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Title:
ELECTRIC CIRCUIT AND DIAGNOSTIC METHOD FOR AN ELECTRIC LOAD
Document Type and Number:
WIPO Patent Application WO/2019/238737
Kind Code:
A1
Abstract:
Electric circuit and diagnostic method for an electric load Electric circuit (1) configured for driving a current through at least one load resistance (4) in a first state and for isolating the at least one load resistance (4) in a second state, the electric circuit (1) comprising at least: at least one first switching unit (2) connecting a first terminal of the at least one load resistance (4) and a first port of the electric circuit (1), which has a first electric potential, at least one second switching unit (3) connecting a second terminal of the at least one load resistance (4) and a second port of the electric circuit (1), which has a second electric potential, which is different from the first electric potential, and at least one auxiliary resistance (5) comprised within a bypass line (6) that bypasses the at least one load resistance (4) and the at least one second switching unit (3), wherein the at least one first switching unit (2) is configured at least for switching a current flowing through the at least one first switching unit (2), and wherein the at least one second switching unit (3) is configured at least for switching a current flowing through the at least one second switching unit (3).

Inventors:
FREDERIKSEN FINN (DE)
BAUER PETER (DE)
MAGUIN GEORGES (DE)
MESMER DENIS (DE)
DIOUF CHEIKH (DE)
Application Number:
EP2019/065315
Publication Date:
December 19, 2019
Filing Date:
June 12, 2019
Export Citation:
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Assignee:
CPT GROUP GMBH (DE)
International Classes:
H05B1/02
Foreign References:
US20020000436A12002-01-03
US6094975A2000-08-01
US20140182272A12014-07-03
Attorney, Agent or Firm:
BARZ, Torsten (DE)
Download PDF:
Claims:
Patent claims

1. Electric circuit (1) configured for driving a current through at least one load resistance (4) in a first state and for isolating the at least one load resistance (4) in a second state, the electric circuit (1) comprising at least :

at least one first switching unit (2) connecting a first terminal (18) of the at least one load resistance (4) and a first port (16) of the electric circuit (1) , which has a first electric potential,

at least one second switching unit (3) connecting a second terminal (19) of the at least one load resistance (4) and a second port (17) of the electric circuit (1) , which has a second electric potential, which is different from the first electric potential, and at least one auxiliary resistance (5) comprised within a bypass line (6) that bypasses the at least one load resistance (4) and the at least one second switching unit (3) ,

wherein the at least one first switching unit (2) is configured at least for switching a current flowing through the at least one first switching unit (2), and wherein the at least one second switching unit (3) is configured at least for switching a current flowing through the at least one second switching unit (3) .

2. Electric circuit (1) according to claim 1, wherein the at least one first switching unit (2) is further configured for generating and outputting a feedback signal depending on the current flowing through the at least one first switching unit (2 ) .

3. Electric circuit (1) according to one of the preceding claims, wherein the at least one first switching unit (2) is connected to a microcontroller (7), and wherein the microcontroller (7) is configured for receiving and processing the feedback signal, and for controlling the at least one first switching unit (2) .

4. Electric circuit (1) according to one of the preceding claims, wherein the at least one second switching unit (3) is connected to a control circuit (8), and wherein the control circuit (8) is configured for controlling the at least one second switching unit (3) .

5. Resistive heater (10) for a selective catalyst reduction (SCR) system (11) being comprised as the load resistance (4) within an electric circuit (1) according to one of the claims 1 to 4.

6. Method for detecting an open circuit failure in an electric circuit (1) according to one of the claims 1 to 4, the method comprising at least monitoring a monitored current, which is the current flowing through the at least one first switching unit (2), wherein an open circuit failure is detected by a deviation of the monitored current form an expected value.

7. Method according to claim 6, wherein the monitored current is detected by means of a feedback signal generated and outputted by the at least one first switching unit (2) .

8. Method according to one of the claims 6 or 7, further comprising performing a diagnostics sequence, wherein the diagnostics sequence comprises at least the following steps :

A) switching the at least one first switching unit (2),

B) switching the at least one second switching unit (3) ,

C) from a first point of time (tl) until a second point of time (t2), detecting the monitored current with the at least one first switching unit (2) being in its on state and the at least one second switching unit (3) being in its off state, wherein the diagnostics sequence is followed by a regular operation of the electric circuit (1) .

9. Method according to claim 8, wherein the diagnostics sequence further comprises the following steps:

a) switching the at least one first switching unit (2), b) switching the at least one second switching unit (3) , c) from an initial point of time (tO) until the first point of time (tl) , detecting the monitored current with both the at least one first switching unit (2) and the at least one second switching unit (3) being in their on states .

10. Method according to one of the claims 8 or 9, wherein the diagnostics sequence further comprises the following steps:

D) switching the at least one second switching unit (3) ,

E) from the second point of time (t2) until the third point of time (t3) , detecting the monitored current with both the at least one first switching unit (2) and the at least one second switching unit (3) being in their on states .

Description:
Description

Electric circuit and diagnostic method for an electric load

The invention relates to an electric circuit configured for driving a current through at least one load resistance in a first state and for isolating the at least one load resistance in a second state. The invention also relates to a method for detecting an open circuit failure in the electric circuit.

Various electric circuit configurations are known in which currents can be driven through electric loads. In particular in automotive applications, it is thereby desired to not only have a means provided for switching the load resistance off if required (e.g. by means of a switch), but also for isolating the load resistance from any other component of the electric circuit. This can be achieved, for example, by having one switch provided on each side of the load resistance. In these configuration both switches are arranged in a serial connection. The load resistance is arranged between both switches.

In some applications, such as those with a resistive heater being the load resistance, one of the two switches may be switched in a frequent manner (in particular periodically) . This is the case, for example with electric resistive heaters in selective catalyst reduction (SCR) systems as load resistance . Such load resistances are often operated by a pulse-width-modulated ( PWM) -signal . This enables to control the electric power transferred to the load resistance in a precise manner. In case of electrical switches on each sides (or in other words on both sides) of the load resistance, it is sometimes not possible to detect an open circuit failure in the electric circuit. An open circuit failure can be any unintended opening in the electric circuit, such as a cable failure, a switch failure or a load resistance failure. With an open circuit failure, no current can be driven through the electric circuit. Hence, the electric circuit cannot be used as intended. With two switches which are arranged in a serial connection, it is much more difficult to identify an open circuit failure can be, because whenever the second switch is switched off, no current can flow through the electric circuit, regardless of any open circuit failures. It cannot be distinguished if no current can flow because of the switch being switched off, and/or because of an open circuit failure.

However, being able to detect an open circuit failure even in a situation as described can be important. In particular, in complex systems such as in an automobile with many different electric components, an automated detection of a failure of one of these electric components can be of great importance. In particular, this is the case for applications which have to be monitored by an On Board Diagnosis System (OBD) .

It is, therefore, an object of the present invention to overcome at least in part the disadvantages known from prior art and, in particular, to provide an electric circuit for driving a current through at least one load resistance in a first state and for isolating the at least one load resistance in a second state, wherein an open circuit failure can be efficiently detected. Further, a method for detecting an open circuit failure in such an electric circuit is provided.

These objects are achieved by the features of the independent claims. The dependent claims are directed to preferred em bodiments of the present invention.

The electric circuit according to the present invention is configured for driving a current through at least one load resistance in a first state and for isolating the at least one load resistance in a second state. The electric circuit comprises at least:

- at least one first switching unit connecting a first terminal of the at least one load resistance and a first port of the electric circuit, which has a first electric potential, - at least one second switching unit connecting a second terminal of the at least one load resistance and a second port of the electric circuit, which has a second electric po tential, which is different from the first electric po tential, and

- at least one auxiliary resistance comprised within a bypass line that bypasses the at least one load resistance and the at least one second switching unit, wherein the at least one first switching unit is configured at least for switching a current flowing through the at least one first switching unit, and wherein the at least one second switching unit is configured at least for switching a current flowing through the at least one second switching unit.

The load resistance may be a resistor element or any other electric device having an electric resistance. The load re sistance may have any resistance value. However, it is preferred that other components of the electric circuit are adapted to the resistance value of the load resistance. In particular, po tentially occurring maximal voltage drops across the load resistance are preferably limited according to the specification of the load resistance. Further, parameters of the electric circuit such as diameters of cables are preferably adapted to the resistance value of the load resistance.

The at least one first switching unit is preferably a switch for switching on and off a connection between two terminals of the switch. It is preferably realized by means of a button, a taster, a toggle switch, an electric switch such as a transistor, or by any other similar means. The same is true for the at least one second switching unit. The at least one first switching unit may, in some applications, be referred to as a "high side switch", while the at least one second switching unit may be referred to as a "low side switch". In the first state a voltage drop across the load resistance causes a current to flow through the load resistance. The voltage drop may be the result of having a voltage source or similar potential reservoir connected to the first terminal of the load resistance, and a drain such as a ground connected to the second terminal of the load resistance. In the second state there is not only no voltage drop across the load resistance, but also both terminals of the load resistance are not connected to any other components of the electric circuit, i.e. the load resistance is isolated to both sides. This may prevent damage from the load resistance. Further, in complex systems, the at least one first switching unit and/or the at least one second switching unit may be used for multiple purposes . The load resistance may be switched off if at least one the switching units is switched off. That is, depending on when the at least one first switching unit and/or the at least one second switching unit are switched off (po tentially for one of the mentioned further purposes) , no current flows through the load resistance.

The electric circuit comprises at least a first port and a second port. Via these two ports, the electric circuit may be included into a larger overall electric circuit. The larger overall electric circuit can, for example, be part of or connected to a control unit of an automobile. It may also be part of an electric power system of an automobile. Whenever the first electric potential is different from the second electric potential, a current may flow through the at least one load resistance depending on the states of the at least one first switching unit and the at least one second switching unit. Thereby, it is preferred that the first electric potential is higher than the second electric potential. It is preferred that the first electric potential is created by a voltage source such as a battery that is connected to the first port of the electric circuit (i.e. the positive terminal of the voltage source or the battery may be connected to the first port of the electric circuit) . However, any another realization of having the first port of the electric circuit at the first electric potential is equally possible. In particular, it is preferred that the larger overall electric circuit has an interface that provides the first electric potential to the first port of the electric circuit. The second electric potential may be generated in a similar way as the first potential. It is preferred that the second port of the electric circuit is connected to a negative terminal of the voltage source, the positive terminal of which being connected to the first port of the electric circuit. Alternatively, it is preferred that the second port of the electric circuit is grounded, i.e. that the second electric potential is zero. Also, it is preferred that the second electric potential is provided by an interface of the larger overall electric circuit.

If the first electric potential is higher than the second electric potential, a current may flow from the first port of the electric circuit through the at least one first switching unit (if switched on) , through the at least one load resistance, through the at least one second switching unit (if switched on) and eventually drain into the second port of the electric circuit (wherein the technical current direction is considered) . If at least one of the switching units is switched off, no such current flows.

As described above, an open circuit failure in an electric circuit comprising only the two switching units and the load resistance may not be detectable with one of the switching units being switched off. Overcoming this issue, the at least one auxiliary resistance comprised within the bypass line provides a pos sibility for the current to flow from the first port to the second port of the electric circuit even with the at least one second switching unit being switched off. As in many applications it is particularly the at least one second switching unit which is switched in a frequent (in particular periodical) manner, the bypass line with the at least one auxiliary resistance may ensure the detectability of open circuit failures in the electric circuit. Further, open circuit failures outside the electric circuit may be detectable . That is, whenever the first port and/or the second port of the electric circuit are not at the first and second electric potential, respectively, the current flowing through the at least one first switching unit deviates from what is expected. If, for example, the second port of the electric circuit is isolated from relevant parts of the larger overall electric circuit due to an open circuit failure within the larger overall electric circuit, the current cannot drain at the second port of the electric circuit. Such a failure may be detectable as well.

The at least one auxiliary resistance is preferably connected in parallel to the at least one load resistance and the at least one second switching unit by means of the bypass line. Therein, the bypass line may be a cable or other electric connection line. The at least one auxiliary resistance may be any resistor element or other electric device having an electric resistance. It is possible that the auxiliary resistance has an additional purpose besides ensuring the detectability of open circuit failures in the electric circuit. For example, the auxiliary resistance may be a (further) resistive heater. The auxiliary resistance may also be referred to as a test resistance or test load. Perferably, the resistance value of the auxiliary resistance is much higher than the resistance value of the load resistance. In particular, preferably it is at least ten times higher or even at least 100 times higher or 1000 times higher. This is in particular the case if the auxiliary resistance has only a test function. Then, the high resistance causes a small loss of electrical power through the auxiliary resistance.

It is preferred that there is exactly one first switching unit. Further, it is preferred that there is exactly one second switching unit. Further, it is preferred that there is exactly one load resistance. Further, it is preferred that there is exactly one auxiliary resistance. However, it is possible that there exists more than one first switching unit, second switching unit or load resistance. In case that there is more than one of these components it is possible that these components are connected in a serial connection or in a parallel connection. In a preferred embodiment of the electric circuit the at least one first switching unit is further configured for generating and outputting a feedback signal depending on the current flowing through the at least one first switching unit.

The feedback signal is preferably an electric signal, in particular an electric current. Further, it is preferred that the feedback signal is related to the strength of the current flowing through the at least one first switching unit. In particular, it is preferred that the feedback signal is proportional to the current flowing through the at least one first switching unit. The feedback signal is preferably generated by the at least one first switching unit, which preferably not only comprises a switching means, but also a current measuring means such as an amperemeter. A measurement signal outputted by the current measuring means is preferably converted into the feedback signal . The feedback signal may be either a digital or an analog signal. The feedback signal may be outputted from the at least one first switching unit via a feedback line. Due to the functionality of generating and outputting the feedback signal the at least one first switching unit may also be referred to as an intelligent (high side) switch.

With the feedback signal representing the current flowing through the at least one first switching unit, an open circuit failure may be revealed in the feedback signal. Hence, an open circuit failure may be detected by monitoring the feedback signal.

In a further preferred embodiment of the electric circuit the at least one first switching unit is connected to a microcontroller, wherein the microcontroller is configured for receiving and processing the feedback signal, and for controlling the at least one first switching unit.

The microcontroller may be any computer chip or electric circuitry, potentially equipped with a software, that is capable of controlling the at least one first switching unit and of receiving and processing the feedback signal. That is, it is preferred that the microcontroller is connected to the feedback line. Further, it is preferred that the microcontroller is capable of converting the feedback signal into a format suitable for further processing. The microcontroller may be an On Board Diagnosis system of an automobile or it may be a part of such an On Board Diagnosis system.

In particular, it is preferred that the microcontroller is configured for monitoring the feedback signal and for detecting an open circuit failure from the feedback signal. It is further preferred that the microcontroller is configured for triggering actions once an open circuit failure has been detected. Such actions preferably include at least one of the following: switching off the at least one first switching unit, switching off the at least one second switching unit, switching off a further electronic component (such as a voltage source that provides the first electric potential) , generating an error signal (in particular for processing within the larger overall electric circuit) , directly emitting a signal (such as an optical or acoustical signal accessible for an operator or control person) , activating a preferably provided substitute for the electric circuit (or parts of it) and causing a recording device to record the detected failure for later analysis.

The controlling of the at least one first switching unit by the microcontroller preferably includes the above mentioned switching off of the at least one first switching unit as a result of an open circuit failure. Further, it is preferred that the microcontroller is configured for causing the at least one first switching unit to be switched whenever this is required. This may depend on the particular application. Therefore, the micro controller preferably processes information. This information may optionally involve inputs from external sources such as measurement values obtained by gauges, or commands generated by other electric components to which the microcontroller is connected to. When the at least one first switching unit is scheduled to be switched (for example due to a time dependent switching schedule stored within the microcontroller) or otherwise required to be switched (for example as a result of processing information or commands as described above) , the microcontroller preferably initiates the switching.

In a further preferred embodiment of the electric circuit the at least one second switching unit is connected to a control circuit, wherein the control circuit is configured for controlling the at least one second switching unit.

It is preferred that the control circuit is configured for switching the at least one second switching unit on and off frequently, in particular periodically. Any on/off signal can be applied to the at least one second switching unit. In particular, it is preferred to have a pulse width modulated (PWM) -signal being applied to the at least one second switching unit by the control circuit. With the frequent or periodic switching of the at least one second switching unit the power in the at least one load resistance can be controlled. Preferably, the at least one second switching unit is realized by means of a transistor having a gate terminal. The control circuit then is preferably connected to the gate terminal of the transistor. The control circuit is preferably controlled by the microcontroller . Alternatively, the control circuit preferably operates autonomously, in particular based on temperature sensing. Temperature sensing means may be a part of the control unit for this purpose.

The controlling of the at least one second switching unit by the control circuit preferably includes the above mentioned switching off of the at least one second switching unit as a result of an open circuit failure. Further, it is preferred that the control circuit is configured for causing the at least one second switching unit to be switched whenever this is required (de pending on the particular application) . Therefore, the control circuit preferably processes information (optionally involving inputs from external sources such as measurement values obtained by gauges, or commands generated by other electric components the control circuit is connected to, including the microcontroller) . When the at least one second switching unit is scheduled to be switched (for example due to a time dependent switching schedule stored within the control circuit) or otherwise required to be switched (for example as a result of processing information or commands as described above) , the control circuit preferably initiates the switching.

It is preferred that the at least one second switching unit, the at least one load resistance, the control circuit and the at least one auxiliary resistance are comprised within a load unit. Further, one or more sensing probes are preferably included within the load unit.

Therein, the functionality of the listed devices is preferably not altered. With the listed devices being comprised within the load unit, only a single device (i.e. the load unit) has to be handled. If an open circuit failure is detected, the load unit can be replaced. This may be more efficient and less expensive than analyzing and optionally repairing the single components. Also, in cases where originally only a load resistance and a control circuit were used without the bypass line with the auxiliary resistance, the old load resistance and the old control circuit may be replaced by the load unit (which, therefore, preferably has an appropriate size, shape and connectability) . That way, the detectability of open circuit failures may be easily retrofitted into existing systems.

The invention may be applied in a resistive heater for selective catalyst reduction (SCR) systems, wherein the resistive heater is comprised as the load resistance within an electric circuit as described.

The details and advantages disclosed for the electric circuit are applicable to the resistive heater, and vice versa. Selective catalytic reduction (SCR) is commonly used in au tomobiles for reducing pollutant substances in the exhaust gas. In particular, SCR units are used to reduce nitrogen oxides (NO, N02) . Therefore, urea is introduced into the exhaust gas system. Thereby, particularly ammonia is included in the process. For efficiently reducing nitrogen oxides, the ammonia is preferably vaporized. Therefore, a resistive heater may be used. In this context, the control circuit preferably causes the at least one second switching unit to be switched in such a way that the heating power generated by the resistive heater corresponds to the heat required for vaporizing a respectively needed amount of ammonia. In particular it is preferred that the control circuit comprises or is connected to a sensor for detecting the temperature generated by the resistive heater. Further, the control circuit preferably comprises or is connected to means for detecting the required amount of vaporized ammonia. Alternatively and/or additionally, the control circuit is preferably coupled to a control unit of the automobile. As commonly there is only one switching unit provided in an SCR system, the above described embodiment comprising the load unit is preferred. In particular in this context, the load unit may be used instead of the load resistance and the control circuit as described above.

A further important application of the electric circuit in the field of selective catalytic reduction (SCR) are resistive heaters as load resistance which have the propose to thaw frozen reducing agent and/or which prevent a freezing of liquid reducing agent. Such liquid reducing agent is often use as ammonia precursor. The reducing agent is used to produce ammonia inside the exhaust system or exhaust system externally in an ammonia generator for this purpose. An important and well known ammonia precursor is AdBlue which is an urea water solution with an urea fraction of 32.5 %.

According to a further aspect of the present invention a method is provided for detecting an open circuit failure in an electric circuit as described. The method comprises at least monitoring a monitored current, which is the current flowing through the at least one first switching unit, wherein an open circuit failure is detected by a deviation of the monitored current from an expected value.

The details and advantages disclosed for the electric circuit and the resistive heater are applicable to the method and vice versa.

Monitoring the monitored current is preferably performed by using the microcontroller. Thereby, the value of the monitored current may be compared to a predefined expected value, which, preferably is stored within the microcontroller. Whenever the monitored current falls below the expected value, this may be an indication for an open circuit failure. Therein, the open circuit failure does not have to be realized in terms of a complete opening of the circuit (such as a wire breakage) . Also, a significant increase in the circuit resistance may be detectable . Preferably, not only the open circuit failure itself, but also the amount of resistance increase is detected (and optionally recorded and/or further processed) .

According to a preferred embodiment of the method, the monitored current is detected by means of a feedback signal generated and outputted by the at least one first switching unit.

As described above, the feedback signal is preferably directly related to the monitored current. That is, the value of the monitored current (strength) can be calculated from the feedback signal. The feedback signal is preferably used for transmitting the information about the value of the monitored current (strength) from the at least one first switching unit to the microcontroller, where it can be further processed.

According to a further preferred embodiment the method further comprises performing a diagnostics sequence, wherein the di agnostics sequence comprises at least the following steps:

A) switching the at least one first switching unit, B) switching the at least one second switching unit,

C) from a first point of time until a second point of time, detecting the monitored current with the at least one first switching unit being in its on state and the at least one second switching unit being in its off state,

wherein the diagnostics sequence is followed by a regular operation of the electric circuit.

As in step C) the at least one first switching unit is supposed to be in its on state and the at least one second switching unit is supposed to be in its off state, steps A) and B) are previously performed as necessary. That is, if the at least one first switching unit is not already in its on state, step A) is performed to switch the at least one first switching unit on. Further, if the at least one second switching unit is not already in its off state, step B) is performed to switch the at least one second switching unit off.

The monitored current detected in step C) has a value corre sponding to only the bypass line with the auxiliary resistance being accessible (i.e. no current can flow through the load resistance) . The value of the current strength of the monitored current detected in step C) can be useful in further operation because the monitored current can only fall below this value if there is a failure, in particular an open circuit failure. That is, the value of the current strength of the monitored current detected in step C) corresponds to the above described expected value for monitored current (that corresponds to the feedback signal) . Hence, it is preferred that value received during step C) is stored (preferably within the microcontroller) and af terwards used as the expected value. If the electric current in step C) is not changing this indicates a short circuit to ground. If the electric current in step C) drop to zero this indicates an open circuit/open loop.

In the regular operation of the electric circuit, the switching units are switched no longer according to the specifications of the diagnostics sequence. In particular, the above described frequent (or periodical) switching of the at least one second switching unit is preferably comprised within the regular operation of the electric circuit.

According to a further preferred embodiment of the method the diagnostics sequence further comprises the following steps: a) switching the at least one first switching unit,

b) switching the at least one second switching unit, c) from an initial point of time until the first point of time, detecting the monitored current with both the at least one first switching unit and the at least one second switching unit being in their on states.

The initial point of time lies preferably before the first and second points of time. Steps a), b) and c) are preferably performed prior to steps A) , B) and C) . Therein, steps a) and b) are performed as needed, similar to what has been described for steps A) and B) . That is, depending on the initial states of the first and second switching units, steps a) and b) are performed in order to bring both the at least one first switching unit and the at least one second switching unit into their on states at the initial point of time. However, it is preferred that before the initial point of time at least the at least one first switching unit is in its off state, so that no current at all can flow through the electric circuit.

With both the at least one first switching unit and the at least one second switching unit being in their on states, both the load resistance and the auxiliary resistance may be flown through by a current. That is, in step c) the maximal current value for the monitored current may be detected. This information may also be valuable in further processing. In particular, the difference between the current strengths detected in steps c) and C) can be used for setting a threshold value for triggering the open circuit failure detection. Further, step c) may initially ensure that there is no failure in the line comprising the load resistance. According to a further preferred embodiment of the method, the diagnostics sequence further comprises the following steps:

D) switching the at least one second switching unit,

E) from the second point of time until the third point of time detecting the monitored current with both the at least one first switching unit and the at least one second switching unit being in their on states.

The third point of time lies preferably after the initial, first and second points of time. Steps a), b) and c) are preferably performed prior to steps A) , B) and C) . Therein, step D) is performed as needed, similar to what has been described for steps A) , B) , a) and b) . With no other steps being performed between steps C) and E) , in step D) the at least one second switching unit has to be switched on. In any case, after step D) the at least one second switching unit is supposed to be in its on state at the second point of time. Preferably from the third point of time onwards, the regular operation of the electric circuit is performed .

Between the second point of time and the third point of time the maximal current as described for step c) may be detected again. This may be valuable for an end of line testing.

Preferably, steps a) and b) happen at a point in time named tO . Preferably, steps A) and B) happen at a point in time named tl . Step c) is active during a time period between tO and tl.

Furthermore preferred, Step D) happens at a point in time named t3. Step c) is active during a time period between tO and tl. Step C) is active during a time period between tl and t2. Subsequent step E) is active during a time period after step t2. End of step E) can be defined by a point in time named t3.

Purpose of step c) (in the period tO to tl) is to make sure that the load is working as intended - it can be switched on and there is no open loop. Purpose of step C) (in the period tl to t2) is to make sure that the second switch can switch off and there is no open loop and nor short circuits to ground. From t2 and onwards the circuit is operating as intended and the current level could be the highest value (second switch is on state) or the lowest value (second switch is off state) .

It should be noted that the individual features specified in the claims may be combined with one another in any desired tech nological reasonable manner and form further embodiments of the invention. The specification, in particular in connection with the figures, explains the invention further and specifies particularly preferred embodiments of the invention. Partic ularly preferred variants of the invention, and also the technological field, will now be explained in more detail on the basis of the enclosed figures. It should be noted that the exemplary embodiments shown in the figures are not intended to restrict the invention. The figures are schematic and may not be to scale. The figures display:

Fig. 1: a circuit diagram of an electric circuit according to the present invention,

Fig. 2: the monitored current as a function of time during the diagnostics sequence.

Fig. 1 shows an electric circuit 1, which is comprised within a selective catalyst reduction (SCR) system 11. The electric circuit 1 comprises a first switching unit 2, which is connected to a positive terminal of a voltage source 12, which is a first port 16 of the electric circuit 1. Further, the first switching unit 2 is connected to a first terminal 18 of a load resistance 4. The first switching unit 2 is further connected to a mi crocontroller 7. This is indicated by dotted arrows, one of which indicates that a feedback signal is transmitted from the first switching unit 2 to the microcontroller 7, another of which indicates that the microcontroller 7 may control the first switching unit 2. The load resistance 4 is realized as the resistive heater 10. Further, a second terminal 19 of the load resistance 4 is connected to a second switching unit 3, which is realized as a transistor 14. The second switching unit 3 is connected to a ground 13, which is a second port 17 of the electric circuit 1. The second switching unit 3 is connected to a control circuit 8. The connection is realized by the control circuit 8 being connected to a gate terminal 15 of the transistor 14. The second switching unit 3 and the load resistance 4 are bypassed by a bypass line 6, which comprises an auxiliary resistance 5. The second switching unit 3, the load resistance 4, the control circuit 8 and the auxiliary resistance 5 are comprised within a load unit 9.

Fig. 2 is a plot of the monitored current, which is the current flowing through the first switching unit, as a function of time. The abbreviation "a.u." stands for "arbitrary units". At an initial point of time tO both the first switching unit and the second switching unit are switched into their on states (before, at least the first switching unit was in its off state) . Ac cordingly, the current may flow through both the load resistance and the auxiliary resistance. The result is a maximum current strength. At the first point of time tl the second switching unit is switched off. Accordingly, the monitored current drops to a value between 0 and the maximum value previously detected. This is the case because only the auxiliary resistance is accessible for the current. At a second point of time t2 the second switching unit is switched on again. The current accordingly rises to its maximum value again. At a third point of time t3 a regular operation of the electric circuit begins. This is not shown further. The period of time between the initial point of time tO and the third point of time t3 is the diagnostics sequence.