Abstract
A method and a device for measuring a current flowing through a switch which has an unknown inner resistance and two connections, a voltage difference being measured at the switch. During operation, the current provided by an AC voltage source, which is part of an AC voltage circuit connected in parallel with the switch, is superimposed on the current to be measured, the current flowing through the switch, by way of the AC voltage source. Both the amplitude and the frequency of the current provided by the AC voltage source are known. An AC voltage component of the voltage difference and the amplitude of the component are ascertained, and the current between the connections is ascertained and output proportionally to the amplitude of the current of the AC voltage source.
Claims
1. A method for measuring a current through a semiconductor switch having an unknown internal resistance, the switch having two connections, the method comprising: providing an AC source as part of an AC circuit connected in parallel with the switch, wherein both an amplitude and a frequency of a current provided by the AC source are known; during an operation, superimposing the current provided by the AC source on a current through the switch to be measured; determining an AC voltage component of a voltage difference across the switch and an amplitude thereof as a maximum voltage value; connecting a gate signal generator for controlling the switch resistance to the switch; interrupting the current provided by the AC source, periodically modulating the switch resistance by the gate signal generator, and determining a further AC voltage component of the voltage difference, wherein the current between the connections is determined proportionally to a product of the amplitude of the current of the AC source and the further AC voltage component; a) selecting a first setting of the switch resistance; b) determining a first AC voltage component of the voltage difference; c) selecting a second setting of the switch resistance; d) determining a second AC voltage component of the voltage difference; e) deactivating the AC source; f) periodically switching over the switch resistance between the first and second settings, corresponding to a square wave signal, by the gate signal generator; g) determining a third AC voltage component of the voltage difference; h) determining and outputting the current between the connections from a maximum current of the AC source and a ratio of the third AC voltage component and a difference between the first and second AC voltage components.
2. The method according to claim 1, which comprises, in addition to the AC voltage component, determining a DC voltage value of the voltage difference, and determining and outputting the current between the connections from the amplitude of the current of the AC source and a ratio of the determined voltage values.
3. The method according to claim 1, which comprises multiplying an AC voltage component of the voltage difference, which AC voltage component is brought about either by superimposing an alternating current or by modulation of the switch resistance, by a reference signal, and determining and outputting a direction of current flow on the basis of the mathematical sign of the product signal.
4. The method according to claim 1, which comprises changing the frequency of the AC source and optionally of the gate signal generator as soon as unfavorable frequency components are identified in the current between the connections.
5. The method according to claim 1, which comprises turning off the switch when the determined current is greater than a predetermined limit value.
6. A device for measuring a current through a semiconductor switch having an unknown internal resistance, the switch having two connections, the device comprising: an AC circuit including an AC source configured to provide an alternating current with a known amplitude and at a known frequency and connected in parallel with the switch, wherein, during operation, the alternating current is superimposed on the current through the switch to be measured; a gate signal generator for controlling the switch resistance connected to the switch; and a measuring circuit for determining an AC voltage component of a voltage present at the switch, said measuring circuit being configured for interrupting a current provided by the AC source, periodically modulating the switch resistance by the gate signal generator, and determining a further AC voltage component of the voltage difference, wherein the current through the switch is determined proportionally to a product of the amplitude of the current of the AC source and the further AC voltage component; a) selecting a first setting of the switch resistance; b) determining a first AC voltage component of the voltage difference; c) selecting a second setting of the switch resistance; d) determining a second AC voltage component of the voltage difference; e) deactivating the AC source; f) periodically switching over the switch resistance between the first and second settings, corresponding to a square wave signal, by the gate signal generator; g) determining a third AC voltage component of the voltage difference; h) determining and outputting the current between the connections from a maximum current of the AC source and a ratio of the third AC voltage component and a difference between the first and second AC voltage components.
7. The device according to claim 6 configured for carrying out the method according to claim 1.
8. The device according to claim 6, wherein said measuring circuit has at least one band pass filter for determining an AC voltage component of the voltage present at the switch.
9. The device according to claim 6, wherein said measuring circuit is configured to determine a DC voltage component of the voltage present at the switch.
10. The device according to claim 9, wherein said measuring circuit has at least one low-pass filter for determining a DC voltage component of the voltage present at the switch.
11. The device according to claim 6, wherein said measuring circuit has a differential amplifier for amplifying the voltage present at the switch.
12. The device according to claim 6, wherein said gate signal generator has a gate voltage switch having at least two switching states, wherein a different switch resistance is assigned to each switching state.
13. The device according to claim 6, wherein said gate signal generator has a clock generator for time controlled, periodic switching over between at least two different gate voltages.
14. The device according to claim 6, which comprises an on/off switch for opening the AC circuit.
15. The device according to claim 6, which comprises an inductive coupling connecting said AC source in said AC circuit.
16. The device according to claim 6, wherein a battery is connected to one of the connections of the switch and a current generator, is connected to the other connection.
17. The device according to claim 6, wherein the switch has at least one insulated gate field effect transistor.
18. The device according to claim 6, which comprises a processing unit connected to said measuring circuit, said processing unit has a multiplier for modulating the AC voltage component with a periodic reference signal, wherein a frequency of the reference signal is substantially identical to the frequency of the alternating current generated by said AC source, and wherein an output of said multiplier is connected to a low pass filter or a plurality of series connected low pass filters.
19. The device according to claim 18, wherein the reference signal is a square wave signal.
20. The device according to claim 6, wherein the frequency of the AC source is selected to be different from any frequency/frequencies superimposed on the current between the connections.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
(1) The invention will be explained in more detail below on the basis of particularly preferred exemplary embodiments, which do not represent any restriction to the invention, however, and with reference to the drawing, in which the following are specifically shown:
(2) FIG. 1 shows a schematic block circuit diagram of a simple basic embodiment of a device for measuring the current through a switch with an unknown internal resistance;
(3) FIG. 2 shows a schematic block circuit diagram of a variant of the device according to the invention comprising a battery and a generator;
(4) FIG. 3 shows a schematic block circuit diagram of a further variant of the device according to the invention comprising a switch having two MOSFETs;
(5) FIG. 4 shows a schematic block circuit diagram as shown in FIG. 1, but with a detailed illustration of the signal processing elements of the measuring circuit;
(6) FIG. 5 shows a group of signal profiles for illustrating the method according to the invention; and
(7) FIG. 6 shows another group of signal profiles for illustrating a further variant of the method according to the invention; and
(8) FIG. 7 shows a further group of signal profiles for illustrating a variant of the method according to the invention for determining the direction of current flow.
DESCRIPTION OF THE INVENTION
(9) The designation of alternating currents and AC voltages is always done using lower case and upper case letters in the figures and the description of the figures below, wherein the lower case letters i and u represent an alternating current and an alternating voltage, respectively, and the upper case letters I and U represent the amplitude of the current and voltage, respectively, designated by the lower case letters. The assignment to circuit elements or nodes is performed on the basis of the indices of the letters. In the case of direct currents and direct voltages, no lower case letters are used, and the upper case letters in this case represent arithmetic means, which can be equated to the amplitude in the case of a perfect direct current or a perfect DC voltage.
(10) The schematic block circuit diagram depicted in FIG. 1 shows a device 1 comprising a switch 2 and two connections 3, 4, which are connected via the switch 2. An AC circuit 5, which is closed via the switch 2, is connected in parallel with the switch 2. The alternating current i.sub.p in the AC circuit 5 is determined by an AC source 6, wherein the alternating current i.sub.p generated by the AC source 6, i.e. the amplitude I.sub.p and frequency f.sub.p of said alternating current, are known or predetermined. In addition, a measuring circuit 7 is connected on both sides to the switch 2, which measuring circuit is designed to determine the voltage u.sub.s present at the switch 2, wherein the determined total voltage u.sub.s or the profile over time thereof is divided into a DC voltage component U.sub.x and AC voltage component u.sub.p. The switch 2 therefore operates as a measuring shunt, but with an unknown resistance R.sub.s. In order to determine the current flowing between the connections 3, 4 or the mean I.sub.x thereof, the measuring circuit 7 is connected to a processing unit 8, which derives the desired current I.sub.x from the known parameter of the AC amplitude I.sub.p and from the measured voltage components U.sub.x, u.sub.p or the amplitude U.sub.p of the AC voltage component u.sub.p, to be precise in accordance with the following equation:
(11)
(12) Knowledge of the internal resistance R.sub.s of the switch 2 is in this case not required. Instead, the knowledge of the AC amplitude I.sub.p and the amplitude U.sub.p of the AC voltage component u.sub.p is sufficient, via which the switch internal resistance R.sub.s is implicitly determined. In this case, it is freely supposed that the internal resistance R.sub.s of the switch 2 is substantially purely resistive, i.e. the switch 2 has a negligibly low or no reactance. Owing to the substantially simultaneous measurement of the two voltage values U.sub.x, U.sub.p, the result is independent of any changes or fluctuations in the internal resistance R.sub.s.
(13) FIG. 2 shows a variant of the device 1, wherein a generator 9, which outputs a voltage u.sub.G, is connected to one connection 3 and a rechargeable battery 10 is connected to the other connection 4. In each case variable local loads 11, 12 are connected both to the generator 9 and to the battery 10, so that only some of the generator or battery current flows via series impedances 13, 14 and the switch 2. The device 1 is intended for determining this partial current I.sub.x. The switch 2 is connected to a control unit 15, for example a switching or gate signal generator 15. The control unit 15 determines the switch position or controls generally changes in the internal resistance R.sub.s of the switch 2.
(14) The AC circuit 5 has an on off switch 17, which is designed to interrupt or open the AC circuit 5 independently of the switch 2. As can also be seen in FIG. 2, the AC source 6 is in this case connected to the actual AC circuit 5 via an inductive coupling 16. In this case, the on off switch 17 is particularly advantageous since it prevents any DC voltage components of the current I.sub.x to be measured from bypassing the switch 2 via the AC circuit 5 and the inductance 16.
(15) In addition, FIG. 2 shows a simple design of the measuring circuit 7. In this case, the measuring circuit 7 comprises a differential amplifier 18, a low pass filter 19 and a band pass filter 20. The two filters 19, 20 are each connected to the output of the differential amplifier 18. The differential amplifier 18 is a simple means for eliminating absolute voltage fluctuations in the current I.sub.x and, if required, brings the voltage differences to be measured into the working range of the two filters 19, 20. In the variant illustrated in FIG. 3, by way of example a detailed design of the switch 2 and the gate signal generator 15 is shown. In this case, the switch 2 has two series connected semiconductor switches 21, 22, preferably MOSFETs. In order to change the internal resistance R.sub.s of the two semiconductor switches 21, 22, a gate voltage source 23 is connected between the source (or bulk) and the gate of both switches 21, 22. The voltage u.sub.g applied by the gate voltage source 23 therefore determines the internal resistance R.sub.s of the switch 2. In this case, the gate voltage source 23 is connected to a gate signal switch 24, which connects the gate voltage source 23 either to a pulsed gate signal generator 25 comprising a clock generator 26 or to a steady-state gate signal generator 27. Preferably at least two steady-state gate signal generators 27 can also be connected to the gate signal switch 24, so that a selection of two different constant internal resistances of the switch 2 is possible. If the gate voltage source 23 is connected to the pulsed gate signal generator 25, the gate voltage and therefore the internal resistance R.sub.s of the switch 2 oscillates corresponding to the pulsed signal. If the pulse frequency of the pulsed gate voltage source 25 substantially corresponds to the frequency of the alternating current, which is intended to be indicated by the connecting line 28, a substantially constant current I.sub.x (i.e. in the case of a switched off AC circuit 5) in an AC voltage component U.sub.p can result in the measuring circuit 7 because the filters 19, 20 of the measuring circuit 7 in this case filter out the signal generated by the modulated gate voltage from the remaining voltage fluctuations so that the determined AC voltage component U.sub.p is proportional to the change in resistance in the switch 2.
(16) As indicated by the linkage 29 in FIG. 3, it is advantageous to switch off the AC circuit 5 when using the pulsed gate signal generator 25. For this purpose, two positions are provided for the on off switch 17: in the first, closed position (depicted in FIG. 3), the AC circuit 5 passes via the switch 2 so that the alternating current i.sub.p is superimposed on the current I.sub.x to be measured; in the second, open position, the on off switch 17 disconnects the switch 2 from the AC circuit 5, so that only the current I.sub.x to be measured flows through the switch 2. The gate signal switch 24 and the on off switch 17 are both controlled by a common operation selection unit 30 for the purpose of simple synchronization in the example illustrated. The operation selection unit 30, which in practice is connected to a super ordinate sequence control system for the measurement, for example, switches over between an operating mode with a constant switch resistance (cf., for example, FIG. 6, interval between t.sub.0 and t.sub.2) and an operating mode with a periodically changing switch resistance (cf. FIG. 6, interval between t.sub.2 and t.sub.x). For this purpose, the operation selection unit 30 is connected to control inputs of the on off switch 17 and the gate signal switch 24 via control lines 30a, 30b, so that a signal output by the operation selection unit 30 (cf. FIG. 6g) simultaneously achieves either switching off of the AC circuit 5 and connection of the gate voltage source 23 to the pulsed gate signal generator 25 or switching on of the AC circuit 5 and connection of the gate voltage source 23 to the steady state gate signal generator 27.
(17) As already indicated in FIG. 1 by the processing unit 8, the device according to the invention can have a number of processing elements, wherein a preferred embodiment of part of the processing unit 18, in addition to the measuring circuit 7, is illustrated in more detail in FIG. 4. The basic elements of the device 1 which have already been discussed in detail above, namely the switch 2 with the connections 3, 4 and the AC circuit 5 together with the AC source 6, are illustrated in simplified form in FIG. 4 for easier orientation (cf. FIG. 1). Similarly to as in FIG. 2, a differential amplifier 18 is provided for determining and amplifying the voltage drop across the switch 2; a low pass filter 19 and, in parallel therewith, two series connected identical band pass filters 20, 20 are connected to the output of the differential amplifier 18. Therefore, an AC voltage u.sub.p corresponding to the AC voltage component of the voltage drop u.sub.s across the switch 2 is present at the output of the second band pass filter 20.
(18) In order to determine the voltage quotient of the DC voltage component and the AC voltage component (see equation 1), it is necessary to determine the amplitude U.sub.p of the AC voltage u.sub.p as precisely as possible. This is achieved in the arrangement illustrated in FIG. 4 by a lock in amplifier circuit 31, wherein the AC voltage u.sub.p is modulated in a multiplier 32 with a reference signal from a signal generator 33. The reference signal is an AC voltage at the same frequency as that of the AC source 6 or the clock generator 26 of the gate signal generator 25. The signal modulated in this way then passes through, for example, two series connected low pass filters 34, 34 (if appropriate, a low pass filter of a higher order can also be used). Prior to being output, the voltage present at the second low pass filter 34 is multiplied by a constant gain voltage 36 for amplification in a further multiplier 35. Therefore, the amplitude or peak voltage U.sub.p of the AC voltage component u.sub.p is present at the output of the multiplier 35.
(19) The amplitude U.sub.p obtained from the amplifier circuit 31 just described has a mathematical sign and therefore allows conclusions to be drawn in respect of the direction of current flow of the current I.sub.x to be measured.
(20) If the current I.sub.x is very low, and in particular when the direction of current flow needs to be established very quickly, the identification on the basis of the amplitude U.sub.p of the AC voltage u.sub.p is insufficient, however, since the signal to noise ratio becomes too small or the time constant of the low pass filter has an unfavorable effect. In FIG. 4, therefore, an additional, slightly modified amplifier circuit 37 is provided in parallel with the amplifier circuit 31 for quick and clear determination of the direction of current flow. This circuit 37, which is again in the form of a lock in amplifier, in this case uses a square-wave reference signal, which is generated by a square wave signal generator 38. In turn, two series connected low pass filters 40, 40 are connected to the output of a multiplier 39, to which the AC voltage u.sub.p and the reference signal are supplied. The voltage present at the output of the second low pass filter 40 is then amplified in a further multiplier 41 with a higher gain voltage 42 than previously described. The multiplier 41 is finally connected to a discriminator 43, which outputs a two value directional signal sign(U.sub.p), which can be used for the quick identification of the direction of current flow.
(21) In order to calculate the result I.sub.res of the current measurement, a calculation unit 8, which is likewise part of the processing unit 8, forms the quotient of the voltage amplitudes U.sub.x and U.sub.p and multiplies this quotient by the known amplitude I.sub.p of the alternating current i.sub.p.
(22) The basic mode of operation of the present invention will be explained in detail with reference to the time sequence illustrated in FIG. 5. The figure comprises seven lines or plots a)-g) of time curves of different parameters of the device or of signal profiles which have been determined on the basis of a circuit simulation. FIG. 5a shows the voltage profile at the two connections 3, 4. The voltage u.sub.G at the first connection 3, which is -tooth fashion at a specific generator frequency f.sub.G. These voltage fluctuations are often referred to as ripple (the term used below), which is characterized by a ripple frequency f.sub.R and a ripple amplitude A.sub.R. The voltage u.sub.B at the second connection 4, which is connected to a battery 10, for example, on the other hand, is substantially constant, which is represented by the continuous line. During a first time segment 44, the voltage u.sub.G at the first connection 3 is higher than the voltage u.sub.B at the second connection 4, with the result that a current I.sub.x flows from the first connection 3 to the second connection 4. This current I.sub.x is plotted in FIG. 5b, wherein the profile of the current I.sub.x is determined by the voltage difference (u.sub.Gu.sub.B) and therefore has the same structure as the voltage u.sub.G at the first connection 3. At a time t.sub.x, the voltage ratios are reversed and the voltage u.sub.B at the second connection 4 now exceeds the voltage u.sub.g at the first connection 3. Accordingly, the direction of current flow also changes, which is illustrated by a change of mathematical sign in the second time segment 44 in FIG. 5b. FIG. 5c shows, parallel to FIGS. 5a and 5b, the time profile of the DC voltage component U.sub.x, i.e. the mean of the voltage drop u.sub.s across the switch 2 (U.sub.x=avg(u.sub.s)). The averaging in this case takes place in a low pass filter 19, as a result of which the voltage fluctuations shown in FIG. 5a are smoothed. However, the low pass filter 19 causes a delay 45 in the case of voltage changes, for example in the case of a change at time t.sub.x, so that the average voltage only converges towards the present mean value or the DC voltage component U.sub.x after a transition period 45.
(23) As already explained many times above, an alternating current i.sub.p from a dedicated AC source 6 is superimposed on the current I.sub.x to be measured, whose profile is shown in FIG. 5b. The total current flowing through the switch 2 as a result (i.sub.3=I.sub.x+i.sub.p) changes the voltage drop u.sub.s across the switch 2. FIG. 5d shows the AC voltage component u.sub.p of such a superimposition. The frequency f.sub.p of the superimposed alternating current i.sub.p in this example is approximately five times the ripple frequency f.sub.R. The visible discrepancy between the AC voltage component u.sub.p and a uniform oscillation is caused by the ripple in the current I.sub.x (cf. FIG. 5a). Since the voltage fluctuations owing to the ripple are approximately of the same order of magnitude as the AC voltage brought about by the AC source 6, the discrepancies are clearly identifiable.
(24) FIG. 5e shows the profile of the gate voltage u.sub.g. The continuous line corresponds to a constant gate voltage U.sub.g and consequently a substantially constant internal resistance R.sub.s of the switch 2, which in the example has a MOSFET, comparable to the device illustrated in FIG. 3. This corresponds to a gate voltage source 23 controlled by a steady-state gate signal generator 27.
(25) The profile over time of the amplitude U.sub.p of the AC voltage component u.sub.p illustrated in FIG. 5d is shown in FIG. 5f. This amplitude U.sub.p can be determined, for example, by the device 1 shown in FIG. 4. Despite the low pass filter 34, 34 used, in this case remainder of the current ripple furthermore takes effect, so that the amplitude U.sub.p illustrated is slightly falsified in comparison with the actual constant amplitude of the alternating current i.sub.p generated by the AC source 6. In a manner comparable to the low pass filter 19 for voltage averaging, the low-pass filters 34, 34 used here also cause a delay 46 during switchover of the direction of current flow of the current I.sub.x. Directly after the switchover time t.sub.x, the amplitude U.sub.p decreases significantly and only diverges back to the same more or less stable profile prior to switchover after a transition period 46.
(26) Taking into consideration the gains used during the signal processing, the current I.sub.x flowing via the switch 2 can be calculated, in accordance with the above cited equation (1), from the illustrated signal profiles for the DC voltage component U.sub.x and the AC voltage u.sub.p or the amplitude U.sub.p thereof. The time profile of the result I.sub.res of this calculation is illustrated in FIG. 5g, wherein it should be emphasized that the value of the internal resistance R.sub.s of the switch 2 has not explicitly been used for the calculation I.sub.res of the desired current I.sub.x (i.e. I.sub.res=I.sub.x). The multiple filtering of the signals effects a delay during the switchover of the direction of current flow and distorts the form of the ripple, but otherwise substantially correctly reproduces the profile of the current I.sub.x.
(27) If the current I.sub.x becomes small, the measurement error increases. In order to improve the accuracy and resolution, therefore, it is possible to convert to a multi step method when a certain measured current is undershot. FIG. 6 illustrates in this connection the signal profiles in the case of an improved three step method. In this case, it is assumed that the internal resistance R.sub.s of the switch 2 is unknown, but is controllable in a targeted manner and in particular a periodic change in resistance is possible (cf. FIG. 3).
(28) In FIG. 6, FIG. 6a shows in detail the voltage u.sub.G, u.sub.B at the two connections 3, 4 of the present device 1; FIG. 6b illustrates schematically the time sequence of the three step method in this example, wherein the first step corresponds to the segment A between t.sub.0 and t.sub.1, the second step corresponds to the segment B between t.sub.1 and t.sub.2, and the third step corresponds to the segment C between t.sub.2 and t.sub.x. The following section C, i.e. the one beginning at time t.sub.x, differs from segment C in terms of a change in the measured variable (cf. FIG. 6a), but not the measurement procedure; FIG. 6c shows the profile of the alternating current i.sub.p in the AC circuit 5; FIG. 6d shows both the current I.sub.x to be measured and the total current i.sub.s which flows via the switch 2; FIG. 6e shows the state of an operation selection unit 30 (cf. FIG. 3); FIG. 6f illustrates the profile of the gate voltage u.sub.g at the switch 2; FIG. 6g and FIG. 6h illustrate the DC voltage component U.sub.x of the voltage drop across the switch and the AC voltage u.sub.p, respectively, and FIG. 6i shows the determined amplitude U.sub.p of the AC voltage u.sub.p.
(29) The three steps or segments A, B, C of the method illustrated here are run through successively, wherein the order is not critical. At time t.sub.x, the direction of the current I.sub.x is reversed (cf. FIG. 6a). Any other desired order can be selected. More important than the order is the proximity in time of the steps since the method sequence is favorably quick in comparison with any changes in the environment parameters. In the plots shown in FIG. 6, the first step A is depicted between times t.sub.0 and t.sub.1 (with the time being plotted on the x axis), the second step B is depicted between t.sub.1 and t.sub.2, and the third step C is depicted between t.sub.2 and t.sub.x. The current I.sub.x flowing between the connections 3, 4 is substantially identical during the entire procedure, from t.sub.0 to t.sub.x (cf. FIG. 6a), apart from a ripple. The first two steps (between t.sub.0 and t.sub.2) differ from the third step (between t.sub.2 and t.sub.x) in particular in that the AC source 6 is inactive during the third step. Therefore, there is no current flowing in the AC circuit 5 between times t.sub.2 and t.sub.x (i.sub.p=0).
(30) During the first two steps, two different gate voltages U.sub.g0, U.sub.g1 (cf. FIG. 6f) are applied to the switch 2. The two gate voltages effect two different internal resistances R.sub.s0 and R.sub.s1 of the switch 2, which results in corresponding changes in the AC voltage u.sub.p. The change (cf. FIGS. 6h and 6i) consists in that the amplitude U.sub.p of the AC voltage u.sub.p is slightly lower during the second step B than during the first step A since the second switch internal resistance R.sub.s1 is lower than the first switch internal resistance R.sub.s0. Owing to the fact that the alternating current i.sub.p or the amplitude I.sub.p thereof in this example is greater than the current I.sub.x between the connections 3, 4, the AC voltage u.sub.p is dominated by the influence of the AC source 6 during the first two method steps, i.e. discrepancies from the uniformly periodic profile are barely identifiable. The effects of the square wave alternating current i.sub.p are also demonstrated in the current i.sub.s flowing via the switch (cf. FIG. 6d), wherein a rectangular component is superimposed on the current I.sub.x with a ripple with a saw tooth waveform. During these two steps, in particular the amplitudes U.sub.p0 and U.sub.p1 of the AC voltage component u.sub.p which are measured during the first step and the second step, respectively, or actually the difference between said amplitudes, i.e. the change U.sub.p=U.sub.p0U.sub.p1 in the amplitude U.sub.p of the AC voltage component u.sub.p in the event of a change in the gate voltage u.sub.g at the switch 2, are relevant for the present measurement.
(31) During the third step C, the AC source 6 is completely deactivated, and possibly the AC circuit 5 is opened, so that only the current I.sub.x to be measured flows via the switch 2. In addition, the gate voltage source 23 is now connected to a periodic gate signal generator 25, so that the gate voltage u.sub.g and therefore the internal resistance R.sub.s of the switch 2 are switched over periodically between values R.sub.s0 and R.sub.s1, which are assumed during the two proceeding steps. The internal resistance R.sub.s of the switch 2 still does not need to be known at any point, however. The switch 2 should be suitable for following the frequency of the changing gate voltage u.sub.g so that the resistances achieved in the case of a periodic change substantially correspond to the constant resistances R.sub.s0 and R.sub.s1 during the first two method steps. The AC frequency of the gate voltage u.sub.g is advantageously equal to the frequency f.sub.p of the alternating current i.sub.p generated by the AC source 6 during the first two steps A, B, so that the AC voltage u.sub.p generated by variation in the resistance (given the same current I.sub.x) can be processed by the same filters as the previously superimposed alternating current i.sub.p. Owing to the changing switch internal resistance R.sub.s, the AC voltage component U.sub.p determined by the measuring circuit 7 is not zero, as the switched-off AC source 6 would have us suppose, but corresponds to the AC voltage drop u.sub.p across the switch 2 in the case of a preset current I.sub.x and changing resistance R.sub.s (cf. FIG. 6h between t.sub.2 and t.sub.x). A valid estimation of the AC voltage component U.sub.p2 only adjusts itself after a short settle time 48 once the AC source 6 has been switched off at time t.sub.2, for reasons which have already been mentioned several times. The amplitude U.sub.p2 of the AC voltage component u.sub.p then determined corresponds to the change in voltage U.sub.x given a constant current I.sub.x (U.sub.x=U.sub.p2) and a changing resistance. The desired current I.sub.x can now be calculated from this change in voltage U.sub.x and the change U.sub.p in the amplitude of the AC voltage component given an active AC source 6 and different constant gate voltages U.sub.g0, U.sub.g1 and therefore different switch resistances R.sub.s0 and R.sub.s1, in accordance with the following equation:
(32)
where
U.sub.x=I.sub.x.Math.R.sub.s0I.sub.x.Math.R.sub.s1 and U.sub.p=U.sub.p0U.sub.p1=I.sub.p.Math.R.sub.s0I.sub.p.Math.R.sub.s1(3).
As can be seen from FIG. 6a, the current I.sub.x remains uninfluenced by the AC source 6 during the third step C. Temporary current peaks 47 are generated, if indirectly, by the changing internal resistance R.sub.s: during switchover from the greater resistance R.sub.s0 to the lower resistance R.sub.s1, a current peak pointing upwards, toward the higher current values, can be seen and in the case of switchover from the lower resistance R.sub.s1 to the higher resistance R.sub.s0, a current peak pointing downwards toward the lower current values can be seen. At time t.sub.x, i.e. in this example after the third and final method step C and at the beginning of the segment C, the direction of the current I.sub.x also changes in the profile illustrated in FIG. 6. An immediate re measurement with the two constant gate voltages U.sub.g0, U.sub.g1 is not absolutely necessary, however. Even during switchover of the direction of current flow, the device according to the invention requires a short settle period 49, in particular as regards the DC voltage component U.sub.x (FIG. 6g) and the amplitude U.sub.p of the AC voltage component u.sub.p (FIG. 6i). In this case, a short-term overshoot may occur, which can be attributed to the energy which is usually stored in the low-pass filters. If the duration of the settle period 49 is acceptable for the respective application, the determination of the DC voltage component U.sub.x is sufficient for identifying the direction of current flow.
(33) The signal profiles shown in FIG. 7 illustrate a variant of the method according to the invention which enables comparatively quick identification of the direction of current flow or of a change in the direction of current flow. The illustrated situation corresponds approximately to the transition between segments C and C in FIG. 6 (cf. FIG. 6b) with the change in direction of current flow at time t.sub.x. In this case, FIG. 7a shows in detail the voltage u.sub.G, u.sub.B at the two connections 3, 4 of the present device 1; FIG. 7b shows the current I.sub.x, whose direction is to be determined; FIG. 7c shows the AC voltage u.sub.p; FIG. 7d illustrates the profile of a reference voltage u.sub.r, whose form, frequency f.sub.p and phase angle corresponds to the gate voltage u.sub.g at the switch 2 (cf. FIG. 6f); and FIG. 7e illustrates both the DC voltage component U.sub.x of the voltage drop across the switch and the directional signal sign(U.sub.p) (cf. FIG. 4).
(34) As already explained in connection with the amplifier circuit 37 (cf. FIG. 4), the AC voltage u.sub.p is multiplied by a preferably rectangular reference signal u.sub.r for directional determination. Owing to the abruptly changed phase angle of the AC voltage u.sub.p at time t.sub.x (cf. FIG. 7c), the product of the AC voltage u.sub.p and the (unchanged) reference signal u.sub.r changes the mathematical sign already within the first half-cycle of the two signals (i.e. the reference signal u.sub.r and the AC voltage u.sub.p), but in any case much more quickly than the DC voltage component U.sub.x likewise illustrated in FIG. 7e. The rate of change of the DC voltage component U.sub.x is preset by the time constant of the low-pass filter.
(35) The relative phase angle between the AC voltage u.sub.p and the reference signal u.sub.r prior to and after the change in direction of current flow at time t.sub.x can be read at the dashed auxiliary lines in FIG. 7. The reversed mathematical sign of the directional signal sign (U.sub.p) in comparison with the DC voltage component U.sub.x results from the phase equality of the gate voltage u.sub.g and the reference signal u.sub.r. For simpler utilization, in practice the directional signal can be output in inverted form or alternatively a phase shift of 180 can be provided between the gate voltage and the reference signal.
