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US 20130249575A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2013/0249575 A1 HERTES (54) (43) Pub. Date: DEVICE AND METHOD FOR MEASURING A VALUE OF A RESISTOR Publication Classi?cation (5 1) Int. Cl. G01R 2 7/02 (52) US. Cl. (71) Applicant: CONTINENTAL AUTOMOTIVE GMBH, HANNOVER (DE) Sep. 26, 2013 (2006.01) CPC .................................... .. G01R 27/02 (2013.01) (72) Inventor: VOLKER HERTES, HAGELSTADT (DE) (73) USPC ........................................................ .. 324/711 (57) ABSTRACT A device for measuring the value of a resistor has a ?rst RC element With a ?rst time constant and a second RC element With a second time constant. The ?rst RC element has a ?rst capacitor and a resistor. The second RC element has a second capacitor and the resistor to be measured. An evaluation cir Assigneez CONTINENTAL AUTOMOTIVE GMBH HANNOVER (DE) (21) APPL NO; 13/849,101 cuit charges the ?rst second capacitors to 'a ?rst and a second voltage, respectively, and m1t1ates their discharge. The evaluation circuit measures the time that elap ses from the (22) Filed; Man 22, 2013 (30) start of the discharge to the instant at Which the voltages across the capacitors are equal, and determines the value of Foreign Application Priority Data Mar. 22, 2012 \ii the resistor from the values. In the event that the ?rst time constant is larger than the second time constant, the ?rst (DE) ....................... .. 1020122045694 -\ Control 0" O? voltage is smaller than the second voltage, and vice versa. |_| SW VR -q— Control v|_ <— Patent Application Publication Sep. 26, 2013 US 2013/0249575 A1 FIG 1 V %L 0 _\ _ n vR toFF i - VL t0FF+T0 t Sep. 26, 2013 US 2013/0249575 A1 DEVICE AND METHOD FOR MEASURING A VALUE OF A RESISTOR CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims the priority, under 35 U.S.C. §119, of German patent application DE 10 2012 204 569.4, ?led Mar. 22, 2012; the prior application is herewith incor porated by reference in its entirety. BACKGROUND OF THE INVENTION Field of the Invention [0002] The value of a resistor often serves as a measure of the temperature, for example of a heating element. Conse quently, the resistance needs to be measured in order to deter mine the temperature. Although in principle this can be the ?rst time constant is greater than the second time constant, the ?rst voltage is smaller than the second voltage, and vice versa. [0011] In other Words, the objects of the invention are achieved by Way of a device for measuring the value of a resistor, having a ?rst RC element, Which has a ?rst time constant and is formed from a ?rst capacitor and a further resistor, Which is connected in parallel With the ?rst capacitor. In addition, the device has a second RC element, Which has a second time constant and is correspondingly formed from a second capacitor and the resistor to be measured, Which is connected in parallel With the second capacitor. The capaci tors are charged to different voltages by means of an evalua tion circuit, Wherein the ?rst time constant is larger than the second time constant When the ?rst voltage is smaller than the second voltage, and vice versa. If appropriate sWitching means ensure at a particular instant that the capacitors can only discharge via the resistors assigned thereto, there is a accomplished by Way of the quotient of the voltage drop particular instant at Which the voltages across the tWo capaci across the resistor and the current ?owing through the resis tors are equal. This instant can be detected. The value of the resistor to be measured can be calculated from knoWledge of this instant and of the values of the tWo capacitors and the further resistor, and also of the ratio of the tWo voltages to tor, this is complex and expensive, since a speci?c shunt resistor is usually required for measuring the current. HoW ever, the value of the shunt has an upper limit for reasons of poWer loss, and the physical siZe of the shunt limits the value doWnWards. The current-measuring circuit must be designed in a correspondingly sensitive manner With corresponding concessions in terms of accuracy. Moreover, large resistors tend to exhibit loWer reliability for the solder joints. In the case of loads With a ?xed ground connection, in Which the supply of poWer is controlled by connecting and disconnect ing the battery voltage, the shunt must be on the battery side of the load and the battery-referenced current-measuring sig nal must be converted in a complex manner into a ground referenced signal so that it can be processed further. SUMMARY OF THE INVENTION Which the capacitors are charged. [0012] In accordance With an added feature of the inven tion, a voltage divider is connected betWeen the ?rst and second capacitors and a voltage source for charging the capacitors, and Wherein the evaluation circuit is con?gured to connect one of the ?rst and second capacitors to the voltage source via a ?rst controllable sWitch and to connect the respectively other capacitor to the voltage source via the ?rst sWitch and the voltage divider, and Wherein the voltage divider is con?gured to draW no current While the ?rst and second capacitors are discharging. [0013] In accordance With an advantageous feature of the invention, a ?rst sWitch connected at an input of the voltage [0003] It is accordingly an object of the invention to provide a method and a device Which overcome a variety of disadvan tages of the heretofore-known devices and methods of this general type and Which provides for a novel a device and a method for measuring the value of a resistor. [0004] With the foregoing and other objects in vieW there is provided, in accordance With the invention, a device for mea suring a value of a resistor, comprising: [0005] a ?rst RC element having a ?rst time constant, the ?rst RC element including a ?rst capacitor and a further resistor connected in parallel With the ?rst capacitor; [0006] a second RC element having a second time con stant, the second RC element including a second capaci tor and the resistor to be measured connected in parallel With the second capacitor; [0007] an evaluation circuit connected to the ?rst and second capacitors and being con?gured: [0008] to charge the ?rst and second capacitors to a ?rst voltage and a second voltage, respectively, and to initiate a discharge thereof through only the further resistor or the resistor to be measured; [0009] to measure a time that elapses from a start of a discharge of the ?rst and second capacitors to an instant at Which the voltages across the ?rst and sec ond capacitors are equal; and [0010] to determine the value of the resistor to be measured from the values, Wherein, in an event that divider and a second sWitch connected at an output of the voltage divider are driven by the evaluation circuit so as to be open during the discharge process. [0014] With the above and other objects in vieW there is also provided, in accordance With the invention, a method of measuring a value of a resistor. The method Which comprises: [0015] providing a measuring device as outlined above and charging the ?rst capacitor of the ?rst RC element to a ?rst voltage and charging the second capacitor of the second RC element to a second voltage; [0016] if the ?rst time constant is greater than the second time constant, the ?rst voltage is smaller than the second voltage, and vice versa, and a ratio of the ?rst voltage to the second voltage is l/K; [0017] With an evaluation circuit, initiating a discharge of the capacitors and measuring a time interval that elapses from a start of the discharge of the capacitors to an instant at Which the ?rst and second voltages across the capacitors are equal, and determining a value of the resistor to be measured from the values of the ?rst and second capacitors, of the further resistor, the ratio of the ?rst voltage to the second voltage, and the measured time. [0018] In accordance With a concomitant feature of the invention, the evaluation circuit is con?gured to determine the value of the resistor according to the formula Sep. 26, 2013 US 2013/0249575 Al 1 TORRCR capacitors CR, CL and the voltage divider l/K is also inter rupted, With the result that the capacitors can discharge only through the resistors RR, RL assigned thereto in the tWo RC elements. wherein: [0019] RL is the value of the resistor to be measured; [0020] [0021] CL is the capacitance of the second capacitor; T0 is the measured time interval; [0022] [0023] RR a resistance of the ?rst resistor; CR is a capacitance of the ?rst capacitor; [0024] K is a ratio of the ?rst voltage to the second voltage. [0025] Other features Which are considered as characteris tic for the invention are set forth in the appended claims. [0026] Although the invention is illustrated and described herein as embodied in a device and method for measuring the value of a resistor, it is nevertheless not intended to be limited to the details shoWn, since various modi?cations and struc [0033] The voltages VR and VL across the capacitors are supplied to the evaluation circuit Control and, for example, have a pro?le as shoWn in FIG. 2. At the instant TO, after the sWitch-off operation at the instant tOFF, the tWo voltages VR and VL have the same value, and this is detected by the evaluation circuit Control. The evaluation circuit Control has a chronometric device in order to record the time period TO. The value of the resistor RL can be determined from the knoWn values of the ?rst capacitor CR, of the further resistor RR, of the second capacitor CL, the determined time period TO and the ratio l/K of the tWo voltages VR and VL to Which the tWo capacitors Were initially charged, according to the for mula tural changes may be made therein Without departing from the spirit of the invention and Within the scope and range of equivalents of the claims. [0027] The construction and method of operation of the l TORRCR invention, hoWever, together With additional objects and [0034] advantages thereof Will be best understood from the folloW ing description of speci?c embodiments When read in con method according to the invention alloW indirect current mea nection With the accompanying draWings. The device according to the invention and the surement using a complex and expensive ampli?er circuit consisting of shunt, operational ampli?er and feedback net Work to be omitted. Likewise, quantitative determination of BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING [0028] FIG. 1 is a basic circuit illustration of a device according to the invention; and [0029] FIG. 2 graph shoWing the pro?le of the voltages across the tWo capacitors. DETAILED DESCRIPTION OF THE INVENTION [0030] Referring noW to the ?gures of the draWing in detail and ?rst, particularly, to FIG. 1 thereof, a ?rst RC element is formed from a ?rst capacitor CR and a further resistor RR, Which is connected in parallel With the ?rst capacitor CR. A second RC element is formed from a second capacitor CL and the resistor RL to be measured. The tWo capacitors CR, CL are connected to a voltage source VQ, and are charged to a par ticular voltage, by operation of a ?rst sWitch SW1. In this case the value of said voltage is not important, rather only the ratio of the ?rst voltage VR across the ?rst capacitor CR to the second voltage VL across the second capacitor CL. This is indicated by a voltage divider symbol l/K, Which represents a circuit by Which the voltage of the voltage source VQ is divided by the factor K. This can be realiZed With a simple voltage divider having resistors; hoWever, it can also be of more complex design. [0031] The voltage divider l/K is con?gured such that in the event that the capacitors CR, CL are discharging, no cur rent can ?oW via the voltage divider l/K, but rather the capaci tors CR, CL can discharge only via the resistor RR, RL respec tively assigned thereto. As is shoWn schematically in FIG. 1, this can be accomplished by means of corresponding sWitches SWR, SWL, Which can be controlled by the evalua tion circuit Control, at the input and output of the voltage divider l/K. the voltage across the resistor is not necessary. In this case the measurement result is independent not only of the voltage to be applied by the voltage source VQ but also of the residual resistances of the closed sWitches SW, SWR, SWC,Wh1Ch are usually realiZed by transistors in Which the sWitch-on resis tances are highly dependent on temperature and current. 1. A device for measuring a value of a resistor, comprising: a ?rst RC element having a ?rst time constant, said ?rst RC element including a ?rst capacitor and a further resistor connected in parallel With said ?rst capacitor; a second RC element having a second time constant, said second RC element including a second capacitor and the resistor to be measured connected in parallel With said second capacitor; an evaluation circuit connected to said ?rst and second capacitors and being con?gured: to charge said ?rst and second capacitors to a ?rst volt age and a second voltage, respectively, and to initiate a discharge thereof through only said further resistor or the resistor to be measured; to measure a time that elapses from a start of a discharge of said ?rst and second capacitors to an instant at Which the voltages across said ?rst and second capaci tors are equal; and to determine the value of the resistor to be measured from the values, Wherein, in an event that the ?rst time constant is greater than the second time constant, the ?rst voltage is smaller than the second voltage, and vice versa. 2. The device according to claim 1, Which further com prises a voltage divider connected betWeen said ?rst and second capacitors and a voltage source for charging said capacitors, and Wherein said evaluation circuit is con?gured As can be seen in FIG. 2, at an instant tOFF the to connect one of said ?rst and second capacitors to said voltage source via a ?rst controllable sWitch and to connect sWitches SW, SWR, SWL are opened by the evaluation circuit Control, Which ensures that the connection betWeen the the respectively other capacitor to said voltage source via said ?rst sWitch and said voltage divider, and Wherein said voltage [0032] Sep. 26, 2013 US 2013/0249575 A1 divider is con?gured to draW no current While the ?rst and second capacitors are discharging. 3. The device according to claim 2, Which comprises a ?rst sWitch connected at an input of said voltage divider and a second sWitch connected at an output of said voltage divider, Wherein said ?rst and second sWitches are connected to said evaluation circuit and said evaluation circuit is con?gured to open said sWitches during the discharge process. 4. A method of measuring a value of a resistor, Which comprises: from a start of the discharge of the capacitors to an instant at Which the ?rst and second voltages across the capacitors are equal, and determining a value of the resistor to be measured from the values of the ?rst and second capacitors, of said further resistor, the ratio of the ?rst voltage to the second voltage, and the measured time. 5. The according to claim 4, Wherein said evaluation circuit is con?gured to determine the value of the resistor according to the formula providing a ?rst RC element, formed of a ?rst capacitor and a further resistor and having a ?rst time constant, and charging the ?rst capacitor to a ?rst voltage; providing a second RC element, formed of a second capacitor and the resistor to be measured and having a second time constant, and charging the second capacitor to a second voltage; if the ?rst time constant is greater than the second time constant, the ?rst voltage is smaller than the second voltage, and vice versa, and a ratio of the ?rst voltage to the second voltage is l/K; With an evaluation circuit, initiating a discharge of the capacitors and measuring a time interval that elapses 1 TORRCR Wherein: RL is the value of the resistor to be measured; CL is the capacitance of the second capacitor; T0 is the measured time interval; RR a resistance of the ?rst resistor; CR is a capacitance of the ?rst capacitor; K is a ratio of the ?rst voltage to the second voltage. * * * * *