Abstract:
An overcurrent protection system giving 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 a non-conducting fault state. The control element may include a PTC device.

Description:
RELATED APPLICATIONS 
     This is a continuation of U.S. patent application filed on Jul. 19, 1999, Ser. No. 09/356,070 (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. 
    
    
     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 “ripped,” 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 useful 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). 
     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 first aspect, this invention provides an electrical protection system which can be connected between an electrical power supply and an electrical load to form an operating circuit, the operating circuit having an on state and an off state and comprising a current carrying line and a return line, and which when so connected protects the circuit from overcurrents, the system having a normal operating condition and a fault condition, and comprising: 
     a. a circuit interruption element, which, when the system is so connected, is connected in series between the power supply and the load, and has 
     (1) a closed state which permits the flow of a normal current, I NORMAL  when the system is in the normal operating condition, and 
     (2) an open state which permits the flow of at most a reduced current, substantially less than I NORMAL  when the system is in the fault condition; 
     b. a sensor element, which, when the system is so connected, is connected in series with the circuit interruption element and the load, and has 
     (1) a normal state, when the current in the system does not exceed the normal current, I NORMAL  by a predetermined current amount, and 
     (2) a fault state, when the current in the system exceeds the normal current, I NORMAL , by the predetermined amount; and 
     c. a control element, which, when the system is so connected, is coupled with the sensor element and with the circuit interruption element, and has a variable resistance which 
     (1) is low when the sensor element is in the normal state, and 
     (2) increases by at least a predetermined resistance amount when the sensor element is in the fault state; 
     the circuit interruption element changing from its closed state to its open state, thereby causing the system to change from its normal operating condition to its fault condition, when the resistance of the control element has increased by the predetermined resistance amount in response to the sensor element changing from its normal state to its fault state. 
     In a second aspect, the invention provides an electrical protection system which can be connected between an electrical power supply and an electrical load to form an operating circuit, the operating circuit having an on state and an off state and comprising a current carrying line and a return line, and which when so connected protects the circuit from overcurrents, the system having a normal operating condition and a fault condition, and comprising: 
     a. a circuit interruption element, which, when the system is so connected, is connected in series between the power supply and the load, and has 
     i. a closed state which permits the flow of a normal current, I NORMAL  when the system is in the normal operating condition, and 
     ii. an open state which permits the flow of at most a reduced current, substantially less than I NORMAL  when the system is in the fault condition; 
     b. a sensor element, which has a variable resistance, and which when the system is so connected, is connected in series with the circuit interruption element and the load, and has 
     i. a normal state, in which its resistance is low, when the current in the system does not exceed the normal current, I NORMAL , by a predetermined current amount, and 
     ii. a fault state, in which its resistance increases by at least a predetermined resistance amount, when the current in the system exceeds the normal current, I NORMAL  by the predetermined amount; and 
     c. a control element, which, when the system is so connected, is coupled with the sensor element and with the circuit interruption element, which causes the circuit interruption element to change from its closed state to its open state when the sensor element changes from its normal state 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 block diagram of a circuit using the first embodiment of the invention. 
     FIG. 2 shows a circuit diagram of an overcurrent protection circuit of the invention. 
     FIG. 3 is an examples of the circuit of FIG. 1 employing PTC devices. 
    
    
     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. FIG. 1 is a block diagram showing an arrangement of such operational elements. 
     Five operational elements depicted in FIG. 1 are the source  102 , sensor element  104 , control element  106 , circuit interruption element  108  and load  112 . The source  102  provides the electrical power to the circuit, and the load  112  performs the intended purpose of the circuit. The sensor element  104  senses the current and determines whether the current delivered to the load  112  is within a normal acceptable range. When the sensor element  104  determines that the current delivered to the load  112  is excessive, the sensor element  104  informs the control element  106  via a first link  114  between the sensor  104  and control  106  elements. Based on information received from the sensor element  104 , the control element  106  controls the state of the circuit interruption element  108  via a second link  116  between the control  106  and interrupt  108  elements. The circuit interruption element  108  interrupts current in the circuit upon receipt of a control signal from the control element  106  when the sensor element  104  senses an overcurrent in the circuit. 
     FIG. 2 shows an example of an overcurrent protection arrangement of the invention  100 . The arrangement  100  in FIG. 2 comprises an electrical power source  2 , a load  4 , a PTC device  8 , a relay coil  12  with associated relay contacts  30   32   34   36  including a center contact  30 , a normally closed contact  32 , a normally open contact  34  and a wiper  36 , and an ON/OFF switch  16 . With the ON/OFF switch  16  initially open, the PTC device  8  in its low resistance state, and the wiper  36  against the normally closed contact  32 , the circuit  100  is in an open state and there is no current through the load  4 . When the ON/OFF switch  16  is closed, a small amount of current is drawn through the relay coil  12 , thereby energizing the relay coil  12  and causing the wiper  36  to move from the normally closed contact  32  to the normally open contact  34 , thereby placing the load  4  in the circuit The PTC device  8  is placed in series with the parallel combination of the relay coil  12  and the load  4 . However, the relay coil  12  draws very little current to keep it energized. In case of an overcurrent, the resistance of the PTC device  8  increases, thereby reducing the current to the load  4  and the relay coil  12 . If the PTC device  8  is chosen properly, its resistance would increase sufficiently to reduce the current through the relay coil  12  enough to deenergize the relay coil  12  thereby causing the wiper  36  to move to the normally closed contact  32  and disconnect the load  4 . If the current through the PTC device  8  and relay coil is  12  sufficient to keep the PTC device  8  tripped in the high impedance state and the relay coil  12  deenergized, the circuit  100  remains in a fault state until the ON/OFF switch  16  is opened and the PTC device  8  allowed to cool. If the current through the PTC device  8  in the high impedance state is not sufficient to keep the PTC device  8  tripped, then the PTC device  8  would cool and reset to its low impedance state. This would allow the current through the relay coil  12  to increase and energize the relay coil  12 , thereby moving the wiper  36  to the normally open contact  34 . If the cause of the fault is still present, then the cycle would continue until the cause of the fault were removed or power were removed, e.g. by opening the ON/OFF switch  16 . 
     However, since the normal circuit current may be many hundred times the current drawn by the relay coil  12 , there is a potential for the PTC device  8  to increase in its resistance and reduce the current to the load  4 , but not reduce the current sufficiently to cause the relay coil  12  to deenergize. This could leave the circuit in a closed state with a fault condition. For example, a PTC device rated to carry 9 amps would typically carry a current of approximately 0.25 amps in the tripped state. Since a typical automotive relay coil current is 0.180 amps, even if the PTC device were tripped, there would still be sufficient current to keep the relay energized. Thus, circuit protection arrangements like that depicted in FIG. 2 would likely require the use of PTC devices with potentially quite precise tolerances. 
     Therefore, it would be preferred to have a circuit protection arrangement in which the PTC device is not placed in the circuit in a position in which the current to both the circuit load and the device controlling the circuit interruption device passes through the PTC device. 
     The circuit in FIG. 3 is an example of an overcurrent protection system in accordance with the fist embodiment of the invention and the block diagram depicted in FIG.  1 . FIG. 3 shows an overcurrent protection circuit  200  employing a certain arrangement of a PTC device  8  with a resistive device  14 , a relay coil  12 , a set of contacts  30   32   34   36  and an ON/OFF switch  16 . In the circuit  200 , the resistive device  14  is placed in series with the load  4  and the PTC device  8  is placed in series with the relay coil  12 , with the latter series combination connected across the power source  2 . With the ON/OFF switch  16  initially open, the PTC device  8  in its low resistance state, and the wiper  36  against the normally closed contact  32 , the circuit  200  is in an open state and there is no current through the load  4 . When the ON/OFF switch  16  is closed, a small amount of current is drawn through the PTC device  8  and the relay coil  12 , thereby energizing the relay coil  12  and causing the wiper  36  to move from the normally closed contact  32  to the normally open contact  34 , thereby placing the load  4  in the circuit. The resistive device  14  and the PTC device  8  are thermally linked, so that in case of an overcurrent in the circuit, the temperature of the resistive device  14  increases and causes the PTC device  8  to heat up to its trip temperature and change to its high impedance state. With the PTC device  8  in its high impedance state, the current through the relay coil  12  reduces, the relay coil  12  deenergizes and causes the wiper  36  to move back to the normally closed contact  32 . The resistive device  14  and PTC device  8  have a combined mass such that the trickle of current through the PTC device  8  and relay coil  12  is not sufficient to keep the temperature of the PTC device  8  high enough to keep the PTC device  8  in the tripped state. Thus, the resistive device  14  and PTC device  8  both cool. When the PTC device  8  cools sufficiently, it resets to its low impedance state and allows sufficient current to again flow through the relay coil  12  to energize the relay coil  12  and move the wiper  36  to the normally open contact  34 . If the cause of the overcurrent remains, the resistance device  14  will heat and the PTC device  8  will again trip to its high impedance state. This cycle continues until either the cause of the overcurrent is removed or power is removed, for example by opening the ON/OFF switch  16 .