Arrangement for monitoring the operation of a heating element
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an arrangement for monitoring the operation of a heating element containing a series connection of a switch and at least one heating element in which the heating element is traversed by an alternating current of a first frequency in the conducting state of the switch, particularly for monitoring the heating element arranged in the fixing station of an electrostatic printer or copying device. 
     2. Description of the Prior Art 
     The monitoring of heating arrangements which contain a heating element, for example a radiant heater, is required, for example, in non-mechanical printing devices or, respectively, copying devices. Heating elements, for example radiant heaters, are employed in such devices in a fixing station in which toner images apply to a paper web are fused into the paper. The heating elements can be arranged within fixing drums or fixing saddles. In order to achieve faultless fixing of the toner images on the paper web, it is necessary to terminate the fixing when a malfunction occurs in the heating arrangement. An arrangement for monitoring the heating arrangement is therefore required. 
     SUMMARY OF THE INVENTION 
     The object of the present invention, therefore, is to provide an arrangement for monitoring the operation of a heating element. 
     The object of the present invention, therefore, is to provide an arrangement for monitoring the operation of a heating element. 
     Given an arrangement of the type initially mentioned, the above object is achieved in that a measuring element is arranged in the heating arrangement; in that an amplifier circuit for identifying the voltage across the measuring element is connected in parallel with the measuring element, the amplifier emitting a pulse-shaped clock signal when the voltage at the measuring element exceeds a threshold value; in that a flip-flop is provided and has its reset input connected to the amplifier; and in that an oscillator is provided which oscillates as a second frequency which is smaller in comparison to the first frequency of the voltage across the measuring element, the oscillator being connected to the dynamic input of the flip-flop. 
     Given an arrangement of the type initially mentioned, the above object is further achieved in that a measuring element is arranged in the heating arrangement; in that an amplifier for identifying the voltage across the measuring element is connected in parallel with the measuring element and emits pulse-shaped clock signals when the voltage across the measuring element exceeds a threshold value; and in that a counter is provided and has its counting input connected to the output of the amplifier and a reset input connected to a sensor which determines the termperature of the heating element and emits a sensor signal when the heating element lies below the desired temperature. 
     It is possible to identify the fault-free operation of the heating element by employing the flip-flop and the oscillator. When, in particular, no current flows through the heating element, although the heating element should be in operation, no clock signals are generated and, therefore, the flip-flop is no longer reset. Therefore, the flip-flop remains in the set state due to the signals from the oscillator. It is advantageous for sampling the output signal of the flip-flop by providing an AND circuit having one input connected to the flip-flop and another input connected to the oscillator by way of an inverter. With this structure, it is achieved in a simple manner that the output signal of the flip-flop is identified upon the occurrence of the trailing edge of the oscillator signal. 
     The monitoring of the heating element should only occur when it is intended that current be flowing through the heating element. For this reason, it is advantageous to block the flip-flop when the heating element is not to be actuated. A sensor can be employed for this purpose, the sensor determining the temperature of the heating element and emitting a sensor signal when the temperature of the heating element has not yet reached the desired value. The output of the sensor is connected to the reset input of the flip-flop by way of an inverter circuit. If, therefore, no sensor signal occurs, then a reset signal is applied to the flip-flop so that the flip-flop is blocked. 
     The counter can be employed for monitoring the switching element which, for example, can comprise a triac. If, in particular, the switching element, for example a triac, has its operation disrupted, there then exists the danger that current will constantly flow through the heating element. Clock signals are also constantly generated by the amplifier circuit in such a case, the signals being supplied to the counting input of the counter. If the sensor no longer emits a sensor signal, if, therefore, the heating element is no longer to be operated, then the counter is no longer reset. The result is that the counter can continue to count. When the counter has reached a specific counter reading, this is an indication that the switch element is no longer working properly. It is advantageous for evaluating the counter reading and therefore, for identifying the malfunction, to provide a second flip-flop which is connected via an AND circuit to two outputs of the counter. Therefore, the AND circuit can emit a signal when the counter has counted, for example, three clock signals and can therefore set the second flip-flop. 
     A line segment between the switch element and the heating element can be employed, for example, as the measuring element. In a simple manner, the amplifier circuit can then comprise a transformer connected parallel to the line segment, an operational amplifier connected to the secondary winding of the transformer, and a Schmitt trigger connected to the output of the operational amplifier. The Schmitt trigger emits a pulse-shaped clock signals. 
     If two heating elements must be connected in series, the heating elements can be employed as the measuring elements. The voltage across each heating element is identified with the assistance of a respective amplifier circuit which comprises an opto-coupler arranged parallel to the heating element and a Schmitt trigger connected to the output of the opto-coupler for generating the clock signals. 
     In order to indicate faultless function of the heating arrangement, a light emitting diode (LED) which is illuminated as long as a heating current flows, can be arranged in a simple manner at the output of the Schmitt trigger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the invention, its organization, construction and modes of operation will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, on which: 
     FIG. 1 is a schematic diagram of a first embodiment of the invention for an arrangement for monitoring a heating arrangement; and 
     FIG. 2 is a schematic diagram of a second embodiment of the invention for an arrangement for monitoring a heating arrangement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a heating arrangement comprises a switch element 10, for example a triac, and a heating element 12, for example a radiant heater, connected in series. The series circuit of the switch element 10 and the heating element 12 is connected between two operating potentials U1 and U2 of, for example, an alternating voltage of 50 Hz. When a signal is applied to a control input 14 of a switch element 10, the switch element 10 is rendered conductive and, therefore, an alternating current flows through the switch element 10 and the heating element 12. The heating element 12 is thus energized. 
     For monitoring the operation of the heating element 12, a measuring element 15 is connected in the series circuit of the switch element 10 and the heating element 12, the measuring element 14 here comprising a line segment. When a current flows through the series circuit, a voltage drop of approximately 50 mVolt occurs across the line segment 15. The voltage drop across the line segment 15 is identified with the assistance of an amplifier circuit 16. 
     The amplifier circuit 16 comprises a transformer 18 having a primary winding connected across the line segment 15 and a secondary winding which is connected to both inputs of an operational amplifier 20. The operational amplifier 20 is, in turn, connected to the input of a Schmitt trigger 22. With this structure, the Schmitt trigger 22 is operated to emit pulse-shaped clock signals at its output when the voltage drop across the line segment 15 exceeds a specific threshold value. The clock signal, therefore, always occurs at the frequency of the alternating current traversing the line segment 15. The clock signal can be derived from the negative half wave or the positive half wave of the alternating current. 
     The clock signals at the output of the amplifier 16, therefore, only occur when an alternating current flows in the series circuit of the switch element 10, the line segment 15 and the heating element 12. The clock signals are supplied to a flip-flop 24, in particular to its reset input R. An oscillator 26 is connected to the dynamic input of the flip-flop 24, the oscillator generating oscillator signals whose frequency is lower than the frequency of the alternating current. The flip-flop 24, therefore, is always set when an oscillator signals occurs at the output of the oscillator 26. However, it is always reset when a clock signal is emitted at the output of the amplifier circuit 16. Since the frequency of the clock signals is higher than the frequency of the oscillator signals, the flip-flop 24 is always reset as long as an alternating current flows through the series connection of the switch element 10, the line segment 15 and the heating element 12. If, in contrast thereto, alternating current does not flow in the series circuit, the flip-flop 24 remains set. 
     The output of the flip-flop 24 is sampled with the assistance of an AND circuit 28. To this end, the AND circuit 28 is connected with one of its inputs by way of an inverter 30 to the oscillator 26. This provides that, with the assistance of the AND circuit 28 and after the occurrence of the trailing edge of the oscillator signal, it is determined whether the flip-flop is set. If this is true, then a malfunction in the heating element 12 exists and the AND gate provides a corresponding signal, i.e. an alternating current should be flowing through the series connection, but is not. 
     Since the monitoring circuit for the heating element 12 is only meant to be switched on during the time in which the current must flow through the heating element 12, a sensor signal is supplied to the reset input R of the flip-flop 24 from a sensor 34 by way of an inverter 36 and an OR circuit 32. The sensor 34 can comprise a negative temperature coefficient (NTC) resistor which is arranged in the proximity of the heating element 12 and whose resistance depends on the temperature of the heating element. As long as the temperature of the heating element has not exceeded a specific value, the sensor 34 emits the sensor signal and thus indicates that an alternating current should flow through the heating element 12. Thereby, the sensor signal can be a binary &#34;1&#34;. The sensor signal is inverted in the inverter element 36 and is applied to the reset input R of the flip-flop 24 by way of the OR circuit 32. Due to the inversion, the &#34;1&#34; becomes a &#34;0&#34; and, therefore, the flip-flop 24 is not influenced by the sensor 34. If, to the contrary, no heating is to be undertaken, i.e. no current should flow through the heating element 12, then the sensor signal is a binary &#34;0&#34;. This signal is inverted by the inverter element 36 into a binary &#34;1&#34; and thus resets the flip-flop 24. Therefore, as long as the sensor signal is a binary &#34;0&#34;, the flip-flop 24 cannot be set by the oscillator 26. With such a control, the monitoring circuit for the heating element 12 is switched off when no current is meant to flow through the heating element 12. 
     Up to this point, that portion of the arrangement has been described which is required for monitoring the heating element 12. The heating arrangement, however, also comprises the switch element 10 which must likewise be monitored with respect to its operation. If the switch element 10 no longer operates faultlessly, if, therefore, it allows current to pass even though the current should be blocked, then a current will also flow through the heating element 12 when there is no intent to heat. In order to be able to identify this malfunction, a counter 38 is provided. The output of the amplifier circuit 16 is connected to the counting input Z of the counter 38. The reset input R of the counter 38 is connected to the sensor 34. 
     As long as alternating current flows through the heating element 12, clock signals are generated at the output of the amplifier circuit 16 and are supplied to the counter 38. When, further, the sensor 34 emits a binary &#34;1&#34; signal and, therefore, indicates that the heating element 12 should be heated, this signal is applied to the reset input R of the counter 38 and constantly resets the counter. In this error-free state, therefore, the counter 38 cannot count. If, in contrast thereto, a current flows through the heating element 12 although the heating element 12 has already exceeded the desired temperature, then the sensor 34 emits a binary &#34;0&#34; signal. The binary &#34;0&#34; signal is, in turn, supplied to the reset input R of the counter 38 and, therefore, the counter 38 is no longer reset. Since clock signals arrive from the amplifier circuit 16 at the same time, the counter 38 is exercised to count up. A malfunction of the switch element is thus indicated. 
     An AND circuit 44 can be connected to the outputs of the counter stages in order to sample the error reading. By an appropriate connection of the counter stages to the AND circuit 44, it can be achieved that the AND circuit emits a signal when the counter 38 has reached a predetermined counter reading. The signal from the AND circuit 44 can be supplied to a further flip-flop 46 at its set input S. The switch element 10 always operates faultlessly when the flip-flop 46 is set. 
     In FIG. 1, a line segment 15 has been employed as the measuring element. Referring to FIG. 2, a heating element itself is employed as the measuring element. This is advantageous when two heating elements must be connected in series. When the switch element 10 is rendered conductive, the series connection of the heating elements 50, 51 is connected between the alternating voltage U1, U2. As long as the heating elements 50, 51 operate faultlessly, the voltage across each heating element 50, 51 corresponds to half the alternating voltage. If, on the other hand, a heating element 50, 51 is defective, then the entire alternating voltage lies across this heating element. The voltage at the other heating element, therefore, has a small value. 
     The voltage across the heating elements 50, 51 is supplied to a respective opto-coupler 52, 53 which respond as long as the voltage across the assigned heating element 50, 51 does not assume the small value. The output of the opto-couplers 52, 53 is connected to a respective Schmitt trigger 54, 55 which emit clock signals which are supplied by way of a NAND circuit 56 to the flip-flop 24 or, respectively, to the counter 38 of FIG. 1. According to FIG. 2, therefore, the amplifier circuit comprises the opto-coupler 52 or, the opto-coupler 53 and the respective Schmitt trigger 54 or 55, and the NAND circuit 56. The further evaluation occurs in the manner described with respect to FIG. 1. 
     A display circuit comprising a resistor 58 and a LED 60 can be arranged at the output of the Schmitt triggers 52, 53, 54, 55. The LED 60 is illuminated as long as clock pulses are emitted, i.e. as long as a heating current flows. 
     Although we have described our invention by reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. We therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of our contribution to the art.