Abstract:
A circuit with at least one catalytic measuring element ( 1 ), in which the at least one catalytic measuring element ( 1 ) is connected to a supply voltage ( 5 ) without a protective resistor and without a thermal safety device arranged upstream. The circuit makes it possible to develop gas-measuring devices with catalytic measuring elements that are characterized by an especially low power consumption.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. §119 of German Patent Application DE 10 2005 022 471.7 filed May 14, 2005, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention pertains to a circuit with at least one catalytic measuring element. 
     BACKGROUND OF THE INVENTION 
     Devices that operate with catalytic measuring elements to perform gas measurements have been used in gas measuring engineering applications. In particular, pellistors are widely used for in gas measuring engineering applications. 
     General conditions must be taken into account when such measuring elements are used. These conditions arise from the fact that the electric properties of such catalytic measuring elements may change during the measurement. Also, they are possibly used in the presence of an explosion hazard. 
     The pellistors are considered to be semiconductor elements especially in conventional circuits with pellistors. Since the resistance of these pellistors may change substantially as a function of the temperature, protective resistors and thermal safety devices are usually used to limit the maximum power of the pellistor and at the same time to limit the maximum current to values at which ignition by spark is ruled out in an environment with explosion hazard. 
     The drawback of such a circuit is, however, that both a protective resistor and a thermal safety device represent separate users and they consume electric power. This represents a considerable drawback, especially in case of portable devices that are battery-operated. The use time is substantially reduced in this manner. Consequently, more energy must be made available than would be necessary for the operation of the pellistor alone. The energy storage means necessary for this, for example, batteries, increase the weight of such devices in a disadvantageous manner. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a circuit with catalytic measuring elements, which can be operated with an especially low energy consumption. 
     According to the invention, a circuit is provided with at least one catalytic measuring element. The at least one catalytic measuring element is connected to a supply voltage without a protective resistor and without a thermal safety device arranged upstream. 
     The present invention is based on the fact that it is possible to dimension a catalytic measuring element such that even though its resistance is a temperature-dependent variable, it does change within limits which ensure a sufficient limitation of the current at any time and thus make it possible to effectively prevent an ignition by spark in an environment with explosion hazard. Furthermore, the catalytic measuring element is to be dimensioned such that it can assume the function of a thermal safety device that limits the power fed to the measuring element. The catalytic measuring element thus also assumes at the same time the protective function for a possibly series-connected electronic unit, for example, a power electronic unit, via which the current flow necessary for the measuring operation through the catalytic measuring element is embodied. 
     It is thus possible according to the present invention to do away with a protective resistor or a thermal safety device arranged upstream. The present invention can be advantageously used in circuits that contain a plurality of, but at least two catalytic measuring elements. In particular, circuits that contain at least one pellistor can be set up in the manner according to the present invention. At least one pellistor is advantageously dimensioned here such that it limits the current at a point in time to a value that prevents ignition by a spark in an environment with explosion hazard. This function is ensured according to the present invention regardless of the operating temperature at which the pellistor is used for measurements. 
     The same pellistor can advantageously be dimensioned such that it acts as a thermal safety device to limit the power that is fed to the pellistor. In particular, pellistors with a wire coil made of platinum or a platinum alloy and a catalytic bead made of γ-aluminum oxide or a mixture of γ-aluminum oxide and zirconium oxide with catalysts from precious metal particles can be dimensioned and used according to the present invention. 
     Advantageous materials of which the precious metal particles of the catalyst consist are platinum, rhodium, palladium or combinations of these precious metals. The circuit according to the present invention can also be used advantageously if two pellistors are used for the measurement. 
     The present invention will be explained in greater detail on the basis of an exemplary embodiment. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a circuit diagram view showing a circuit with two pellistors according to the state of the art; and 
         FIG. 2  is a circuit designed according to the present invention with two pellistors dimensioned according to the present invention without a protective resistor and without a thermal safety device; and 
         FIG. 3  is a schematic perspective view showing the design of each pellistor of the circuit of  FIG. 2  with a catalytic bead with carrier material and catalyst shown in section. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in particular,  FIG. 1  shows a circuit with two pellistors as catalytic measuring elements  1 ,  1 ′. These are connected in series with a power electronic assembly unit  2 , a protective resistor  3  and a thermal safety device  4  as a protection for the protective resistor  3  and all series-connected elements. The output voltage of a power source  5  is applied to this series-connected arrangement. An electronic assembly unit with an evaluating electronic unit  6  is used to evaluate the measured signals, which are sent by the arrangement of pellistors. An assembly unit with actuating electronic unit  7  is used to actuate the power electronic assembly unit  2 . The evaluating electronic unit and the actuating electronic unit are likewise supplied by the power source  5 , but are connected to the power source  5 , the output signal of the pellistors and the power electronic assembly unit  2  via high-ohmic resistors R 1 , R 2 , R 3 . 
       FIG. 2  shows a circuit designed according to the present invention with two pellistors dimensioned according to the present invention without a protective resistor and without a thermal safety device. The series-connected arrangement, which is supplied with the output voltage of the power source  5 , contains only an arrangement of two pellistors as catalytic measuring elements  10 ,  10 ′ and a power electronic assembly unit  2 , which is necessary for the operation of the pellistors. An-assembly unit with an actuating electronic unit  7  and an assembly unit with an evaluating electronic unit  6 , which are again uncoupled from the rest of the circuit by high-ohmic resistors R 1 , R 2 , R 3 , are likewise present again. 
     The pellistors  10 ,  10 ′ are designed according to the present invention such that they act, on the one hand, as a thermal safety device to limit the power that can be fed to the pellistors and have, on the other hand, a minimum resistance that prevents ignition by sparks at any time. At the same time, they assume the protective function for the series-connected power electronic assembly unit  2 . 
     It is possible to do away with a protective resistor and a thermal safety device for the first time because of this functionality. This arrangement according to the present invention is made possible by the specification of the pellistors  10 ,  10 ′. As shown in  FIG. 3 , every individual pellistor  10 ,  10 ′ contains a wire coil  20  consisting (or consisting essentially) of a platinum alloy with a wire diameter of 0.01 mm to 0.08 mm. The wire coil  20  is connected by mounting pins to the circuit board  1 . The material of the catalytic bead  30  consists either of γ-aluminum oxide or a mixture of γ-aluminum oxide and zirconium oxide, using catalysts from precious metal particles such as platinum, rhodium, palladium or combinations of these precious metals. The individual pellistor  10 ,  10 ′ is dimensioned such that its resistance is in a range between 3 and 25 Ohm at an applied voltage of 500 mV to 2,500 mV. The actual voltage does not drop below nor exceed this range even at different operating temperatures, which may become necessary for the measurement of the gases to be detected. It can thus be ensured that ignition by sparks is effectively prevented from occurring in any state of operation. 
     Based on the circuit according to the present invention, the pellistors necessary for the measurement form the only users themselves that consume energy to an appreciable extent. Due to this circuit, portable gas-measuring devices with a substantially lower energy consumption can be designed, which means, in return, a substantially longer use time at a given weight. 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.