Abstract:
A circuit and method protect switching devices from alternating current overload. By protecting the switching devices from AC overload, the reliability of the equipment using the switching devices is improved, cost for repair is decreased and information regarding the cause of the AC overload is available.

Description:
This patent application is a U.S. nationalization under 35 U.S.C. §371 of International Application No. PCT/US2009/035681, filed Mar. 2, 2009, which claims the benefit of priority under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 61/032,225, filed Feb. 28, 2008. The disclosures set forth in the referenced applications are incorporated herein by reference in their entireties, including all information as originally submitted to the United States Patent and Trademark Office. 
    
    
     BACKGROUND 
     Many electromechanical actuators are powered by an Alternating Current (AC) voltage source. Control of these devices is often accomplished with thyristor switches including triacs and Silicon Controlled Rectifiers (SCRs). 
     In an illustrated embodiment, a circuit is provided that protects such switching components from damage in case of a severe overcurrent or short circuit resulting from an AC overload. By protecting the switching components from AC overload, the reliability of the equipment using the switching components is improved, cost for repair is decreased, and information regarding the cause of the AC overload may be obtained. 
     Additional features will become apparent to those skilled in the art upon consideration of the following detailed description of drawings exemplifying the best mode as presently perceived. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description particularly refers to the accompanying figures in which: 
         FIG. 1  is a functional circuit diagram illustrating an apparatus utilizing an illustrated embodiment; 
         FIG. 2  is a more detailed functional circuit diagram illustrating a portion of the overcurrent protection circuit in accordance with illustrated embodiments; 
         FIG. 3  is a circuit diagram of a rectifier bridge illustrated in accordance with an illustrated embodiment; 
         FIG. 4  is a schematic circuit diagram illustrating the overcurrent protection circuit in accordance with an illustrated embodiment; and 
         FIG. 5  is a functional flow chart illustrating operations performed in accordance with at least one embodiment. 
     
    
    
     While embodiments have been illustrated and described in the drawings and foregoing description, such illustrations and descriptions are considered to be exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. 
     DESCRIPTION 
     While the present disclosure may be susceptible to embodiment in different forms, there is shown in the drawings, and herein will be described in detail, embodiments with the understanding that the present description is to be considered an exemplification of the principles of the disclosure and is not intended to limit the disclosure to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. 
     A well-known characteristic of thyristors is their ability to handle very high currents for a short time without suffering damage. For instance, a triac rated for 4 amps continuous current can tolerate a one-time surge of 30 amps that lasts for 20 milliseconds. Much larger currents can be survived if the duration is shortened further. Accordingly, the illustrated embodiments provide a mechanism and operations for protecting thyristors against overcurrent. 
     With reference to  FIG. 1 , an illustrated embodiment provides an overcurrent protection circuit  145  placed in series with the thyristors  110 ,  120 ,  130  to be protected. As explained in connection with  FIG. 2 , this overcurrent protection circuit  145  includes another switching device  205 . In accordance with an illustrated embodiment, the switching device may be a transistor or Field Effect Transistor (FET) or any component with like functionality. It should be appreciated that the overcurrent protection circuit  145  can be placed either in the supply or return side of the power source  140 . 
     As should be readily apparent from  FIG. 1 , the controller circuit  105  also loses power when overcurrent of the thyristors  110 ,  120 ,  130  occurs. The controller circuit  105  can be configured to reset and remove command signals from the thyristors  110 ,  120 ,  130  after a power interruption. This functionality prevents repeated cycling of shorted loads ( 115 ,  125 ,  135 ) because power may be restored within seconds after shutting down due to an original short. Alternatively, the controller circuit  105  may be configured to retain power (e.g., including of coupled to a backup power source) and an auxiliary control signal (such as that generated by microcontroller  230  described in connection with  FIG. 2 ) may be used to reset the controller circuit  105 . 
     As illustrated in  FIG. 2 , the switch  205  is provided in combination with resistance in the form of resistor Rs  210  and a threshold detection circuit  215 , which is configured to detect the current flowing through the switch  205 . This current detection is performed by simply measuring the voltage experienced by the threshold detection circuit (and, under Ohm&#39;s law, the resistance Rs  210 ); as a result of knowing both the resistance and the voltage, the current flowing through Rs may be determined. In turn, based on Ohm&#39;s law, an indication of the current experienced by both the switch  205  and the thyristors  110 ,  120 ,  130  illustrated in  FIG. 1  may also be known. Accordingly, the threshold detection circuit  215  may provide an indication of whether and when the thyristors  110 ,  120 ,  130  are experiencing overcurrent. 
     The switch  205  cannot be thyristor; this is because, inherently, thyristors cannot achieve a rapid turn-off. Thus, another DC switching device, such as a FET or transistor, must be used. In order to switch AC using a DC device, it is necessary to implement a bridge rectifier circuit as shown in  FIG. 3 , described below. 
     The threshold detection circuit  215  outputs a control signal  220  to the switch  205  in a feedback manner such that the switch operation is controlled by the controls signal  220 . Thus, the threshold detection circuit  215  may be configured to control the circuit  205  to open, thereby ceasing operation of the circuit illustrated in  FIG. 1 , and protecting against overcurrent induced damage of the thyristors  110 ,  120 ,  130 . 
     In accordance with at least one illustrated embodiment, the overcurrent protection circuit  145  may also be configured to generate a control signal  225  in the case of overcurrent detection. Thus, when the switch  205  (e.g., a SCR) turns on, voltage may be supplied to a Light Emitting Diode (LED) coupled to an optocoupler (e.g., optocoupler  410 , as illustrated in  FIG. 4 ). Accordingly, an external microcontroller  230  may use a signal to determine which load is on, when the overcurrent condition occurred. The microcontroller may also be configured to use this signal  225  to display the condition to the operator as well as initiate a diagnostic routine. 
     In implementation, the overcurrent protection circuit  145  may also include a bridge rectifier circuit  305  as illustrated in  FIG. 3 , to switch AC using the DC switching device. As illustrated in  FIG. 3 , the bridge rectifier includes Q 1   315 , configured to act as a DC switch. 
       FIG. 4  includes the components of  FIGS. 1-3  in combination showing the bridge rectifier  315  ( FIG. 3 ) coupled to the thyristors  110 ,  120 ,  130  ( FIG. 1 ) and DC switching device  205  ( FIG. 2 ). It should be appreciated that the particular circuit components utilized in an overcurrent protection circuit are based on the constraints of the apparatus in which the circuit is utilized. Therefore, the following illustrative values are provided only as a frame of reference for one of ordinary skill in the art. 
     It should be appreciated that, with the disclosed component configuration and values of  FIG. 4 , the overcurrent protection circuit will interrupt the current within 10 microseconds. 
     The DC switch is Q 1   315 , the bridge  305 , and the threshold detector is an SCR designated as Q 2   460 . Thus, in implementation the current is detected by measurement of the voltage developed across Rs  210 . When this voltage reaches the threshold detector gate turn-on voltage threshold, Q 2   460  turns on and pulls down the voltage at the gate of Q 1   315 , causing it to turn off. Thus, it should be appreciated that changing the value of current sensing resistor Rs  210  or the ratio of resistor  415  to resistor  420  can vary the threshold. 
     Further, it should be appreciated that various alternative circuit configurations may be provided to provide various alternative functionality. For example, the resistance in the form of resistor  420  may be omitted entirely; alternatively, the values of resistances  415  and  420  may be selected so as to provide a voltage divider configuration so as to vary the threshold voltage required to trigger operation of Q 2 . Additionally, resistance in the form or resistor  415  and capacitance in the form of capacitor  465  may operate as a low pass filter configuration. The other components ( 470 ,  475 ) provided in  FIG. 4  may also provide various voltage and current managing operations, and, therefore, are not required for embodiments of the invention. 
     In operation of the circuit illustrated in  FIG. 4 , when there is no overcurrent, the gate of Q 1   315  is positive due to the voltage developed by diode  405 , resistors  425 ,  430  and capacitor  435 . Likewise, zener diode  440  limits this voltage to a predetermined maximum. Thus, it should be understood that components  405 - 455  are utilized for implementation of the overcurrent protection circuit and voltage and current balancing of the circuit. Therefore, these components and their relative configuration are not essential to the functionality of the invention. Accordingly, various component configurations may be provided in the overcurrent protection circuit without departing from the scope of the invention 
       FIG. 5  illustrates a series of operations performed in accordance with an illustrated embodiment. As shown in  FIG. 5 , operations begin at  505  and control proceeds to  510  at which a current value is detected by measurement of the voltage developed across a resistor (e.g., Rs  210 ). Control then proceeds to  515 , when the detected voltage reaches a threshold detector gate turn-on voltage threshold. Control then proceeds to  520 , at which Q 2  is turned on. As a result, at  525 , Q 1  is turned off. Control then proceeds to  530 , at which a control signal is generated indicating overcurrent detection. Control then proceeds to  535 , wherein the control signal is provided to an external microcontroller for use for maintenance personnel use and/or diagnostic routine analysis. Control then proceeds to  540 , at which operations end. 
     For example, illustrative embodiments include a resistive element (e.g., resistor  210  illustrated in  FIG. 2 ) in series with the apparatus switching devices to be protected, wherein the threshold detection circuit is configured to detect the voltage across the resistance to detect the overcurrent. However, it should be understood that the resistance may be replaced with any type of component that enables detection of voltage or current to determine whether overload current is occurring. For example, such components may include a hall effect sensor or a current transformer. Those particular examples are galvanically isolated, they could also be located anywhere in series with the X and Y connections with their outputs connected to the threshold detector  215 ; in this case resistor  210  would be replaced with a short circuit. It should be appreciated that, depending on the type of component used in this manner, costs of associated circuits may be increased and may necessitate additional supporting resistors, capacitors, etc. 
     The applicants have provided description and figures which are intended as illustrations of embodiments of the disclosure, and are not intended to be construed as containing or implying limitation of the disclosure to those embodiments. There are a plurality of advantages of the present disclosure arising from various features set forth in the description. It will be noted that alternative embodiments of the disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the disclosure and associated methods, without undue experimentation, that incorporate one or more of the features of the disclosure and fall within the spirit and scope of the present disclosure and the appended claims.