EP0200099B1 - Control circuit for an electromagnetic relay to interrupt an ac circuit under tension - Google Patents

Abstract

In a control circuit for a relay for switching an AC load, the zero transition of the AC voltage is sampled and is used to control the relay at a predetermined response time before the next zero transition. In this case, the relay is connected via a timer (R1, C1) to a supply voltage (UN) which is significantly greater than the rated voltage of the relay, such that when the semi conductor switch is switched on, a voltage surge initially occurs on the relay winding which reduces the range of variation of the response time to a minimum level. In this way, the load circuit can be switched on and off fairly precisely in the vicinity of the zero transition. <IMAGE>

Description

The invention relates to a control circuit for an electromagnetic relay according to the preamble of claim 1.

Electromagnetic relays are generally ideal for switching electrical loads on the AC network, their high dielectric strength for separating the control and load circuits and their insensitivity to voltage peaks and current peaks in the switched load circuit having a particularly advantageous effect. The errors that usually occur in semiconductors in the load circuit cannot arise with relays, which often reduces the monitoring effort. However, relays that switch AC loads also have problems and disadvantages. The contact life is severely impaired by high cold currents, especially when switching on incandescent lamp loads, and when switching off due to arcing until the following mains voltage zero crossing.

Attempts have already been made repeatedly to extend the contact life of relays when switching AC voltage loads by moving the switching time to the zero point of the AC voltage. Because the relay each after switching on or switching off the excitation voltage, a certain response time or fall time is required until the contact of the load circuit closes or opens, in this case the relay winding must be applied to the control voltage or disconnected from it at this time before the zero crossing. The zero crossing of the AC voltage can be determined exactly, and an exact switching point in time before the zero crossing could also be determined via timers, but such circuits have so far not been successfully used because the response times of the relays vary widely, even within and the same type. These variations in the response times are often of the order of magnitude of an AC half-wave, so that the response time determined for one relay cannot be transferred to the next of the same type. However, the response times also fluctuate with one and the same relay, in particular due to the ambient conditions and due to aging.

In order to avoid the problem of contact material migration with the same polarity of the switched voltage at the moment of switching, it is proposed in US Pat. No. 3,707,634 to switch a relay independently of the actual zero crossing of the alternating voltage in the load circuit in such a way that each switching operation has one switch over the previous one There is a phase shift of 180 °. This is to ensure that the material migration of the contact surfaces is reversed after each switching operation and so the contact wear is compensated to a certain extent. However, such a circuit not only requires an increased outlay on measuring and switching elements, but it is also known that the material contact in two opposite directions does not nevertheless restore the original contact surface, so that the extension of the service life is not achieved in a satisfactory manner becomes.

A zero crossing circuit is known from European patent application EP-A-108 538, in which one of two solenoids (electromagnets of a contactor) is optionally driven with a short pulse in order to either switch a load on or off. This pulse is an output signal which is synchronized with the zero crossing point when a corresponding switch is actuated. For this purpose, an electronic circuit is provided, which brings about a predetermined delay. These pulses arrive from the outputs of the IC via an electronic circuit arrangement made up of several transistors for pulse amplification to the windings of the electromagnets. With the known circuit it is possible to switch a load as close as possible to the zero point, but it is not intended for the control of a conventional relay, which should have the fastest possible response behavior, in order to prevent scattering of the relays with different response characteristics from being effective.

DE-A-2 929 261 describes a device for actuating a relay which has a drive circuit with a transistor at one point of the relay winding and a capacitor and resistor at the other relay winding. However, this known circuit causes the relay to respond very slowly in order to achieve the lowest possible current consumption. An accelerated response of a relay with a DC supply voltage that is higher than the nominal voltage of the relay winding is therefore not possible.

The object of the invention is therefore to provide a control circuit of the type mentioned, the switching of the relay can be achieved with a relatively small amount of circuitry even with a large spread of the response sides within the same relay types close to the zero crossing, so that the life of the relay contacts are considerably increased can.

According to the invention, this object is achieved in that the one connection point of the relay winding is connected to ground potential via the switching path of the semiconductor switch and to a DC supply voltage via a reverse polarized diode, in that the other connection point of the relay winding is connected to ground potential via a capacitor and via a resistor is connected to the DC supply voltage and that the DC supply voltage is significantly higher than the nominal voltage of the relay winding.

The invention makes use of the knowledge that a relay not only responds faster when overexcited, that is, when excited with an excessive control voltage, but that the spread of the response times can also be greatly reduced. With the circuit arrangement according to the invention, the relay winding is applied to such an overvoltage, the special type of circuit simultaneously ensuring that the overvoltage is dissipated very quickly, so that at the latest after the relay armature has been tightened, only a voltage is present on the winding at most corresponds to the nominal voltage or is preferably lower than this. Due to the small fluctuation in the response times, the point in time for the activation can then be determined quite precisely, which can be done with conventional semiconductor switching means.

A comparable result is obtained when the control voltage is switched off Effect, since at the time of the switch-off according to the law of induction there is initially an increased voltage in the opposite direction at the relay winding, which is rapidly reduced by the current through the diode and the resistor and via the diode and the capacitor, which shortens the fall time. This means that the switching time of the contact can be moved shortly before the zero point even when the AC load is switched off, so that no or only slight arcs can occur.

The dimensioning of the DC supply voltage and the circuit components naturally depends on the properties of the relay and also on how exactly the zero point control is desired. However, an embodiment has proven to be advantageous in which the DC supply voltage is twice as high as the nominal voltage of the relay winding, a fluctuation range of the response time of 2 ms being set by appropriately dimensioning the capacitor and the resistor. The resistance can be coordinated with the relay winding so that, for example, only a holding voltage of approximately 2/3 of the nominal voltage is applied to the relay winding after the response.

• Fig. 1 eine erfindungsgemäße Ansteuerschaltung für ein Relais,
• Fig. 2 ein Zeitdiagramm für den an der Relaiswicklung anliegenden Spannungs- und Stromverlauf.

The invention is explained in more detail using an exemplary embodiment with reference to the drawing. It shows

• 1 shows an inventive control circuit for a relay,
• Fig. 2 is a timing diagram for the voltage and current applied to the relay winding.

1 shows a load circuit with an incandescent lamp L which is connected between a phase conductor P and a neutral conductor N via a contact k. An AC voltage U _{w is present} at the phase conductor P.

The contact k is part of a relay with the winding K, which is connected between the two connection points 1 and 2. A diode D1 is connected in the reverse direction between a supply voltage U _v and point 1, while a resistor R1 is connected between point 2 and supply voltage U _v . In addition, the collector-emitter path of a transistor TR lies between point 1 and the ground potential, while at the other terminal of the relay winding a capacitor C1 is connected between point 2 and the ground potential. The relay is driven by a lamp signal ls, which is applied to the base of the transistor TR via a synchronization memory SP in the form of a D flip-flop and a series resistor R2.

The synchronous memory SP is driven by a clock which is derived from a zero crossing sampling of the alternating voltage U _w . For this purpose, the alternating voltage U _{w is} supplied to a phase detector PD, which generates a pulse i1 at each zero crossing of the voltage. These pulses i1 are delayed by a certain time via a delay element VG, so that the pulses i2 emitted at the output of the delay element VG are each before the next zero crossing by the expected response time of the relay. With such a pulse i2, the lamp signal ls present at input D of memory SP is switched through to output Q, making transistor TR conductive.

Fig. 2 shows the voltage and current profile at the relay in one embodiment with the dimensions given in parentheses in Fig. 1. Thereafter, a DC supply voltage U _v of 24 V is used for a relay K with the nominal voltage 12 V and a winding resistance of 215 ohms. Resistor R1 has a value of 390 ohms and capacitor C1 has a capacitance of 22 µF.

2 shows the state of the memory output Q, corresponding to the on-state of the transistor TR, over the time axis t. If Q takes the value 1, the transistor is conductive, if Q = 0, the transistor TR is blocked. Below this, the voltage curves U 1 at point 1 of the relay winding K and U 2 at point 2 of the relay winding are shown, as well as the current curve I _K through the relay winding. Finally, the switching state of the contact k is specified; in state 0 the contact is open, in state 1 it is closed.

Various times are indicated on the time axis as a function of the respective zero crossing of the alternating voltage U _w , the time of the zero crossing being denoted in each case by T₀. Assuming that an AC voltage of 50 Hz is used, a zero crossing occurs every 10 ms. The specified overvoltage ensures that the relay contact closes approximately 2.5 ms to 4.5 ms after the transistor TR has become conductive; bounce times and tolerances are already included. The circuit with the delay element VG is thus set such that the transistor is controlled in each case 6.5 ms after a zero crossing of the AC voltage. Then the relay contact closes between 1 ms before and 1 ms after the next zero crossing.

As long as the transistor TR is blocked, the capacitor C1 is fully charged, so that the full voltage of 24 V is applied to both ends of the relay winding K (U 1 = U 2 = 24 V). At the time 6.5 ms after T₀, the transistor TR becomes conductive, and the voltage U 1 at point 1 practically drops to 0. Thus, the charged capacitor C1 discharges via the winding K to approximately 8.5 V in accordance with the voltage divider ratio between R1 and K. During this discharge, a current spike initially occurs of the current I _K - with a saddle at the moment of the armature movement - which allows the relay to pick up quickly. Then the excitation current I _{K decays} to the value determined by the direct voltage U _v and the resistors R1 and K and reaches approximately the response current of the relay (approx. 40 mA). As mentioned, the relay contact k closes approx. 2.5 ms to 4.5 ms after the time T _s at which the transistor became conductive (6.5 ms after T₀). The next zero crossing at time T₀ thus coincides approximately with the closing of the contact.

When the lamp L is switched off, the conductive transistor TR is blocked by the signal Q which has fallen to 0, again at the time T _s , ie 6.5 ms after the zero crossing at the time T₀. The energy stored in the relay has due to the initially high voltage at the relay (U₂ - U₁ = 24 V - 8 V = 16 V) through the relay winding via D1 and R1 or C1 and D1 a rapidly decaying current I _K , which the relay drops quickly. Relay tolerances and tolerances of the supply voltage U _v have little influence on the fall time. Despite the higher voltage at K, the collector-emitter voltage U 1 increases only by a (negligible) diode voltage above the supply voltage U _v of 24 V. In this case, the relay contact opens approx. 2.0 ms to 3.0 ms after transistor TR is blocked (tolerances included) and thus approx. 1.5 to 0.5 ms before the next zero crossing at time T₀ of the mains voltage wave. This ensures that the lamp load is in any case switched off shortly before the zero crossing, so that an arc which may still occur is extinguished in the zero crossing of the mains half-wave, ie can no longer be present.

The inrush current of the incandescent lamp can be reduced to approximately 1/10 of the maximum by the control circuit according to the invention The value is reduced and the arcing is shortened to the short duration (in the example, a maximum of 1.5 ms) until the zero crossing. In this way, the contact life can be increased significantly.

Claims (5)

1. Control circuit for an electromagnetic relay for switching an alternating voltage load circuit near to the zero crossing point which is scanned by means of a phase detector (PD), a semiconductor switch (TR) located in the control circuit of the relay being controlled in chronological dependence on the phase relation of the alternating voltage (U_w), characterised in that one connecting point (1) of the relay winding (K) is connected to earth potential via the switching path of the semiconductor switch (TR) and to a direct supply voltage (U_v) via a diode (D1) poled in the blocking direction, in that the other connecting point (2) of the relay winding (K) is connected to earth potential via a capacitor (C1) and to the direct supply voltage (U_v) via a resistor (R1), and in that the direct supply voltage (U_v) is substantially higher than the nominal voltage (U_N) of the relay winding (K).
2. Control circuit according to Claim 1, characterised in that the direct supply voltage (U_v), the capacitor (C1) and the resistor (R1) are dimensioned such that the operate time of the relay (K) has a deviation range of less than 2 ms, and in that the semiconductor switch (TR) is controlled with such a time delay with respect to a zero crossing point of the alternating voltage (U_w) that the closing of the contact (k) occurs in the period between 1 ms before and 1 ms after the next zero crossing point of the alternating voltage (U_w).
3. Control circuit according to Claim 1 or 2, characterised in that the direct supply voltage (U_v), the capacitor (C1) and the resistor (R1) are dimensioned such that the drop-out time of the relay (K) has a deviation range of less than 1 ms, and in that the semiconductor switch (TR) is controlled with such a time delay with respect to a zero crossing point of the alternating voltage (U_w) that the opening of the contact (k) occurs in the period between 1.5 and 0.5 ms before the next zero crossing point of the alternating voltage (U_w).
4. Control circuit according to one of Claims 1 to 3, characterised in that the supply voltage (U_v) is approximately twice as high as the nominal voltage of the relay winding (K).
5. Control circuit according to one of Claims 1 to 4, characterised in that the resistance values of the resistor (R1) and of the relay winding (K) are matched to one another such that when the transistor is switched through, a voltage of approximately 1/3 of the nominal voltage is present at the relay winding.

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