Patent Publication Number: US-2007095378-A1

Title: Thermoelectric transducer

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based on Japanese Patent Applications No. 2005-313358 filed on Oct. 27, 2005, and No. 2006-102396 filed on Apr. 3, 2006, the contents of which are incorporated herein by reference in its entirety.  
     FIELD OF THE INVENTION  
      The present invention relates to a thermoelectric transducer in which a direct current is passed through a series circuit including N-type thermoelectric elements and P-type thermoelectric elements to thereby absorb or radiate heat. The thermoelectric transducer can suitably monitor a failure of the thermoelectric elements connected in series.  
     BACKGROUND OF THE INVENTION  
      In a conventional thermoelectric transducer described in U.S. Pat. No. 5,254,178 (corresponding to JP Patent No. 3166228), a plurality of sets of N-type thermoelectric element and P-type thermoelectric element are connected in series in this order to construct a group of thermoelectric elements. These groups of thermoelectric elements are connected sequentially in series by heat absorbing electrode members and heat radiating electrode members. Furthermore, heat absorbing heat-exchange members are bonded in a protruding manner to the heat absorbing electrode members of the groups of thermoelectric elements, and heat radiating heat-exchange members are bonded in a protruding manner to the heat radiating electrode members of the groups of thermoelectric elements, respectively, so as to construct heat absorbing heat-exchange portions and heat radiating heat-exchange portions, respectively.  
      However, in the thermoelectric transducer disclosed in U.S. Pat. No. 5,254,178, all of the thermoelectric elements are electrically connected to each other in series via the heat absorbing electrode members or the heat radiating electrode members. For this reason, the thermoelectric elements which are adjacent to each other, the electrode members and the heat-exchange members are arranged in a state where they are electrically insulated from each other.  
      In the thermoelectric transducer like this, a failure that the thermoelectric element abnormally generates heat to melt parts around the thermoelectric element is known as one of the failure modes. This failure is caused by micro cracks produced in the thermoelectric element itself by the thermal stress of expansion or contraction developed when the thermoelectric element itself generates heat or is cooled. When the micro cracks grow, the thermoelectric element may be broken and brought completely out of conduction or may generate heat abnormally by contact resistance before it is completely broken.  
      When the thermoelectric element generates heat abnormally, there is presented a problem that the electrode member and the heat exchange member, which are bonded to the thermoelectric element, generate heat abnormally to melt a case member around them to thereby produce a bad smell.  
      In order to eliminate this problem, it is necessary to fix temperature sensors for detecting abnormal heat generation to all of the heat exchange members, which is not practical. In addition, this raises also a problem that the selection of positions where the temperature sensors are to be fixed so as to reduce the number of temperature sensors cannot be easily made.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above-described problems. The object of the present invention is to provide a thermoelectric transducer capable of detecting a failure of a thermoelectric element in an early stage and of taking measures against abnormalities.  
      According to an aspect of the present invention, a thermoelectric transducer includes a thermoelectric element module and a control device for controlling the thermoelectric transducer. In the thermoelectric element module, a plurality of pairs of P-type and N-type thermoelectric elements are arranged and all of the thermoelectric elements are electrically connected in series. Furthermore, the thermoelectric element module includes a first terminal connected to an electric power input side of the thermoelectric elements for inputting electric power, a second terminal for outputting electric power and connected to an electric power output side of the thermoelectric elements, and a third terminal arranged at one position or plural positions between the first terminal and the second terminal and used for detecting electric potential at the one position or the plural positions. In this thermoelectric transducer, the control device controls the thermoelectric element module on the basis of voltage between the respective terminals determined by electric potentials from the respective terminals when electric power is applied between the first terminal and the second terminal.  
      Accordingly, when the thermoelectric element causes an abnormality, the voltages between the respective terminals are thrown out of balance (e.g., a relationship) and hence a failure of the thermoelectric element can be detected by monitoring voltages between the respective terminals. Therefore, the failure of the thermoelectric element can be detected without using a complex construction.  
      Moreover, resistance values between the respective terminals are widely varied by variations in the characteristic of the thermoelectric element itself, distribution of wind speed, and distribution of temperature. Thus, variations in the voltages between the respective terminals can be reduced by arranging a plurality of (two or more) third terminals. This can improve the accuracies of the voltages between the respective terminals.  
      According to another aspect of the present invention, a thermoelectric transducer includes: a plurality of thermoelectric element modules electrically connected in series, each of which includes a plurality of pairs of P-type and N-type thermoelectric elements arranged to be electrically connected in series; a first terminal connected to an electric power input side of one of the thermoelectric element modules for inputting electric power; a second terminal connected to an electric power output side of another one of the thermoelectric element modules for outputting electric power; a third terminal arranged at one position or plural positions between the first terminal and the second terminal and used for detecting electric potential at the one position or the plural positions; and a control device that controls the thermoelectric element modules on the basis of voltage between the respective terminals determined by electric potentials from the respective terminals when electric power is applied between the first terminal and the second terminal.  
      Accordingly, even when the plurality of thermoelectric element modules are used, a failure of the thermoelectric element can be detected at an early stage by monitoring the voltages between the respective terminals. For example, a plurality of the third terminals may be arranged at the plural positions between the first terminal and the second terminal, or a single third terminal may be arranged at a predetermined position where voltage between the first and third terminals is approximately equal to voltage between the second and third terminals. In this case, an electric current passing through the thermoelectric elements can be stopped quickly before the case member near a heat exchange member is melted by heat to produce a bad smell or before a case member of the thermoelectric element module is broken. As an example, the control device may stop an electric current passing through the thermoelectric element module when a difference in voltages between the respective terminals is larger than a predetermined value.  
      The control device may include a thermoelectric element driving member for driving the thermoelectric element module by PWM control and a voltage detecting means for detecting voltage between the respective terminals. In this case, the control device controls the thermoelectric element driving member and the voltage detecting means in such a way that the voltage detecting means detects voltage between the respective terminals in synchronization with timing when the thermoelectric element driving member drives the thermoelectric element module. Accordingly, the thermoelectric element driving member can drives the thermoelectric element module by the control of changing the ratio between ON and OFF in a pulse width. Hence, when the thermoelectric element module is ON, the voltages between the respective terminals can be monitored.  
      For example, there is a case where when the frequency of the thermoelectric element driving member is fast and the processing of A/D converting of the voltage detected by the voltage detecting means is slow, the time that elapses before the voltage is stabilized becomes short and hence the A/D conversion timing is not in time. In this case, the control device controls the thermoelectric element driving member periodically for a predetermined time, thereby being able to synchronize the A/D conversion timing correctly with the ON timing outputted by the thermoelectric element driving member.  
      The thermoelectric transducer may be suitably used for a heating/cooling device for an air conditioner, e.g., a seat air-conditioning device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments when taken together with the accompanying drawings. In the drawings:  
       FIG. 1  is a schematic diagram showing a general construction of a thermoelectric element module according to a first embodiment of the present invention;  
       FIG. 2  is a cross-sectional view taken on a line II-II shown in  FIG. 1 ;  
       FIG. 3  is a schematic diagram showing an example of mounting in which the thermoelectric element module according to the first embodiment of the present invention is used for a seat air-conditioning device;  
       FIG. 4  is a cross-sectional view taken on a line IV-IV shown in  FIG. 1 ;  
       FIG. 5  is a flowchart showing a control process of a control device according to the first embodiment of the present invention;  
       FIG. 6  is a schematic diagram for determining voltage between terminals in the first embodiment of the present invention;  
       FIG. 7  is a graph showing a relationship between a change in a resistance R 1  and a temperature of a heat exchange portion on a heat radiating side when air volume is used as a parameter;  
       FIG. 8  is a schematic diagram for determining voltage between terminals in a second embodiment of the present invention;  
       FIG. 9  is a schematic diagram showing a general construction of a seat air-conditioning device when a plurality of heating/cooling devices according to a third embodiment of the present invention are mounted in a seat;  
       FIG. 10  is an electric circuit diagram showing an electric circuit of a control device and a plurality of thermoelectric element modules according to the third embodiment of the present invention;  
       FIG. 11  is a flowchart showing a control process of a control device according to the third embodiment of the present invention;  
       FIG. 12  is a characteristic diagram showing a relationship between a target air-cooling capacity and duty ratios of a thermoelectric element module and a blower;  
       FIG. 13  is a timing chart showing ON/OFF timing of a thermoelectric element driving member and A/D conversion timing of a voltage detecting means according to the third embodiment of the present invention; and  
       FIG. 14  is a timing chart showing the ON/OFF timing of thermoelectric element driving member and the A/D conversion timing of the voltage detecting means according to a modification of the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
      Hereinafter, a thermoelectric transducer according to the first embodiment of the present invention will be described on the basis of  FIG. 1  to  FIG. 7 .  
       FIG. 1  is a schematic diagram showing a general construction of a thermoelectric element module  30 , and  FIG. 2  is a cross-sectional view taken on the line II-II shown in  FIG. 1 . In this embodiment, the thermoelectric transducer is typically used for a cooling device or/and a heating device mounted on a vehicle. For example, as shown in  FIG. 3 , the thermoelectric transducer is used for a seat air-conditioning device in which the thermoelectric element module  30  is arranged in a seating portion  1   b  of a vehicle seat  1  and in which cold air cooled by the thermoelectric element module  30  is blown off from the surface of the seat  1 .  
      This seat air-conditioning device has the seat  1  having a backing portion  1   a  and the seating portion  1   b,  a heating/cooling device  5  arranged in a space  4  formed under the seat  1 , and a control device  40  (ECU) for controlling this heating/cooling device  5 .  
      The backing portion  1   a  is provided with a first duct  3   a  communicating with the space  4  and a plurality of air blowing openings  2  communicating with the first duct  3   a.  The seating portion  1   b  is provided with a second duct  3   b  communicating with the space  4  and a plurality of air blowing openings  2  communicating with the second duct  3   b.    
      The heating/cooling device  5  is constructed of a blower  50  and the thermoelectric element module  30 . The blower  50  introduces air (inside air) in a vehicle compartment into the seat  1  and blows the air to the air blowing openings  2  via the thermoelectric element module  30 .  
      The thermoelectric element module  30  is a well-known Peltier element for converting electricity to heat, and is constructed of electrode members  16  connected to thermoelectric semiconductors arranged inside and a plurality of heat radiating/absorbing heat exchange portions  25   b  arranged outside so as to heat or cool air in the vehicle compartment introduced by the blower  50  by changing the passing direction of an electric current (this will be described in detail).  
      The space  4  has an exhaust duct  3   c  communicating with the outside of the seat  1 , and the exhaust duct  3   c  is partitioned by a partition plate (not shown) arranged between the first duct  3   a  and the second duct  3   b  described above. In other words, the space  4  is formed so as to prevent air-conditioned air heated or cooled by one heat exchange portion  25   b  from mixing with exhaust air heated or cooled by the other heat exchange portion  25   b.    
      Moreover, reference symbols  7  and  8  indicated in  FIG. 3  denote temperature sensors. Specifically, the temperature sensor  7  senses the temperature of air-conditioned air to be blown off from the air blowing openings  2  and the temperature sensor  8  senses the temperature of exhaust air blown off from the exhaust duct  3   c.  Temperature information sensed by these temperature sensors  7 ,  8  are inputted to the control device  40 .  
      The thermoelectric element module  30 , as shown in  FIG. 1 ,  FIG. 2  and  FIG. 4 , is constructed of: a thermoelectric element substrate  10  having a plurality of P-type and N-type thermoelectric elements  12 ,  13  arranged thereon; electrode members  16  for electrically connecting the adjacent thermoelectric elements  12 ,  13  in series; a plurality of heat exchange members  25  bonded to the electrode members  16  so as to transfer heat; and a case member  28 .  
      The thermoelectric element substrate  10  is integrally constructed of: the plurality of P-type and N-type thermoelectric elements  12 ,  13 ; a holding plate  11  for holding these thermoelectric elements  12 ,  13 ; a waterproof film member  14  forming a waterproof film on the surface of this holding plate  11 ; and the electrode members  16  (electrode elements).  
      Specifically, the thermoelectric element substrate  10  is integrally constructed as follows: a group of thermoelectric elements, in which a plurality of pairs of P-type thermoelectric element  12  and N-type thermoelectric element  13  are arranged alternately in a lattice pattern, are arranged on the holding plate  11  made of a plate-shaped insulating material (for example, glass epoxy, PPS resin, LCP resin, or PET resin); and the electrode members  16  are bonded to both end surfaces of the pair of adjacent thermoelectric elements  12 ,  13 , respectively.  
      The P-type thermoelectric element  12  is an extremely small component constructed of a P-type semiconductor made of a Bi—Te based compound, and the N-type thermoelectric element  13  is an extremely small component constructed of an N-type semiconductor made of a Bi—Te based compound. The holding plate  11  is formed so as to have a thickness nearly equal to the element heights of the thermoelectric elements  12 ,  13 .  
      As shown in  FIG. 4 , an electric power input terminal  24   a  and an electric power output terminal  24   b  are fixed to the thermoelectric elements  12 ,  13  arranged on the left and right upper ends, respectively. The electric power input terminal  24   a  has the positive terminal of a direct current power source (not shown) connected thereto and the electric power input terminal  24   b  has the negative terminal of the direct current power source connected thereto.  
      The electrode member  16  made of the electrode element is a plate-shaped electrode formed of a conductive metal such as copper material and for connecting electrically in series a pair of P-type thermoelectric element  12  and N-type thermoelectric element  13 , which are adjacent to each other, of the group of thermoelectric elements arranged on the thermoelectric element substrate  10 .  
      Specifically, as shown in  FIG. 1 , the electrode member  16  arranged on the upper side is an electrode for passing an electric current from the N-type thermoelectric element  13  to the P-type thermoelectric element  12 , which are adjacent to each other, and the electrode member  16  arranged on the lower side is an electrode for passing an electric current from the P-type thermoelectric element  12  to the N-type thermoelectric element  13 , which are adjacent to each other.  
      All of the electrode members  16 , as shown in  FIG. 4 , are unified in their planar shapes and are formed in the same rectangular shape enough to cover the end surfaces of a pair of adjacent thermoelectric elements  12 ,  13 . The electrode members  16  are arranged at the predetermined positions corresponding to the state of arrangement of the thermoelectric elements  12 ,  13  arranged on the thermoelectric element substrate  10 . A paste solder or the like is applied thinly uniformly to the end surfaces of the thermoelectric elements  12 ,  13  by screen printing and then the electrode members  16  are bonded to the end surfaces by the use of solder.  
      With this, all of the thermoelectric elements  12 ,  13  are connected electrically in series to each other via the electrode members  16 . In other words, when electric power is applied between the electric power input terminal  24   a  and the electric power output terminal  24   b,  as shown by a single dot and dash line in  FIG. 4 , an electric current flows from the electric power input terminal  24   a  on the left side to the electric power output terminal  24   b  on the right side while snaking repeatedly in a direction along the group of thermoelectric elements.  
      In this embodiment, a middle terminal  24   c  (a terminal between the input terminal  24   a  and the output terminal  24   b ) is fixed to the thermoelectric element  12  arranged nearly in the middle position between the thermoelectric element  12  connected to the electric power input terminal  24   a  and the thermoelectric element  13  connected to the electric power output terminal  24   b.    
      More specifically, the middle terminal  24   c  is fixed to the thermoelectric element  12  arranged in a position where when a predetermined voltage is applied between the electric power input terminal  24   a  and the electric power output terminal  24 b, voltage between the electric power input terminal  24   a  and the middle terminal  24   c  is nearly equal to voltage between the middle terminal  24   c  and the electric power output terminal  24   b.    
      The electric power input terminal  24   a,  the electric power output terminal  24   b  and the middle terminal  24   c  are electrically connected to the control device  40  to be described later so as to output electric potential information at their terminal positions to the control device  40 . That is, these terminals  24   a,    24   b  and  24   c  are terminals for detecting electric potentials at an electric power input portion, a middle portion, and an electric power output portion.  
      With this, voltage between the electric power input terminal  24   a  and the middle terminal  24   c  and voltage between the middle terminal  24   c  and the electric power output terminal  24   b  can be determined (this will be described hereinafter in detail).  
      The above-described electrode member  16  is integrally formed with the waterproof film member  14 . The waterproof film members  14  are arranged on one surface and the other surface of the holding plate  11 , whereby the electrode members  16  are arranged on the end surfaces of the pair of thermoelectric elements  12 , 13  which are adjacent to each other, respectively.  
      The waterproof film member  14  is a sheet formed in the shape of a thin film made of a laminate of a thermoplastic polyimide thin film and a thermosetting polyimide thin film, and has a copper foil layer made of a copper foil integrally formed on one surface thereof. The copper foil layer is etched off to form the electrode members  16  at predetermined positions of arrangement and in predetermined shapes.  
      The waterproof member  14  is arranged on the entire surface of one surface and the other surface of the holding plate  11  to form waterproof films thereon. Further, the waterproof member  14  has openings  14   a  formed at the positions where the electrode members  16  are arranged opposite to the waterproof member  14 , that is, at the positions corresponding to the respective end surfaces of the thermoelectric elements  12 ,  13 . The openings  14   a  are nearly equal in size and shape to the end surfaces of the thermoelectric elements  12 ,  13 . The electrode members  16  and the end surfaces of the thermoelectric elements  12 ,  13  are bonded to each other at peripheries of these openings  14   a  by the use of solder.  
      Therefore, when the openings  14   a  of the waterproof film member  14  are sealed by the solder, condensed water does not enter into bonding portions of the thermoelectric elements  12 ,  13  and the electrode members  16  from the heat exchange member  25  to be described later.  
      Next, the heat exchange member  25  is formed of a thin plate made of a conductive metal such as copper material. The heat exchange member  25 , as shown in  FIG. 2 , has a cross section formed nearly in the shape of a letter U. The heat exchange member  25  includes a plane-shaped electrode portion  25   a  formed at the bottom, and a heat exchange portion  25   b  shaped like a louver formed at a plane extended outward from the electrode portion  25   a.    
      The heat exchange portion  25   b  is a fin for absorbing and radiating heat transferred from the electrode portion  25   a  and is formed integrally with the electrode portion  25   a  by a forming process such as a cutting and bending process. The plane-shaped electrode portions  25   a  are arranged at the predetermined positions corresponding to the state of arrangement of the electrode members  16  arranged on the thermoelectric element substrate  10  and are bonded to one end surfaces of the electrode members  16  by the use of solder.  
      Moreover, a reference numeral  22  denotes a fixing plate and a holding member for holding the other end sides of the plurality of heat exchange members  25 . With this, predetermined spaces are formed between the adjacent heat exchange members  25 , and the adjacent heat exchange members  25  are electrically insulated from each other.  
      The fixing plate  22  is made of a plate-shaped insulating material (for example, glass epoxy, PPS resin, LCP resin, or PET resin), just as with the holding plate  11 , and has fixing openings (not shown) through which the other end sides of the electrode portions  25   a  are passed.  
      The direct-current electric power inputted from the electric power input terminal  24   a,  as shown in  FIG. 1 , flows from the electrode member  16  arranged at the upper end of the P-type thermoelectric element  12  on the left end shown in the drawing to the P-type thermoelectric element  12 , and then flows in series to the N-type thermoelectric element  13  on the right adjacent side via the electrode member  16  on the lower side, and then flows in series to the P-type thermoelectric element  12  on the right adjacent side via the electrode member  16  on the upper side.  
      At this time, the electrode member  16  arranged on the upper side in  FIG. 1  and constructing an N-P junction is brought into the state of low temperature by the Peltier effect and the electrode member  16  arranged on the lower side in  FIG. 1  and constructing a P-N junction is brought into the state of high temperature. In other words, the heat exchange portion  25   b  arranged on the upper side in  FIG. 1  forms a heat absorbing heat-exchange portion of a heat absorbing side; and heat of low temperature is transferred to the heat exchange portion  25   b  and a cooling fluid is put into contact with the heat exchange portion  25 b. By contrast, the heat exchange portion  25   b  arranged on the lower side in  FIG. 1  forms a heat radiating heat-exchange portion of a heat radiating side; and heat of high temperature is transferred to the heat exchange portion  25   b  and fluid to be cooled is put into contact with the heat exchange portion  26   b.    
      The case members  28  are arranged on both sides of the thermoelectric element substrate  10  by using the thermoelectric element substrate  10  as a partition wall to form air flowing passages so that air flows through the air flowing passages to exchange heat between the heat exchange portions  25   b  and the air. With this, the air can be cooled by the heat exchange portions  25   b  on the upper side in  FIG. 1  and the air can be heated by the heat exchange portions  25   b  on the lower side in  FIG. 1 , for example. The case members  28  are integrally formed of appropriate resin, for example, polypropylene having reinforcing member mixed therein (for example, PBT-M20GF20).  
      In this embodiment, the positive terminal of the direct-current electric power is connected to the electric power input terminal  24   a  and the negative terminal thereof is connected to the electric power output terminal  24   b  to input the direct-current electric power to the electric power input terminal  24   a.  However, the positive terminal of the direct-current electric power may be connected to the electric power output terminal  24   b  and the negative terminal of the direct-current electric power may be connected to the electric power input terminal  24   a  to input the direct-current electric power to the electric power input terminal  24   a  to thereby reverse the passing direction of the electric current.  
      However, at this time, the heat exchange portions  25   b  on the upper side of  FIG. 1  form the heat radiating heat-exchange portions and the heat exchange portions  25   b  on the lower side of  FIG. 1  form the heat absorbing heat-exchange portions. In this case, the cooling/heating device  5  is used as a heating device.  
      In the thermoelectric element module  30  constructed in the above-described manner, a failure that the thermoelectric elements  12 ,  13  abnormally generate heat and melt parts arranged around them is known as one of the failure modes. This failure is caused by micro cracks produced in the elements  12 ,  13  themselves by the thermal stress of expansion or contraction developed when the thermoelectric elements  12 ,  13  themselves generate heat or are cooled. When the micro cracks grow, the thermoelectric elements  12 ,  13  may be broken and brought completely out of conduction or may generate heat abnormally by contact resistance before they are completely broken.  
      In particular, when the thermoelectric elements  12 ,  13  generate heat abnormally, there is presented a problem that the heat generated abnormally is transferred to the electrode member  16 , bonded to the thermoelectric elements  12 ,  13 , and also to the heat exchange member  25  to melt the case member  28  near the heat exchange member  25  to thereby produce a bad smell.  
      This embodiment can detect the failure of the thermoelectric elements  12 ,  13  such as abnormal heat generation at an early stage and can take measures against the abnormality by a simple construction. More specifically, as shown in  FIG. 3  and  FIG. 4 , this embodiment is provided with the control device  40  of control means for controlling the thermoelectric element module  30  and the blower  50 .  
      The control device  40  is constructed mainly of a microcomputer and stores a previously set control program in a built-in ROM (not shown) and controls the thermoelectric element module  30  and the blower  50  on the basis of not only temperature information from the temperature sensors  7 ,  8  and an inside temperature sensor (not shown) for detecting temperature in the vehicle compartment, but also electric potential information from the above-described respective terminals  24   a,    24   b,  and  24   c  and operating information from an operating panel (not shown).  
      The control device  40  is operated to have an air cooling mode, an air heating mode, and an air blowing mode, as usual operating modes. The air cooling mode is a mode of cooling air in the vehicle compartment introduced by the blower  50  by the thermoelectric element module  30  and of blowing off the cooled air-conditioned air from the air blowing openings  2 .  
      In the control at this time, the positive terminal of the electric power is connected to the electric power input terminal  24   a  and the negative terminal of the electric power is connected to the electric power output terminal  24   b  to apply a predetermined voltage between these terminals  24   a,    24   b  and the blower  50  is operated. With this, air in the vehicle compartment introduced by the blower  50  is cooled by the thermoelectric element module  30  and cold air is blown off from the air blowing openings  2 .  
      The air heating mode is a mode of heating air in the vehicle compartment introduced by the blower  50  by the thermoelectric element module  30  and of blowing off the heated air-conditioned air from the air blowing openings  2 . In this case, the negative terminal of the electric power is connected to the electric power input terminal  24   a  and the positive terminal of the electric power is connected to the electric power output terminal  24   b  to apply a predetermined voltage between these terminals  24   a,    24   b,  and the blower  50  is operated.  
      With this, air in the vehicle compartment introduced by the blower  50  is heated by the thermoelectric element module  30  and hot air is blown off from the air blowing openings  2 . Further, the air blowing mode is a mode of blowing off air in the vehicle compartment introduced by the blower  50  from the air blowing openings  2 . In this case, only the blower  50  is operated to blow off the air in the vehicle compartment from the air blowing openings  2 .  
      The predetermined voltage applied between the terminals  24   a  and  24   b  are controlled by the control device  40 . In other words, the amount of electricity is variably controlled on the basis of the operating information of a temperature setting/adjusting switch (not shown) set on an operating panel (not shown). Hence, for example, the predetermined voltage applied between the terminals  24   a  and  24   b  is determined from the amount of electricity determined by PWM control on the basis of the operating information.  
      In the above-described operating modes, abnormality measure control means for controlling the thermoelectric element module  30  and the blower  50  is performed on the basis of electric potential information from the respective terminals  24   a,    24   b,  and  24   c.  Specifically, this abnormality measure control means is a flowchart of control processing shown in  FIG. 5  and will be described below on the basis of this flowchart.  
      When the electric power is inputted to the cooling/heating device  5 , the control processing of the abnormality measure control means is started and initialization is performed in step  410 . Here, a flag in step  480  to be described later is initialized. In step  420 , the operating information of the operating switch (not shown) is read. In step  430 , it is determined whether or not the operating switch is ON. Here, if the operating switch is OFF, the processing is repeatedly performed until the operating switch is turned to ON.  
      If the operating switch is ON, in step  440 , the electric potential information v 0 , v 1 , and v 2  of the respective terminals  24   a,    24   b,  and  24   c  are read. Step  440  corresponds to voltage detecting means. In step  450 , voltages between the respective terminals  24   a,    24   b,  and  24   c  are computed.  
      More specifically, as shown in  FIG. 6 , voltage V 1  between the electric power input terminal  24   a  and the middle terminal  24   c  and voltage V 2  between the middle terminal  24   c  and the electric power output terminal  24   b  are computed. Here, it is known that the resistance values of the thermoelectric elements  12 ,  13  are widely changed by applied voltage, ambient temperature, the amount of heat radiation, and air volume.  
      However, resistance R 1  between the electric power input terminal  24   a  and the middle terminal  24   c  and resistance R 2  between the middle terminal  24   c  and the electric power output terminal  24   b  are in the same atmosphere and hence are nearly equal to each other in the amount of change, even if their absolute values are changed, so that the predetermined voltage V 0 =V 1 +V 2  and voltage V 1 ≅voltage V 2 . In other words, in this case, the thermoelectric elements  12 ,  13  operate normally.  
      When the thermoelectric elements  12 ,  13  between the electric power input terminal  24   a  and the middle terminal  24   c  causes a failure such as abnormal heat generation, the resistance R 1  is changed. That is, as shown in  FIG. 7 , when the thermoelectric elements  12 ,  13  generate heat abnormally, the amount of generation of heat is proportional to the resistance value R 1 . This was found by experiments by the inventors. The graph in  FIG. 7  shows a relationship between temperature of the heat exchange part and a change in the resistance R 1  by using air volume Va (Va 1 , Va 2 , Va 3 ) as a parameter. Here, Va 1 &lt;Va 2 &lt;Va 3 .  
      Hence, in this case, when the resistance R 1  and the resistance R 2  are thrown out of balance, the computed voltages V 1  and V 2  are thrown out of balance.  
      Next, in step  460 , it is determined whether or not the thermoelectric element module  30  operates normally. If the thermoelectric element module  30  operates normally, it is determined in step  470  whether or not the absolute value of the difference between the voltage V 1  and the voltage V 2  is not smaller than a predetermined value X. Here, the predetermined valueX is determined by taking into account factors such as variations in the element itself of the thermoelectric elements  12 ,  13  and variations in the temperature of a pair of thermoelectric elements  12 ,  13 .  
      Next, when it is determined in step  470  that the difference (absolute value) between the voltage V 1  and the voltage V 2  is smaller than the predetermined value X, it is determined that there is no abnormality and a normal control is continuously performed in step  480 . Here, if the difference (absolute value) between the voltage V 1  and the voltage V 2  is not smaller than the predetermined value X, it is determined that there is an abnormality and, first, a flag is set NG in step  490  and then the passage of an electric current between the terminals  24 a and  24   b  is stopped in step  500 . That is, in step  500 , the electric current applied between the terminals  24   a  and  24   b  is stopped, and the operation of the blower  50  is continued.  
      In this case, the blower  50  is controlled so as to continue operating, but the blower  50  may be controlled so as to continue operating only for a predetermined time and then to stop operating. When an abnormality occurs and the blower  50  and the thermoelectric element module  30  are stopped, a temperature increase is caused around the thermoelectric elements  12 ,  13  by overshoot. However, this temperature increase can be stopped by taking the above-described measures, that is, by continuing the operation of the blower  50 .  
      Moreover, in order to prevent erroneous determination, the determination means in step  470  may be constructed as follows: if it is determined in the first determination that there is an abnormality, the routine returns to step  440  and the control processing from step  440  to step  470  is performed several times and then it is determined that there is an abnormality.  
      With the above-described control, the failure of abnormal heat generation of the thermoelectric elements  12 ,  13  can be detected by the fact that the voltages V 1  and V 2  between the respective terminals  24   a,    24   b,  and  24   c  are thrown out of balance. Hence, the failure of the thermoelectric elements  12 ,  13  can be detected at an early stage even without using a complex construction.  
      The above-described change in the resistances R 1  and R 2  is caused by various failure modes including not only the abnormal heat generation but also a clogged filer, a reduced air volume caused by the failure of the blower  50 , a change in suction temperature, and a change in the voltage of electric power. The failure of the thermoelectric elements  12 ,  13  can be detected at an early stage by a simple construction using the voltages between the respective terminals  24   a,    24   b,  and  24   c  as determination values.  
      Since the failure of the thermoelectric elements  12 ,  13  can be detected at the early stage, the failure of the thermoelectric elements  12 ,  13  can be stopped at the early stage before the case member  28  near the heat exchange members  25  is melted by heat to cause a bad smell or before the case member  28  is broken.  
      When a seat air-conditioning device using a thermoelectric element module  30  is being operated in an air cooling mode and thermoelectric elements  12 ,  13  fail, a humid feeling can be dissipated by controlling a blower  50  so as to continue the operation of the blower  50 .  
      The thermoelectric transducer of the first embodiment described above has the electric power input terminal  24   a,  the electric power output terminal  24   b,  and the middle terminal  24   c  arranged at a position between the electric power input terminal  24   a  and the electric power output terminal  24   b  and used for detecting electric potential at the position. Further, the thermoelectric transducer has the control device  40  that controls the thermoelectric element module  30  on the basis of such voltages between the respective terminals  24   a,    24   b,  and  24   c  that are determined by the electric potential information from the respective terminals  24   a,    24   b,  and  24   c  when electric power is applied between the electric power input terminal  24   a  and the electric power output terminal  24   b.    
      According to this, the failure of the thermoelectric elements  12 ,  13  can be detected by monitoring the voltages between the respective terminals  24   a,    24   b,  and  24   c.  For example, if an abnormality occurs, the voltages between the respective terminals  24   a,    24   b,  and  24   c  loss balance. Hence, the failure of the thermoelectric elements  12 ,  13  can be detected at an early stage even without using a complex construction.  
      The middle terminal  24   c  is arranged at the predetermined position where the voltages between the respective terminals  24   a,    24   b,  and  24   c  are nearly equal to each other. The thermoelectric element module  30  is varied by the external factors of, for example, electric power voltage, air volume, and ambient temperature.  
      However, when the middle terminal  24   c  is arranged at the middle position of the thermoelectric element module  30 , the external factors of, for example, electric power source voltage, air volume, and ambient temperature have the same effect on two divided modules of the thermoelectric element module  30 . For this reason, variations in the two divided modules caused by these external factors can be cancelled and hence the failure of the thermoelectric elements  12 ,  13  can be correctly determined.  
      When the absolute values of the differences between the respective terminals  24   a,    24   b,  and  24   c  are not smaller than the predetermined value, the control device  40  stops passing an electric current through the thermoelectric element module  30 . With this, the control device  40  can stop passing the electric current through the thermoelectric elements  12 ,  13  at an early stage before the case member  28  near the heat exchange member  25  is melted by heat to cause a bad smell or before the case member  28  is broken.  
      Moreover, the thermoelectric element module  30  is used as a cooling device or a heating device mounted in a vehicle in combination with the blower  50 . When the absolute value of a difference in voltages between the respective terminals  24   a,    24   b,  and  24   c  is not smaller than the predetermined value, the control device  40  stops passing an electric current through the thermoelectric element module  30  and continues operating the blower  50 .  
      According to this, when the thermoelectric elements  12 ,  13  fail, if the blower  50  and the thermoelectric element module  30  are stopped, a temperature increase is caused near the thermoelectric elements  12 ,  13  by overshoot. However, this temperature increase can be stopped by continuing the operation of the blower  50 .  
      Moreover, in a cooling device for a vehicle, for example, a seat air-conditioning device for blowing off cold air from the air blowing openings  2  of a seat for the vehicle, when the thermoelectric elements  12 ,  13  fail, air is blown off in place of cold air, which can more dissipate a humid feeling as compared with a case where the blower  50  is stopped.  
     Second Embodiment  
      In the above-described first embodiment, the middle terminal  24   c  is arranged approximately at the middle position between the electric power input terminal  24   a  and the electric power output terminal  24 b. However, the position of the middle terminal  24   c  is not limited to this, but three middle terminals  24   c  may be arranged at suitable positions to divide the distance between the electric power input terminal  24   a  and the electric power output terminal  24   b  into quarters.  
      In this case, if the thermoelectric elements  12 ,  13  operate normally, the predetermined voltage V 0 =V 1 +V 2 +V 3 +V 4  and voltage V 1 ≅voltage V 2  voltage V 3 ≅voltage V 4 . According to this, the resistance values between the respective terminals  24   a,    24   b,  and  24   c  are widely varied by variations in the characteristics of the element itself, distribution of wind speed, and distribution of temperature. However, the variations in the voltages between the respective terminals  24   a,    24   b,  and  24   c  can be decreased by arranging three middle terminals  24   c.  With this, the accuracies of the voltages between the respective terminals  24   a,    24   b,  and  24   c  can be enhanced.  
     Third Embodiment  
      In the above-described embodiments, the thermoelectric transducer is used for the seat air-conditioning device in which one heating device  5  is arranged in the seating part  1   b  and in which heated or cooled air-conditioned air is blown off into the first duct  3   a  communicating with the air blowing openings  2  on the backing part  1   a  side and the second duct  3   b  communicating with the air blowing openings  2  on the seating part  1   b  side. However, the present invention may be applied to a seat air-conditioning device in which a plurality of heating/cooling devices  5  are arranged in the seating part  1   b  and the backing part  1   a  and in which air-conditioned air is blown off out of the air blowing openings  2 .  
      In other words, this embodiment is an example for seat air-conditioning means and abnormality measure controlling means when a plurality of thermoelectric element modules  30  are used and will be described on the basis of  FIG. 9  to  FIG. 14 .  FIG. 9  is a schematic diagram showing the general construction when a plurality of heating/cooling devices  5  are arranged in the seat  1 .  FIG. 10  is an electric circuit diagram showing an electric circuit of the control device  40  and the plurality of thermoelectric element modules  30 .  FIG. 11  is a flowchart showing the control processing of the control device  40 .  
       FIG. 12  is a graph showing a relationship between a target air-cooling capacity and the duty ratios of the thermoelectric element module  30  and the blower  50 .  FIG. 13  is a timing chart showing the ON/OFF timing of thermoelectric element driving member  42  and the A/D conversion timing of voltage detecting means. Further,  FIG. 14  is a timing chart showing the ON/OFF timing of the thermoelectric element driving member  42  and the A/D conversion timing of the voltage detecting means in a modification.  
      The thermoelectric transducer of this embodiment, as shown in  FIG. 9 , includes: the seat  1  having the backing part  1   a  and the seating part  1   b;  a plurality of (for example, two) heating/cooling devices  5  arranged in the spaces  4  formed in the seating part  1  ba and the backing part  1   a;  and the control device  40  as control means for controlling the plurality of heating/cooling devices  5 .  
      For example, the thermoelectric transducer is constructed so as to control the two thermoelectric element modules  30  and the two blowers  50  by using one control device  40 . Thus, the two thermoelectric modules  30 , as shown in  FIG. 10 , are provided with: the electric power input terminal  24   a  connected to the electric power input side of one thermoelectric element module  30 ; the electric power input terminal  24   b  connected to the electric power output side of the other thermoelectric element module  30 ; and middle terminals  24   c  arranged at two or more positions between the electric power input terminal  24   a  and the electric power input terminal  24   b  and used for detecting electric potentials at these positions. These terminals  24   a,    24   b,  and  24   c  are electrically connected to the control device  40 .  
      In other words, the two thermoelectric electric element modules  30  are electrically connected in series and the middle terminals  24   c  are arranged in such a way that if the two thermoelectric electric element modules  30  operate normally, the predetermined voltage V 0 =V 1 +V 2 +V 3 +V 4  and voltage V 1 ≅voltage V 2 ≅voltage V 3 ≅voltage V 4 . Here, as shown in  FIG. 10 , the voltage V 1  is the absolute value of the voltage difference between the terminals  24   a  and  24   b,  the voltage V 2 , V 3  is the absolute value of the voltage difference between adjacent the terminals  24   c  and  24   c,  and the voltage V 4  is the absolute value of the voltage difference between the terminals  24   c  and  24   b.    
      Of these respective terminals  24   a,    24   b,  and  24   c,  the electric power input terminal  24   a  is connected to the thermoelectric element driving member  42  arranged in the control device  40 . Two blowers  50  are connected to two blower driving members  43 , which are arranged in the control device  40  and will be described later, respectively.  
      The control device  40  of this embodiment includes a computing circuit  41  by a computer, the thermoelectric element driving member  42  for driving the thermoelectric element modules  30 , and the blower driving members  43  for driving the blowers  50 . The respective terminals  24   a,    24   b  and  24   c  and the output terminals  7   a,    8   a  of the respective temperature sensors  7 ,  8  are connected to the computing circuit  41 .  
      The computing circuit  41  determines a target air-cooling capacity on the basis of set information such as a set temperature set by an occupant by the use of an operating panel (not shown), and computes the duty ratios of indication values of the thermoelectric element module  30  and the blower  50  from a relationship, shown in  FIG. 12 , between the target air-cooling capacity and the duty ratios of the thermoelectric element module  30  and the blower  50 .  
      Moreover, electric potential information from the respective terminals  24   a,    24   b,  and  24   c  and temperature information from the terminals  7   a,    8   a  are A/D converted and inputted to the computing circuit  41 . The thermoelectric element driving member  42  and the blower driving members  43  are devices each including a FET and a current detecting circuit and output duty ratios at which the thermoelectric element module  30  and the blower  50  are operated by PWM control on the basis of indication values computed by the computing circuit  41 , respectively.  
      Here, the thermoelectric element driving member  42  outputs a voltage applied between the electric power input terminal  24   a  and the electric power output terminal  24   b  according to the duty ratio, and the blower driving members  43  outputs the number of revolutions according to the duty ratio.  
      The control device  40  of this embodiment having the above-described construction performs abnormality measure control means for controlling the thermoelectric element module  30  and the blower  50  on the basis of electric potential information from the respective terminals  24   a,    24   b,  and  24 c. This abnormality measure control means is a flowchart shown in  FIG. 11  and will be described below on the basis of this flowchart.  
      When the electric power is inputted to the cooling/heating devices  5 , the control processing of the abnormality measure control means is started. In step  410 , initialization is performed. In step  421 , set information set by an occupant from the operating panel (not shown) is read. Here, the abnormality measure control means may be constructed in such a way that an indication value from an air-conditioning control device (not shown) used for an air-conditioning device mounted in a vehicle is inputted as a target air-cooling capacity.  
      In step  423 , a Peltier duty ratio (duty ratio for module  30 ) and a blower duty ratio (duty ratio for fan) are computed. More specifically, the duty ratios of the indication values of the thermoelectric element module  30  and the blower  50  are computed from the relationship, shown in  FIG. 12 , between the target air-cooling capacity and the duty ratios of the thermoelectric element module  30  and the blower  50 . With this, a predetermined voltage to be applied between the electric power input terminal  24   a  and the electric power output terminal  24   b  and the number of revolutions of the blower  50  are determined.  
      In step  424 , the thermoelectric element driving member  42  and the blower driving members  43  output the duty ratios. More specifically, for example, 40 Hz is outputted as the Peltier duty and 200 Hz is outputted as the blower duty. With this, the blower  50  is driven at a predetermined number of revolutions, and a predetermined voltage is applied between the electric power input terminal  24   a  and the electric power output terminal  24   b  to drive the thermoelectric element modules  30 .  
      In step  431 , temperature information sensed by the temperature sensors  7 ,  8  are monitored. Here, for example, if Peltier temperature from the heat exchange portions  25   b  on the heat absorbing side is not higher than a first predetermined temperature (for example, 15° C.), the waist portion and the buttocks portion of an occupant of the vehicle are too cooled and hence the routine proceeds to step  500   a  so that an electric current passing between the terminals  24   a  and  24   b  is stopped.  
      If Peltier temperature from the heat exchange portions  25   b  is not lower than a second predetermined temperature (for example, 70° C.) higher than the first predetermined temperature, the temperatures of the thermoelectric elements  12 ,  13  are increased for some reason (for example, heat generation caused by a tracking phenomenon developed by migration) and hence the routine proceeds to step  500   a  such that an electric current passing between the terminals  24   a  and  24   b  is stopped. Here, if the Peltier temperature is not lower than 15° C. or not higher than 70° C., the routine proceeds to step  432 . That is, if the Peltier temperature is between the first predetermined temperature and the second predetermined temperature, the routine proceeds to step  432 .  
      In step  432 , a driving current detected by a current detecting circuit (not shown) arranged in the thermoelectric element driving member  42  is monitored. For example, it is determined whether or not the driving current detected by the current detecting circuit is not smaller than a predetermined value (for example,  5 A). Here, if the driving current is not smaller than the predetermined value (for example,  5 A), the routine proceeds to step  500   a  such that an electric current passing between the terminals  24   a  and  24   b  is stopped. With this, a failure such as a short circuit in the thermoelectric element module  30  or a short circuit caused by a bitten electric wire can be detected.  
      Here, if the driving current is not larger than a predetermined value (for example,  5 A), the routine proceeds to step  440  where electric potential information V 0 , V 1 , and V 2  of the respective terminals  24   a,    24   b,  and  24   c  are read. Here, the electric potential information V 0 , V 1 , and V 2  of the respective terminals  24   a,    24   b,  and  24   c  are A/D converted and then are read.  
      Since the Peltier duty ratio is outputted to the thermoelectric element module  30  by the thermoelectric element driving member  42 . Hence, as shown in  FIG. 13 , voltage applied between the electric power input terminal  24   a  and the electric power output terminal  24   b  is outputted at ON/OFF timing. Hence, it is recommended that in the A/D conversion, voltage be detected in synchronization with the timing when ON is outputted to the electric power input terminal  24   a.    
      Since the Peltier duty ratio shown in  FIG. 13  is 50%, the length of time that the thermoelectric element driving member  42  outputs ON continuously is long. However, when the Peltier duty ratio is shorter than this and the A/D conversion of slow conversion speed is used, the time that elapses before the voltage is stabilized becomes shorter, which presents a problem that A/D conversion is not in time.  
      In this case, as shown in  FIG. 14 , the thermoelectric element driving member  42  may be constructed so as to generate a predetermined ON time periodically in place of using the Peltier duty ratio and to synchronize the timing of the AND conversion with the ON time. In addition to this, the minimum value of the Peltier duty ratio may be previously set at a predetermined value (for example, 10%) or more to prevent a shorter Peltier duty ratio from being outputted. The control processing in step  440  corresponds to voltage detecting means.  
      In step  450 , the voltages between the respective terminals  24   a,    24   b  and  24   c  are computed. More specifically, voltage V 1  between the electric power input terminal  24   a  and the middle terminal  24   c,  voltages V 2  between the middle terminal  24   c  and the middle terminal  24   c,  voltages V 3  between the middle terminal  24   c  and the middle terminal  24   c,  voltage V 4  between the middle terminal  24   c  and the electric power output terminal  24   b,  and voltage V 0  between the electric power input terminal  24   a  and the electric power output terminal  24   b  are computed.  
      Next, in step  470   a,  it is determined whether or not the absolute value of (voltage V 1 +voltage V 2 )/voltage V 0  is from 0.45 to 0.55. Here, the absolute value of the ratio of voltages to be applied to the two thermoelectric element modules  30  is compared with a predetermined value. Here, when the absolute value of the ratio of voltages, which are supposed to be equal to each other, is not smaller than the predetermined value, it is determined that air is not blown because one thermoelectric element module  30  fails or some abnormality occurs in one air blowing system (for example, a clogged filter or a separated duct occurs).  
      When the absolute value of (voltage V 1 +voltage V 2 )/voltage V 0  is not in the range from 0.45 to 0.55, it is determined that there is an abnormality, and the routine proceeds to step  500   a  where an electric current passing between the terminals  24   a  and  24   b  is stopped. If there is no abnormality in step  470   a,  it is determined in step  470   b  whether or not the absolute value of voltage V 1 /(voltage V 1 +voltage V 2 ) is from 0.45 to 0.55. This step is means for determining a failure in the thermoelectric element module  30  arranged in the seating part  1   b.    
      Here, usually, the ratio between the voltage V 1  and the voltage V 2  is nearly equal to 1. However, for example, when a failure caused by micro cracks occurs in the thermoelectric elements  12 ,  13 , this ratio of voltage becomes not smaller than a predetermined value. With this, a failure in the thermoelectric element module  30  on the seating part  1   b  side can be found.  
      If the absolute value of voltage V 1 /(voltage V 1 +voltage V 2 ) is not in the range from 0.45 to 0.55 in step  470   b,  there is an abnormality, and the routine proceeds to step  500   a  where an electric current passing between the terminals  24   a  and  24   b  is stopped. If there is no abnormality in step  470   b,  it is determined in step  470   c  whether or not the absolute value of voltage V 3 /(voltage V 3 +voltage V 4 ) is from 0.45 to 0.55. This step is means for determining a failure in the thermoelectric element module  30  arranged on the backing part  1   a.  The ratio between the voltage V 3  and the voltage V 4  is nearly equal to 1, just as with the step  470   b.  However, for example, when a failure caused by micro cracks occurs in the thermoelectric elements  12 ,  13 , this ratio of voltages becomes not smaller than a predetermined value. With this, a failure in the thermoelectric element module  30  on the backing part  1   a  side can be found. Similarly, when the absolute value of voltage V 3 /(voltage V 3 +voltage V 4 ) is not in the range from 0.45 to 0.55, the control processing processes to step  500   a.    
      In step  500   a,  an electric current passing between the terminals  24   a  and  24   b  is stopped but operating the blower  50  is continued. Moreover, the blower duty ratio may be set at 100% to drive the blower  50  at a maximum number of revolutions. If an abnormality occurs and hence the blower  50  and the thermoelectric element module  30  are stopped, a temperature increase is developed near the thermoelectric elements  12 ,  13  by overshoot. However, in this embodiment, this temperature increase can be stopped by continuing the operation of the blower  50 .  
      According to the above-described control processing, when the voltages between the respective terminals  24   a,    24   b  and  24   c  are thrown out of balance, a failure caused by the abnormal heart generation of the thermoelectric elements  12 ,  13  can be detected. For example, when a relationship of the voltages between the respective terminals  24   a,    24   b  and  24   c  does not stay in a predetermined range, a failure caused by the abnormal heart generation of the thermoelectric elements  12 ,  13  can be detected. Hence, the failure of the thermoelectric elements  12 ,  13  can be detected at an early stage even without using a complex construction.  
      Moreover, in the thermoelectric transducer according to the above-described third embodiment, the thermoelectric element modules  30  are driven based on the control for changing the ratio between ON and OFF in a pulse width by the thermoelectric element driving member  42 . Hence, when the thermoelectric element module  30  is ON, the voltages between the respective terminals  24   a,    24   b,  and  24   c  can be monitored.  
      Furthermore, after the thermoelectric element driving member  42  starts supplying electric power to the thermoelectric element module  30  and then a predetermined time elapses, the control circuit  40  detects the voltages between the respective terminals  24   a,    24   b,  and  24   c  by the voltage detecting means  440 . Hence, after the thermoelectric element module  30  is driven, the voltage detecting means  440  can detect the failure of the thermoelectric element module  30  and the thermoelectric elements  12 ,  13  at an earlier stage and more correctly.  
      For example, there is a case where when the frequency of the thermoelectric element driving member  42  is fast and the processing of A/D converting of the voltage detected by the voltage detecting means  440  is slow, the time that elapses before the voltage is stabilized becomes short and hence the A/D conversion timing is not in time. Even in this time, the control device  40  controls the thermoelectric element driving member  42  periodically for a predetermined time, thereby being able to synchronize the A/D conversion timing correctly with the ON timing outputted by the thermoelectric element driving member  42 .  
     Other Embodiments  
      Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art.  
      For example, the above-described first embodiment, one middle terminal  24   c  is arranged at a position between the electric power input terminal  24   a  and the electric power output terminal  24 b. In the above-described second embodiment, three middle terminals  24   c  are arranged at the positions between the electric power input terminal  24   a  and the electric power output terminal  24   b.  However, the number of middle terminals  24   c  is not limited to these, but a plurality of (two or more) middle terminals may be arranged between the electric power input terminal  24   a  and the electric power output terminal  24   b.    
      Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.