Abstract:
An overcurrent protection system gives rapid response to relatively small overcurrents has a sensor element and an interruption element in series between a power source and a load. The sensor element is functionally linked to the circuit interruption element via a control element. When current exceeds a predetermined amount, the sensor element transfers heat to the control element which causes the circuit interruption element to change from a conducting state to non-conducting fault state. A latching arrangement causes the protection system to remain in the non-conducting fault state until reset. The control element may include a PTC device or a bimetal switch.

Description:
RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 09/357,520, filed Jul. 19, 1999, now abandoned, which is a continuation of U.S. patent application Ser. No. 09/156,933, filed on Sep. 18, 1998, now abandoned, which is a continuation of U.S. patent application Ser. No. 08/682,067, filed on Jul. 16, 1996, now abandoned, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electrical circuit overcurrent protection. 
     2. Introduction to the Invention 
     Positive temperature coefficient (PTC) circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively, and/or the ambient temperature around the PTC device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, then the PTC device will be “tripped,” i.e. converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tripped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly usefill PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) dispersed, or otherwise distributed, in the polymer, a particulate conductive filler, preferably carbon black. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, the disclosures of which are incorporated herein by reference for all purposes. 
     In a batch of PTC devices made by the same manufacturing process, uncontrollable variations in the process can cause substantial variation in the conditions which will trip any individual device. The largest steady state current which will not cause any of the devices in the batch to trip is referred to herein as the “pass current” (I PASS ) or “hold current”, and the smallest steady state current which will cause all of the devices to trip is referred to as the “trip current” (I TRIP ). In general, the difference between I PASS  and I TRIP  decreases slowly as the ambient temperature increases. Depending on the particular type of device, I TRIP  may for example be 1.5 to 2.5 times I PASS  at 20° C. For any individual device, the pass current and the trip current are the same. However, in this specification, reference is made to a PTC device having an I PASS  and a different I TRIP , because as a practical matter, the manufacturer of an electrical switch must make use of PTC devices taken from a batch of such devices. Generally, the higher the ambient temperature, the lower the pass current and the trip current. This phenomenon is referred to as “thermal derating”, and the term “derating curve” is used to denote a graph of temperature against pass current. 
     A limitation on the known uses of PTC protection devices is that when a PTC device is placed in series with the load and sized to conduct the normal circuit current, the PTC device can take a relatively long time to convert to its tripped state on an overcurrent which is, e.g., up to a few times the normal circuit current. 
     SUMMARY OF THE INVENTION 
     The invention provides a new overcurrent protection system which will give a rapid response to even relatively small overcurrents. In the new system, a sensor element and circuit interruption element are placed in series with the load. The sensor element is functionally linked to the circuit interruption element via a control element, so that, when the current in the circuit exceeds a predetermined amount, the sensor element senses the overcurrent and communicates with the control element. The control element causes the circuit interruption element to change from a relatively conductive normal state to a relatively non-conductive fault state (including a completely open state) and remain latched in the fault state until reset. 
     In an example of a preferred embodiment of circuit arrangements of the invention, the sensor element comprises a resistive device connected in series with the load, and the control element comprises a PTC device which is thermally linked to the resistive device and is electrically connected to the circuit interruption element. When an overcurrent passes through such a system, the resistive device increases in temperature causing the PTC device to heat up and trip to its high resistance state. The PTC device is linked to the circuit interruption element so that the increased resistance of the PTC device causes the circuit interruption element to switch into its fault state. The PTC device is not placed in series with the load and therefore may operate at current levels much less than the normal circuit current which passes through the load. 
     The thermal linking of a resistive device with a PTC device is known in the art. A current to be measured and/or controlled passes through the resistive device. I 2 R heating of the resistive device causes the PTC device to heat up and its resistance increases accordingly. Such resistive devices may comprise resistors, heaters, high resistance wire (e.g. NiChrome), PTC devices and the like. It is known that in order to obtain the desired current/temperature performance of such combinations, certain characteristics of the resistive device must be controlled, particularly in the zone adjacent to the PTC device. Some of the characteristics to be controlled include the resistivity, shape and cross sectional area of the material. The resistive device should be chosen to minimize system impedance while achieving sufficient temperature rise under overcurrent conditions to cause the PTC device to heat up and trip to its high impedance state. 
     In a second example of a preferred embodiment of the invention, the sensor element comprises a resistive device connected in series with the load, and the control element comprises a bimetal switch which is thermally linked to the resistive device and is electrically connected to the circuit interruption element. When an overcurrent passes through such a system, the resistive device increases in temperature causing the bimetal switch to heat up and trip to its open state. The bimetal switch is linked to the circuit interruption element so that the open condition of the bimetal switch causes the circuit interruption element to switch to its fault state. The bimetal switch is not placed in series with the load and therefore may operate at current levels much less than the normal circuit current which passes through the load. 
     In a third example of a preferred embodiment of the invention, the function of the sensor element is provided by a bimetal switch which is placed in series with the parallel combination of the load and the control element. When an overcurrent passes through such a system, the bimetal switch increases in temperature and trips to its open state. The control element senses the state change of the sensor element and causes the circuit interruption element to switch to its fault state. 
     It will be apparent that polymeric PTC devices, ceramic PTC devices, other PTC devices such as bimetal devices, metallic PTC devices, arrangements of solid state devices with PTC characteristics, and devices displaying similar characteristics may be used in the circuit arrangements of this invention to provide reliable overcurrent protection. It will likewise be apparent to those of ordinary skill in the art that mechanical switches used in the circuit arrangements of this invention may include switches, relays, circuit breakers, isolators, bimetal devices and other devices. In addition, a solid state device or combination of solid state devices which provide disconnecting characteristics similar to those provided by mechanical switches may be used in place of the mechanical switches. Bimetal devices have also been referred to as bimetallic devices, electrothermal relays, thermally activated switches and/or electrothermal mechanisms with bimetal elements. 
     It will be apparent that in the preferred embodiments, this invention permits the use of PTC devices and bimetal switches to be arranged with mechanical switches and other electrical devices to provide reliable protection which protection was not previously available in the art. These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiments of the invention as illustrated in the various drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description of the preferred embodiments of the invention, taken in conjunction with the accompanying drawings, in which like components are given the same reference numerals in each FIG. in which they appear, and in which: 
     FIG. 1 is a first embodiment of the present invention employing a PTC device. 
     FIG. 2 is a second embodiment of the present invention employing a PTC device. 
     FIG. 3 is a third embodiment of the present invention employing a PTC device. 
     FIG. 4 is a fourth embodiment of the present invention employing a bimetal switch. 
     FIG. 5 is a fifth embodiment of the present invention employing a bimetal switch. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Overcurrent protection circuits arranged in accordance with the principles of this invention generally perform the functions of sensing the current, issuing a control signal to interrupt the circuit, interrupting the circuit and partially or completely isolating the load from the power source. The overcurrent protection circuits may be viewed as comprising operational elements which work cooperatively to perform the overcurrent protection functions. 
     In some applications, it is preferred that the overcurrent protection circuit not attempt to reset itself, but rather latch open in a fault state. The circuit  300  in FIG. 1 is an example of a circuit protection arrangement in accordance with the first embodiment of the invention, and is an overcurrent protection circuit which will latch open in the fault state. The circuit  300  employs an arrangement of a PTC device  8  with a resistive device  14 , a relay coil  12  and a set of relay contacts  30 , 32 , 34 , 36 . In the circuit  300 , the relay coil  12  is connected to the normally closed contact  32 , the load  4  is connected to the center contact  30 , and the normally open contact  34  is connected to the resistive element  14 . The circuit  300  is initially energized by closing the ON/OFF switch  16 . Current flows through the PTC device  8  and the relay coil  12 . The relay coil  12  energizes, causing the wiper  36  to move to the normally open contact  34 . This places the resistive element  14  in the current path with the load  4 . In the event of an overcurrent, the resistive element  14  heats up causing the PTC device  8  to heat up and trip. The relay coil  12  then deenergizes, causing the wiper  36  to return to the normally closed contact  32 . The load  4  remains in the circuit, however, with the PTC device  8  in its tripped state, the “trickle current” through the PTC device  8  and the parallel combination of the relay coil  12  and the load  4  is very little, but is sufficient to keep the PTC device  8  from returning to its low impedance state. The ON/OFF switch  16  would have to be opened to permit the PTC device  8  to cool and reset the circuit  300 . 
     The circuit  400  in FIG. 2 is a second example of a circuit protection arrangement in accordance with the invention. The circuit  400  will also latch open in the fault state. The circuit  400  employs an arrangement of a PTC device  8  with a resistive device  14 , a relay coil  12  and a set of relay contacts  30 , 32 , 34 , 36 , and is similar to the circuit  300  shown in FIG.  1 . The ON/OFF switch  16  has been moved to lie between ground  6  and the junction between the relay coil  12  and the load  4 . The operation of the circuit  400  shown in FIG. 2 is the same as for the circuit  300  shown in FIG.  1 . However, in certain applications, e.g. in the automotive industry, it may be preferred to employ “ground switching” techniques as shown in FIG.  5 . 
     The overcurrent protection circuits shown in FIGS. 1 and 2 are both capable of latching, i.e. not attempting to reset, when they are in a fault state. Both arrangements require a “trickle current” to keep them latched. In some applications it is preferred to have the overcurrent protection circuit latch open, but not require a trickle current to stay latched. For example, in automobile and other battery-powered applications, a trickle current could run down the battery if allowed to continue for an extended period of time. FIG. 3 shows a third example of an embodiment of an overcurrent protection circuit  500  which will latch open in the fault state. However, unlike the circuits  300 , 400  shown in FIGS. 1 and 2, respectively, the circuit  500  shown in FIG. 3 does not require a “trickle current” to latch the circuit open in case of an overcurrent. The circuit  500  employs an arrangement of a PTC device  8  with a resistive device  14 , a relay coil  12  and a set of relay contacts  30 , 32 , 34 , 36 . It also includes a diode  22 , a momentary ON switch  18  and a momentary OFF switch  20 . The relay contacts  30 , 32 , 34 , 36  are positioned between the power source  2  and the resistive element  14 , with the center contact  30  connected to the power source  2  and the normally open contact  32  connected to the resistive element  14 . With the circuit  500  in an OFF state, and the PTC device  8  in its low impedance state, the circuit  500  is turned on by momentarily pressing the ON switch  18 . Current flows through the relay coil  12 , energizing the relay coil  12  and causing the wiper  36  to move to the normally open contact  34 . The diode  22  prevents full operating current from power source  2  from flowing through the PTC device  8 , the resistive element  14 , and the load  4  when the ON switch  18  is closed. With the ON switch  18  released, current flows through the resistive element  14  to the load  4 , and also through the series combination of the PTC device  8 , the diode  22  and the relay coil  12 , thereby keeping the relay coil  12  energized. In case of an overcurrent, the resistive element  14  heats up causing the PTC device  8  to heat up and trip to its high impedance state. The reduced current causes the relay coil  12  to deenergize and the wiper  36  to move to the normally closed contact  32 . Current ceases to flow in the circuit  500 , and the PTC device  8  cools and returns to its low resistance state. The momentary OFF switch  20  is used to turn the circuit OFF under normal operating conditions. 
     In each of the circuits shown in FIGS. 1,  2  and  3 , the PTC device  8  is shown connected so that it provides overcurrent protection to the respective circuit, but does not have to carry the load current. Thus in circuits protection arrangements according to the invention, a PTC device can be used to control a load current which is higher than the current rating of the PTC device. 
     The circuit  600  shown in FIG. 4 is an alternate embodiment of the circuit shown in FIG. 3 in which the PTC device  8  is replaced by a bimetal switch  42 . With the circuit  600  in an OFF is state, and the bimetal switch  42  in its closed state, the circuit  600  is turned on by momentarily pressing the ON switch  18 . Current flows through the relay coil  12 , energizing the relay coil  12  and causing the wiper  36  to move to the normally open contact  34 . The diode  22  prevents current from flowing up through the bimetal switch  42  to the resistive element  14  and the load  4 . With the ON switch  18  released, current flows through the resistive element  14  to the load  4 , and also through the series combination of the bimetal switch  42 , the diode  22  and the relay coil  12 , thereby keeping the relay coil  12  energized. In case of an overcurrent, the resistive element  14  heats up causing the bimetal switch  42  to heat up and trip to its open state. The relay coil  12  deenergizes and the wiper  36  moves to the normally closed contact  32 . Current ceases to flow in the circuit  600 , and the bimetal switch  42  cools and returns to its closed state. With the wiper  36  against the normally closed contact  32  and the relay coil  12  deenergized, the circuit  600  remains, i.e. latches, in the fault state and draws no “trickle current.” The momentary OFF switch  20  is used to turn the circuit OFF under normal operating conditions. 
     FIG. 5 is an embodiment of an overcurrent protection circuit  700  employing a bimetal switch  42 . However, in the circuit  700  in FIG. 5, the bimetal switch  42  provides the functionality of the sensor element, and the relay coil  12  and diode  22  provide the functionality of the control element. The circuit  700  employs an arrangement of a bimetal switch  42  with a relay coil  12  and a set of relay contacts  30   32   34   36 . It also includes a diode  22 , a momentary ON switch  18  and a momentary OFF switch  20 . The relay contacts  30   32   34   36  are positioned between the power source  2  and the bimetal switch  42 , with the center contact  30  connected to the power source  2  and the normally open contact  34  connected to the bimetal switch  42 . In the circuit  700  the bimetal switch  42  is in series with the parallel combination of the load  4  and relay coil  12 . With the circuit  700  in an OFF state, and the bimetal switch  42  in its closed state, the circuit  700  is turned on by momentarily pressing the ON switch  18 . Current flows through the relay coil  12 , energizing the relay coil  12  and causing the wiper  36  to move to the normally open contact  34 . The diode  22  prevents full operating current from flowing through the ON switch to the load  4  when the ON switch  18  is closed. With the ON switch  18  released, current flows through the bimetal switch  42  to the load  4 , and also through the series combination of the diode  22  and the relay coil  12 , thereby keeping the relay coil  12  energized. In case of an overcurrent, the bimetal switch  42  heats up and trips to its open state. The relay coil  12  deenergizes and the wiper  36  moves to the normally closed contact  32 . Current ceases to flow in the circuit  700 , and the bimetal switch  42  cools and returns to its closed state. The circuit  700  latches in the fault state with no current flowing in the circuit  700 . This circuit  700  has an advantage in that since the bimetal switch  42  opens when it heats up, rather than just increase in impedance, the bimetal switch  42  may be placed in series with both the relay coil  12  and the load  4  with confidence that the relay coil  12  will deenergize when the bimetal switch  42  opens. The momentary OFF switch  20  is used to turn the circuit OFF under normal operating conditions.