Abstract:
The invention relates to a circuit arrangement for operation of a relay which provides a switching-on current for the field coil of the relay and subsequently, after a predetermined time, a holding current. In order that any possible discrepancies in the parameters of the relay from their nominal values have no negative influence and need not be taken into account when designing the circuit, and in order that the circuit arrangement can be used with operating voltages of different magnitude, at least one constant current source supplies the relay switching-on current and the relay holding current.

Description:
CLAIM FOR PRIORITY 
     This application claims priority to International Application No. PCT/DE00102436 which was published in the German language on Jul. 19, 2000. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a circuit arrangement for operation of a relay, and in particular, to a current having a timer unit which provides a relay switching-on current for a time which is predetermined by the timer unit and which provides a relay holding current, which is less than the relay switching-on current for a subsequent holding period. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,107,391 discloses a circuit arrangement in which the current flowing through at least one relay (i.e. its field coil) is controlled by means of an electronic switch in the form of a field-effect transistor. When it is switched on, the relay is supplied with a holding current throughout its holding period. The magnitude of the holding current is governed by the duty ratio of electrical pulses which drive the electronic switch. The temperature of the relay is measured by means of a temperature sensor, and the voltage which is applied to the field coil of the relay is measured by means of a voltage sensor. These measurement variables, as well as information which is stored in a function memory and relates to the nominal values of the relay, are used to define the duty ratio of the pulses, and thus to define the magnitude of the holding current. The relay nominal values must therefore be known in order to properly operate the circuit. A voltage change which occurs on the field coil of the relay is used to identify that the relay has been switched on, and causes a timer unit to be started. This timer unit uses a continuous pulse to switch on the electronic switch, so that a switching-on current flows which is sufficient to ensure that the relay is switched on. Once the time which is predetermined by the timer unit, and which must be longer than the time required for the relay that is being used to be switched on has elapsed, the continuous pulse ends, and the holding current, which is governed by the duty ratio of the pulses, still flows through the relay. 
     In order to ensure that a sufficient holding current flows to hold the relay in the switched-on state, the circuit takes account of the voltage across the field coil, the temperature and the nominal values for the relay. Individual discrepancies from the nominal values for the relay, in particular discrepancies in the coil resistance, are ignored, however. Discrepancies such as these can occur, for example, during the manufacture of the relay, due to aging processes during operation, or due to oxidation of conductors and contacts of the field coil. 
     Furthermore, although the circuit is able to take account of fluctuations in the operating voltage by measuring the voltage across the field coil of the relay, the circuit is, however, intended for operation from a voltage source with a predetermined nominal voltage, for example from a motor vehicle battery with a voltage of 12 V. Circuits with components of different sizes are therefore required to operate relays from different operating voltages. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention discloses a circuit arrangement for operation of a relay. Discrepancies in the relay from its nominal values have no effect on the magnitude of the switching-on current and holding current, and need not be taken into account with regard to the size of the components in the circuit. Furthermore, it is possible to use the cireuit, with one and the same relay, from operating voltages of different magnitude. 
     In this embodiment, the relay switching-on current and the relay holding current are constant currents, which are supplied from at least one constant current source. A constant current is thus used both as the relay switching-on current and as the relay holding current, whose magnitude is not influenced either by discrepancies in the relay from its nominal values or by operation of the cireuit arrangement from operating voltages of different magnitude. 
     A constant current source whose constant current magnitude is variable can be used to supply the constant relay switching-on current and the constant, lower relay holding current. The constant current source supplies the relay switching-on current from the start of the switching-on process. Once the relay has been switched on and the time which is predetermined by the timer unit has elapsed, the constant current is reduced to the relay holding current. 
     In another embodiment of the invention, the cireuit arrangement includes, for example, a first constant current source which provides the relay holding current during the time which is predetermined by the timer unit and during the subsequent holding period, and a second constant current source which provides a constant current, which is superimposed on the relay holding current in order to form the relay switching-on current, during the time which is predetermined by the timer unit. This has the advantage that relatively simple constant current sources can be used, whose constant currents need not be variable. 
     In one aspect of the invention, a switch, which is closed during the time that is predetermined by the timer unit, can advantageously be located in the current path of the constant current provided by the second constant current source. Once this time has elapsed, the switch is opened, thus making it easy to switch between the relay switching-on current and the relay holding current. 
     In another aspect of the invention, the timer unit and/or the switch may require an auxiliary voltage. A voltage drop across at least one electrical component that is connected in series with one of the constant current sources can advantageously be used for this auxiliary voltage. 
     In one embodiment, a resistor may be used as the electrical component. Since the current flowing through the resistor is constant, the voltage which is dropped across this resistor is also constant, and may be used as an auxiliary voltage. 
     In another embodiment, a zener diode can advantageously be used as the electrical component. A zener diode has the advantage that the voltage drop which occurs across it is constant even when the current flowing through the zener diode changes. By way of example, a situation may arise during operation of a relay which requires a change to the switching-on current and/or holding current, and hence appropriate changes to the constant currents. The advantage just mentioned is also applicable when series-connected diodes are used as electrical components. 
     In the circuit arrangement according to the invention, both constant current sources may, for example, supply a constant current of the same magnitude. In this case, the relay switching-on current is twice the magnitude of the relay holding current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an exemplary embodiment of a cireuit arrangement according to the invention for operation of a relay. 
     FIG. 2 shows a diagram of the state of elements in the cireuit arrangement, plotted against time. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The circuit arrangement  1  illustrated in FIG. 1 for operation of a relay is connected in series with a field coil SP of a relay. A voltage U is applied to the series cireuit. The voltage U is a switching voltage, that is to say application of the voltage U is intended to cause the relay to switch. At the same time, the voltage U is used as the operating voltage for the cireuit arrangement and the relay. As long as no switching voltage U is applied, the cireuit arrangement is in a quiescent state with no current flowing, and a switch SCH is closed. 
     When the switching voltage U is applied, the cireuit arrangement starts to operate. A constant current source KS 1  drives a constant current IK 1  through a zener diode ZD and through the field coil SP of the relay. Since the switch SCH is closed, a constant current source KS 2  also drives a constant current IK 2  through the switch. This current is added to the constant current IK 1  at a node  2 ; a current whose magnitude is (IK 1 +IK 2 ) flows, as a coil current ISP, through the field coil SP of the relay. The constant current source KS 1  is designed such that it supplies a constant holding current. The constant current source KS 2  supplies the difference to make up the necessary switching-on current for the relay, in this case, the difference is of precisely the same magnitude as the relay holding current. The switching-on current now flows through the field coil SP of the relay and the relay pulls in, that is to say it switches. The constant current IK 1  which is flowing through the zener diode ZD results in an auxiliary voltage being dropped across the zener diode ZD, and this is supplied via the conductors  3  and  4  to a timer unit ZE as a supply voltage UTH. When the switching voltage U is applied, the timer unit ZE starts to operate and, after an adjustable time which is greater than the switching-on time of the relay that is being used, opens the switch SCH via a link  5 . Since the switch SCH Since the switch SCH is open, the constant current IK 2  from the constant current source KS 2  can no longer flow. Rather, the constant current IK 1  from the constant current source KS 1  flows through the field coil SP of the relay, as a holding current. This means that, once the switching-on process has been completed, the holding current is applied to the field coil SP and, in this case, this holding current is half the relay switching-on current. 
     Thus, during the switching-on process, the constant current IK 1  from the constant current source KS 1  and the constant current IK 2  from the constant current source KS 2  flow through the relay with the field coil SP. After completion of the switching-on process, the constant current IK 1  from the constant current source KS 1  flows through the relay. The magnitude of the currents IK 1  and IK 2  is governed by the constant current sources KS 1  and KS 2 . Any discrepancies, for example between the coil parameters and their nominal values, do not influence the magnitude of the currents. 
     Since the currents IK 1  and IK 2  which are present are constant, the voltage drops across the field coil SP of the relay and across the zener diode ZD are also constant. If the cireuit arrangement is operated with switching voltages U of different magnitude, then the difference between the switching voltage U and the voltage drops just mentioned is dropped across the constant current sources KS 1  and KS 2 . The cireuit arrangement can thus be operated from a switching voltage U which varies within wide limits but without any change to the magnitude of the relay switching-on current or the relay holding current. 
     When the circuit is disconnected from the switching voltage U, the circuit changes back to its quiescent state, with no current flowing. The timer unit ZE is reset, and the switch SCH is closed. The relay changes back to its quiescent position. Immediately after completion of these processes, the circuit can be actuated by applying a switching voltage U once again. 
     The state of the switching voltage U is shown at the top of the diagram in FIG. 2, with the state of the output from the timer unit ZE being shown underneath this, followed by the state of the switch SCH and, underneath this, the profile of the coil current ISP, plotted against time. Three times t 1 , t 2  and t 3  are marked on a horizontal time axis. The switching voltage U is applied to the circuit arrangement at the time t 1 ; the time (t 1 ), which is predetermined by the timer unit ZE, lapses at the time t 2 , and the circuit arrangement is disconnected from the switching voltage U once again at the time t 3 . 
     During the time interval between the times t 1  and t 2 , the switching voltage U is applied to the circuit arrangement, the timer unit is operating, and the time which is predetermined by the timer unit is counting down; the switch is switched on, and a coil current ISP, which is composed of the sum of the constant currents IK 1  and IK 2 , flows through the relay. This coil current is the switching-on current ES. 
     The switching voltage is likewise applied to the cireuit arrangement in the time interval between the times t 2  and t 3 . The timer unit switches off the switch SCH at the time t 2 , and the current IK 1 , which forms the holding current HS, flows through the relay, as the coil current ISP. 
     The circuit is in the quiescent state, with no current flowing, at times before t 1  and at times after t 3   
     The circuit can drive a relay reliably over a wide temperature range, since the constant current sources KS 1  and KS 2  provide the constant currents IK 1  and IK 2  irrespective of the temperature level. Temperature-dependent changes in the resistance of the field coil SP likewise do not influence the current levels. Since the constant holding current during the holding period of the relay is less than the constant switching-on current (for example half of it), the cireuit arrangement requires a fraction (for example approximately half) of the energy which would be required for operation just with a current of the same magnitude as the switching-on current. The power losses are reduced, the thermal loads on the relay are reduced, and the life of the relay coil is increased.