Abstract:
A control device includes a control unit and an evaluation unit that is configured to generate a control signal by which the control unit is impinged upon. The control unit is provided with a voltage source and a reference resistor that can be connected in series to a sensor resistor whose value depends on the temperature thereof. An output voltage of the voltage source drops on the sensor resistor and the reference resistor in the connected state. The reference resistor is dimensioned in such a way that the maximum power loss of the sensor resistor lies in the specified value range of the sensor resistor.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a control unit and a control device comprising the control unit. Such a control unit or such a control device is configured to activate a sensor resistor. They are used in particular to detect the oil level of an internal combustion engine of a motor vehicle. 
   If a motor vehicle, in which an internal combustion engine is disposed, is not equipped with an oil level sensor, the owner of the vehicle must check at regular intervals whether their vehicle is filled with an adequate quantity of engine oil. An oil level sensor can be used to ensure that the driver does not have to use a dipstick to check the oil level in the motor vehicle at regular intervals, which is on the one hand more user-friendly and on the other hand ensures that the owner of the vehicle is informed when the oil level is too high or too low and can then top up or drain the engine oil accordingly. Motor vehicle manufacturers can protect themselves against unjustified warranty claims based on too low an oil level by registering the measured values of the oil level sensor accordingly. 
   The sensor element of the oil level sensor can be a wire, which is disposed in an oil pan of the internal combustion engine between two supports such that the oil level can be concluded from the proportion of the total length of the wire that is in the oil. The oil level is then determined by means of an electro-thermal measuring principle. 
   Depending on the oil level there is oil round a varying length of the wire, the remainder of the wire being in a gaseous medium, preferably air. If a current is passed through the wire, the electrical power in the wire is converted to heat. This heat is given off to the medium surrounding the wire. The electro-thermal measuring principle makes use of the fact that the heat conductivity values of engine oil and air are very different and the electrical resistance of the wire is temperature-dependent. The thermal transfer resistance from wire to oil is significantly lower than from wire to air. This means that the part of the wire in the engine oil is cooled much more efficiently and therefore gives off heat more effectively than the part in air. 
   With regard to the electro-thermal measuring principle, it is known that a predefined current can be passed through the wire for a predefined time period, causing the wire and its surroundings to be heated. This causes the value of the resistance of the wire to change as a function of the current oil level over the predefined time period. Depending on the voltages, which drop at the measuring wire when the current is first passed and at the end of the predefined time period, it is known that the oil level can be determined from a set of characteristics. The power loss that is converted in the wire during the predefined time period of current passage is highly dependent on the temperature of the wire when the current is first passed and therefore also the ambient temperature. This means that sensitivity is very much a function of ambient temperature. 
   A mechanism for improving the accuracy of a sensing resistor for an NTC resistor used as a temperature sensor is known from WO 91/08441. It comprises a circuit arrangement with a network of resistors. A computing mechanism influences the network of resistors such that the measuring range for the NTC resistor is displaced. The overall resistance is changed to this end. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to create a control unit and a control device comprising the control unit, which are simple and can be adjusted in a precise manner by means of the one power loss in a sensor resistor. 
   The object is achieved by the features of the independent claims. Advantageous embodiments of the invention are characterized in the subclaims. 
   In respect of the control unit the invention is characterized by a control unit with a voltage source and a reference resistor, which can be connected in the required manner in series with a sensor resistor, the value of which is a function of its temperature. The control unit is configured such that in the connected state the output voltage of the voltage source drops at the sensor resistor and the reference resistor. The reference resistor is dimensioned such that the maximum power loss of the sensor resistor is in the required value range of the sensor resistor. 
   As far as the control device is concerned, the invention is characterized by the control device comprising the control unit and an evaluation unit, which is configured to generate a control signal. 
   Both the claimed control unit and the claimed control device have the advantage that while a voltage is being applied to the sensor resistor by the voltage source, the power loss that is converted in the sensor resistor remains approximately identical within the required value range of the sensor resistor. This means that when the electro- thermal measuring principle is applied, the sensitivity is almost independent of the temperature of the sensor resistor when voltage is first applied to the sensor resistor. 
   In one advantageous embodiment of the control unit the voltage source is configured to amplify the input voltage. This has the advantage that the output voltage of the voltage source can be greater than its maximum input voltage. It is thus possible to modify the power loss that is converted in the sensor resistor to a high value in a simple manner. 
   The object is achieved by the features of the independent claims. Advantageous embodiments of the invention are characterized in the subclaims. 
   In respect of the control unit the invention is characterized by a control unit with a voltage source and a reference resistor, which can be connected in series with a sensor resistor, the value of which is a function of its temperature. The control unit is configured such that in the connected state the output voltage of the voltage source drops at the sensor resistor and the reference resistor. The reference resistor is dimensioned such that the maximum power loss of the sensor resistor is in the required value range of the sensor resistor. 
   As far as the control device is concerned, the invention is characterized by a control device comprising the control unit and an evaluation unit, which is configured to generate a control signal. 
   Both the claimed control unit and the claimed control device have the advantage that while a voltage is being applied to the sensor resistor by the voltage source, the power loss that is converted in the sensor resistor remains approximately identical within the required value range of the sensor resistor. This means that when the electro-thermal measuring principle is applied, the sensitivity is almost independent of the temperature of the sensor resistor when voltage is first applied to the sensor resistor. 
   In one advantageous embodiment of the control unit the voltage source is configured to amplify the input voltage. This has the advantage that the output voltage of the voltage source can be greater than its maximum input voltage. It is thus possible to adjust the power loss that is converted in the sensor resistor to a high value in a simple manner, thereby allowing the sensor resistor to give off a large amount of heat to its surroundings. A change in the sensor resistor can thus be enhanced, thereby increasing the sensitivity of the measurement. 
   In a further advantageous embodiment of the control unit the voltage source has a limiter for the output voltage. It can thus be ensured in a simple manner that the sensor resistor is not damaged if the voltage source is activated incorrectly. The limiter can be configured as a Zener diode in a particularly simple manner. 
   In a further advantageous embodiment of the control unit the voltage source comprises three transistors with a common emitter. The first transistor is connected such that its base current is a function of a control signal, which can be applied to the control unit. The base of the second transistor is connected to the collector of the first transistor and the base of the third transistor is connected to the collector of the second transistor. This has the advantage that the voltage source is intrinsically safe. In other words if the voltage source is not activated, the output voltage of the voltage source is zero. 
   In a further advantageous embodiment of the control unit a low-pass filter is disposed between the first and second transistors of the voltage source. This allows a high direct component to be achieved in the output voltage of the voltage source in a simple manner, even if the input voltage of the voltage source has a high alternating component. 
   In a further advantageous embodiment of the control unit the low-pass filter is formed by a capacitor, which is connected to the collectors of the first and second transistors, a resistor, which is connected both to the collector of the first transistor and to a voltage supply of the voltage source, and a further resistor, which is connected both to the collector of the second transistor and to the voltage supply of the voltage source. Such a low-pass filter is characterized by its simplicity. 
   In a further advantageous embodiment of the control unit the reference resistor is connected both to the output of the voltage source and to the sensor resistor. This has the advantage that the voltage source is able to withstand a short circuit when the sensor resistor short circuits to ground. 
   In a further advantageous embodiment the control unit is configured such that it outputs a variable characterizing the voltage drop at the sensor resistor and the reference resistor at a first output and that it outputs a variable characterizing the potential between the sensor resistor and the reference resistor at a second output. This configuration allows very precise determination of the value of the sensor resistor as errors are eliminated when adjusting the voltage that drops at the sensor resistor and the reference resistor and in the case of an analog-digital conversion of the characterizing variables in the evaluation unit, errors due to fluctuations in the supply voltage of the analog-digital converter(s), which is at the same time the reference voltage of the analog-digital converter(s), are eliminated. 
   In a further advantageous embodiment of the control unit a voltage divider is provided, to which the voltage drop at the sensor resistor and the reference resistor is applied on the input side and which is connected to the first output on the output side. A reduced voltage is therefore output at the first output, corresponding to the division ratio of the voltage divider. Appropriate dimensioning of the voltage divider allows the converter range of an analog-digital converter to be utilized as fully as possible and it can also be ensured that the voltage present at the first output is not greater than the supply voltage of the analog-digital converter. 
   In a further advantageous embodiment of the control unit a switch is provided, which is used to control whether the voltage drop at the sensor resistor and the reference resistor is applied to the voltage divider on the input side or a supply voltage of the evaluation unit. If the control device is equipped with such a control unit, the actual voltage divider ratio can be determined precisely by controlling the switch to the position, in which the supply voltage of the evaluation unit is present on the input side of the voltage divider. This means that manufacturing, temperature and age-induced fluctuations in the values of the voltage divider resistors can be compensated for in a simple manner. 
   In one advantageous embodiment of the control device the evaluation device has a regulator, the regulated variable of which is the voltage drop at the sensor resistor and the reference resistor and the actuating signal of which is the control signal. This means that the output voltage of the voltage source can be adjusted even more accurately. If the evaluation unit is a microcontroller, the control signal can be pulse-width modulated very simply. 
   Exemplary embodiments of the invention are described below with reference to the schematic drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a control device comprising a control unit, 
       FIG. 2  shows a flow diagram of a program for determining an oil level, 
       FIG. 3  shows a flow diagram of a program providing a regulator, 
       FIG. 4  shows a further embodiment of the control device and 
       FIG. 5  shows the pattern of different variables over values of the sensor resistor Rsens. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Elements with the same structure and function are marked with the same reference characters in all the figures. 
   A control device ( FIG. 1 ) comprises a control unit  1  and an evaluation unit  3 . It is also assigned a first voltage supply  4 , which is preferably the vehicle electrical system voltage supply where the control device is being used for an internal combustion engine of a motor vehicle, said vehicle electrical system voltage supply being supplied by the vehicle battery and a generator. The control device also comprises a second voltage supply  5 , which transforms and preferably adjusts the vehicle electrical system voltage Vbat to a supply voltage VCC of the evaluation unit  3 . The vehicle electrical system voltage Vbat is generally 12 V, while the supply voltage VCC of the evaluation unit  3  is generally 5 V. The evaluation unit  3  is preferably configured as a microcontroller. 
   The control unit  1  can be configured separately from the evaluation unit  3  and the second voltage supply  5 . It can for example be configured on a chip as an integrated circuit. The control device is preferably part of an engine control device, to which different further measured variables, e.g. an air mass flowing through the intake tract of the internal combustion engine, the position of a gas pedal or even the current air/fuel ratio are received. As a function of these measured variables the engine controller then determines actuating signals for the actuators of the internal combustion engine, which are for example a throttle valve or an injection valve. 
   The control unit  1  has a control input  11 , to which a control signal CTRL can be applied, which is generated in the evaluation unit  3 , said control input  11  being connected to the input of a first low-pass filter  14 . 
   The control unit also has a first and second output  12 ,  13 , which are connected to an analog-digital converter  31  of the evaluation unit. 
   In a simple embodiment the first and second outputs  12 ,  13  of the control unit  1  are connected via a multiplexer to a single analog-digital converter  31 . The outputs are however each preferably connected to their own analog-digital converter  31 . This has the advantage that the voltages present at the terminals  12  and  13  can undergo analog-digital conversion at the same time. The analog-digital converter(s)  31  has/have a conversion range, which corresponds to the supply voltage VCC of the evaluation unit  3 . 
   The first low-pass  14  comprises resistors R 4   a , R 4   b  and a capacitor C 4 . The first low-pass  14  is connected on the output side to the base of a first transistor Q 1  of a voltage source  15 . A resistor R 3  is also provided, which is connected both on the output side to the low-pass and to the base of the first transistor Q 1  and also to ground GND. The resistor R 3  causes the first transistor Q 1  to remain disconnected when there is no control signal CTRL. 
   The voltage source  15  comprises the first transistor Q 1 , a second transistor Q 2 , a third transistor Q 3 , a second low-pass filter  16  and a Zener diode D 2 . The emitter of the first transistor Q 1  is connected to ground GND. The collector of the first transistor Q 1  is connected both to the base of a second transistor Q 2  and to a second low-pass, via which it is connected to the first voltage supply  4  and thus to the vehicle electrical system voltage Vbat. 
   The emitter of the second transistor Q 2  is connected to ground GND and its collector is connected both to the base of a third transistor Q 3  and to the second low-pass  16  and via this to the first voltage supply  4  and thus to the vehicle electrical system voltage Vbat. 
   The anode of the Zener diode D 2  is connected to ground GND and its cathode is connected to the base of the third transistor Q 3 . The collector of the third transistor Q 3  is connected to the cathode of a protective diode D 1 , the anode of which is connected to the first voltage supply  4  and thus to the vehicle electrical system voltage Vbat. The emitter of the third transistor Q 3  forms an output  17  of the voltage source  15 . 
   The output  17  of the voltage source  15  is connected both to a first terminal for a sensor resistor Rsens and to a voltage divider on the input side. The voltage divider comprises a resistor  7   a  and  7   b . A capacitor C 1  is connected parallel to the resistor  7   b . The first output  12  is connected to the connecting line between the resistor R 7   a  and the resistor R 7   b . The capacitor C 1  brings about voltage stabilization at the first output  12 . A second terminal  19  for the sensor resistor Rsens is connected to a reference resistor Rref, which is also connected to ground GND. The reference resistor Rref is preferably a so-called shunt resistor. Such shunt resistors have relatively low ohmic values of 0.001Ω up to around 100Ω and a high current carrying capacity of 1 mA up to 100 A. 
   The second terminal  19  is also connected to a resistor R 8 , which is connected to the second output  13  of the control unit  1  and to a capacitor C 2 , which in turn is connected to ground GND. The resistor R 8  is configured to be high-resistance and preferably has a value from 3 to 8 kΩ. The capacitor C 2  is used for voltage stabilization at the second output  13 . 
   The sensor resistor Rsens is preferably a resistance wire, which is disposed vertically in an oil pan of the internal combustion engine. That is to say the resistance wire is disposed in the oil pan such that the proportion of the resistance wire in the oil is a measure of the oil level of the internal combustion engine. During the required operation of the control device the sensor resistor Rsens is connected to the first and second terminals  18 ,  19 . 
   If there is a high potential present at the base of the first transistor Q 1 , for example the supply voltage VCC of the evaluation unit  3  minus a corresponding voltage drop at the resistors R 4   a  and R 4   b , the first transistor Q 3  is at saturation, that is to say ground GND is almost present at its collector. Almost the entire vehicle electrical system voltage Vbat then drops at the resistor R 2 . The second transistor Q 2  is correspondingly blocked. In the stationary state the vehicle electrical system voltage Vbat is present at the collector of the second transistor or, if the vehicle electrical system voltage Vbat is greater than the breakdown voltage of the Zener diode D 2 , the breakdown voltage of the Zener diode D 2  is present at the collector of the second transistor Q 2 . Therefore the vehicle electrical system voltage Vbat or the breakdown voltage of the Zener diode D 2  is also present at the base of the third transistor Q 3 . In this instance the vehicle electrical system voltage Vbat minus the base emitter voltage of the third transistor Q 3  or the breakdown voltage of the Zener diode D 2  also minus the base emitter voltage of the third transistor Q 3  is present at the output  17  of the voltage source  15 . 
   The Zener diode D 2  ensures that the output voltage of the voltage source  15  does not exceed the breakdown voltage of the Zener diode D 2  minus the base emitter voltage of the third transistor. By defining the breakdown voltage of the Zener diode D 2  correspondingly it is thus possible to adjust the maximum output voltage present at the output  17  of the voltage source  15 . This ensures in a simple manner that circuit elements connected downstream are not damaged in the event of a fault. 
   The diode D 1  protects the voltage source  15  against polarity reversal of the first voltage supply  4 . 
   If however the control signal CTRL has a low level, for example that of ground GND, the first transistor Q 1  also blocks in stationary mode, with the result that the base of the second transistor Q 2  receives approximately all the current flowing through the resistor R 2 , as a result of which the second transistor Q 2  is conductive and at saturation. This in turn means that the third transistor Q 3  blocks. In this instance ground GND is present as potential at the output  17  of the voltage source  15 . 
   If however a voltage passed via the resistors R 4   a , R 4   b  is present at the base of the first transistor Q 1 , the potential of said voltage being between the two extremes described above, the transistor Q 1  is operated in proportional mode and the transistor Q 2  is also operated in proportional mode in reverse proportion to the transistor Q 1 . The third transistor Q 3  is operated in proportional mode. Its emitter voltage follows the collector voltage of the second transistor Q 2  minus its base emitter voltage. The output voltage at the output  17  of the voltage source  15  can in this instance thus be varied continuously and thus adjusted. 
   A second low-pass  16  smooths the base voltage of the third transistor Q 3 , thereby reducing the alternating component of the output voltage, which is present at the output  17  of the voltage source  15 . 
   If an additional resistor (not shown) is provided, which is both connected to the base of the third transistor Q 3  and is also connected to the cathode of the Zener diode and the collector of the second transistor Q 2 , it can be ensured by dimensioning said resistor appropriately that the third transistor Q 3  is not damaged in the event of a short circuit at the output  17  of the voltage source  15 . Alternatively the protective diode D 1  can also be disposed between the emitter of the third transistor Q 3  and the output  17  of the voltage source  15 . 
   The transistors Q 1  to Q 3  of the voltage source  15  are preferably integrated monolithically. This results in a particularly appropriate set of characteristics for the transistors Q 1 , Q 2 , Q 3  and more even temperature distribution in the transistors Q 1  to Q 3 . 
   A program ( FIG. 2 ) for determining an oil level L_OIL of the engine oil in the internal combustion engine is started in a step S 1 . It preferably starts at approximately the same time as the internal combustion engine, as the oil is distributed in the internal combustion engine and its level in the oil pan sinks as time continues to pass after the start time. An informative oil level measurement is therefore simply effected very close to the time when the internal combustion engine starts up. 
   Also—starting in step S 1 —a control signal CTRL is generated for a predefined time period, e.g. 600 ms. The subsequent steps of the program are processed parallel to the generation of the control signal CTRL. The control signal CTRL is preferably generated by means of a regulator, which is described in more detail below with reference to the flow diagram in  FIG. 3 . The control signal CTRL is preferably pulse-width modulated. In a simple embodiment of the control device however the regulator can be omitted and the control signal CTRL need only be output for the predefined time period with a voltage level of the supply voltage VCC of the evaluation unit  3 . In this instance the resistors R 4   a , R 4   b  and R 3  must then be correspondingly dimensioned, such that the required voltage is present at the base of the first transistor Q 1 . 
   The output voltage present at the output  17  of the voltage source is preferably between 6 and 8 volts maximum. 
   In a step S 2  the analog-digital converter(s)  31  is/are used to determine digital values ADC_A 1 , ADC_A 2  of the voltages present at the first and second outputs  12 ,  13 . Almost the entire converter range of the analog-digital converter(s)  31  can be utilized in conjunction with appropriate dimensioning of the resistors R 7   a  and R 7   b  of the voltage divider and the reference resistor Rref. 
   In a step S 3  the value of the sensor resistor Rsens at time t 0  is then determined as a function of the value of the reference resistor Rref, the resistors R 7   a  and R 7   b  and the digital values ADC_A 1 , ADC_A 2  of the voltages at the first and second output  12 ,  13 . By determining the value of the resistor Rsens as a function of the relationship of the digital values ADC_A 1  and ADC_A 2  of the voltages at the first and second output  12 ,  13 , fluctuations of the supply voltage VCC of the evaluation unit  3  do not affect the value of the sensor resistor Rsens. 
   The program is then continued in a step S 5 , in which it is verified whether the current time t is greater than or equal to the time t 0  plus a predefined delay time period dt. If the condition of step S 5  is not satisfied, the program remains at step S 7  for a predefined waiting time period T_W, which is shorter than the delay time period dt. If however the condition of step S 5  is satisfied, the program branches to a step S 9 . The delay time period dt and the waiting time period T_W are preferably selected such that the step S 9  is processed in a time t 1  which is delayed by the predefined time period for the presence of the second control signal CTRL 2  at time t 0 . This time period is approximately 600 ms. 
   In step S 9  the analog-digital converter(s)  31  is/are used again to determine the digital values ADC_A 1  and ADC_A 2  of the voltages at the first output  12  and the second output  13 . The time sequences of the steps S 5 , S 7  and S 9  are selected such that the control signal CTRL is still being generated at the time when step S 9  is being processed. 
   In a step S 11  the value of the sensor resistor at time t 1  is determined from the digital values ACD_A 1  and ADC_A 2  determined in step S 9 , the reference resistor Rref and the values of the resistors R 7   a  and R 7   b.    
   In a subsequent step S 13  the oil level L_OIL is determined as a function of the values of the sensor resistor Rsens at times t 0  and t 1  as determined in steps S 3  and S 11 . This is preferably done using a set of characteristics, which was determined previously by means of corresponding tests and measurements. The program is then terminated in a step S 15 . 
   The evaluation unit  3  preferably also comprises a regulator, which is deployed in the form of a program. The program is stored in the evaluation unit  3  and downloaded for the operation of the evaluation unit  3  and processed at regular intervals. The program is preferably processed parallel to the processing of steps S 1  to S 9  according to the program in  FIG. 2 . 
   In a step S 20  ( FIG. 3 ) the program is started and variables are optionally initialized. In a step S 22  the digital value ADC_A 1  of the voltage at the first output  12  is determined. 
   In a step S 24  an actual value U_REF_AV of the voltage, which drops at the reference resistor Rref and the sensor resistor Rsens, is determined as a function of the digital value ADC_A 1 , the maximum value ADC_A 1 _MAX of the digital value ADC_A 1  of the supply voltage VCC of the evaluation unit  4  and the reverse voltage divider ratio of the voltage divider. 
   In a step S 26  a target value U_REF_SP is determined of the voltage, which drops over the sensor resistor Rsens and the reference resistor Rref. 
   In a step S 28  the control signal is generated as a function of the determined target value and actual value of the voltage drop at the sensor resistor Rsens and the reference resistor Rref. The control signal CTRL is preferably pulse-width modulated, the pulse width being a function of the difference between the target value U_REF_SP and the actual value U_REF_AV. It is possible in this manner to regulate the output voltage very precisely at the output  17  of the voltage source  15 . 
   In an alternative embodiment of the control device ( FIG. 4 ) the reference resistor Rref is connected both to the output  17  of the voltage source  15  and to the first terminal  18  for the sensor resistor Rsens. The second terminal  19  for the sensor resistor Rsens is connected directly to ground GND. This circuit arrangement has the advantage compared with the one in  FIG. 1  that due to the arrangement of the reference resistor Rref it is resistant to short circuits when the sensor resistor Rsens short circuits to ground GND. With this embodiment of the control device it is therefore possible optionally to omit the resistor between the cathode of the Zener diode D 2  and the base of the third transistor Q 3 . 
     FIG. 5  shows patterns of different variables over the value range of the sensor resistor Rsens in the event that the output voltage at the output  17  of the voltage source  15  is 6 volts and the reference resistor has a value of 10Ω. The required value range of the sensor resistor Rsens is thereby between 17 and 37Ω for example. A curve  91  represents the pattern of the voltage drop at the sensor resistor Rsens. A curve  92  represents the current through the sensor resistor Rsens. A curve  93  represents the power loss in the sensor resistor Rsens. By comparison a curve  94  shows the power loss in the sensor resistor Rsens, when there is a constant current regulator present instead of the voltage regulator. The curve  91  is scaled in respect of the right ordinates. The curves  92 ,  93  and  94  are scaled in respect of the left ordinates. 
   It can be seen from the curve  93  of the power loss in the sensor resistor Rsens that its maximum is within the required value range of the sensor resistor Rsens and that the pattern of the curve in this range is extremely flat, almost horizontal. The power loss in the sensor resistor is thus almost constant in the required value range of the sensor resistor Rsens. This means that irrespective of the temperature of the sensor resistor Rsens at the start of the application of voltage to the sensor resistor Rsens, an approximately identical heat is converted in the sensor resistor Rsens within the predefined time period. The sensitivity of the oil level measurement is therefore almost independent of the start temperature. 
   The voltage divider, formed by the resistors R 7   a  and R 7   b , is preferably connected on the input side to a switch  19   a , which connects the voltage divider as a function of its switch position either to the first terminal  17  of the sensor resistor Rsens or to the second voltage supply  5  and therefore the supply voltage VCC of the evaluation unit  3 . Thus by corresponding detection of the digital value ADC_A 1  of the voltage at the first output  12 , when the switch  19  connects the input of the voltage divider to the second voltage supply  5 , it is possible to determine the actual voltage divider ratio of the resistors R 7   a  and R 7   b  and take it into account when determining the value of the sensor resistor Rsens in steps S 3  and S 11  of the program according to  FIG. 2 . It is thus possible to increase the accuracy of the determination of the value of the sensor resistor Rsens in steps S 3  and S 11  further in this manner. 
   The accuracy of the determination of the value of the sensor resistor Rsens can also be further increased by measuring the reference resistor Rref individually when producing the control device and storing the value of the reference resistor thus determined in the evaluation unit  3 . 
   The sensor resistor Rsens is preferably configured as a resistance wire but it can also be in the form of any other resistor, to which a power that has to be adjusted precisely is to be fed. The transistors can also be field effect transistors, in particular MOS-FET transistors. 
   It is possible to identify an error using the digital value(s) ADC_A 1 , ADC_A 2  by means of plausibilization and to adjust the control signal CTRL such that a predefined potential, preferably ground, is present at the output  17  of the voltage source  15 .