ELECTRICAL SYSTEMS

Abstract: Electrical protection systems which contain a resistor and a relay in series with a load, and a PTC device which is connected in parallel with the load and in series with a coil which controls the relay. The PTC device is thermally linked to the resistor and in consequence the relay opens if an overcurrent heats the resistor. The relay coil can be activated remotely by means of an RC circuit, and a diode or other electrical component permits current to pass through the RC network to the relay coil but prevents it from flowing in the opposite direction.

Description: -1- ELECTRICAL SYSTEMS This invention relates to electrical systems which contain positive temperature coefficient (PTC) circuit protection devices, particularly such systems in automobiles and other wheeled vehicles. PTC devices are well known. Particularly useful devices contain PTC elements composed of a PTC conductive polymer, i.e. a composition comprising an organic polymer and, dispersed or otherwise distributed therein, a particulate conductive filler, e.g. carbon black, or a metal or a conductive metal compound. Such devices are referred to herein as polymer PTC, or PPTC, devices. Other PTC materials are also known, e.g. doped ceramics, but are not as generally useful as PTC conductive polymer, in particular because they have higher resistivities. PTC devices can be used in a number of different ways, and are particularly useful in circuit protection applications, in which they function as remotely resettable fuses to protect electrical components from excessive currents and/or temperatures. Components which can be protected in this way include motors, batteries, loudspeakers and wiring harnesses in automobiles. The use of PPTC devices in this way has grown rapidly over recent years, and continues to increase. Reference may be made for example to U.S. Patent 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, 5,378,407, 5,451,919, 5,451,921, 5,831,510, 5,852,397 and 5,864,281. More recently, it has been proposed to combine PTC devices with other electrical components to provide circuit protection systems which respond to faults in ways which make use of the characteristics of both the PTC device and the electrical component. Reference may be made for example to U.S. Patent Nos. 5,666,254 and 5,689,395, and to International Publication Nos. WO 97/10636, WO 97/20372, WO 98/31084, WO 98/02946, WO 98/02947 and WO 98/56014, all of which International Publications were published after the priority date of this application. PCT Publication Nos. WO 98/02946 and WO 98/02947 describe protection systems which comprise a sensor element (typically a resistor) and a circuit interruption element (typically a relay) in series with the load, and a control element (typically a PTC device in parallel with the sensor element) which links the sensor element and the circuit interruption element so that an overcurrent is detected by the -2- sensor element, and causes a change in the control element, which in turn causes a change in (typically opens) the circuit interruption element. Depending on the arrangement of these and other components, the system can (a) latch in an open state, either with or without a trickle current to keep the PTC device in a tripped condition, or (b) reclose if the overcurrent has gone away or cycle between open and closed positions if the overcurrent remains. PCT Publication No. WO 98/56014 describes particularly useful device- for use in the protection systems of PCT Publication Nos. WO 98/02946 and WO 98/02947. In the copending commonly assigned U.S. Application Serial No. 09/019,767 filed February 6, 1998, and the corresponding PCT application filed contemporaneously with this application, there are described a number of improved protection systems based on those disclosed in PCT Publication Nos. WO 98/56014, WO 98/02946 and WO 98/02947. The present invention provides further improvements in the systems disclosed in that patent application and those International Publication Numbers. As disclosed in U.S. Application Serial No. 09/019,767, and the corresponding PCT application, the manually-operated relay switch disclosed in PCT Publication Nos. WO 98/02946 and WO 98/02947 can advantageously be replaced by a remotely operated system, preferably an RC circuit in the line to the relay coil. However, we have discovered that if there is a short circuit in the load when the relay contacts are closed, there are some conditions under which the relay contacts can chatter (i.e. continually open and close) and, which can, therefore, lead to the contacts welding shut, and to catastrophic failure. This occurs when the fault resistance is such that the current which can flow through the short circuit exceeds the current that the power source can supply. Under these circumstances, when the relay closes onto the short circuit, the system voltage (e.g. battery voltage), drops drastically. This voltage drop causes the relay to drop out, thus restoring the battery voltage. Immediately after this, since the battery voltage has been recovered, the relay picks up, again reducing the battery voltage, and the cycle continues. The chattering is primarily due to the fact that as the system voltage drops, the voltage across the capacitor in the RC network discharges in the reverse direction, causing the relay coil to drop out. We have realized, in accordance with the present invention, that this problem can be solved by adding to the system a diode which will prevent the capacitor from discharging in this way, and thus will keep the relay coil energized, and the relay contacts closed, for long enough to trip the PPTC and open the relay in the desired way. An embodiment of the invention comprises 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 set of relay contacts 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, IN O R M AL, 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 resistive device which, when the system is so connected, is connected in series with the set of relay contacts and the load, and has i. a normal state, when the current in the system does not exceed the normal current, I NO R MAL , by a predetermined current amount, and ii. a fault state, when the current in the system exceeds the normal current, IN OR MAL, by the predetermined amount; c. a control element which comprises a series combination of i. a PTC device thermally coupled with the resistive device; and ii. a relay coil coupled with the relay contacts; the series combination being connected across the power supply, between the current carrying line and the return line, with the PTC device connected to the current carrying line and the relay coil coupled to the return line; the set of relay contacts 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 PTC device increases by a predetermined resistance amount in response to the resistive element changing from its normal state to its fault state; d. a resistance-capacitance network through which current can be supplied to the relay coil, so that if the relay contacts are open, the only way in which they can be closed is by supplying current to the relay coil through the resistance-capacitance network; and e. a diode or other electrical component which permits current to pass through the resistance-capacitance network to the relay coil but which prevents it from flowing in the opposite direction. As illustrated in Figure 1, 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 set of relay contacts 16, 17 and 18 which, when the system is so connected, is connected in series between the power supply V+ and the load R L , 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 NOR M A L, when the system is in the fault condition; -5- b. a resistive device Rs which, when the system is so connected, is connected in series with the set of relay contacts 16, 17 and 18 and the load R L , and has i. a normal state, when the current in the system does not exceed the normal current, I NORMAL , by a predetermined current amount, and ii. a fault state, when the current in the system exceeds the normal current, I NORM A L , by the predetermined amount; c. a control element 20 which comprises a series combination of i. a PTC device 24 thermally coupled with the resistive device Rg; and ii. a relay coil 25 coupled with the relay contacts; the series combination being connected across the power supply, between the current carrying line and the return line, with the PTC device connected to the current carrying line and the relay coil coupled to the return line; the set of relay contacts 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 PTC device increases by a predetermined resistance amount in response to the resistive element changing from its normal state to its fault state; d. a resistance-capacitance network 28 through which current can be supplied to the relay coil, so that if the relay contacts are open, the only way in which they can be closed is by supplying current to the relay coil through the resistance-capacitance network; and e. a diode 42 or other electrical component which permits current to pass through the resistance-capacitance network to the relay coil but which prevents it from flowing in the opposite direction. As shown in Figure 2, it is particularly preferred that this system should be in the form of a device 50 which comprises components (a), (b), (c), (d) and (e) set out above, and which also comprises five terminals which can be connected to a power source, a normally open side of the relay, a normally closed side of the relay, a load, and a switching source (e.g. an electronic control module and associated FET as described in U.S. Patent Application Serial No. 09/019,767 and the corresponding PCT application). The device may also have a diagnostics pin out which provides logic level voltage when the PTC device has tripped. The logic level voltage is achieved by dividing the coil in appropriate proportions. Preferably the device is a pluggable device. Various components can be packaged in the device 50 shown in Figure 2. For example, in Figure 2 PTC device 24, diode 44, RC network 28 and diode 42 are packaged in device 52. Relay contacts 16, 17, 18, relay coil 25 and resistor 45 are packaged in device 54. Devices 52 and 54 are packaged in device 50.

Claims: -7- CLAIMS 1. 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 set of relay contacts 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 NORMA L, 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 IN OR M AL , when the system is in the fault condition; b. a resistive device which, when the system is so connected, is connected in series with the set of relay contacts and the load, and has i. a normal state, when the current in the system does not exceed the normal current, IN ORMAL , by a predetermined current amount, and ii. a fault state, when the current in the system exceeds the normal current, IN O RM A L, by the predetermined amount; c. a control element which comprises a series combination of i. a PTC device thermally coupled with the resistive device; and ii. a relay coil coupled with the relay contacts; the series combination being connected across the power supply, between the current carrying line and the return line, with the PTC device connected to the current carrying line and the relay coil coupled to the return line; the set of relay contacts 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 PTC device increases by a predetermined resistance amount in response to the resistive element changing from its normal state to its fault state; d. a resistance-capacitance network through which current can be supplied to the relay coil, so that if the relay contacts are open, the only way in which they can -8- be closed is by supplying current to the relay coil through the resistance- capacitance network; and ;. an electrical component which permits current to pass through the resistance- capacitance network to the relay coil but which prevents it from flowing in the opposite direction. A system according to claim 1 wherein the electrical component (e) is a diode.

Inventors: MYONG INHO, MYONG, INHO
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1999
Main classification: H02H3
IPC codes: H02H3