EP1366502B1 - Electrical circuit for preventing an arc across an electrical contact - Google Patents

Abstract

The invention relates to an electrical circuit for preventing an arc across an electrical contact on opening the contact, whereby the electrical circuit comprises a timer which forces a time-delayed increase in the contact voltage in comparison with an unconnected contact. According to the invention, an electrical circuit for preventing an arc across an electrical contact, which very probably restricts the formation of an arc on opening said circuit contact and is furthermore economically produced and may be miniaturised may be achieved, whereby said electrical circuit comprises a transistor, wired in parallel to the circuit contact. Said transistor may be, for example, a power MOSFET, operated in the source circuit, or a Darlington transistor, formed by a bipolar transistor and a field effect transistor.

Description

The present invention relates to an electrical circuit for avoiding an arc over an electrical contact when opening the contact according to the preamble of patent claim 1.

If in a circuit, in particular a circuit with an inductive load component, the flowing current is interrupted by means of a mechanically moving contact, as a result of current injection by the inductive component on the opening switching contact an arc can occur, via which the current flow is maintained at least for a short time. Such an arc can greatly reduce the life of the switching contact or - at higher voltages than stationary arc - lead to destruction of the contact. To form such an arc, a certain minimum voltage is required, depending on the contact spacing and contact material. If this minimum voltage is exceeded, no arc is formed. Since the opening mechanical switching contact due to its finite speed of movement increases the contact distance only comparatively slowly, the voltage across the contact must fall below this minimum voltage at any time.

• Mechanische Schalter oder Relais mit Doppel- oder Mehrfachkontakten in Reihe,
• mechanische Schalter oder Relais mit magnetischer Lichtbogenlöschung (z. B. Blasmagnet oder Löschkammer),
• mechanische Schalter oder Relais mit Magnet und spannungsbegrenzendem elektronischem Bauteil,
• elektronische Schalter.

For switching inductive loads in particular the following switches are considered:

• Mechanical switches or relays with double or multiple contacts in series,
• mechanical switches or relays with magnetic arc quenching (eg blowing magnet or quenching chamber),
• mechanical switches or relays with magnet and voltage-limiting electronic component,
• electronic switches.

When switching small loads by means of a mechanical switching contact, the prior art by the insertion of an RC element, the cut-off voltage lowered over the switching contact for the period of the contact opening under the voltage necessary for the ignition of an arc voltage. After complete opening of the contact, the necessary breakdown voltage at a contact distance of 0.2 mm is about 600 V, so that in general an ignition of an arc can then no longer occur. Such a conventional RC circuit is z. B. in the monograph "Relay technique: Basis and Latest Developments", Verlag Moderne Industrie, 1998, described on page 57 under the chapter "reduction of switching arcs" and is shown in Figure 12.

An alternative to the conventional RC circuit shown in FIG. 18 is the additional use of a voltage limiting element (eg, a Zener diode or a varistor) shown in FIG. 19, which achieves that the RC element only a very small portion of the Shutdown must take energy and the main part after reaching the Z voltage (80 V in the present example) is converted by the Zener diode. The capacitance C can be designed correspondingly smaller: In the present example, 1 μF is sufficient instead of the 1000 μF used in FIG. All other components shown in Figure 13 have the same reference numerals and have the same value as the corresponding components in Figure 18.

However, these prior art circuits have the disadvantage that they are no longer practicable at higher loads (such as occur in the automotive application) due to the necessary size of the capacitor and the associated high cost. In addition, when the contact is switched on again by the discharge of the high capacitance via the low-resistance resistor, a high inrush current that can lead to welding of the contact pieces in conjunction with the switch-on contact bounce results.

Document US 4658320 shows a circuit according to the preamble of claim 1.

Object of the present invention is therefore to provide an electrical circuit for avoiding an arc over a switching contact, which effectively prevents the formation of an arc when opening the switching contact and also inexpensive to produce and largely miniaturized.

This object is achieved by an electrical circuit with the features of claim 1 Advantageous developments of the invention are the subject of several subclaims.

The invention is based on the finding that, due to the electronic device provided parallel to the electrical contact according to the invention, even when switching off higher inductive loads, for example in DC circuits with operating voltages above 20 V, the instantaneous sudden increase of the voltage above the contact to supercritical values after opening of the Contact is prevented and thus reliable opening arc can be avoided. The electrical circuit according to the invention also does not require its own power supply and is only connected via two electrical connections with a switch or relay. In contrast to other electronic circuits connected in parallel with the contact, the electric circuit according to the invention can also be used to prevent the arc at a changeover switch for the braked shutdown of an electric motor or in the case of a polarity reversal circuit. A particular advantage of the electrical circuit according to the invention is the possibility of a very compact construction, which is of great advantage, inter alia, in the automotive industry. The life of the mechanical contacts can be significantly increased, since only a relatively low mechanical wear of the contacts occurs. Finally, switching contacts with the electrical circuit according to the invention represent a particularly cost-effective solution since single contacts can be used with low contact thicknesses, no great demands must be placed on the dynamic properties of the switching contact and other costly arc extinguishing devices can be completely dispensed with.

In order to discharge the capacitance C in the circuit as quickly as possible, when the switch is closed again, and thereby to ensure good dynamic properties of the electronic device, according to an advantageous development parallel to the resistor R2, a discharge diode V1 is connected, the cathode with the Capacitor C is connected The diode V1 simultaneously protects the gate of the amplifier V3 from negative gate-source voltages.

Another resistor R1, which is connected between the capacitor C and the switching contact, limits the discharge current of the capacitor C and improves the dynamic properties of the circuit.

According to an advantageous embodiment, a voltage-limiting component V2 can be connected in parallel to the capacitance C for protecting the amplifier.

A particularly simple and cost-effective solution for such a voltage-limiting component is a Zener diode.

According to an alternative embodiment, as overvoltage protection for the amplifier when switching off an inductive load, a voltage-limiting component V4 may also be connected in parallel with the amplifier V3.

Again, the use of a Zener diode is advantageous because it is a very low cost component

According to an advantageous embodiment, a power MOSFET is used as amplifier V3. The advantage of this solution is that MOS field effect transistors can be controlled with low power and the overall structure can be highly miniaturized. In this case, the MOSFET is operated, for example, in source circuit, d. H. the source terminal is connected to the terminal of the resistor R102 connected to the switch contact, and the gate terminal is connected to the common terminal of capacitance C and resistor R102. Capacitance C is in feedback branch between drain and gate. This embodiment offers the advantage that the capacitance C can have a comparatively low value and nevertheless has the effect of a much greater capacity (Miller effect).

If a Darlington transistor (FIG. 1) having a first and a second transistor is used as the amplifier, the voltage across the opening contact can advantageously be kept below the minimum voltage for arcing for a certain time, and then sufficiently large contact distance quickly increase to the appropriate value for demagnetization of the load circuit. The time for which the voltage across the contact is at a constant Value is kept below the minimum voltage, determined by the series connection of a resistor and a capacitor

According to an advantageous embodiment, the Darlington transistor comprises two bipolar transistors. As a result, the voltage across the opening contact for a certain adjustable time can be advantageously maintained at the value of the base-emitter voltage of the Darlington transistor and then increased rapidly to the voltage necessary for the demagnetization of the load inductance. The adjustable time is determined by the charging of the capacity. During this time, the Darlington transistor carries the current of the DC circuit, first at the low voltage level, then at a higher voltage level. The charging of the capacitance is essentially determined by the base-emitter voltage of the first transistor

According to an alternative advantageous embodiment, the Darlington transistor comprises a field effect transistor as first transistor and a bipolar transistor as second transistor. As a result, the voltage of the opening contact can be advantageously maintained at the value of the gate-source voltage, for example, of the logic power MOSFET, and these can be rapidly increased again after the settable time has elapsed. In this embodiment, the MOSFET carries the current of the DC circuit and the charging of the capacitance is determined by the gate-source voltage of the MOSFET. Another advantage of this solution is that the substrate diode of the MOSFET can take over the task of the freewheeling diode D1 according to FIG.

As overvoltage protection for the amplifier when switching off an inductive load, a voltage-limiting component may be connected in parallel to the output of the amplifier. A particularly simple and cost-effective solution for such a voltage-limiting component is a Zener diode. Their breakdown voltage should be well above the operating voltage of the DC circuit to allow a fast Abkommutieren the inductive load circuit. Advantageously, the required Z-voltage can also be adjusted by a series connection of a plurality of Z-diodes of smaller Z-voltage, so that the voltage-limiting element in the sum of a smaller differential resistance and also a smaller temperature coefficient receives as well as the distribution of a possible power dissipation better dissipate.

In a particularly effective manner, the base of the Darlington transistor is controlled by a second amplifier T9, which has a Darlington transistor complementary conductivity type.

In order to keep the amplifier T9 in saturation during the adjustable time, it is controlled by a third amplifier, the transistor T10, which has the same conductivity type as the Darlington transistor.

In order to prevent large cross currents occurring when, in the case of a relay application, a closing normally closed contact meets the circuit according to the invention in the reset state, a thyristor structure may be provided parallel to the input of the Darlington transistor.

The advantageous properties of the electrical circuit according to the invention can be used particularly effectively in an electromagnetic relay, wherein the electrical circuit is connected in parallel to a normally open contact of the relay.

Likewise, the electrical circuit according to the invention can be used in an advantageous manner in an electrical connector to avoid an arc when releasing the connector.

Figur 1

ein Schaltbild einer elektrischen Schaltung zur Vermeidung eines Lichtbogens gemäß einer dritten Ausführungsform;

Figur 2

ein Schaltbild einer elektrischen Schaltung zur Vermeidung eines Lichtbogens gemäß einer vierten Ausführungsform;

Figur 3

ein Ersatzschaltbild eines Relais mit einer elektrischen Schaltung zur Vermeidung eines Lichtbogens gemäß einer fünften Ausführungsform;

Figur 4

ein Schaltbild einer Thyristorstruktur aus einer elektrischen Schaltung zur Vermeidung eines Lichtbogens gemäß einer sechsten Ausführungsform;

Figur 5

einen zeitlichen Verlauf der Spannung über einem Arbeitskontakt, des Stromes durch eine Lastinduktivität sowie des Kollektorstromes eines ersten Transistors bei einem Schaltvorgang in der erfindungsgemäßen Schaltung gemäß der dritten Ausführungsform;

Figur 6

einen vergrößerten Ausschnitt aus dem Zeitdiagramm der Figur 11;

Figur 7

einen Zeitverlauf eines Schaltvorgangs bei Verwendung einer elektrischen Schaltung gemäß einer vierten Ausführungsform;

Figur 8

ein Zeitdiagramm für den Schaltvorgang in einer Schaltung gemäß der fünften Ausführungsform für eine Betriebsspannung von 60 V;

Figur 9

ein Zeitdiagramm eines Schaltvorgangs in einer Schaltung gemäß der fünften Ausführungsform bei einer Betriebsspannung von 42 V;

Figur 10

ein Zeitdiagramm eines Schaltvorgangs in einer Schaltung gemäß der fünften Ausführungsform bei einer Betriebsspannung von 24 V;

Figur 11

ein Zeitdiagramm zur Erläuterung der Wirkung einer Thyristorstruktur gemäß Figur 9;

Figur 12

eine konventionelle RC-Kontaktbeschaltung nach dem Stand der Technik;

Figur 13

eine weitere konventionelle RC-Kontaktbeschaltung nach dem Stand der Technik.

With reference to the embodiments shown in the accompanying drawings, the invention will be explained in more detail below. Similar or corresponding details of the electrical circuit according to the invention are provided in the figures with the same reference numerals. Show it:

FIG. 1

a circuit diagram of an electrical circuit for avoiding an arc according to a third embodiment;

FIG. 2

a circuit diagram of an electrical circuit for avoiding an arc according to a fourth embodiment;

FIG. 3

an equivalent circuit diagram of a relay with an electric circuit for avoiding an arc according to a fifth embodiment;

FIG. 4

a circuit diagram of a thyristor structure of an electric circuit for avoiding an arc according to a sixth embodiment;

FIG. 5

a time profile of the voltage across a normally open contact, the current through a load inductance and the collector current of a first transistor in a switching operation in the inventive circuit according to the third embodiment;

FIG. 6

an enlarged section of the timing diagram of Figure 11;

FIG. 7

a time course of a switching operation using an electric circuit according to a fourth embodiment;

FIG. 8

a timing diagram for the switching operation in a circuit according to the fifth embodiment for an operating voltage of 60 V;

FIG. 9

a timing chart of a switching operation in a circuit according to the fifth embodiment at an operating voltage of 42 V;

FIG. 10

a timing chart of a switching operation in a circuit according to the fifth embodiment at an operating voltage of 24 V;

FIG. 11

a timing diagram for explaining the effect of a thyristor structure of Figure 9;

FIG. 12

a conventional RC contact circuit according to the prior art;

FIG. 13

another conventional RC contact circuit according to the prior art.

The shutdown of inductive DC loads at voltages above 20 V occurs, inter alia, in automotive technology (24 V and 42 V electrical system).

Figure 1 shows schematically an equivalent circuit diagram of a circuit with a switching contact 101 and an inductive DC load. The load circuit should in turn be interrupted by opening the switch contact 101. Without a suitable measure, as with the circuit diagram shown in FIG. 1, an arc would open when the switching contact 101 was opened. Therefore, an electric circuit 100 according to a third advantageous embodiment is switched to avoid an arc parallel to the switching contact 101. In this embodiment and in the embodiments shown in FIGS. 8 to 10, the formation of an arc is prevented by first maintaining the voltage across the contact 101 constant at a low level and increasing it to its final value only when the contact is opened so far is that no more arc ignites.

The electrical circuit 100 according to the invention has, according to this third embodiment, a Darlington transistor, which is formed by the transistors T1 and T2. The base of this Darlington transistor is controlled by a transistor T9 with Darlington transistor complementary conductivity type. The transistor T9, in turn, is controlled by a transistor T10 of the same type as the Darlington transistor, such that the transistor T9 is kept in saturation for an adjustable time. This adjustable time is determined by a timer R10, C1, which is in the E-Mitterzweig of the transistor T10. During this time, the Darlington transistor is fully conductive at the low voltage level. The capacitor C1 is charged only by the difference of the base voltages of the Darlington transistor on the one hand and the transistor T10 on the other hand. If the charging of the capacitor C1 has progressed so far that the collector current Ic of the transistor T10 drops and as a result the transistor T9 can no longer be kept in saturation, the Darlington transistor is increasingly blocked and the voltage across it rises and is driven through the load inductance, on. The contact distance should now be widened so that the subsequent rapid increase in voltage can no longer lead to the ignition of an arc.

The phase of the almost constant low voltage across the Darlington transistor is now followed by a rapid increase in voltage until the breakdown voltage of the Zener diode Z1 again controls the transistor T1 and is followed by a second phase of almost constant high voltage, during which the inductive part of the Load circuit can abkommutieren quickly. The voltage increase between the two phases is slightly delayed due to the parasitic collector capacitance of all transistors (Mlller effect).

As is apparent from the associated time courses, which are shown in FIGS. 5 and 6, an increase in the voltage across the switching contact 101 to approximately 2 takes place directly after the opening time of the switching contact 101 (assumed in the diagrams at t = 50 ms) to 5 V in order to bring the prior art energy-free inventive circuit in less than 1 microseconds (due to the Miller effect of the transistors T2, T9 and T10) before the opening of the switching contact 101 in the operating point. This is followed by a voltage plateau, which is at about 1.4 V. At this level, the voltage remains above the switching contact 101, until the capacitor C1 due to the difference voltage from the base-emitter voltage of T1 / 2 and the base-emitter Voltage of T10 is charged so far that the decaying collector current of the transistor T10 can no longer keep the transistor T9 in saturation. This is followed by a relatively steep, only by the Miller capacitors braked voltage increase to take over by the Zener diode Z1. As the respective collector-base capacitances decrease with increasing voltage, the voltage increase also takes place with increasing speed. At the high level defined by the Zener diode, the load inductance LL commutes and a damped oscillation ensues, while the parasitic energies are degraded.

Figure 2 shows a fourth embodiment in which the transistor T1 of the Darlington transistor is formed by a logic power MOSFET instead of a bipolar transistor as shown in Figure 1. The characteristic base-emitter voltage of the Darlington transistor (about 1.5 V) is essentially in the gate-source voltage at the operating point of the MOSFET over (about 3.5 V). Again, the transistor T9 is initially saturated, thereby essentially connecting the drain and gate of the MOSFET across the base-emitter path of the transistor T2 until the capacitor C1 is charged. Thereafter, the voltage increase decelerated by the Miller effect continues until Z-voltage. The breakdown voltage of the Zener diode should be well above the operating voltage of the DC circuit to allow a fast Abkommutieren the inductive load circuit. As shown in Figure 3, the required Z-voltage can be adjusted by series connection of a plurality of Z-diodes smaller Z-voltages, which in sum have a smaller differential resistance and a smaller temperature coefficient and can better dissipate a possible power dissipation through the division ,

In Figure 3, an operating case is simulated in the left part of the diagram, as it is often found in relay applications: About the contact 101, the normally open or normally open, a complex load is turned on. With the switches 102 to 108 a thrice bouncing normally closed contact (NC) of a changeover (changer) is modeled, the load, z. B. shorts when braking an engine.

In this case, it is possible in principle for the closing normally closed contact to strike a discharged circuit arrangement according to FIG. 1 or 2. This resulted in very large cross currents over the normally closed contact and the circuit arrangement, since this normally attempts to prevent an immediate rapid increase in voltage, even across the normally open contact 101 open in this case. However, a larger current through the Darlington transistor or the MOSFET requires a higher control voltage at the base or at the gate. This higher control voltage is used to ignite a thyristor structure formed of transistors T3 and T4 in conjunction with transistor T8, so that the current supplied by transistor T9 is lower than the normal control voltage due to the lower firing voltage of the thyristor (≤1 V) Darlington transistor (≥ 1.2V) or MOSFET (≥ 3.5V). Therefore, the current drops in the output stage T1, T2 and the voltage can, only by the Miller capacitors braked, increase to a maximum of the Z voltage. FIG. 9 shows a specific embodiment of the thyristor structure. The ignition of this thyristor is via the sum of the voltages of the transistors connected as diodes T6 and T7 as a reference voltage in conjunction with the Rert R7 to the normal control voltage adapted to the Darlington transistor. If, instead of a bipolar transistor for the transistor T1, a MOSFET is used, a higher reference voltage and thus possibly a Zener diode are required due to the higher control voltage.

In the example shown in FIG. 3, it is assumed that the switching contact 101 opens at t = 50 ms, that the switching contact 102 closes at t = 50.35 ms and the switching contact 103 opens at t = 50.40 ms, that the switching contact 104 closes at t = 50.45 ms and the switching contact 105 opens at t = 50.50 ms, that the switching contact 106 closes at t = 50.55 ms and the switching contact 107 opens at t = 50.60 ms, and that the switching contact 108 closes at t = 50.65 ms.

FIG. 4 shows a further possible embodiment of the thyristor. Here takes place after the ignition of the thyristor, the leadership of the current to be dissipated via the transistor T3, the Schottky diode DS and the transistor T4. The capacitors Cv1 and Cv2 are used to charge balance in the start phase of the circuit immediately after the opening of the normally open contact 101. The diodes T6 and T7 again form the reference voltage and control the transistor T4 here.

FIG. 5 shows a complete wear-down process: in the left-hand part, the voltage across the make contact (curve 110) is only at a low level, namely, after starting the circuit, the base-emitter voltage of the Darlington transistor of less than 2 V. FIG Time range after 50.05 ms to recognize the rapid increase in voltage to about 75 V, this value is essentially determined by the Z voltage. Over time, a voltage level now closes, during which the load inductance abkommutiert. In the region of the commutation of the load inductance, the current I at the inductance (curve 112) decreases linearly to zero. This area is followed by a decaying vibration of the voltage across the make contact, while the parasitic energy is dissipated.

A partial enlargement of the left part of the illustration from FIG. 5 is shown in FIG. Immediately after the opening of the contact 101 (at time T = 50 ms), the collector current of the transistor T1 increases (curve 114), while the voltage remains over the contact 101 after a slight voltage spike at the low level of below 1.5V, the base-emitter voltage of the Darlington transistor (see trace 110).

If, instead of a bipolar transistor for the transistor T1, a MOSFET is used, as shown in FIG. 2, the time profile shown in FIG. 7 results. The curve 110 again means the voltage across the switching contact 101, the curve 112 the current through the load inductance and the curve 114 the current flowing in the electrical circuit according to the invention. The plateau voltage in the low range is here by the use of the MOSFET at a value of about 5 V. Later in the time course, the Abmagnetisierungskurve is identical to the course shown in Figure 12.

FIGS. 8, 9 and 10 show the demagnetization curves for different operating voltages, namely 60 V, 42 V and 24 V. After the time of flight of an armature of a relay closes the normally closed contact of a changeover (changer) and forces the circuit to a voltage which is below the Z voltage on. Even during the bounce time of the normally closed contact there is a demagnetization of the load inductance (as can be seen from the curve of the curve 112), but the circuit follows the voltage changes, delayed only by the Miller effect of the semiconductor. A reset of the circuit arrangement takes place only when the capacitor C1 is discharged again. This is done via the resistor R10 and the diode D2 when, for example, the make contact 101 closes. After a renewed opening of the make contact 101, the circuit arrangement reacts again in the manner shown above and delays the voltage increase.

FIG. 11 shows in extracts the effect of the thyristor structure under changed load conditions. After a first demagnetization of the load inductance capacitor C1 is reset due to overshoot and freewheeling through the diode D1. A subsequent closing of the normally closed contact (here at t = 50.35 ms) forces a steep current increase in transistor T1 (curve 114 in the left part of the illustration, to the left of the auxiliary line) until at a value of about 16 A the thyristor fires , This value can be set with the reference voltage. The curve 116 shows with a rapid increase in current to about 2.5 A, an assumption of the control current supplied by the transistor T9 and, associated therewith, a rapid rise in voltage at the emitter of the Transistor T1 (curve 110 to the right of the auxiliary line). The further increase of the current in the Darlington transistor to about 20 A is due to the now beginning Miller effect of the rapidly increasing voltage. At the same time, however, the transistor T10 is also actively blocked so that only the Miller effect of the transistor T9 is effective. This can be recognized by the receding current in the thyristor (see drop of curve 116 below 1 A). In the following, a positive coupling becomes effective due to the decreasing firing voltage of the thyristor structure.

Claims (8)

1. Electrical circuit for preventing an arc over an electrical contact when the contact opens, the electrical circuit (100) being connected in parallel to the electrical contact (101) and including at least one transistor (T1) and at least one timing element (C1, R10),
   characterised in that
   the transistor (T1) takes over the current over the contact (101) immediately after the contact opens, and holds the voltage over the contact, for a time which is determined by the timing element (C1, R10), to a constant low voltage level below the arc voltage of an arc, until the contact is so widely opened that an arc no longer strikes,
   and in that the voltage then rises to a higher voltage level which is necessary to demagnetise a load inductor.
2. Electrical circuit according to claim 1, characterised in that the transistor (T1) is a bipolar transistor, the collector of which is connected to a first terminal, and the emitter of which is connected to a second terminal, of the contact (101), and the base of which is connected with low resistance, for a time which is determined by the timing element (C1, R10), via a second transistor (T2), to the first terminal of the contact (101).
3. Electrical circuit according to claim 1, characterised in that the transistor (T1) is a charge-controlled transistor, the collector/drain of which is connected to a first terminal, and the emitter/source of which is connected to a second terminal, of the contact (101), and the gate of which is connected, for a time which is determined by the timing element (C1, R10), via a second transistor (T2), to the first terminal of the contact (101).
4. Electrical circuit according to claims 1 and 2, characterised in that the second transistor (T2) is a bipolar transistor, the emitter of which is connected to the base of the bipolar transistor (T1), and the collector of which is connected to the first terminal of the contact (101), and the base of which is connected with low resistance, for a time which is determined by the timing element (C1, R10), via a monostable trigger circuit (T9, T10, C1, R10), to the first terminal of the contact (101).
5. Electrical circuit according to claims 1 and 3, characterised in that the second transistor (T2) is a bipolar transistor, the emitter of which is connected to the gate of the charge-controlled transistor (T1), and the collector of which is connected to the first terminal of the contact (101), and the base of which is connected, for a time which is determined by the timing element (C1, R10), via a monostable trigger circuit (T9, T10, C1, R10), to the first terminal of the contact (101).
6. Electrical circuit according to claims 4 and 5, characterised in that the monostable trigger circuit basically consists of a flipflop of mutually complementary transistors (T9, T10), the transistor (T9) being connected with its emitter to the first terminal of the contact (101), with its collector to the base of one of the complementary transistors (T10) and the base of the second transistor (T2), and with its base to the collector of one of the complementary transistors (T10), and the emitter of one of the complementary transistors (T10) being connected via a series circuit of a resistor (R10) and a capacitor (C1) to the second terminal of the contact (101), in that the capacitor (C1) is discharged before the contact (101) opens, and in that the flipflop is triggered by the sudden voltage rise over the contact (101) when the contact opens, by the reaction capacitances of the complementary transistors (T9, T10).
7. Electrical circuit according to claims 2 to 6, characterised in that the base/gate of the power transistor (T1) is connected via a Z diode (Z1) to the first terminal of the contact (101), to limit the voltage over the circuit (100).
8. Electrical circuit according to claim 6, characterised in that the emitter of one of the complementary transistors (T10) is connected via a diode (D2) to the first terminal of the contact (101), in such a way that the capacitor (C1) is discharged when the contact (101) closes again.

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