Abstract:
A main heater, auxiliary heater and guard heater arranged on an insulator substrate to absorb leakage current, thereby maintaining precise sensing temperature. The main heater is parallel with a bridge circuit containing the auxiliary heater, and the main and auxiliary heater s are typically adjacently arranged with a guard heater there between, electrically interconnected to maintain target temperature of the main heater, regardless of leakage current typically resultant from insulation substrate deterioration.

Description:
TECHNICAL FIELD 
     The invention relates to a heated type sensor such as an oxygen or a NOx gas sensor. 
     BACKGROUND OF THE INVENTION 
     In a heated type sensor in which a sensing portion of an oxygen or a NOx sensor is heated by a heater, the characteristics of the sensing portion are dependent on its temperature which will vary due to the variation of temperature of the environment or the heater&#39;s own temperature variation. A conventional temperature control method for heater in which the heater temperature is maintained to a predetermined value, for example, 300-400° C. is described in the JP-A-60 114,758 official gazette. 
     The heated type sensor maintains constant temperature of a detective part ( 3 ) with a main heater ( 30 ) and an auxiliary heater ( 32 ). A leakage current originating in the deterioration of the insulation substrate ( 2 ) is absorbed by a guard heater. 
     The good thermal conductivity (high thermal conductivity) insulation substrate or film ( 2 ) contains the detective part that is maintained at the high temperature, for example, 400° C., and main and auxiliary heaters are nearly arranged. Alternatively, a guard heater for the leakage current protection is arranged between the main and auxiliary heaters adjacent to the auxiliary heater. A bridge circuit ( 18 ) contains the auxiliary heater in one side, and first, second and third resistors ( 12 ,  14 ,  16 ) in the remaining three sides. The pyro-control circuit comprises an amplifier or a voltage follower ( 20 ) which supplies a voltage to the main heater, and another amplifier ( 24 ) having an inverting input connected to a serial node between the first resistor and the auxiliary heater, a non-inverting input connected to another serial node between the second and third resistors, and an output end connected to a control input end of the voltage follower. This amplifier controls the power supply voltage to the bridge circuit and the main heater based on the output of the bridge circuit. The second amplifier ( 52 ) controls the voltage of a node ( 50 ) with the guard heater to be identical to that of the node ( 13 ) with the auxiliary heater, based on an output of the second bridge circuit ( 44 ) containing auxiliary heater and guard heater ( 40 ). (See FIG.  4 ). 
     In the aforementioned temperature control method, a thermometric element is disposed adjacent to the heated type sensor with a space therebetween containing gas or ambient air to measure its environmental temperature, and the temperature of the sensing portion is constantly maintained by controlling a power supply to the heater based on the measurement value. The method necessitates the thermometric element by which the temperature of ambient air should be measured, and complex heated type sensor structure and heat control circuitry resulting in higher cost. 
     Further, an indirect thermal measurement of the sensing portion cannot result in high accuracy because under the condition of gas or air flow through the space, the temperature of the sensing portion is not precisely transferred to the element, and also a delay of temperature transfer to the element can possibly lead to uncontrollable situation such as temperature convergence or divergence. 
     As another conventional example without any auxiliary heater, a tin oxide film gas sensor is disclosed in 111 page of Nikkei Electronics issued on Jan. 20, 1992 by Nikkei BP publisher. The gas sensor provides four silicon film substrates projected inwardly from a comer of a center-scooped rectangular silicon dioxide substrate chip, and the tin oxide film and platinum heater are deposited on one side of each of the silicon film substrates. The present inventor proposed some improvement regarding the above method in Japanese Patent Application No. 7-328,395. 
     FIG. 1 shows an example of a well-known bridge type heat control circuit. The circuit comprises a bridge circuit  18  having, for example, a platinum film heater  10  on one side thereof and the resistors  12 ,  14  and  16  on the remaining three sides thereof, respectively, an emitter follower  20  which supplies a voltage to the bridge circuit  18 , and an amplifier  24  having its inverting input connected to a serial node  13  between the resistor  12  and the heater  10 , a non-inverting input connected to another serial node  17  between the resistors  14  and  16 , and an output connected to the base of the emitter follower  20  through a resistor  22 . 
     Resistance value of the platinum film heater  10  is changed with temperature as the temperature T—resistor R characteristics of FIG.  2 . If electric potential values of the serial nodes  13  and  17  are e1 and e2, respectively, the amplifier  24  and the emitter follower connected transistor are so operated that the electric potential e1 is identical to the electric potential e2, i.e., the resistance ratio of the resistor  12  to the heater  10  is identical to that of the resistors  14  to  16  to maintain the temperature of the heater  10  to a predetermined value. 
     Then, when the temperature of the heater  10  is lower than the predetermined value, output voltage of the amplifier  24  and emitter follower  20  are increased under the condition of e1&lt;e2 to filter increase the power supply to the heater  10 . When the temperature of the heater  10  is higher than the predetermined value, output voltage of the amplifier  24  and emitter follower  20  are decreased under the condition of e1&gt;e2 to decrease the power supply to the heater  10 . 
     As described above, it is obvious that the bridge type heat control circuit shown in FIG. 1 has following problems. In order to detect temperature change, i.e., resistance change of the heater  10  precisely, the resistor  12  having higher resistance value than that of the heater  10  may be used. In that case, however, the heat quantity generated in the resistor  12  is also increased more than that in the heater  10 . Thus the energy loss by the resistor  12  is increased, the temperature increase of the resistor  12  simultaneously becomes excessive, and then dangerous. 
     Since the resistance value of the reference resister  12  changes according to its temperature coefficient, it is difficult to obtain an appropriate heater temperature. Moreover, in case that the resister  12  has a positive temperature coefficient of resistance, the temperature and thus the resistance value of the resister  12  are increased when the heater  10  is heated. Therefore, the settling time to e1=e2 is delayed. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, it is a first object of the present invention to provide a high reliable and low power consumption heated type sensor which is enabled to maintain a predetermined heater temperature by using main and auxiliary heaters even if an environment temperature is changed. 
     A second object of the present invention is to provide high reliable heaters and those heat control circuits for the heated type sensor in which an leakage current of the main heater does not influence an auxiliary heater even if the electric insulation of the substrate is deteriorated, and thus the main heater can hold a predetermined temperature. 
     According to the invention, the heated type sensor comprises main and auxiliary heaters arranged adjacent to the sensing portion to heat the sensing portion to a predetermined temperature. These resistance values of the heaters are so determined that a current to the main heater is, for example 10 times, more than that of the auxiliary heater. 
     In one preferred form of the present invention, the heated type sensor further includes a guard heater arranged on the insulating substrate adjacent to the auxiliary heater between the main and auxiliary heaters. Though the resistance values of the auxiliary and guard heaters are determined higher than that of the main heater, the heater materials are same as that of the main heater. Therefore, these heaters are formed by different mask patterns in case that a predetermined heating element metal is applied or deposited for the same deposition time. Alternatively, the thickness of the main heater may be provided thicker than that of the auxiliary or guard heater by changing deposition time for each heater. 
     In a heat control circuit, the main heater is connected in parallel to the bridge circuit in which one side is composed of the auxiliary heater and the power is supplied to the bridge circuit and the main heater based on the output of said bridge circuit. 
     According to the present invention, the heated type contains an insulating substrate on which the sensing portion, the main and auxiliary heaters are adjacently arranged The bridge circuit comprises the auxiliary heater in its one side, and first, second and third resistors in the remaining three sides thereof. The heat control circuit comprises a voltage follower which supplies a voltage to the main heater, and an amplifier having an inverting input connected to a serial node between the first resistor and the auxiliary heater, a non-inverting input connected to another serial node between the second and third resistors, and an output connected to a control input end of the voltage follower. 
     The insulating substrate has a good thermal conductivity. The sensing portion is formed on its one surface of the insulating substrate and the main heater is formed on the other surface, corresponding to the position of the sensing portion. The auxiliary heater is arranged around the main heater on the other surface of the insulating substrate. 
     The insulated substrate is a ceramic substrate such as aluminum nitride or silicon carbide whose thermal conductivity is similar to that of metal or silicon dioxide layer for use as a support plate of the sensing portion. 
     Therefore, the auxiliary heater forms one side of the bridge circuit to mainly detect temperature. The main heater is connected in parallel to the bridge circuit to heat the sensing portion so that an out put of the bridge circuit may be balanced. Because the heat quantity generated in the auxiliary heater is remarkably small compared with that of the main heater, the heat by the first resistor part of FIG. 1 is very small and then a rapid and accurate temperature control becomes possible. 
     The heat control circuit according to the present invention comprises a main heater connected in parallel to abridge circuit containing an auxiliary heater, a first amplifier controlling a first voltage supplied to the bridge circuit and the main heater, based on the output of the bridge circuit, and a second amplifier controlling a second voltage supplied to the guard heater being identical to a third voltage applied to the auxiliary heater, based on the output of a second bridge circuit containing the auxiliary and said guard heaters. 
     As for the amplifier, its inverting input is connected to a first node between the first resistor and the auxiliary heater, its non-inverting input is connected to the second node between the second and third resistors, and its output is connected to the main heater and the bridge circuit, for example, through a voltage follower. Power is initially supplied to the bridge circuit by a pull-up resistor. 
     Alternatively, in the second amplifier, the non-inverting input is connected to the second node, and the inverting input is connected to the guard heater, and the output is connected to the guard heater to supply a current. The second bridge circuit contains the transistor connected to a feeding point between the guard heater and the bridge circuit. A base or gate of the transistor is controlled by the second amplifier to which the voltage of the guard heater is applied. 
    
    
     BRIEF SUMMARY OF THE DRAWINGS 
     FIG. 1 shows a conventional bridge type heat control circuit. 
     FIG. 2 is a temperature (T)—resistor (R) characteristic of platinum film heater. 
     FIG. 3 shows an embodiment of a gas sensor according to the invention and is a schematic plan view in which a sensing portion is arranged on a front side (not shown), and main and auxiliary heaters are arranged on a back side. 
     FIG. 4 is a circuit diagram showing an embodiment of the heat control circuit according to the invention. 
     FIG. 5 is a plan view of the heaters for the heated type sensor according to the invention. 
     FIG. 6 is a circuit diagram showing a second embodiment of the heat control circuit according to the invention. 
     FIG. 7 is another circuit diagram showing a third embodiment of the heat control circuit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention are described in detail with reference to the drawings. 
     Firstly, a heated type sensor of the present invention adds an auxiliary heater to the conventional sensor shown in FIG. 2 of Japanese Patent Application No. 7-328,395. For example, a gas sensing portion is placed between two porous electrode substrates, one being in contact with the gas to be examined, and the other being in contact with the atmosphere. On each porous electrode substrate, a main heater of platinum film or wire is laid, and an auxiliary heater of platinum film or wire is also laid with a space to the main heater. 
     An electric insulation film with heat resistance and preferable good thermal conductivity is applied on those heaters, and an electrode is further formed on that insulation film. These two porous substrates constructed like this are assembled by facing the electrode as an inside each other. Gas detecting material for example, sintered porous tin oxide or lead oxide, is filled in the inside between two electrodes. 
     The main and auxiliary heaters are heated to, for example, 410° C., to maintain the entire gas sensor at 400° C. The thinner above electric insulation film is made, the smaller its heat capacity is. Use of the electric insulation film with small heat capacity and good thermal conductivity will make its measurement error small. 
     In a first embodiment of the heated type sensor as shown in FIG. 3, a gas sensing portion (not shown) to be heated to high temperature is formed on the front side of the insulating substrate  2  with good thermal conductivity and two electrodes (not shown) are formed on its both ends. On the back side of the insulating substrate  2 , a main heater  30  of platinum film is formed corresponding to the position of the gas sensing portion so that a heating area may be slightly larger than the gas sensing area. A platinum film auxiliary heater  32  which for example has a target resistance value at 410° C. is arranged around the main heater  30 . 
     In FIG. 3, oblique lines portions show the heating area The main heater  30  and auxiliary heater  32  are preferably made of same material or metal having a same temperature coefficient of resistance, i.e., nickel chromium alloy, platinum or platinum alloy. In this case, temperature change, i.e., resistance change of the main heater  30  is identical to that of the auxiliary heater  32 . The main and auxiliary heaters may be platinum films or wires and arranged in meander or concentric circles form. 
     In order to provide the resistance value of the auxiliary heater higher than that of the main heater using same heater material, these heaters are made using different mask patterns by deposition of a predetermined metal or alloy under the same deposition time, or the thickness of the auxiliary heater may be thinner than that of the main heater by setting the deposition time of the auxiliary heater shorter than that of the main heater. 
     FIG. 4 is a circuit diagram showing an embodiment of the heat control circuit used for the heated type sensor according to the present invention. In FIG. 4, same numerals are denoted to those similar components shown in FIG.  1 . The heat control circuit according to the invention differs from the conventional circuit of FIG. 1 in the following points; the main heater  30  is provided in parallel to a bridge circuit  18 , and an auxiliary heater  32  is placed in the position which the conventional heater is placed in the bridge circuit  18 . Thus a non-dividable voltage is directly supplied to the main heater  30  in the heat control circuit of the invention. 
     In other words, in a transistor  20  of the emitter follower connection, a collector is connected to a positive line voltage+Vc line, and an emitter is connected to the bridge circuit  18  and main heater  30 . The main heater  30  is grounded. On the other hand, in the bridge circuit  18 , a first resistor  12  is connected between the emitter of the transistor  20  and a serial node  13 , and the auxiliary heater  32  is connected between the serial node  13  and ground. 
     Moreover, a second resistor  14  is connected between the emitter of the transistor  20  and a serial node  17 , and a third resistor  16  is connected between the serial node  17  and ground through a variable resistor  34 . These serial nodes  13  and  17  are respectively connected to inverting and non-inverting inputs of a field effect transistor (FET) or bipolar transistor input-operational amplifier  24 , of which input current is negligible. An output of the amplifier  24  is connected to the base of the transistor  20  through a protection resistor  22 . Connected between the collector and emitter of the transistor  20  is a pull-up resistor  36  through which an initial voltage is supplied to the bridge circuit  18  in operation when the emitter follower  20  is in an OFF state. 
     In the basic action of this circuit, because an unbalanced output voltage is not generated in bridge circuit  18  at the initial stage of the power charge, the resistor  36  is necessary to supply an initial voltage, for example, 1 volt to the bridge circuit. A resistance value of this pull-up resistor  36  can be very high because current to the main heater  30  is mainly supplied through the emitter follower  20 . Thus power consumption in the resistor  36  can be ignored. Moreover,both main heater  30  and auxiliary heater  32  with positive temperature coefficient of resistance have resistance values that are low at room temperature and gradually increased to reach target resistance values at, for example, 400° C. when current is supplied. Therefore, the bridge circuit  18 , amplifier  24  and emitter follower  20  increase the power supply voltage to the main heater  30  and bridge circuit  18  to reach the target voltage by the unbalanced output voltage from the bridge circuit  18  due to the increase in the resistance value of the auxiliary heater  32 . 
     When the temperature of the auxiliary heater  32  is lower than the predetermined value, the output voltage of the amplifier  24  and emitter voltage of the transistor follower  20  are increased under the condition of e1&lt;e2. Thus the power supply to the main heater  30  is increased. When the temperature of the auxiliary heater  32  is higher than the predetermined value, the output voltage of the amplifier  24  is decreased under the condition of e1&gt;e2, and the power supply to the main heater  30  and auxiliary heater  32  is decreased. 
     Variable resistor  34  is a potentiometer or rheostat which adjusts heater temperature to a desirable value. In the circuit of FIG. 4, a target temperature of the main and auxiliary heaters is provided so that the resistance ratio of the first resistance  12  to the auxiliary heater  32  is set to the value which is identical to that of the second resistor  14  to the sum of third resistor  16  and variable resistor  34 . 
     Therefore, when the temperature of the auxiliary heater  32  is 400° C., the temperature of the main heater  30  is simultaneously regulated to 400° C. according to the power supply. The temperature variation of the main heater  30  influences the temperature of the auxiliary heater  32 , and thus the power supply voltage varies in order to make the temperature return to the target value. The target temperature may be set to, for example, 410° C. in consideration of the heat loss by the thermal capacity of the film or substrate placed between the gas sensing portion and heaters. 
     In the heated type sensor shown in FIG. 3, when the electric insulation characteristics of the substrate on which the main and auxiliary heaters are arranged deteriorate, for example, in aged change, an leakage current is likely to flow between the main and auxiliary heaters. By this leakage current, the electric potential of e1 may be increased, and there may occur a problem that the temperature of the auxiliary heater  32  is lowered from the predetermined value. 
     FIG. 5 is a plan view showing a second embodiment of the heated type sensor according to the invention to improve the above problem. In FIG. 5, same numerals are denoted to those similar components shown in FIG.  3 . The gas sensing portion (not shown) to be heated to high temperature is formed on the front side of zirconia substrate  2  having a good thermal conductivity, and two electrodes (not shown) are formed at its both ends. 
     On the back side of the substrate  2 , a main heater  30  of platinum film is outwardly arranged corresponding to the sensing area. A guard heater  40  of platinum film is surrounded with the main heater  30 , and an auxiliary heater  32  of platinum film is surrounded with the guard heater  40 . Therefore, the guard heater  40  is arranged between the main heater  30  and the auxiliary heater  32  on the zirconia substrate  2  adjacent to the auxiliary heater  32 . If electric potential of each position of the auxiliary heater  32  is identical to that of the guard heater  40  correspondingly adjacent thereto, leakage current from the main heater  30  does not influence the auxiliary heater  32 . 
     Of course, though resistance values of the se auxiliary and guard heaters are provided higher than that of the main heater, heater materials are same as that of the main heater. Therefore, these heaters are formed by different mask patterns in case that a predetermined heating element metal or alloy is applied or deposited the same deposition time. Alternatively, the thickness of the main heater may be provided thicker than that of the auxiliary or guard heater by changing deposition time for each heater. 
     FIG. 6 shows a second embodiment of the heat control circuit used for the heated type sensor according to the invention. In FIG. 6, same numerals are denoted to those similar components shown in FIG.  4 . In the first transistor  20  having an emitter follower connection, a collector is connected to the positive line voltage+Vc line, and an emitter is grounded through the bridge circuit  18  and main heater  30  which are connected in parallel. 
     Because the internal resistance of the bridge circuit  18  is much higher than the resistance of the main heater  30 , the most of current is supplied to the main heater  30 . In this bridge circuit  18 , a first resistor  12  is connected in series to the auxiliary heater  32  at anode  13 , and a second resistor  14  is connected in series to a third resistor  16  and a variable resistor  34  through a node  17 . 
     These nodes  13  and  17  are connected to the inverting and non-inverting inputs, respectively, of a first operational amplifier  24 . An output of the amplifier  24  is connected to the base of the first transistor  20  through a protection resistor  22 . A pull-up resistor  36  is connected between collector and emitter of the first transistor  20  in order to supply initial voltage to the bridge circuit  18  for starting. 
     The first resistor  12  and auxiliary heater  32  compose a second bridge circuit  44  associated with guard heater  40  and second transistor  42 . In the second transistor  42 , a collector is connected to feeding point  46  of the main heater  30 , and an emitter is connected to the guard heater  40  through a resistor  48 . 
     An inverting input of the second operational amplifier  52  is connected to the node  50  between resistor  48  and auxiliary heater  40 . In the second operational amplifier  52 , a non-inverting input is connected to the node  13 , and output is connected to the base of the second transistor  42  through the resistor  54 . 
     FIG. 7 shows a third embodiment of the heat control circuit of the heated type sensor according to the invention. In FIG. 7, same numerals are denoted to those similar components shown in FIG.  6 . Because the current supplied to the guard heater  40  is fairly less than that supplied to the main heater  30 , the second transistor  42  of the emitter follower connection can be omitted. Therefore, the output of the second amplifier  52  is connected to its inverting input to compose the second bridge circuit  44 . 
     The basic action of heat control circuit according to the second and third embodiments of the invention is as follows. Because unbalanced output voltage is not generated in the bridge circuit  18  at the initial stage of the power charge, for example, an starting voltage of 1 volt is initially supplied to the bridge circuit  18  through a resistor  36 . The resistance value of the pull-up resistor  36  can be very high because current to the main heater  30  is mainly supplied tough the emitter follower  20 during sensing operation Thus power consumption in the resistor  36  can be ignored. 
     Moreover, both main heater  30  and auxiliary heater  32  with positive temperature coefficient of resistance have resistance values that are low at room temperature and gradually increase to reach target resistance values at, for example, 400° C. when current is supplied. Therefore, the amplifier  24  and emitter follower  20  increase those power supply voltage, and reach a balanced power supply voltage by an unbalanced output voltage of the bridge circuit  18  due to the increase in the resistance value of the auxiliary heater  32 . In other words, the first transistor  20  is controlled by the amplifier  24  so that the electric potential e1 of node  13  is identical to e2 of the node  17  to maintain temperature of the auxiliary heater  32  to the predetermined value. 
     Therefore, the output of amplifier  24  is increased under e1&lt;e2 when the temperature of the auxiliary heater  32  is lower than the predetermined temperature. Therefore, the voltage supplied to the main heater  30 , auxiliary heater  32  and guard heater  40  is increased through the first transistor  20 . The output of the amplifier  24  is decreased under e1&gt;e2 when temperature of auxiliary heater  32  is higher than the predetermined temperature. Therefore, the voltage supplied to the main heater  30 , auxiliary heater  32  and guard heater  40  is decreased. Of course, almost all current flows through the main heater  30  to mainly radiate heat. 
     Variable resistor  34  is a potentiometer or rheostat which adjusts the temperature of the auxiliary heater  32  to a desirable value. In the circuit example of FIG. 6 or  7 , the auxiliary heater  32  is settled to the temperature defined by R1:rH=R3:(R4+VR1). 
     Next, the action of the guard heater  40  is describe. Defining the electric potential of the nodes  13  and  50  connected to the auxiliary heater  32  and guard heater  40  are e1and e3, respectively, the output voltage of the second amplifier  52  and the emitter voltage of the second transistor  42  are increased and the electric potential e3 of the guard heater  40  is also increased when e3&lt;e1. 
     When e3&gt;e1, the output voltage of the second amplifier  52  and the emitter voltage of the second transistor  42  are decreased and e3 is decreased. Therefore, the second transistor  42  is controlled by the second amplifier  52  so that the relation of e3=e1 may be always maintained, and the leakage current influence of the auxiliary heater  32  is avoided. 
     Even if the leakage current flows from the main heater  30  to the guard heater  40  and electric potential of e3 is increased because the electric insulation characteristic of zirconia substrate  2  is deteriorated, the auxiliary heater  32  is not influenced because e3 becomes equal to e1 at once by the second amplifier  52 . In other words, its heater temperature is not varied. The resistor  36  is a pull-up resistor which prevents e1, e2 or e3 from falling to zero electric potential, when the first transistor  20  is turned off 
     In the circuit of FIG. 6 or  7 , target temperatures of the main and auxiliary heaters are provided so that the resistance ratio of the first resistor  12  to the auxiliary heater is set to the value which is identical to that of the second resistor  14  to the sum of third resistor  16  plus variable resistor  34 ). The guard heater  40  has a function that prevents the auxiliary heater  32  from being influenced by the leakage current even if the leakage current caused by the insulation deterioration of the heater substrate flows. 
     Therefore, when the auxiliary heater  32  is heated up to, for example, 400° C., the target voltage is supplied to the main heater  30 , and then the temperature of the main heater  30  simultaneously becomes 400° C. The temperature variation of the main heater  30  influences the temperature of the auxiliary heater  32  and guard heater  40 , and thus the power supply voltage varies in order to make temperature return to the target value. The target temperature may be set to, for example, 410° C. in consideration of the heat loss by the thermal capacity of the film or substrate that is positioned between the gas sensing portion and heater 
     The insulating substrate is a ceramic substrate such as aluminum nitride or silicon carbide whose thermal conductivity is similar to that of metal, or silicon dioxide layer for use as a support plate of the sensing portion. Of course MOSFET may be used as the transistor in addition to bipolar type. As described the above, in the heat control circuit of the heated type sensor of the invention, the resistance values of the firs resistor and auxiliary heater can be determined higher than those (the resistance values of the first resistor and main heater) of the conventional bridge type heat control circuit, its ratio can be set to roughly about 1:1 to obtain the maximum sensitivity at predetermined temperature, and its heat quantity generated in the circuit can be extremely reduced. Therefore, the output voltage change (change in e1) due to the resistance or temperature change of the main heater or auxiliary heater can widely range and is capable of more accurate temperature control. 
     Moreover, in the heat control circuit, its energy loss is decreased and internal temperature rise is also safely limited. Further, because the resistance change of the first resistor served as a reference resistor becomes extremely small, the convergent time to predetermined temperature is shortened as well as accurate temperature control function can be provided. 
     Therefore, the sensing portion in the ambient air which varies or the heater temperature can be constantly maintained at the high temperature, for example 400° C., and its structure can be simply composed at a low cost as well as its reliability is enhanced. Moreover, since the substrate or film provided between the sensing portion and the heater has a constant thermal conductivity similar to that of metal, and the heat transfer in the substrate is different from the gaseous convection. Therefore, temperature variation of the gas sensing portion or the heater is compensated for its variation, and the sensitivity characteristics of the gas sensing portion become stable. 
     In the second embodiment of the heated type sensor according to the invention, the second bridge circuit comprises components on the side of the auxiliary heater of the bridge circuit and the guard heater. Because the second amplifier is provided so that the applied voltage (e3) of the guard heater  40  is identical to the applied voltage (e1) of the auxiliary heater  32 , even if the electric insulation of the heater substrate such as zirconia is deteriorated, its heater temperature is not varied as the first embodiment of the heated type sensor. 
     Because the resistance value of the guard heater can be set to large, the output voltage change due to the temperature variation of the main heater can widely range. The temperature rise in the heat control circuit is also restrained. 
     The auxiliary heater mainly has a temperature sensing function. The main heater is connected in parallel to the bridge circuit to heat the sensing portion so that the bridge circuit may be balanced. Because the guard heater absorbs a leakage current originated from the main heater, a rapid and accurate temperature control becomes possible.