EP1034553B1 - Circuit for monitoring an alternative current power switch - Google Patents

Abstract

A circuit (1) for monitoring the state (on or off) of an alternative current power switch has an input (4) which is connected to a high-resistance input (11) of a first digital module (12) by means of a diode (9) and a first resistance (10). One connection of the alternative current power switch (2) is connected to the phase (P) of a supply voltage (UPN). The other connection is connected to the input (4) of the switch (1). The high-resistance input (11) of the first digital module (12) is connected to an output (14) of the second digital module by means of a second resistance (13). After a specific time period, the output (14) of the second digital module (15) is connected to the zero point (N) of the power supply (UPN) or guides a voltage which is larger than the threshold voltage (US) of the input (11) of the first digital module (12). A signal is provided at the output of the first digital module (12) associated with the input (11). Using said signal it is possible to deduce whether the alternative power switch (2) is off and whether the diode (9) and the second resistance are intact or whether said switch is on.

Description

The invention relates to a circuit for monitoring an AC switch of the type mentioned in the preamble of claim 1.

Such circuits are used, for example, in devices for the control and monitoring of the burner and the ignition device of oil and gas firing systems and for monitoring switches for actuators such as fuel valves and ventilation flaps, with a microprocessor evaluating the information supplied via line voltage-carrying signal lines and issuing corresponding control commands. In particular because of the safety required when switching on and when operating oil and gas burners, the switch-off capability of the switching devices that switch safety-critical loads such as a fuel valve must be checked frequently in order to be able to detect a malfunction of the switching device before a dangerous situation arises.

From DE-PS 30 44 047 and the older DE-PS 30 41 521 C2 a control device for oil burners according to the preamble of claim 1 is known, in which information about the switching states of relay and sensor contacts are transmitted to a microprocessor by means of an amplifier. The switching states of the relay contacts are fed via signaling lines carrying line voltage to an amplifier which is connected on the output side to an input of the microprocessor, so that the microprocessor must have a number of inputs corresponding to the number of amplifiers. Isolators such as e.g. are used for the electrical isolation of the signal lines and the microprocessor. Optocoupler or transmitter used. There is one isolator per signal voltage. The microprocessor is programmed to perform a number of tests to determine whether a system with switched consumers is actually going through a switch-on phase in the correct way. For this purpose, signals are read in by the microprocessor and compared with setpoints. In the event of a faulty consumer status, the microprocessor switches the consumers off.

Furthermore, in an arrangement known from DE-OS 41 37 204 for monitoring AC switches, mains voltage-carrying signal lines are connected via optocouplers to an interrogation unit of an AC voltage detector. The signal lines are each connected to the optocoupler via a low-pass filter, which consists of a resistor and a capacitor connected in series with it. The switching states of the AC switches are queried and saved via the signal lines. In an evaluation unit connected downstream of the interrogation unit, the switching states are compared with a target state - open or closed - and then a switch state signal is formed, which contains at least one piece of information - error or no error - in total for all AC switches that occur. It cannot be determined from the switch status signal which AC switch can no longer be switched off, so that a simple display for diagnosis is not possible.

Circuits for monitoring AC switches are also known from European patents EP 660 043 and EP 660 044. However, these circuits cannot be checked for the correct functionality of their safety-relevant components in continuous operation.

The invention has for its object to provide a circuit for monitoring an AC switch with which a load can be switched to a mains voltage, which can be fed relative to the zero point of the mains voltage and whose safety-relevant components can be checked for correct functionality at any time during continuous operation of the circuit.

The invention consists in the features specified in claim 1. Advantageous configurations result from the dependent claims.

Exemplary embodiments of the invention are explained in more detail below with reference to the drawing.

Fig. 1

eine Schaltung zur Überwachung eines Wechselstromschalters,

Fig. 2, 3, 4

Spannungs- und Signaldiagramme,

Fig. 5

eine erste Erweiterung der Schaltung, und

Fig. 6

eine zweite Erweiterung der Schaltung,

Show it:

Fig. 1

a circuit for monitoring an AC switch,

2, 3, 4

Voltage and signal diagrams,

Fig. 5

a first extension of the circuit, and

Fig. 6

a second extension of the circuit,

1 shows a circuit 1 for monitoring an AC switch 2. The circuit 1 is fed by a mains voltage U _PN lying between a phase P and a zero point N. The AC switch 2 can, for example, as shown in dashed lines in FIG. 1, switch a load 3 to the mains voltage U _PN . The AC switch 2 and the load 3 are connected in series, a connection of the AC switch 2 being connected to the phase P and a connection of the load 3 being connected to the zero point N. The tap between the AC switch 2 and the load 3 is connected to an input 4 of the circuit 1.

The AC switch 2 can also be a switch whose position - open or closed - serves as a signal or control signal. As an example, it should be mentioned that the AC switch 2 is an overtemperature switch that opens as soon as a predetermined temperature is exceeded, or that the AC switch 2 is a limit switch that opens (or closes) as soon as a device reaches a predetermined position. In this case, the load 3 is simply to be imagined in FIG. 1.

In all cases, one connection of the AC switch 2 is thus connected to the phase P of the mains voltage U _PN and the other connection is connected to the input 4 of the circuit 1.

The circuit 1, which has the usual supply voltage connections V _DD and V _SS , is supplied in a known manner, for example with a voltage part formed from a diode 5, a resistor 6, a Zener diode 7 and a capacitor 8, with the voltage U _PN the supply voltage connection V _{SS is} connected to the zero point N. The supply of the circuit 1 can also on another type, for example by means of a transformer with subsequent rectification, stabilization and galvanic coupling to the zero point N.

The input 4 is connected via a diode 9 and a first resistor 10 to a high-resistance input 11 of a first digital module 12. A second resistor 13 connects the high-impedance input 11 to an output 14 of a second digital module 15. The input 11 and the output 14 of the two digital modules 12 and 15 are, as usual, connected to the supply voltage V _DD and the zero point N via protective diodes 16.

For safety reasons, component errors must not lead to circuit 1 interpreting an open AC switch 2 as a closed AC switch. All component faults that could lead to a result that is dangerous in terms of safety must therefore be automatically recognized by the circuit 1. For this purpose, a component test is planned. For example, if the diode 9 is short-circuited due to a defect, then capacitively coupled signals at the input 4 of the circuit 1 must not lead to the circuit 1 erroneously reporting that the AC switch 2 is closed. An interruption of the resistor 13 also leads to a change in the voltage at the input 11 of the first digital component 12. This must not lead to a result that is dangerous in terms of safety technology with regard to the position of the AC switch 2. Resistors 10 and 13 are manufactured using a technology that only allows an interruption, but not a short circuit, as a defect. It is therefore sufficient to test the resistors 10 and 13 for an open.

The interrogation of the position of the alternating current switch 2 and the execution of a component test on the functionality of the components 9, 10 and 13 can be carried out in time-separated methods or by means of a common method.

• a) den Spannungsverlauf am Eingang der Diode 9,
• b) den Spannungsverlauf am Ausgang der Diode 9,
• c) den Spannungsverlauf am Eingang 11 des ersten Digitalbausteins 12,
• d) die Abtastimpulse,
• e) binäre Signale 0 oder 1 am Ausgang 17 (Fig. 1) des ersten Digitalbausteins 12, die durch die Abtastung der am Eingang 11 des ersten Digitalbausteins 12 anliegenden Spannung mit den Abtastimpulsen entstehen.

The state of the AC switch 2 - open or closed - can be queried without checking the components by connecting the output 14 of the digital module 15 to the zero point N. 2 shows for the two states AC switch 2 closed or open as a function of time t:

• a) the voltage curve at the input of the diode 9,
• b) the voltage curve at the output of the diode 9,
• c) the voltage curve at the input 11 of the first digital module 12,
• d) the scanning pulses,
• e) binary signals 0 or 1 at the output 17 (FIG. 1) of the first digital module 12, which result from the scanning of the voltage present at the input 11 of the first digital module 12 with the scanning pulses.

Because of the protective diodes 16 present in the digital modules 12 and 15 (FIG. 1), the voltage profile when the AC switch 2 is closed is at the input 11 of the first digital module 12 practically rectangular and in phase with the voltage U _PN . The resistors 10 and 13 act as voltage dividers. With each scanning pulse, a binary signal 0 or 1 appears at the output 17 of the first digital component 12, which indicates whether the voltage at the input 11 is lower or higher than a threshold voltage U _S of 2.5 V, for example, specified by the input 11. The further evaluation of the sampling can take place, for example, by summing the signals 0 or 1 occurring during a certain time period, the time period being longer than half a network period. When AC switch 2 is open, this sum must be zero. When AC switch 2 is closed, this sum must on the one hand result in a finite, non-zero value, and on the other hand the values of the signal must contain both values 0 and 1 within the said time period.

To carry out the component test, with which it can be checked whether the diode 9 is not short-circuited or one of the resistors 10 or 13 is interrupted, a positive voltage is applied to the output 14 of the second digital module 15, which is greater than the threshold voltage voltage U _S. If the resistor 13 is intact, then the voltage at the input 11 of the first digital module 12 is also greater than the threshold voltage U _S. The period of time during which the positive voltage is present at the output 14 is greater than a half wave and shorter than a full wave. A component fault occurs when the signals appearing at the output 17 of the first digital module 12 do not correspond to the expected signals, as will now be explained in more detail.

• a) den Spannungsverlauf am Ausgang der Diode 9,
• b) den Spannungsverlauf am Ausgang 14 des zweiten Digitalbausteins 15,
• c) den Spannungsverlauf am Eingang 11 des ersten Digitalbausteins 12,
• d) die Abtastimpulse, und
• e) die Signale am Ausgang 17 des ersten Digitalbausteins 12.

3a and 3b show in the case of the intact or the short-circuited diode 9 again for the two states of the AC switch 2 closed or open as a function of time t:

• a) the voltage curve at the output of the diode 9,
• b) the voltage curve at the output 14 of the second digital module 15,
• c) the voltage curve at the input 11 of the first digital module 12,
• d) the sampling pulses, and
• e) the signals at the output 17 of the first digital module 12.

If the AC switch 2 is closed and the diode 9 and the resistors 10 and 13 are intact (FIG. 3a), signals with the value 1 appear at the output 17 of the first digital module 12 when the voltage at the input 11 is due to a current positive mains half-wave or as a result of a positive voltage at the output 14 of the second digital module 15, the threshold voltage U _s exceeds.

In the event of a short circuit of the diode 9 (FIG. 3b), on the other hand, signals with the value 1 only appear during the positive mains half-wave: During the negative mains half-wave, the voltage at the input 11 of the first digital module 12 is below the threshold value U _S , so that at the output 17 of first digital module 12 signals with the value 0 appear.

If the resistor 10 is interrupted, then the voltage at the input 11 of the first digital module 12 regardless of the position of the AC switch 2 and equal to the voltage at the output 14 of the second digital module 15, so signals 1 or 0 appear at the output 17 of the first digital module 12, which are in phase with the voltage at the output 14 of the second digital module 15.

If the resistor 13 is interrupted, then the voltage at the input 11 of the first digital module 12 is independent of the voltage at the output 14 of the second digital module 15. The signals at the output 17 of the first digital module 12 are then in phase with the voltage at the input 4 Circuit 1.

If the AC switch 2 is open, then the voltage at the input 11 of the first digital module 12 depends only on the voltage at the output 14 of the second digital module 15 and on whether the resistor 13 is intact or interrupted. If the resistor 13 is intact, then the voltage at the output 17 of the first digital module 12 must be in phase with the voltage at the output 14 of the second digital module 15. If the resistor 13 is interrupted, then only signals 0 may appear at the output 17 of the first digital module 12.

The following table gives an overview of the different options. The first column symbolically shows whether the AC switch 2 is open or closed and whether the diode 9 and the resistors 10 and 13 are intact or not. The number N in the second column indicates how many signals with the value 1 appear at the output 17 of the first digital module 12 if there are twenty scanning pulses per full wave and if the voltage at the output 14 of the second digital module 15 during a period of time, the fourteen scanning pulses includes, above the threshold voltage U _S.

The component test must therefore result in a number N that is greater than or equal to 14. If the AC switch 2 is closed, the diode 9 and the resistors 10 and 13 are intact if the number N ≥ 14. If the AC switch 2 is open, only the resistor 13 can be checked. It is intact if the number N = 14. If the component test results in a number N that is less than 14, then there is a component error. If the component test yields a number N that is greater than or equal to 14, the state - open or closed - of the AC switch 2 can now be determined, as explained above, by a query in which the output 14 of the second digital module 15 is connected to the zero point N. is.

• a) den Spannungsverlauf am Ausgang der Diode 9,
• b) den Spannungsverlauf am Ausgang 14 des zweiten Digitalbausteins 15,
• c) den Spannungsverlauf am Eingang 11 des ersten Digitalbausteins 12,
• d) die Abtastimpulse, und
• e) die Signale am Ausgang 17 des ersten Digitalbausteins 12.

Die folgende Tabelle zeigt die möglichen Werte der Summe Z und deren Bedeutung, wobei neben den allgemein gültigen Angaben auch Angaben für das konkrete Beispiel mit M=32 und R=8 angegeben sind. Summe Z Summe Z für M=32 und R=8 Wechselstromschalter 2 Zustand der Bauteile Z=0 Z=0 ? Widerstand 13 unterbrochen Z=R Z=8 offen Widerstand 13 intakt;
Diode 9 und Widerstand 10 unbekannt Z = $\frac{\text{M}}{\text{2}}$ Z=16 geschlossen Kurzschluss der Diode 9 oder Unterbruch des Widerstandes 13; Widerstand 10 intakt Z = $\frac{\text{M}}{\text{2}}$ + $\frac{\text{R}}{\text{2}}$ Z=20 geschlossen Diode 9, Widerstände 10 und 13 intakt Z=M Z=32 ? Eingang 11 oder Ausgang 14 ist permanent mit V_DD verbunden Z = $\frac{\text{M}}{\text{2}}$ + $\frac{\text{R}}{\text{1}}$ bedeutet also, dass der Wechselstromschalter 2 "geschlossen" ist und dass die Bauteile 9, 10 und 13 intakt sind. Z=R bedeutet, dass der Wechselstromschalter 2 "offen" ist, wobei der Zustand der Diode 9 und des Widerstandes 10 allerdings nicht bestimmt werden kann. Wird jedoch zusätzlich immer dann, wenn die Spannung am Ausgang 14 über der Schwellwertspannung U_S liegt, überprüft, ob der Ausgang 17 den Abtastwert "1" führt, dann kann daraus ermittelt werden, ob die Diode 9 kurzgeschlossen oder der Widerstand 13 unterbrochen ist. Ein Unterbruch des Widerstandes 10 hingegen wird immer als Wechselstromschalter 2 "offen" interpretiert. Dieses Verfahren hat den Vorteil, dass innerhalb einer Netzwelle feststellbar ist, ob ein die Sicherheit beeinträchtigender Bauteilfehler vorliegt und welche Stellung der Wechselstromschalter 2 einnimmt. Es ist aus sicherheitstechnischer Sicht tolerierbar, wenn die Werte für Z um ± 1 vom korrekten Wert abweichen. Es ist dann nämlich nicht nötig, die Frequenz der Abtastimpulse mit der Netzspannung zu synchronisieren.4, an embodiment is explained in which the query of the position of the AC switch 2 and the functionality of the components is checked in a single process. The output 14 of the second digital module 15 normally carries the level of the zero point N, but is set to the higher level with the frequency R at regular intervals for the duration of a single scanning pulse, at which the voltage at the input 11 of the first digital module 12 exceeds the threshold voltage U _S exceeds. During a full network wave TN, M sampling pulses are generated and the binary samples at the output 17 of the first digital module 12 are summed up to the sum Z. The diagrams show for the two states AC switch 2 closed or open as a function of time t:

• a) the voltage curve at the output of the diode 9,
• b) the voltage curve at the output 14 of the second digital module 15,
• c) the voltage curve at the input 11 of the first digital module 12,
• d) the sampling pulses, and
• e) the signals at the output 17 of the first digital module 12.

The following table shows the possible values of the sum Z and their meaning, whereby in addition to the generally applicable information, information for the specific example is given with M = 32 and R = 8. Sum Z Sum Z for M = 32 and R = 8 AC switch 2 Condition of the components Z = 0 Z = 0 ? Resistor 13 interrupted Z = R Z = 8 open Resistor 13 intact;
Diode 9 and resistor 10 unknown Z = $\frac{\text{M}}{\text{2}}$ Z = 16 closed Short circuit of the diode 9 or interruption of the resistor 13; Resistor 10 intact Z = $\frac{\text{M}}{\text{2}}$ + $\frac{\text{R}}{\text{2}}$ Z = 20 closed Diode 9, resistors 10 and 13 intact Z = M Z = 32 ? Input 11 or output 14 is permanently connected to V _DD Z = $\frac{\text{M}}{\text{2}}$ + $\frac{\text{R}}{\text{1}}$ means that the AC switch 2 is "closed" and that the components 9, 10 and 13 are intact. Z = R means that the AC switch 2 is "open", although the state of the diode 9 and the resistor 10 cannot be determined. However, if the voltage at the output 14 is above the threshold voltage U _{S, it} is additionally checked whether the Output 17 carries the sample value "1", it can then be determined whether the diode 9 is short-circuited or the resistor 13 is interrupted. An interruption of the resistor 10, however, is always interpreted as an AC switch 2 "open". This method has the advantage that it can be determined within a network shaft whether there is a component fault which affects safety and which position the AC switch 2 is in. From a safety point of view, it is tolerable if the values for Z deviate from the correct value by ± 1. It is then not necessary to synchronize the frequency of the scanning pulses with the mains voltage.

5 shows an extension of the circuit 1, with which several AC switches 2 can be monitored. The special feature of this circuit is that the second resistors 13 are all routed to a common output 14. Instead of the digital components 12, 15 (FIG. 1), which could also be used, transistor stages 18, 19 are used here.

6 shows an expansion of the circuit 1 in which the input 11 of the first digital module 12 can also be switched as an output and the output 14 of the second digital module 15 can also be switched as an input. The input 11 and the output 14 are thus bidirectional ports 20, 21. The connection of the first resistor 10 is now connected to the first port 20 via a resistor 22 and to the second port 21 via a further resistor 23. The circuit structure with respect to the ports 20, 21 is therefore symmetrical, so that the function of the ports 20, 21 - "input" or "output" - is interchangeable. Therefore, by repeating the tests, with the wiring of the two ports 20, 21 swapped as the input or output, the functionality of the digital modules 12 and 15 can also be checked.

The circuit 1 (FIGS. 1 and 6) offers the advantage over known circuits that, when the AC switch 2 is open, capacitively coupled AC voltages are suppressed as a result of parasitic line capacitances. The injected AC voltage is rectified by the diode 9. The line capacitances are thereby polarized, so that the injected AC voltage is shifted in terms of DC by the peak value of the injected AC voltage. The diode 9 is arranged in such a way that the injected AC voltage has a negative DC voltage component. AC voltages coupled in capacitively can therefore not influence the signals at the output 17 of the first digital module 12.

Claims (4)

1. A circuit (1) for monitoring the state - open or closed - of an AC switch (2) of which one terminal is connected to the phase (P) of a mains voltage (U_PN) and the other terminal is connected to an input (4) of the circuit (1), characterised in that the input (4) is connected by way of a diode (9) and a first resistor (10) to a high-ohmic input (11) of a first digital component (12), that the high-ohmic input (11) of the first digital component (12) is connected by way of a second resistor (13) to an output (14) of a second digital component (15), and that the output (14) of the second digital component (15), in accordance with a predetermined configuration in respect of time, is either connected to the neutral point (N) of the mains voltage (U_PN) or carries a voltage which is greater than the threshold voltage (U_S) of the input (11) of the first digital component (12) so that from the signal at an output (17), associated with the input (11), of the first digital component (12), it is possible to deduce whether the AC switch (2) is closed and the diode (9) and the second resistor (13) are intact or whether the AC switch (2) is open.
2. A circuit (1) according to claim 1 which is designed for monitoring a plurality of AC switches (2), characterised in that associated with each AC switch (2) is its own high-ohmic input (11) of the first digital component (12) and that the output (14) of the second digital component (15) serves as a common output (14) for all AC switches (2).
3. A circuit (1) according to claim 1 or claim 2 characterised in that the input (11) of the first digital component (12) and the output (14) of the second digital component (15) are bidirectional ports (20; 21).
4. A circuit (1) according to one of claims 1 to 3 characterised in that transistor stages (18; 19) are used instead of the digital components (12;15).