Electronic switching arrangement
BACKGROUND OF THE INVENTION 
     The present invention refers to an electronic switching arrangement, especially for controlling a coil of an electromagnet for moving an armature. In particular, the present invention relates to an electronic switching arrangement of the type having a voltage supply source charging a capacitor for activating a power transistor after comparison of the charging voltage with a reference voltage for closing the circuit through the coil. 
     It is generally known that a solenoid or electromagnet requires much more power for drawing the armature than for maintaining the attractive position. Proposals were made to generate a momentary current flow by a capacitor which is charged to a charging voltage in order to provide the electromagnet with the required attractive force for drawing the armature. After generating the attractive force, the coil is supplied only with a lower holding current. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved switching arrangement of the above mentioned type by which the power consumption of the electromagnet is reduced and unintentional armature movements are detected and evaluated. 
     This object, and others which will become apparent hereinafter, is attained in accordance with the present invention by incorporating a switching circuit parallel to the coil by which an unintentional movement of the armature is detected and suitable countermeasures are initiated. 
     Preferably, the switching circuit includes a voltage divider in form of two resistors arranged parallel to the coil. The medium voltage of the voltage divider is connected to the base of a transistor, the emitter of which is connected to the positive supply of a power source and the collector of which is connected to the negative supply thereof. 
     Through incorporation of a further transistor and voltage divider parallel to the coil, the position of the armature can be maintained, i.e. in the drawing mode, and possible undesired movements thereof can be detected and suitably corrected. 
     Preferably, a resistor is connected between the negative supply, on the one hand, and the collector of the transistor as well as the gate of the power transistor, on the other hand. 
     Preferably, the switching arrangement further includes a voltage divider in form of two resistors in parallel relationship to the capacitor. The medium voltage of this voltage divider is applied across one input of an operational amplifier, with the other input thereof being connected to a reference voltage source in form of a zener diode. The output of the operational amplifier is connected via a diode to the gate of the power transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of the present invention will now be described in more detail with reference to the accompanying drawing in which the sole FIG. 1 is a schematic circuit block diagram of a switching arrangement in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a schematic circuit block diagram of an electronic switching arrangement or circuitry according to the invention, including a coil L1 which is part of a solenoid or electromagnet and is energizable to move or draw an armature (A) for actuating a suitable element (not shown). The coil L1 is connected via lines 12, 14 to a power supply, such as a DC source, to supply a voltage U o , with line 12 leading to the positive supply +U o  and line 14 leading to the negative supply or ground -U o  and being connected to the source of a power transistor (MOS transistor or MOSFET) V3. 
     Arranged between the lines 12 and 14 is a capacitor C1 and a voltage divider in form of two resistors R1 and R2 which are connected in series and arranged parallel to the capacitor C1. In parallel relationship to the voltage divider R1, R2 between the lines 12, 14 is a reference voltage source in form of a zener diode V1 which provides a reference voltage, and a resistor R3 which is in series with the zener diode V1 and serves for current limitation and thus for a temperature-independent voltage stabilization of the zener diode V1. 
     Further connected to the lines 12, 14 is an operational amplifier N1 which has one input connected to a point 16 between both resistors R1 and R2 and another input connected to a point 18 between the resistor R3 and the zener diode V1. The output of the operational amplifier N1 is connected to the gate of the power transistor V3 via a diode V2. 
     Further incorporated in parallel relationship to the coil L1 is a switching circuit which includes a voltage divider in form of resistors R4, R5 arranged in series and a transistor V4, the base of which is connected to the voltage divider at point 20 between both resistors R4, R5. The emitter of the transistor is connected via the line 12 to the positive supply +U o  and the collector is connected via a resistor R6 to the line 14 and thus to the negative supply -U o . The line leading from the diode V2 to the gate of the power transistor V3 is linked at point 22 with the line leading from the transistor V4 to the resistor R6 so that the gate of the power transistor V3 is connected to the negative supply -U o  via the resistor R6. 
     A protective or clamping diode V5 is further provided parallel to the coil L1 in order to avoid voltage peaks during switch off and thus to prevent destruction of the power transistor. 
     After having described the individual parts of the electronic switching arrangement according to the invention, its mode of operation is now set forth. 
     Through application of the operating voltage U o  from the power source across the capacitor C1, the capacitor C1 is charged. When the capacitor C1 reaches its charging voltage, i.e. when the voltage at point 16 exceeds the reference voltage at point 18 as applied by the zener diode V1, the operational amplifier N1 is actuated to activate the power transistor V3. 
     The resistor R3 merely serves for current limitation and thus for a temperature-independent voltage stabilization of the reference zener diode V1. 
     The power transistor V3 thus closes the circuit through the coil L1. The stored charge of the capacitor C1 is sufficient to allow flow of necessary current through the coil L1 for a limited period in order to provide the attractive force for the armature A. The activation of the power transistor V3 and the inclusion of the coil L1 in the circuit results in the voltage U o  dropping to the coil voltage UL across the coil L1 and in a voltage drop across the power transistor V3. 
     The operational amplifier N1 would now block the power transistor V3 since the medium voltage of the voltage divider R1, R2 would become smaller than the voltage of the zener diode V1. 
     However, the voltage divider R4, R5 now activates the transistor V4, with the collector of the transistor V4 being connected via point 22 with the gate of the power transistor V3 to ensure activation of the power transistor V3 after drop of the voltage across the power transistor V3 whereby the diode V2 prevents the operational amplifier N1 from controlling the power transistor V3. In this manner, the attractive position of the armature A of the solenoid is maintained. 
     The voltage divider R4, R5 serves however also another purpose. If, for some reason, the armature A of the solenoid is unintentionally moved during operation, e.g. through shaking or vibrations, the movement induces a current in the coil L1. As a result of this induced current, a voltage fluctuation is encountered across the voltage divider R4, R5. When the voltage drop across the resistor R4 becomes smaller than the base-emitter-voltage of the transistor V4, then the transistor V4 is OFF, with the power transistor V3 now being connected to the negative supply -U o  only via the resistor R6 and thus is OFF as well. The switch-off of the power transistor V3 enables the capacitor C1 to recharge. 
     In this manner, the switching circuit is able to recognize a movement of the armature, e.g. caused through external influence, with voltage peaks which are generated by self-induction being utilized to detect whether or not the armature is still in its attractive position. 
     As a result of the evaluation of the armature movement, a new charging step is initiated which causes the dropped armature of the solenoid to be automatically drawn again. 
     Thus, the switching arrangement according to the invention is capable of recognizing and automatically evaluating an unintentional movement of the armature. 
     Moreover, the voltage divider R4, R5 is capable of monitoring a minimum current with regard to the coil L1. If the current in the coil L1 drops below a predetermined value, the voltage divider R4, R5 responds to the voltage drop across the coil L1 in a same manner as described above in order to automatically draw again the armature A. 
     While the invention has been illustrated and described as embodied in an electronic switching arrangement, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.