BACKGROUND TO THE INVENTION 
       [0001]    The present invention relates generally to a system and method for protecting a coil structure used in a controlled switch. More specifically, the present invention relates to a system for protecting a coil structure that provides a switching force used to actuate one or more load switches. The coil structure generates the switching force in response to a sizable current. However, continual application of the current at the same level to the coil structure can cause damage to the coil. 
       SUMMARY OF THE INVENTION 
       [0002]    Systems and methods are described for protecting a coil structure in a controlled switch. In many embodiments, the coil structure includes a switching configuration and a hold configuration. A substantial current can be provided to the coil structure in the switching configuration while a smaller current can be provided in the hold configuration. To protect against damage to the coil structure in the switching configuration caused by the substantial current, a resettable current limiting device can used to substantially limit current when the current exceeds a threshold over a predetermined period of time. 
         [0003]    In one embodiment, the invention relates to a circuit including at least one load switch connectable between a power source and a load and at least one auxiliary switch, each switch actuable between a default position and a switched position, a coil structure configured to actuate the at least one load switch and at least one auxiliary switch in response to a control input, the coil structure switchable between an actuating configuration for actuating the at least one load switch and the at least one auxiliary switch from the default position, and a holding configuration for holding the at least one load switch and the at least one auxiliary switch in the switched position, and a resettable current limiting device configured to limit the current flow in the actuating configuration of the coil structure if the current flow exceeds a threshold for a predetermined period of time, wherein the at least one auxiliary switch is configured to switch the coil structure from the actuating configuration to the holding configuration. 
         [0004]    In another embodiment, the invention relates to a switch circuit for actuating one or more load switches and at least an auxiliary switch, each of the one or more load switches and the auxiliary switches having a default position and a switched position, where the default position of the auxiliary switch is closed, the switch circuit including a main coil configured to receive a current from a control power source and to provide a force for holding the one or more load switches and the auxiliary switch in the switched position, where the main coil comprises a first end, a second end and a tap, a resettable current limiting device connected to the first end of the main coil and to the auxiliary switch, the resettable device configured to limit current flow when the current exceeds a threshold for a predetermined period of time, and a component coil comprising a portion of the main coil from the second end to the tap of the main coil, where the tap is connected to the auxiliary switch, and where the component coil is configured to provide a force for actuating the one or more load switches and the auxiliary switch from the default position. 
         [0005]    In yet another embodiment, the invention relates to a method of operating a switch circuit including a coil structure configured to actuate at least one load switch and at least one auxiliary switch in response to a control input, the coil structure switchable between an actuating configuration for actuating the at least one load switch and the at least one auxiliary switch from a default position, and a holding configuration for holding the at least one load switch and the at least one auxiliary switch in a switched position, and a resettable current limiting device having a current threshold, the method including providing a switching current to the coil structure, generating, in response to the switching current, a force for switching the at least one load switch and the at least one auxiliary switch, and limiting the switching current flowing through the coil structure using the resettable current limiting device when the switching current exceeds the current threshold for a predetermined period of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic block diagram of a power control system in accordance with an embodiment of the present invention. 
           [0007]      FIG. 2  is a schematic diagram of a power control system including a controlled switch with a current limiting device in accordance with an embodiment of the present invention. 
           [0008]      FIG. 3  is a schematic diagram of a power control system including a controlled switch having normally closed load switches in accordance with an embodiment of the present invention. 
           [0009]      FIG. 4  is a schematic diagram of a power control system including a controlled switch having three load switches in accordance with an embodiment of the present invention. 
           [0010]      FIG. 5  is a schematic diagram of a power control system including a controlled switch having one load switch in accordance with an embodiment of the present invention. 
           [0011]      FIG. 6  is a flowchart of a process for operating a controlled switch in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Turning now to the drawings, embodiments of systems and methods for protecting a coil in a controlled switch such as a contactor or a relay are illustrated. Controlled switches can be used to connect or disconnect loads of various sizes and phases from sources of power. Such controlled switches typically include a coil acting as an electromagnet to provide a force to switch the load when current is applied to the coil. This force can be used to switch one or more load switches together with an auxiliary switch. Substantial currents are provided to enable the coil to switch the load and the auxiliary switches. A hold or “economizer” coil is often used to decrease the amount of current needed to hold the armature(s) of the controlled switch in the switched position once the load and auxiliary switches have been actuated. In such case, a portion of the economizer coil can be used as an actuator or “pull-in” coil. 
         [0013]    In operation, the control power source applies a voltage to generate a substantial current in the pull-in coil. The hold coil is placed in parallel with the normally closed auxiliary switch, causing it to short during the switching period. The pull-in coil can be placed in series with the auxiliary switch and can produce a switching force proportional to that substantial current supplied by the control power source. When the controlled switch is actuated by the switching force, the auxiliary contacts open and the economizer (hold) coil is placed in the current path. The economizer coil generally has a larger impedance than the pull-in coil, decreasing the current required from the control power source to keep the controlled switch in the switched position. 
         [0014]    In the event that the auxiliary switch fails to operate, continuation of the substantial actuation current can cause permanent damage to, and the subsequent failure of, the coils. To avoid destruction of the coils and the controlled switch (e.g. relay) when the auxiliary switch fails to operate, a resettable current limiting device is placed in series with the pull-in coil and auxiliary switch. This resettable current limiting device substantially limits the current passing through the device when the current exceeds a threshold for a predetermined period of time. When the current drops below the threshold, the resettable device returns to its original state in which it conducts current with minimal limitation. 
         [0015]    In one embodiment, the controlled switch can control the distribution of power in an aircraft electrical system. Power can be distributed using any of DC or AC (single, two or three phase) systems, or any combination thereof. In one embodiment, the controlled switch has two load switches that switch DC power sources. In several embodiments, the DC power sources operate at 28 volts, 26 volts or 270 volts. In one embodiment, DC power sources operate in the range of 11 to 28 volts. In other embodiments, the controlled switch has three load switches that switch AC power sources. In one embodiment, the AC power source operates at 115 volts and at a frequency of 400 hertz. In other embodiments, the controlled switch has a single load switch that can switch a DC power source or a single phase of an AC power source. In other embodiments, the power sources operate at other voltages and other frequencies. In one embodiment, the DC power sources can include batteries, auxiliary power units and/or external DC power sources. In one embodiment, the AC power sources can include generators, ram air turbines and/or external AC power sources. 
         [0016]    A schematic block diagram of a power control system in accordance with an embodiment of the present invention is shown in  FIG. 1 . The power control system  200  includes a controlled switch  202  connected to a power source  204 , a load  206  and a control power source  208 . In operation, if the controlled switch  202  receives a signal from the control power source  208 , the switch  202  connects or disconnects the load  206  from the power source  204 . 
         [0017]    In one embodiment, the power control system  200  is a subsystem of an aircraft electrical system. The power source  204  can be a generator, a battery or other AC or DC power sources. The load  206  can include aircraft flight instruments, essential systems such as landing gear, exterior lights, aircraft motor controls and the like, and/or passenger services such as lights, air conditioning and entertainment systems. In one embodiment, the loads range from requiring 1 to 400 amps. In another embodiment, the loads typically require 50 to 60 amps. The control power source  208  can be a signal generated in response to either automated systems or manual systems. In many embodiments, the control power source is capable of supplying an amount of current adequate to operate the controlled switch  202 . In one embodiment, 6 to 7 amps from the control power source is sufficient to cause the controlled switch to connect or disconnect the load. In one embodiment, the controlled switch is a contactor. In another embodiment, the controlled switch is a relay. 
         [0018]    A schematic block diagram of a power control system including a controlled switch with a current limiting device in accordance with an embodiment of the present invention is shown in  FIG. 2 . The power control system  300  includes a controlled switch  302 , a power source  304 , a load  306  and a control power source  308 . The controlled switch  302  includes a pull-in coil  310 , an economizer coil  312 , an auxiliary switch  320 , a first load switch  322 , a second load switch  324 , and a current limiting device  330 . The economizer coil  312  includes a first end  314 , a tap  316 , and a second end  318 , where the pull-in coil consists of the portion of the economizer coil extending from the second end  318  to the tap  316 . 
         [0019]    The power source  304  is connected to the first load switch  322  and the second load switch  324 . The first load switch  322  and the second load switch  324  are connected to the load  306 . The control power source  308  is connected to both the first end  314  and the second end  318  of the economizer coil  312 . The auxiliary switch and first and second load switches each include a switch arm (armature) and a switch contact. The tap  316  is connected to the auxiliary switch  320 . In the illustrated embodiment, the auxiliary switch  320  is normally closed and operates complimentary to the load switches that are normally open. The three switches are arranged such that all three are switched together. The auxiliary switch  320  is connected to the current limiting device  330 . The first end  314  of the economizer coil  312  is connected to the current limiting device  330 . 
         [0020]    In operation, the control power source  308  provides a voltage and switching current to the pull-in coil  310  at the second end  318 . At this time, the current limiting device  330  has minimal resistance to current flowing through the device. Thus, nearly all of the switching current initially travels through the pull-in coil  310  and out of the tap  316 , through the auxiliary switch  320  and current limiting device  330 , and returns to the current power source  308  via the first end  314 . In one embodiment, the switching current provided by the control power source  308  enters at the first end  314  of the economizer coil  312  and returns via the second end  318 . 
         [0021]    The initial switching current can be substantial in order to facilitate switching using the pull-in coil. In one embodiment, the initial switching current or inrush current is approximately 6 to 7 amps. The pull-in coil acts as an electromagnet to create a force that physically switches or actuates the auxiliary switch and both of the load switches. Upon successful switching of the auxiliary switch, the switching current supplied by the control power source can be reduced as the force required to hold the switch in the switched position is generally less than the force required to operate the switch. This is accomplished by opening the auxiliary contact in parallel to the hold coil and therefore placing the hold coil, with greater impedance, in the path of the current. 
         [0022]    The auxiliary switch can fail to open because of failed switch contacts, low voltage from the control power source or another reason. In any such case, the current limiting device senses the continuing switching current and acts to limit the current flowing through the device. In one embodiment, the current limiting device is a resettable fuse such as a thermistor or a polymer positive temperature coefficient (PPTC) fuse. In such case, the resettable fuse has a threshold or trip current such that when the threshold is exceeded for a predetermined period of time, the resettable fuse begins to heat up quickly. The heat changes the resistive properties of the resettable fuse such that the resistance of the device increases dramatically when the threshold is exceeded. In one embodiment, the resistance increases by at least an order of magnitude. In another embodiment, the resistance increases exponentially. However, when the current through the resettable fuse returns to a level below the threshold, the resistance of the fuse becomes minimal again as though it had been reset. In this way, the resettable fuse can be used any number of times to prevent switch failures from destroying the switching coil. 
         [0023]    In one embodiment, the current limiting device can be a combination of components capable of comparing an input current level and a threshold over some period of time, and changing the overall impedance seen by the input current based on the comparison. The combination can include any number of devices including integrated circuits, processors and/or discrete devices coupled to one another. 
         [0024]    In one embodiment, the threshold current must be exceeded for a period of time greater than 20 to 30 milliseconds before the resettable fuse heats up quickly to limit current flow. In one embodiment, the resettable fuse can be a POLYFUSE® resettable positive temperature coefficient fuse such as any of the 60R250, 60R300, and 60R375 fuses made by Littlefuse, Inc. of Des Plaines, Ill. In such case, the threshold current can be 5, 6, and 7.5 amps, respectively. In one embodiment, the inrush current is greater than 6 to 7 amps. In another embodiment, the threshold current must be exceeded for a period of time much greater than 20 to 30 milliseconds. In one embodiment, two or more resettable fuses can be used together in parallel to increase the current capability and to extend the time for the combination of favor to substantially change their impedance. In another embodiment, two or more resettable fuses can be used together in series. 
         [0025]    Returning briefly to  FIG. 1 , the controlled switch  202  functions in the same manner described above for the controlled switch  302  of  FIG. 2 , in one embodiment of the invention. 
         [0026]    A schematic block diagram of a power control system including a controlled switch having normally closed load switches in accordance with an embodiment of the present invention is shown in  FIG. 3 . The power control system  400  includes a controlled switch  402 , a power source  404 , a load  406  and a control power source  408 . The controlled switch  402  includes a pull-in coil  410 , an economizer coil  412 , an auxiliary switch  420 , a first load switch  422 , a second load switch  424 , and a current limiting device  430 . The economizer coil  412  includes a first end  414 , a tap  416 , and a second end  418 , where the pull-in coil consists of the portion of the economizer coil extending from the second end  418  to the tap  416 . 
         [0027]    The power source  404  is connected to the first load switch  422  and the second load switch  424 . The first load switch  422  and the second load switch  424  are connected to the load  406 . The control power source  408  is connected to both the first end  414  and the second end  418  of the economizer coil  412 . The auxiliary switch and first and second load switches each include a switch arm and a switch contact. The tap  416  is connected to the auxiliary switch  420 . In the illustrated embodiment, the auxiliary switch  420  and the load switches are normally closed. The three switches are arranged such that all three are switched together. The auxiliary switch is connected to the current limiting device  430 . The first end  414  of the economizer coil  412  is connected to the current limiting device  430 . 
         [0028]    The power control system  400  of  FIG. 4  is identical to the embodiment shown in  FIG. 3 , except that load switch  422  and load switch  424  are normally closed switches. The power control system  400  can operate as described previously for the embodiment shown in  FIG. 3 . 
         [0029]    A schematic block diagram of a power control system including a controlled switch having three load switches in accordance with an embodiment of the present invention is shown in  FIG. 4 . The power control system  500  includes a controlled switch  502 , a power source  504 , a load  506  and a control power source  508 . The controlled switch  502  includes a pull-in coil  510 , an economizer coil  512 , an auxiliary switch  520 , a first load switch  522 , a second load switch  524 , a third load switch  526 , and a current limiting device  530 . The economizer coil  512  includes a first end  514 , a tap  516 , and a second end  518 , where the pull-in coil consists of the portion of the economizer coil extending from the second end  518  to the tap  516 . 
         [0030]    The power source  504  is connected to the first load switch  522 , the second load switch  524 , and the third load switch  526 . The first load switch  522 , the second load switch  524  and the third load switch  526  are connected to the load  506 . The control power source  508  is connected to both the first end  514  and the second end  518  of the economizer coil  512 . The auxiliary switch and first and second load switches each include a switch arm and a switch contact. The tap  516  is connected to the auxiliary switch  520 . In the illustrated embodiment, the auxiliary switch  520  is normally closed and the load switches are normally open. The four switches are arranged such that all three are switched together. The auxiliary switch is connected to the current limiting device  530 . The first end  514  of the economizer coil  512  is connected to the current limiting device  530 . 
         [0031]    The power control system  500  of  FIG. 5  is nearly identical to the embodiment shown in  FIG. 3 , except that it includes an additional load switch. The power control system  500  can operate as described previously for the embodiment shown in  FIG. 3 . 
         [0032]    A schematic block diagram of a power control system including a controlled switch having one load switch in accordance with an embodiment of the present invention is shown in  FIG. 5 . The power control system  600  includes a controlled switch  602 , a power source  604 , a load  606  and a control power source  608 . The controlled switch  602  includes a pull-in coil  610 , an economizer coil  612 , an auxiliary switch  620 , a load switch  622 , and a current limiting device  630 . The economizer coil  612  includes a first end  614 , a tap  616 , and a second end  618 , where the pull-in coil consists of the portion of the economizer coil extending from the second end  618  to the tap  616 . 
         [0033]    The power source  604  is connected to the load switch  622 . The load switch  622  is connected to the load  606 . The control power source  608  is connected to both the first end  614  and the second end  618  of the economizer coil  612 . The auxiliary switch and the load switch each include a switch arm and a switch contact. The tap  616  is connected to the auxiliary switch  620 . In the illustrated embodiment, the auxiliary switch  620  is normally closed and the load switch is normally open. The two switches are arranged such that they are both switched together. The auxiliary switch is connected to the current limiting device  630 . The first end  614  of the economizer coil  612  is connected to the current limiting device  630 . 
         [0034]    The power control system  600  of  FIG. 5  is nearly identical to the embodiment shown in  FIG. 2 , except that it includes only one load switch. The power control system  600  can operate as described previously for the embodiment shown in  FIG. 2 . 
         [0035]    In one embodiment, any number of load switches can be used in conjunction with at least one auxiliary switch in any number of arrangements of normally open or normally closed default switch settings. 
         [0036]    A flowchart of a process for operating a controlled switch in accordance with an embodiment of the present invention is shown in  FIG. 6 . The process  750  begins when it provides ( 752 ) switching current to a coil (e.g. a pull-in coil). The process then generates ( 754 ) a force for switching one or more load switches and an auxiliary switch. In one embodiment, the force is proportional to, or a function of, the switching current. The process then determines ( 756 ) whether the auxiliary switch failed to operate. In one embodiment, the determination is made based on the amount of current flowing through the current limiting device and the coil. If the auxiliary switch fails to operate, then the process limits ( 758 ) the current flowing through the coil and returns to determining ( 756 ) whether the auxiliary switch failed to open. In one embodiment, the current flowing through the coil is limited using a resettable fuse. If the auxiliary switch does not fail to operate, then the process allows ( 760 ) the full amount of current to flow through the coil. 
         [0037]    While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of one embodiment thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.
A system and method for protecting a coil structure in a controlled switch. In one embodiment, the invention relates to a circuit including at least one load switch connectable between a power source and a load and at least one auxiliary switch, each switch actuable between a default position and a switched position, a coil structure configured to actuate the at least one load switch and at least one auxiliary switch in response to a control input, the coil structure switchable between an actuating configuration for actuating the at least one load switch and the at least one auxiliary switch from the default position, and a holding configuration for holding the at least one load switch and the at least one auxiliary switch in the switched position, and a resettable current limiting device configured to limit the current flow in the actuating configuration of the coil structure if the current flow exceeds a threshold for a predetermined period of time, wherein the at least one auxiliary switch is configured to switch the coil structure from the actuating configuration to the holding configuration.