EP3855467B1 - Method and device for controlling a relay - Google Patents

Description

Area of application and state of the art

The invention relates to a method for controlling a relay with relay contact, preferably a conventional relay. Furthermore, the invention relates to a device which is designed to carry out this method and which has the relay, preferably also a load to be connected from the relay to a switching voltage, and connections to a controller in order to control the relay, which preferably has a microcontroller .

From the DE 19916778 A1 a method for controlling the switching operations of load switches, for example a contactor, is known. The current flow is monitored.

Another method for improved production of a switching connection is from DE 102005051762 A1 known. A switch-on delay and a switch-off delay can be recorded.

From the US 2017/229269 A1 a method and a system for controlling the timing of activation of a relay are known. The voltage or current is monitored on a relay. In order to be able to switch as close as possible to the voltage zero crossing, a switching delay is recorded. A signal for switching the relay is generated offset by the detected switching delay so that switching occurs at zero voltage crossing if possible.

From the US 2005/012505 A1 It is known to influence a switching process on a relay, for which a microcontroller is used. The microcontroller can control the relay in such a way that it switches when a certain voltage is present, for example at zero voltage crossing.

From the WO 2005/074321 A2 A hob is known as an electrical household appliance in which the power of heating devices for hotplates is switched using relays. A control with a microcontroller is provided to control the relays. This means that these relays can be switched as desired, especially for clocking the heating devices. This involves switching relays as de-energized as possible, but not necessarily switching at a zero voltage crossing.

It is difficult to improve such a control of a relay in practical operation with little effort and high safety in operation.

Task and solution

The invention is based on the object of creating a method mentioned at the beginning and a device mentioned at the beginning designed to carry out the method, with which problems of the prior art can be solved and in particular it is possible to be able to control a relay safely and reliably as well as a Opening or closing of the relay contact should, if possible, be achieved at zero switching voltage crossing, whereby the design effort should preferably remain small.

This object is achieved by a method with the features of claim 1 and by a device with the features of claim 10. Advantageous and preferred embodiments of the invention are the subject of the further claims and are explained in more detail below. Some of the features are described only for the method or only for the device. However, they should be able to apply independently and independently of each other to both a method and a device. The The wording of the claims is incorporated into the content of the description by express reference.

The procedure requires the following steps to be carried out. In general, switching is timed as an opening process or as a closing process on the relay with regard to precise opening and precise closing of the relay contact in comparison to a switching voltage zero crossing of a switching voltage to be switched. The switching voltage to be switched is advantageously monitored and it is also monitored whether the relay contact is open or closed. This is determined as a stop or start of a switching current flow, which is therefore also recorded or monitored indirectly by monitoring the voltage change. The duration between activating the relay with a switching trigger signal and an actual opening or closing of the relay contact is referred to as the switching delay and is recorded or determined. In practice it can be a few milliseconds. With a frequency of the switching voltage to be switched at 230 V of 50 Hz as the normal mains frequency, a half wave lasts 10 msec. The switching delay is therefore preferably less than the duration of a half wave.

The switching voltage to be switched by the relay is therefore monitored or recorded, preferably permanently. After a command for switching has been released, the relay is controlled with a switching trigger signal in order to switch or to open or close the relay contact. However, the relay is activated with the switching trigger signal before the or a switching voltage zero crossing, namely shifted forward in time by the switching delay mentioned or earlier than a switching voltage zero crossing occurs or as the next or following switching voltage zero crossing follows. If the duration of the switching delay is longer than the duration of a half wave, then after the relay has been activated, the opening or closing process will only be carried out with the actual opening or closing of the relay contact at the next but one switching voltage zero crossing.

Then the actual switching time, i.e. when the relay contact is opened or closed and the switching current flow stops or starts, is recorded. In addition, the switching voltage zero crossing is recorded. The switching time actually carried out and the switching voltage zero crossing are then placed in relation to each other or it is checked whether the actual switching time took place before or after the switching voltage zero crossing. Here, a so-called switching difference is determined as a duration, whereby the actual switching time was before or after the switching voltage zero crossing by this switching difference or occurred too late or too early. Only if the switching difference were zero and the actual switching time occurred at the same time as the switching voltage zero crossing the desired optimal switching would have occurred. For subsequent similar switching, the switching delay is corrected with this switching difference. Similar switching here means switching off or on or an opening process or a closing process. During the correction, the switching difference is added to the switching delay if the actual switching time was too late by the switching difference or only occurred with this duration after the switching voltage zero crossing. If the actual switching time was too early before the switching voltage zero crossing or by this duration of the switching difference, the switching difference is deducted from the switching delay. For the subsequent similar switching, a significantly smaller switching difference or a significantly smaller difference between the actual switching time and the switching voltage zero crossing can then be assumed; ideally, they are exactly the same.

In the invention, a microcontroller is used to detect the switching or the opening process and the closing process as well as the switching voltage zero crossing. Various aforementioned values can also be saved there. The signals are advantageously recorded digitally on the microcontroller, i.e. at an A/D input, and converted or processed there.

The invention therefore makes it possible for switching to occur as accurately as possible at the switching voltage zero crossing. It is therefore easy to adapt to different relays as well as to possibly different switching behavior during the opening and closing processes.

In order to protect a relay or its relay contact as much as possible when switching, it can advantageously be provided that the switching difference is corrected so precisely that switching takes place within a time range of ± 0.4 msec around the switching voltage zero crossing. This is considered sufficient to avoid an arc when switching, so that a relay contact lasts significantly longer or can be used. This is guaranteed with the switching range mentioned of ± 0.4 msec around the switching voltage zero crossing.

In an advantageous embodiment of the invention, ohmic loads are switched during switching or during the method. Preferably, only ohmic loads are switched because the current and voltage are exactly in phase. This makes the process much easier.

In an embodiment of the invention it can be provided that the possible switching delays are recorded separately. A closing switching delay and an opening switching delay can therefore be determined separately for the closing process and the opening process become. These two delays do not necessarily have to be the same and can differ by up to 50% or up to 70%. The opening process is usually faster than the closing process. This is one of the reasons why the two switching delays are recorded and used separately.

In the method, it can be provided that after a command for switching has been released or after activation with the switching trigger signal, the relay is shifted forward in time by the switching delay or is activated earlier than a subsequent switching voltage zero crossing by the duration of the switching delay. Switching thus takes place within a duration of a half-wave of the switching voltage and after a last detected switching voltage zero crossing. An alternative calculation can provide that the duration of the switching delay is subtracted from the duration of a half-wave and the relay is activated with the switching trigger signal by this difference or switching time after the switching voltage zero crossing, so that switching actually takes place in the subsequent switching voltage zero crossing if possible . The result is the same.

In the invention, a closing process and an opening process are carried out alternately at the end of a positive half-wave and at the end of a negative half-wave of the switching voltage. This means that the closing takes place alternately after the end of a positive half-wave and then a negative half-wave and then another positive half-wave. The opening also takes place alternately after the end of a positive half-wave and then a negative half-wave. This can reduce what is known as material migration at the relay contact, which would permanently damage the relay contact. Every similar closing process and every similar opening process is therefore carried out with directly alternating current directions, whereby this material migration is reduced or avoided.

In practice, it can be provided for a method that after an electrical device containing the relay is switched on for the first time, this relay is switched once at any time at any time, in particular is switched exactly once at any time. The duration of the switching delay of this relay can be roughly recorded as the initial switching delay. Subsequently, the next time the relay switches, usually a closing process, this initial switching delay is used as the duration for triggering the switching trigger signal at the relay before a switching voltage zero crossing. Then, as explained above, the switching difference is recorded as the time difference between the actual switching time and the switching voltage zero crossing. This corrects the initial switching delay as described above and obtains a switching delay that replaces the initial switching delay is used for further operation of the electrical device when the relay is switched on. This corrected switching delay of the relay is then used for a subsequent opening process of the relay. Since, as explained above, the opening and closing of the relay contact can occur at different speeds, a noticeable switching difference will again occur between the actual switching time and the switching voltage zero crossing. With this switching difference, the aforementioned switching delay is corrected in order to obtain an opening switching delay. The next time the relay is switched as a closing process, i.e. when it is to be closed, the switching difference is recorded again and the switching delay is corrected in order to obtain a closing switching delay. The relay can then be further activated using these two switching delays.

It can be provided that the switching delay is only corrected at certain intervals by means of a detected switching difference, for example every fifth, every tenth or every fiftieth switching and not every switching. Alternatively and advantageously, the switching delay is corrected for each switching or each subsequent opening and closing process of the relay. Actually, after two or three corrections, there should be no or no significant switching difference, at least none above the aforementioned 0.4 msec. However, under certain circumstances it is possible that the switching delay changes slightly due to heating of the relay.

In a possible further embodiment of the invention, a switching delay from earlier switching, in particular in the first hours of operation of the electrical device with the relay in it, can be saved. This is advantageously done for both an opening process and a closing process. In the event that currently recorded values for the switching delay deviate from this by more than 20%, alternatively by more than 30% or 40%, a fault is detected in the relay or in a control for the relay. Such an error can be signaled to an operator visually and/or acoustically. Under certain circumstances it can even be provided that a closing process of the relay is prevented, so that it is not switched on at the electrical device. Such a deviation from currently recorded values for the switching delay by more than the aforementioned proportion, especially if the currently recorded switching delay lasts correspondingly longer, is a strong indication of a fault in the relay with a possible risk of sudden failure, so that the relay can no longer open, for example can. That's why it shouldn't be turned on in the first place.

It is possible that the microcontroller uses between 1,000 and 20,000 measuring points per second as the sampling frequency. These are preferably between 5,000 and 8,000 measuring points. This means that measurements are taken approximately every 150 µsec or every 160 µsec or there is this time interval between two measuring points. This is well below the aforementioned target of 0.4 msec as the desired upper limit for a time interval from the switching voltage zero crossing. When using the microcontroller, digital recording can be made to the sense line of the microcontroller. The microcontroller then triggers the relay with the switching trigger signal.

An electrical device that is designed to carry out the method is advantageously an electrical household appliance or a cooking appliance. The method is therefore integrated into its control with a microcontroller, for example in a hob. The method can advantageously be used for switching any type of ohmic load, including in an oven or for use in industry.

The division of the application into individual sections and subheadings does not limit the general validity of the statements made under them.

Brief description of the drawings

Fig. 1

eine schematische vereinfachte Darstellung eines Kochfelds zur Durchführung des erfindungsgemäßen Verfahrens mit einer Steuerung als erfindungsgemäßer Vorrichtung,

Fig. 2

eine Darstellung einer Verschaltung in der Steuerung mit Taktrelais und Trennrelais,

Fig. 3

einen weiteren Teil der Verschaltung mit Anschluss der Schaltung aus Fig. 2 an einen Mikrocontroller der Steuerung,

Fig. 4

einen zeitlichen Verlauf der Schalt-Spannung, eines Ansteuersignals für eines der Relais sowie eines Kontrollsignals zum ersten Ermitteln der Schaltverzögerungen,

Fig. 5

den zeitlichen Verlauf der Schalt-Spannung mit Anwendung der ermittelten Schaltverzögerungen, um möglichst im Schalt-Spannungsnulldurchgang zu schalten und

Fig. 6

eine vergrößerte Darstellung des Schaltbereichs und der Schaltdifferenz.

Embodiments of the invention are shown schematically in the drawings and are explained in more detail below. Shown in the drawings:

Fig. 1

a schematic simplified representation of a hob for carrying out the method according to the invention with a control as a device according to the invention,

Fig. 2

a representation of a connection in the control with clock relays and isolating relays,

Fig. 3

another part of the circuit with connection of the circuit Fig. 2 to a microcontroller of the control,

Fig. 4

a time course of the switching voltage, a control signal for one of the relays and a control signal for first determining the switching delays,

Fig. 5

the time course of the switching voltage with application of the determined switching delays in order to switch, if possible, at the switching voltage zero crossing and

Fig. 6

an enlarged view of the switching range and the switching difference.

Detailed description of the exemplary embodiments

In the Fig. 1 A hob 11 is shown schematically and in simplified form with a hob plate 12. Under the hob plate 12, for example, two heating devices 13a and 13b are arranged, which are designed here as known and usual radiant heating devices. They are therefore resistance heaters and therefore represent an exclusively ohmic load. Above the right heating device 13a, a pot 14 is placed on the hob plate 12 in order to be heated.

The hob 11 has a control 16 in order to be operated, in particular in order to supply the heating devices 13a and 13b with power during operation. The control 16 has an operating device 18 on the underside of the hob plate 12, advantageously designed with operating elements in the form of touch switches and displays in the form of LED or illuminated displays. Furthermore, the controller 16 has a microcontroller 20 as so-called intelligence. The microcontroller 20 controls clock relays 22a and 22b, which each switch the switching voltage for the heating devices 13a and 13b and thus take over the power supply. Furthermore, an isolating relay 23 is provided and a switching power supply 28, possibly designed as a switched mode power supply. For power supply, the control 16 has a connection cable 25 with a plug 26 at the free end as a power connection to 230 V/50 Hz. This represents the electrical connection of the hob 11, which is normally made in a junction box.

In the Fig. 2 A circuit diagram is shown with a conductor L and a neutral conductor of the switching voltage U, which virtually correspond to the connecting cable 25. The conductor L goes to a cut-off relay 23, which essentially serves to be opened in emergencies and to switch off the hob 11, for example because a serious error has been detected. Such a switching process is very rare. Otherwise, the isolating relay 23 only switches in the de-energized state, so that its control or switching behavior is not critical for the subject of the present application. A resistor R1 is connected to the isolating relay 23 in the upper branch upper circuit connected. Using two diodes, the signal lies behind the resistor R1 between ground and the potential of the neutral conductor N. Another resistor R2 and a capacitor C1 are then connected in parallel to ground. The S_DLB signal applied to it is required, especially for dynamic load balancing.

The clock relay 22 is arranged in the lower branch of the neutral conductor N. In the lower branch there is also a resistor R3 and a diode connection similar to that in the upper branch. Furthermore, the lower branch is connected to ground with a capacitance C2 and to the neutral conductor N or its potential with a resistor R4. A signal S_Takt can be tapped here.

Above all, however, a heating device 13 is connected as an ohmic load between the two branches behind the clock relay 22 and the isolating relay 23. If both relays 22 and 23 are closed, the heating device 13 is connected to the switching voltage and is operated or heated with it. To set a specific power in the clocking process, as is common for radiant heating devices in a hob, the microcontroller 20 controls the clock relay 22 to open and close its relay contact based on a power specification from an operator on the operating device 18 or based on a cooking program. The clock relay 22 switches the switching voltage between L and N. The isolating relay 23 is always closed. It is only opened by the microcontroller 20 in the aforementioned serious incident.

In the Fig. 3 is now shown how the switching power supply 28 is also connected to the neutral conductor and to the conductor L according to the switching voltage. The switching power supply 28 supplies the microcontroller 20 with a supply voltage in the usual way. This can be chosen differently, for example 3.3 V or 5 V, depending on how the microcontroller 20 is designed. Behind the switching power supply you can see that the upper branch is at the potential of the neutral conductor N, while ground compared to this potential of N is then at -3.3 V or -5 V. This results in the aforementioned supply voltage at the microcontroller 20. This negative ground, so to speak, also applies to the interconnection of the Fig. 2 . The microcontroller 20 in turn is connected to the isolating relay 23 and to the clock relays 22a and 22b, as well as to other clock relays if they are present.

The two signals S_DLB and S_Takt are also fed into the microcontroller 20, namely the signal S_Takt on a digital senseline. The S_DLB signal is also mandatory as it is necessary for zero crossing detection. The signals serve to like from the Fig. 2 It can be seen that the microcontroller 20 can, on the one hand, monitor the switching voltage U in order to carry out the method according to the invention and, on the other hand, sees or detects when the clock relay 22 switches or when its actual switching time occurs.

In the Fig. 4 The curve for the switching voltage U is now shown over time t, below that for a switching trigger signal and again below that for the signal S_Takt. The switching voltage U here is 230 V and 50 Hz. At the beginning of the method, the microcontroller 20 switches the clock relay 22 with a switching trigger signal at any time 1, so that the clock relay closes or a closing process takes place on its relay contact. After a certain closing switching delay SV _on , which can last for example 7 msec, the clock relay or its relay contact is actually closed and the switching current flow has started. This can be seen in the S_Takt signal, when it is at 1 again. During the closing process it is at zero.

This switching delay SV _on is detected by the microcontroller 20 and stored for use in the next closing process on the clock relay 22.

At an already planned time of opening the clock relay 22, for example because this is desired due to a power level of the power supply for the heating device 13, here as time 3, the microcontroller 20 generates a switching trigger signal for opening the clock relay as an opening process at the relay contact. For example, this occurs at the zero crossing of the switching voltage, but this does not have to be the case. At time 4, after an opening switching delay SV _off , the relay contact is actually opened or the switching voltage is switched, the switching current flow has stopped. This switching delay SV _off is around 5 msec and is therefore slightly shorter than the switching delay when switching on SV _on . The microcontroller 20 detects the switching delay SV _off of the opening process. On the one hand, the heating device 13 is now switched off. On the other hand, this can be recognized by the S_Takt signal. This switching delay is saved for next use.

The jump in the S_Takt signal when the clock relay actually closes results from the relay contact bouncing during the closing process. This is not taken into account.

In the Fig. 5 now shows how these switching delays are actually used, for example the next time the clock relay switches. If the power supply to the heating device is to be switched on again, this will occur before a zero crossing the switching voltage, here from negative to positive, is shifted forward by the switching delay SV _on , a switching trigger signal is generated at the clock relay. In practice, this can also be easily achieved by the microcontroller 20 subtracting this switching delay SV _on from the duration of a half wave, thereby resulting in a switch-on time t _on . In the oppositely polarized zero crossing of the switching voltage U, the microcontroller 20 waits for this switch-on time t _on after the zero crossing from positive to negative in order to then give the switching trigger signal to the clock relay, i.e. to cause it to switch. This is again time 1. The clock relay begins the closing process at the relay contact, which is completed after a certain period of time, which is approximately the aforementioned switching delay SV _on . The actual switching time is now recognized in the S_Takt signal when it increases again. Here this is a little before the zero crossing shown in dashed lines, but still within the desired switching window SB, which can be around 800 µsec. The microcontroller 20 recognizes this switching difference between the actual switching time and the switching voltage zero crossing, which can be 200 µsec, for example. The clock relay switched in the switching range SB, but slightly before the actual switching voltage zero crossing, so that the switching delay is corrected. The switching difference mentioned is subtracted from the switching delay SV _on , so the switching trigger signal for the closing process will be triggered a little later the next time or the switch-on time t _on will be slightly longer until the switching trigger signal is sent to the clock relay.

After a certain time, the microcontroller 20 should open the clock relay 22 again as part of the power supply. A switching trigger signal for the opening process should therefore be generated or given to the clock relay 22 by the switching delay SV _off before a voltage zero crossing, here from negative to positive. For this purpose, the microcontroller 20 subtracts the switching delay SV _off from the duration of a half-wave of the switching voltage and obtains the switch-off time t _off . After the oppositely polarized switching voltage zero crossing from positive to negative, the switch-off time t _off is waited and then the switching trigger signal to open the relay contact is generated at time 3.

After the switching delay SV _off has expired at time 4, the relay contact is opened or is actually opened and the switching current flow stops. This switching can also be recognized in the S_Takt signal; it is also within the desired switching range SB. However, the actual switching time is slightly before the switching voltage zero crossing, which again results in a switching difference. This switching difference is subtracted from the switching delay SV _off , which therefore becomes shorter, so that the switching trigger signal is generated a little later during the next opening process or less long before the switching voltage zero crossing, This makes the switch-off time t _off slightly longer. If the actual switching time were a little after the switching voltage zero crossing, the resulting switching difference would be added to the switching delay, and the switching trigger signal would be generated a little earlier during the next opening process.

The switching delays SV _on and SV _off corrected in this way are used for the next closing process and the next opening process. The respective switching difference can then be recorded again and the switching delays SV _on and SV _off can be corrected again. Furthermore, the microcontroller 20 can take into account that the next closing process and also the next opening process take place at the oppositely polarized switching voltage zero crossing, i.e. from positive to negative. Then switching may occur a half-wave later, but this does not represent any significant difference or problem for the power supply.

In the Fig. 6 is greatly enlarged accordingly Fig. 5 shown how switching actually takes place after the switching delay SV _on has expired at time 2. This occurs slightly earlier than the switching voltage zero crossing, which is shown in dashed lines. The switching difference SD can be around 100 µsec here and is still well within the switching range SB or the actual switching time is within this switching range SB around the switching voltage zero crossing. The switching difference SD shown is added to the switching delay SV _on as the aforementioned correction because the actual closing process on relay 22 actually occurred a little too early.

The special feature of the invention with the simple interconnection according to Figs. 2 and 3 as well as the simple control or wiring of the microcontroller 20 show how the invention can be advantageously implemented. The wiring effort according to Fig. 2 is low. The above-described continuous correction of the switching delays SV _on and SV _off ensures that the switching time is actually carried out as close as possible to the switching voltage zero crossing, i.e. with a low switching current, at least in the aforementioned switching range SB of less than 1 msec or less than 0.8 msec .

Furthermore, it can be provided that not only the previous and the new or corrected switching delay are stored in the microcontroller 20, but also previous switching delays. These switching delays are advantageously stored from the first time the hob 11 is put into operation, so that a history of the duration of these switching delays can be recorded over a very long period of time. For this purpose, it may be sufficient if not every value for switching delays is saved, but rather just each value, for example tenth, every fiftieth or every hundredth switching delay. Over a period of several months or even several years of operation of the hob 11, it can then be monitored how the switching delay behaves in the long term. As a rule, it will probably increase somewhat. However, if this increase becomes too strong or the rate of increase increases too much, for example exceeds certain predetermined limit values, the controller 16 or the microcontroller 20 can generate a type of warning or a request to replace a specific clock relay.

Claims (11)

1. A method of driving a relay (22a, 22b) having a relay contact, wherein the following steps are performed:

   - a switching as an opening operation or as a closing operation on the relay (22a, 22b) is detected in time with respect to accurate opening or closing of the relay contact compared to a switching voltage zero crossing of a switching voltage (U) to be switched,

   - the duration between energizing the relay (22a, 22b) with a switching trigger signal and an actual opening or closing of the relay contact with stop or start of a switching current flow is detected as a switching delay (SV_on, SV_off),

   - the switching voltage (U) to be switched by the relay (22a, 22b) is monitored,

   - after release of a command for switching, the relay (22a, 22b) is triggered for switching with a switching release signal after being shifted forward in time by the switching delay (SV_on, SV_off) before a subsequent switching voltage zero crossing,

   - the actual switching time is detected in addition to the switching voltage zero crossing,

   - it is checked whether the actual switching time was too late or too early by a switching difference (SD) relative to the switching voltage zero crossing, the switching delay (SV_on, SV_off) being corrected with the switching difference (SD) for a subsequent similar switching:

   - by adding the switching difference (SD) to the switching delay (SV_on, SV_off) if the actual switching time was too late by the switching difference (SD) relative to the switching voltage zero crossing, or

   - by subtracting the switching difference (SD) from the switching delay (SV_on, SV_off) if the actual switching time was too early by the switching difference (SD) relative to the switching voltage zero crossing,

   - wherein a microcontroller (20) is used for detecting the switching or opening and closing operation and the switching voltage zero crossing,

   characterized in that:
   an opening operation and a closing operation are performed alternately at the end of a positive half-wave and at the end of a negative half-wave of the switching voltage (U), respectively.
2. Method according to claim 1, characterized in that switching takes place in a time range of +/- 0.4 msec around the switching voltage zero crossing.
3. Method according to claim 1 or 2, char characterized acterised in that resistive loads (13a, 13b) are switched during the switching.
4. Method according to one of the preceding claims, characterized in that a closing switching delay (SV_on) and an opening switching delay (SV_off) are determined separately for the closing operation and the opening operation, respectively.
5. Method according to one of the preceding claims, characterized in that the relay (22a, 22b), after release of a command for switching, is triggered with a time offset forward by the switching delay (SV_on, SV_off) before a following switching voltage zero crossing for switching within a duration of a half-wave of the switching voltage (U) and after a last detected switching voltage zero crossing.
6. Method according to one of the preceding claims, characterized in that after a first switching on of an electric apparatus (11) with the relay (22a, 22b) therein, the relay (22a, 22b) is switched once arbitrarily at an arbitrary time to roughly detect a duration (SD) of the switching delay (SV_on, SV_off) as an initial switching delay, wherein subsequently, at the first switching of the relay (22a, 22b), this first switching delay being subsequently used as the duration for activating the relay (22a, 22b) with the switching trigger signal before a switching voltage zero crossing, the then detected switching delay (SV_on, SV_off) being used for further operation of the electrical apparatus (11) with switching of the relay (22a, 22b) instead of the first switching delay.
7. Method according to one of the preceding claims, characterized in that the switching delay (SV_on, SV_off) is corrected for each subsequent switching or each subsequent opening and closing operation.
8. Method according to one of the preceding claims, characterized in that a switching delay (SV_on, SV_off) of previous switching operations is stored both for an opening operation and for a closing operation, and in the event that currently detected values for the switching delay (SV_on, SV_off) deviate therefrom by more than 20%, a fault is detected at the relay (22a, 22b) or at a control for the relay (22a, 22b) and is signalled visually and/or acoustically to an operator.
9. Method according to one of the preceding claims, characterized in that between 1,000 and 20,000 measuring points are used per second as the sampling frequency for the microcontroller.
10. Device designed for carrying out the method according to one of the preceding claims, characterized by:

    - a relay (22a, 22b) with a relay contact,

    - a load (13a, 13b) to be connected by the relay (22a, 22b) to a switching voltage (U),

    - connections to a controller (16) for controlling the relay (22a, 22b),

    - a microcontroller (20) connected at signal inputs with a signal corresponding to the switching voltage (U) and a signal corresponding to the actual opening or closing of the relay contact.
11. The device according to claim 10, characterized in that the device is integrated in a controller (16) in a household electrical appliance or in a cooking appliance (11).

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