Patent Publication Number: US-9837219-B2

Title: Switch contact wetting with low peak instantaneous current draw

Description:
FIELD 
     The present disclosure relates to the field of dry contacts of a switch used in an electronic device, and more particularly, to a method and circuit for supplying a wetting current to the dry contacts of a switch while reducing power consumption from the power supply of the electronic device. 
     BACKGROUND 
     “Dry contacts” include contacts of a type of switch which does not carry power on a normal basis, i.e. not intended to carry power as a part of an operational circuit. Dry contact switches are used in a variety of applications as an input method to an electronic device. For example, dry contacts are used to select settings on switches such as dual-in-line packages (DIP) switches for an electronic device. Dry contact switches are polled with a wetting current to determine their state (e.g., ON or CLOSED and OFF or OPEN) in order to perform certain operations. The wetting current is a current that is used to clean surface oxidation, if present, on the dry contacts of the switch. A wetting current parameter defines a minimum current, for the wetting current, that is necessary for cleaning surface oxidation on the dry contacts of the switch to be properly conductive. 
     Circuits, such as current sources and multiplexors, are used to supply wetting current for dry contacts, but are costly and consume a significant amount of power. Furthermore, it may not be desirable or even possible to supply wetting current to the dry contacts of a switch directly from the power supply of a low-powered and/or self-powered electronic device. A low-powered electronic device includes devices that draw around tenths of a watt or less of power. A self-powered electronic device includes devices that have a separate power supply which may be used in case of power failure or shut down of the main lines as part of the protective function of the electronic device, to maintain the protective capability of the electronic device in the absence of line power. These types of electronic devices may need to limit the average current consumed when reading or sensing the state of the switch. The terms “read” and “sense” and their derivatives are used interchangeably herein. 
     For example, a self-powered electronic device, such as a self-powered circuit breaker or motor overload relay, may employ a current transformer (CT) to supply operating current (also known as “supply current”) to the device inductively as well as to produce a measurement signal used for polling an operational parameter of the device which is set by the position of the dry contact switch. The electronic device may perform one or more functions, such as load protection (e.g., overcurrent or overload protection), based on the operational parameter. However, measurement error of the measurement signal can be a non-linear function of the power drawn by the electronic device and its components. Thus, if the electronic device draws too much power to produce the wetting current, it may increase the possibility of an erroneous measurement signal, which in turn impacts the functions of the electronic device that depend on the measurement signal. 
     Accordingly, there is a need to provide a simple and cost effective wetting current circuit for an electronic device. There is also a need to provide a wetting current circuit that limits or reduces power consumption from a power supply of the electronic device. 
     SUMMARY 
     To address these and other shortcomings, a contact wetting circuit and method thereof, particularly useful for a self-powered electronic device, are disclosed for supplying an appropriate wetting current to dry contacts of a switch used in an electronic device, for sensing the state of the switch setting. The contact wetting circuit is able to supply a wetting current for the switch, while limiting an average current or peak current (also known as “peak instantaneous current”) drawn from a power supply of the electronic device to below the wetting current parameter of the dry contacts of the switch. 
     In accordance with an exemplary embodiment, the contact wetting circuit includes a Resistor-Capacitor (RC) circuit and a controller connected to a power supply. The controller is configured to supply a first voltage to the RC circuit during a first time period. The first voltage produces a charging current through the RC circuit. The charging current has an average current or a peak current, which is limited below a wetting current parameter of the dry contacts of the switch. The charging current is used to charge a capacitor of the RC circuit during the first time period. Thereafter, the controller stops supplying the first voltage to the RC circuit, which in turn stops the flow of the charging current through the RC circuit. The capacitor, which is now charged (“charged capacitor”), is then allowed to supply a second voltage across the switch. The second voltage is used to produce a wetting current for the switch during a second time period. After the second time period, the controller senses the state of the switch (e.g., ON or CLOSED position, or OFF or OPEN position) or, in other words, the switch setting. The switch may include one or more switches. The controller may then perform certain operations or functions, according to the state of the switch. The state of the switch may reflect settings for operational modes or parameters of the electronic device. Thus, in addition to controlling the supply of wetting current, the controller may be used in the electronic device to implement other functions or features, depending on the nature or purpose of the electronic device. These functions or features may involve load protection. 
     The disclosed contact wetting circuit is low in complexity and cost, and does not require the use of active devices, such as current sources and multiplexors which consume a significant amount of power. Furthermore, the contact wetting circuit can be used with motor starters (e.g., DIP switch contact controlled settings in a motor starter control circuit), motor overload relays, circuit breakers, sensors, or other low-powered electronic devices or self-powered electronic devices that include a current transformer (CT) or other energy harvesting system. 
     To further reduce the complexity of the contact wetting circuit, the controller may interact with the RC circuit and the switch via a single lead, such as a general purpose input output (GPIO) pin. The controller changes the configuration of the GPIO pin to output a high logic level (e.g., 2 volt or 5 volt) to charge the capacitor of the RC circuit, and to receive as input a state of the switch after wetting current is applied to the dry contacts of the switch. After sensing the state of the switch, the controller can set the GPIO pin to output a low logic level, until the next time a switch reading operation is to be performed. In this way, the electronic device can minimize or reduce current consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description of the various exemplary embodiments is explained in conjunction with the appended drawings, in which: 
         FIG. 1  illustrates a diagram of an electronic device with a contact wetting circuit for supplying wetting current to dry contacts of a switch. 
         FIG. 2  illustrates a timing diagram of a switch reading process implemented by the components, such as a controller, of the electronic device of  FIG. 1 . 
         FIG. 3  illustrates a timing diagram according to  FIG. 2 , in which the switch is in the ON or in the closed position. 
         FIG. 4  illustrates a timing diagram according to  FIG. 2 , in which the switch is in the OFF or in the open position. 
         FIG. 5  illustrates an exemplary flow diagram of a process by which a state of the dry contacts of a switch, such as in  FIG. 1 , is sensed. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary contact wetting circuit and method are disclosed for use in supplying wetting current to dry contacts (e.g., a pair or multiple pairs of dry contacts) of a switch used in an electronic device. As will be described in detail further below in conjunction with  FIGS. 1-5 , the contact wetting circuit produces a charging current to charge an energy storage device, such as a capacitor, over a time period using power drawn from a power supply for the electronic device. The charging current has an average current and/or a peak current below a wetting current parameter of the dry contacts. The capacitor builds up charge over time. There is eventually sufficient energy stored in the capacitor to create the wetting current. The production of charging current is stopped, and wetting current is available for wetting the dry contacts using power supplied from the charged capacitor. The wetting current may be available before the charging current is stopped. The state of the switch is then sensed. Thereafter, the electronic device may perform functions according to the sensed switch setting. The electronic device can include motor starters, motor overload relays, circuit breakers, sensors, or other low-powered electronic devices and/or self-powered electronic devices that include a current transformer (CT) or other energy harvesting system. 
     Turning to  FIG. 1 , an electronic device  10  includes a switch SW 1  having dry contacts, a power supply  130  and protection and/or other circuitry  140 . The electronic device  10  also includes a contact wetting circuit  100  for supplying wetting current to the switch SW 1 . The switch SW 1  may include one or more switches. The switch SW 1  can include a type of switch that does not carry power on a normal basis, i.e. not intended to carry power as a part of an operational circuit. Examples of the switch SW 1  can include a DIP switch (e.g., a DIP style toggle switch, rotary-dial switch, sliding switch, etc.) or other types of relays with dry contacts that need to be wetted with a wetting current. 
     The setting of the switch SW 1  may define operational modes or parameters for the electronic device  10 . For example, the operational modes or parameters may include: a trip class selection (e.g., a parameter of the trip curve) implemented through a rotary-dial, sliding or toggle switch; a trip current setting implemented through a rotary-dial switch; a setting to enable, disable or configure other functions such as underload protection, undercurrent protection, ground fault protection and automatic reset (all of which are implementable through a toggle switch); a setting for a network communications address; or other settings involving an operation of the electronic device  10 . 
     The contact wetting circuit  100  includes a Resistor-Capacitor (RC) circuit  110  and a controller  120 , which is connected to the power supply  130 . The RC circuit  110  includes a resistor R 1  and a capacitor C 1 , and is connected between the controller  120  and the switch SW 1 . For example, the resistor R 1  of the RC circuit  110  may be connected to a lead, such as a GPIO pin  122 , of the controller  120 . In this example, the use of the GPIO pin  122  allows the controller  120  to interact with the RC circuit and the switch SW 1 , through a single lead. The capacitor C 1  of the RC circuit  110  is connected in parallel to the switch SW 1 . 
     The controller  120  can be a microcontroller, microprocessor, field programmable gate array (FPGA), application specific integrated circuit (ASIC) or other processing or control system. The controller  120  may include an internal memory  124  or may be connected to an external memory (not shown) for storing data and computer executable program or code. In one example, the computer executable program or code, when executed by the controller  120 , controls certain operations or functions of the electronic device  10 , such as the contact wetting function, load protection function or other operations of the electronic device  10 . 
     For example, the controller  120  is configured to supply a first voltage to the RC circuit  110  by drawing power from the power supply  130 ; to allow a wetting current for the dry contacts of the switch SW 1  to be produced from a second voltage supplied from the capacitor C 1  of the RC circuit  110 ; to sense a state of the switch SW 1 ; and to perform or control other functions or operations of the electronic device  10 , including those performed through the protection and/or other circuitry  140 . If the electronic device  10  is a load protection device (e.g., a circuit breaker or motor overload relay), the circuitry  140  may provide for load protection under the control of the controller  120 . 
     The power supply  130  supplies operating power to the various components of the electronic device  10 . The power supply  130  may include an energy harvesting system, such as a current transformer (CT), which is able to inductively generate operating current from a current carrying conductor connected between a power line and a load. The operating current also serves as a measurement signal, which reflects current levels on the conductor connected to the load. If the electronic device  10  is a load protection device, the controller  120  may utilize the measurement signal as part of the load protection function. For example, if the measurement signal exceeds a current threshold (e.g., reflects abnormal operating conditions), the controller  120  may issue a trip command to the circuitry  140  (e.g., a trip mechanism), which interrupts the flow of current to the load. 
       FIG. 2  illustrates a timing diagram  200  of a switch reading process implemented by an electronic device, such as the electronic device  10  in  FIG. 1 . For example, as shown in  FIG. 2 , the controller  120  initiates the process by configuring the GPIO pin  122  as an output, and then outputs a high logic level (e.g., 2 volt or 5 volt) to supply a first voltage to the RC circuit  110  during a first time period (e.g., delay D 1 ). During the first time period, the first voltage produces a charging current across the RC circuit  110  to charge the capacitor C 1  of the RC circuit  110 . After the first time period, the controller  120  configures the GPIO pin  122  as an input. The first time period is preferably at least of a minimum duration to charge the capacitor C 1  so that the capacitor C 1 , when charged, is able to produce a second voltage that is at or above a voltage, which is considered a high logic level input by the controller  120 . 
     By configuring the GPIO pin  122  as an input, the controller  120  stops the supply of the first voltage to the RC circuit  110  after the first time period or sufficient capacitor charging. The controller  120  allows the capacitor C 1 , which is now charged, to supply a second voltage across the switch SW 1  during a second time period (e.g., delay D 2 ). The second voltage is used to produce a wetting current for the switch SW 1 . The capacitor C 1  may already be available to provide wetting current during the charging time period (e.g., the first time period) if the capacitor C 1  is sufficiently charged. After the second time period, the controller  120  senses the state of the switch SW 1 , via the GPIO pin  122 , any time during a third time period (e.g., delay D 3 ). The second time period has a maximum duration that is preferably insufficient for the capacitor C 1  to discharge, through intrinsic leakage and the leakage current of the GPIO pin  122 , to below the high logic level input voltage of the controller  120 . 
     As shown in an exemplary timing diagram  300  of  FIG. 3 , the GPIO pin  122  of the controller  120  is sensing a signal, e.g., a low logic level, corresponding to a state of the switch SW 1  in the ON or in the closed position during the third time period (e.g., delay D 3 ).  FIG. 4  shows an exemplary timing diagram  400  in which the GPIO pin  122  of the controller  120  senses a signal, e.g., a high logic level, corresponding to the switch SW 1  in the OFF or in the open position during the third time period. The input logic levels shown in  FIGS. 3 and 4  are exemplary logic levels representing the ON or OFF state of the switch SW 1 , respectively. Turning back to  FIG. 2 , after the third time period, the controller  120  may configure the GPIO pin  122  as an output, which is then driven to a low logic level. By driving the GPIO pin  122  to output a low logic level, it is possible to further reduce current consumption by the switching circuitry of the electronic device  10 , which is particularly beneficial in low-power or self-powered applications. 
       FIG. 5  illustrates an exemplary flow diagram of a process  500  by which a state of the dry contacts of a switch is sensed. For the purpose of explanation, the process  500  will be described below with reference to the components of the electronic device  10  of  FIG. 1 . 
     At reference  502 , the controller  120  supplies a first voltage to the RC circuit  110  to produce a charging current having an average current and/or a peak current below a wetting current parameter of the dry contacts of the switch SW 1  during a first time period. The level or amount of the charging current can be controlled based on a combination of the resistance of the RC circuit  110 , e.g., the resistance of the Resistor R 1 , and the first voltage supplied by the controller  120 . At reference  504 , the capacitor C 1  of the RC circuit  110  is charged with the charging current during the first time period. 
     At reference  506 , the controller  120  stops the supply of the first voltage after sufficient capacitor charging to allow a wetting current for the dry contacts of the switch SW 1  to be produced from a second voltage supplied from the charged capacitor C 1  of the RC circuit  110  during a second time period. As previously discussed, the capacitor C 1  may already be available to provide wetting current during the charging time period (e.g., the first time period) if the capacitor C 1  is sufficiently charged. At reference  508 , the controller  120  senses a state of the switch (e.g., ON or CLOSED and OFF or OPEN), during a third time period after the second time period. After the third time period, the controller  120  outputs a third voltage to the RC circuit  110 , e.g., a low voltage or logic signal, to discharge the capacitor C 1 , at reference  510 . At reference  512 , the controller  120  may perform operations or functions according to the sensed state of the switch SW 1 . As previously discussed, the function can include, among other things, load protection. 
     The exemplary contact wetting circuit is described in  FIG. 1  with an exemplary R-C circuit for use in accumulating energy to limit average and/or peak power consumption over time, and using the accumulated energy to produce a wetting current for a switch. Circuit configurations, other than the RC circuit of  FIG. 1 , may also be used to accumulate energy and produce a wetting current in accordance with the contact wetting circuit and method of the present disclosure. For example, these circuit configurations can include resistor(s), capacitor(s) or other electronic elements or combinations thereof to limit the charging current and to store energy over a period of time for use in supplying a wetting current. Furthermore, the electronic device may also include protective circuitry, such as a diode, which can be connected across the lead(s) of the controller to protect the controller against floating voltages. 
     Furthermore, although the exemplary controller of  FIG. 1  employs a single lead for input and output, e.g., a GPIO pin, to interact with the RC circuit and the switch, the controller may be configured, at some expense, to use separate leads for input and output when interacting with the RC circuit and the switch. 
     While particular embodiments and applications of the present disclosure have been illustrated and described, it is to be understood that the present disclosure is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.