Abstract:
Apparatus and method for monitoring a self-regulating heater comprising a PTC conductive polymer while it is being powered by a power supply signal. The apparatus is connected to a power supply and to the heater, the heater then producing a heater signal which is fed to a comparator. The comparator deterines the phasal relationship between the power supply signal and the heater signal and indicates when the signals are out of phase by a predetermined magnitude.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to conductive polymer devices and in particular to apparatus and method for testing and detecting a fault condition in such devices. 
     2. Introduction to the Invention 
     Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, are well known. Reference may be made, for example, to U.S. Pat. Nos. 3,793,716, 4,177,376, 4,177,466, and 4,421,582, the disclosures of which are incorporated herein by reference. 
     Apparatus and method for detecting fault conditions in conductive polymer devices are known and are disclosed in commonly assigned U.S. Ser. No. 324,470 (Rhodes), now U.S. Pat. No. 4,506,259, in which a device is tested by evaluating the high frequency noise which it generates. 
     SUMMARY OF THE INVENTION 
     I have now discovered that excellent testing results for conductive polymer devices can be obtained through the use of my apparatus and method. 
     In one aspect, the present invention provides apparatus for detecting a fault condition in a conductive polymer device while it is being powered by a power supply signal, the apparatus comprising 
     (1) means for connecting the apparatus to the power supply; 
     (2) means for connecting the apparatus to the device; and 
     (3) a comparator which, when the apparatus is connected to the power supply and to the device, the device then producing a device signal which is fed to the comparator, 
     (a) determines the phasal relationship between the power supply signal and the device signal for signals that have a predetermined amplitude; and 
     (b) indicates when the power supply signal and device signal are out of phase by a predetermined magnitude. 
     Preferably, the power supply signal is a voltage signal and the device signal is a current signal, although the power supply signal may be a current signal and the device signal may be a voltage signal. 
     The device may be, for example, a self-regulating conductive polymer heater comprising a PTC conductive polymer, and may be part of a device that is heat shrinkable. The heat may be part of a device that is heat shrinkable. The heat may be adapted to be connected to a DC or AC power supply. In the latter case, the AC power supply signal is periodic and may be, for example, sinusoidal, triangular or sawtooth. The comparator can determine the phasal relationship between the power supply signal and the device signal between a preselected portion of a 360° period, for example, 0° to 180°. 
     Preferably, the apparatus comprises means for disconnecting the device from the apparatus when the power supply signal and the device signal are out of phase by a predetermined magnitude, for example, at least 15 degrees. 
     In another aspect, the present invention provides an assembly comprising 
     (a) a power supply; 
     (b) a conductive polymer device; and 
     (c) an apparatus comprising 
     (1) means for connecting the apparatus to the power supply; 
     (2) means for connecting the apparatus to the device; and 
     (3) a comparator which, when the apparatus is connected to the power supply and to the device, the device then producing a device signal which is fed to the comparator, 
     (a) determines the phasal relationship between the power supply signal and the device signal for signals that have a predetermined amplitude; and 
     (b) indicates when the power supply signal and device signal are out of phase by a predetermined magnitude. 
     In another aspect, the present invention provides a method for monitoring a conductive polymer device to determine when a fault condition has occurred in the device, which method comprises 
     (1) impressing a power signal from an external source across the device, the device then producing a device signal; and 
     (2) determining the phasal relationship between the power signal and the device signal. 
     In another aspect, the present invention provides a method for monitoring a self-regulating heater comprising a PTC conductive polymer, which method comprises 
     (1) impressing a periodic power signal from an external power source across the heater, the heater then producing a heater signal, the power signal being a voltage signal and the heater signal being a current signal; 
     (2) filtering the voltage and current signals to produce filtered voltage and current signals; 
     (3) establishing a minimum reference magnitude of said filtered voltage and current signals; 
     (4) comparing said filtered voltage and current signals for phase differences within a preselected portion of the period of the power signal, but only for signals above said reference magnitude; and 
     (5) providing an alarm when the voltage and current signals are out of phase by a predetermined magnitude. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is illustrated in the accompanying drawing in which 
     FIG. 1 is an electrical circuit of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Attention is now directed to FIG. 1 which provides an electrical circuit 10 of the invention. The circuit 10 includes a heater 12, a power supply 14, transducers 16 and 18, filters 20 and 22, and a comparator 24. Conventional components can be used for this purpose. 
     As indicated above, the power supply signal preferably is a voltage signal and the device signal preferably is a current signal. These signals are shown in FIG. 1 as they are developed in the primary circuit of the transducers 16 and 18 (shown as transformers) and then inputted from the secondary circuit of the transducers 16 and 18 to the filter circuits 20 and 22. The transducers 16 and 18 preferably have a capability to handle both the power supply signal, typically 60 Hz, and, a fault signal (i.e., the current signal), which may typically appear within a 10 Khz to 100 Khz spectrum. The filter circuits 20,22, on the other hand, are preferably high pass filters, but may be band pass filters having a lower pass level of at least 10 Khz and an upper pass level of at most 100 Khz. 
     The power supply signal and the device signal outputted by the filters 20 and 22 preferably are inputted to the comparator 24 through capacitive coupling i.e. capacitors 26 and 28. The comparator 24 comprises the following assembly: 
     (a) a first operational amplifier 30 that receives device signal inputs and provides a first digital logic output signal for input to a device signal inverter 32; 
     (b) a second operational amplifier 34 that receives device signal inputs and provides a second digital logic output signal for input to a first NAND-gate module 36; 
     (c) a third operational amplifier 38 that receives power supply signal inputs and provides a third digital logic output signal for input to a power supply signal inverter 40; and 
     (d) a fourth operational amplifier 42 that receives power supply signal inputs and provides a fourth digital logic output signal for input to a second NAND-gate module 44. 
     The device signal inverter 32 receives the first digital logic output signal from the first operational amplifier 30 and provides an inverted first digital logic output signal for input to the second NAND-gate module 44. 
     The power supply signal inverter 40 receives the third digital logic output signal and provides an inverted third digital logic output signal for input to the first NAND-gate module 36. 
     The first NAND-gate module 36 receives the second digital logic output signal and the inverted third digital logic output signal and outputs a first module signal for input to a third NAND-gate module 46; and, the second NAND-gate module 44 receives the fourth digital logic output signal and the inverted first digital logic output signal and outputs a second module signal for input to the third NAND-gate module 46. The third NAND-gate module 46 receives the first and second module signals and outputs a third module signal which is indicative of the phasal relationship between the power supply signal and the device signal. 
     As indicated above, the comparator can determine the phasal relationship between the power supply signal and the device signal for signals that have a predetermined amplitude. This feature accommodates a &#34;dead band&#34; of noise and it works as follows. For all dead band signals having a magnitude less than a predetermined magnitude and in or out of phase, over any part of the entire 360° period of the power supply signal, the &#34;hi&#34; reference of operational amplifiers 30 and 38 ensures a logic 1 output while the &#34;lo&#34; reference of the operational amplifiers 34 and 42 ensures the logic 0 output. The outputs of the device signal inverter 32 and power supply signal inverter 40 are, accordingly, logic 0. Consequently, the logic inputs to the first and second NAND-gate modules 36 and 44 are all 0, so that their outputs are logic 1. Finally, the NAND-gate module&#39;s 36 and 38 logic 1 output provides inputs to the third NAND-gate module 46. Here, logic 1 inputs become, uniquely, a logic 0 output, thus indicating a &#34;no-alarm&#34; situation. 
     Now consider, on the other hand, the case where, over a preselected portion of a 360° period, e.g. 0°-180°, the power supply signal and the device signal are out of the dead band, but in phase (and hence in a &#34;no alarm&#34; situation). Here, the &#34;hi&#34; reference of operational amplifiers 30 and 38 ensures a logic 0 output, and the &#34;lo&#34; reference of the operational amplifiers 34 and 42 ensures a logic 0 output. (not shown). The outputs of the device signal inverter 32 and power supply signal inverter 40, are, accordingly, logic 1. Consequently, the logic inputs to the first and second NAND-gate modules 36 and 44 are combinations of 1&#39;s and 0&#39;s, so that their outputs are logic 1 and, finally, their inputs to the third NAND-gate module 46 become, uniquely, a logic 0 output, thus again indicating a no-alarm situation. 
     It follows from the preceeding description that, for the case where the power supply signal and the device signal are out of the dead band and out of phase, the output of the third NAND-gate module 46 must be a logic 1. 
     Although the present invention preferably employs the three NAND-gate modules 36, 44 and 46, it is possible to provide alternative, equivalent logic. For example, the NAND-gate modules 44 and 36 may be replaced by AND-gate modules, while the NAND-gate module 46 is replaced by an OR-gate module (not shown). Also, in order to enhance the sensitivity of the circuit 10, the output signal of the NAND-gate module 46 (or the OR-gate module) may be further processed by a low pass filter 48. The output of the low pass filter 48, may, in turn, be inputted to a conventional alarm circuit 50.