flows into the compressor line  739  as well as to the fan  742 . Thus, the compressor  721  is turned on. At the same time, the fan  742  is also turned on. The fan  742  blows the air that is chilled by the evaporator  724 . This chilled air circulates in the refrigeration room  713 . Therefore, more homogenous refrigeration is possible in the refrigerator  700  having the second electric system  730   b . When the temperature in the refrigeration room  713  is lower than a certain temperature, the thermostat  732  becomes off. In this case, the AC current doesn&#39;t flow into the defrost timer  100 , the compressor line  739  and the fan  742 . Thus, the defrost timer  100  doesn&#39;t count the time while the compressor  721  is turned off. In addition, since the defrost timer  100  includes the flash memories  632 , it may not lose the time information counted by the controller U 1  up until the time when the thermostat  732  becomes off. Therefore, the defrost timer  100  may resume counting after the thermostat  732  becomes on again. Thus, the time counted by defrost timer  100  reflects the running time, during which the compressor  721  is turned on by the thermostat  732 , quite accurately. 
     While the defrost timer  100  selectively couples the active terminal TAB 1  to the heater terminal TAB 2 , the AC current flows into the heater line  749 . In other words, the AC current flows to the defrost thermostat  743 , the heater  744 , and the thermal fuse  745 . Thus, the heater  744  is turned on. Thereby, the heater  744  warms up the evaporator  724 . Thereby, frost and ice on the evaporator  724  will be melted down and removed. When the temperature of the heater  744  is higher than a certain temperature, the defrost thermostat  743  becomes off. This shuts off the current to the heater  744 , preventing the temperature of the heater  744  from becoming too high. If the temperature of the heater  744  is too high, the thermal fuse  745  fuses. Thereby, the temperature of the heater  744  is prevented from being extremely high. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Current Time 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1:00- 
                 3:00- 
                 5:00- 
                 7:00- 
                 9:00- 
                 11:00- 
                 13:00- 
                 15:00- 
                 17:00- 
                 19:00- 
                 21:00- 
                 23:00- 
               
               
                   
                 2:59 
                 4:59 
                 6:59 
                 8:59 
                 10:59 
                 12:59 
                 14:59 
                 16:59 
                 18:59 
                 20:59 
                 22:59 
                 0:59 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Setting 
                 2:00 
                 4:00 
                 6:00 
                 8:00 
                 10:00 
                 12:00 
                 14:00 
                 16:00 
                 18:00 
                 20:00 
                 22:00 
                 24:00 
               
               
                 Time 
               
               
                 Switch 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
               
               
                 Pushing 
               
               
                 Times