Abstract:
Methods and systems for selecting and connecting an electronics package to a power source from one or more of a plurality of possible power source paths. An embodiment of the invention uses inexpensive analog components to select a power path by testing the possible power source paths for a desired potential. The invention interrupts the alternate power path while maintaining the connection of the electronics package to the power source through the selected power path. An optional removal circuit removes the testing and selection circuitry once the alternate power path is interrupted. In actual embodiment of the invention, the methods and systems described are included in a reversible circuit breaker.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to electrical circuit breakers and, more specifically, to electrical circuit breakers including electronic components. 
     BACKGROUND OF THE INVENTION 
     Electrical circuit breakers interrupt the current flow in electrical circuits when the circuit breaker detects a fault in the electrical circuit. Most circuit breakers rely on the heat induced in a bimetal conductor by excess current to deform the bimetal conductor, which induces a mechanical action that physically breaks the circuit. These circuit breakers are often referred to as “thermal” circuit breakers. Recent improvements to circuit breakers utilize electronics to detect circuit faults that a purely thermal circuit breaker may not respond to, in part because the faults do not necessarily result in a sustained over-current situation. Examples of these circuit faults include arc faults and ground faults. If the electronics detect a fault condition, the electronics generate a signal that “trips” the circuit breaker, generally by activating a solenoid that induces a mechanical action to physically break the circuit. Not only do the electronics provide the means to detect these fault conditions, they permit the circuit breaker to respond to these conditions long before an over-current situation develops—if such an over-current situation develops at all. 
     No longer simple electromechanical devices, the electronics in present day circuit breakers must be powered. A challenge is that the electronics should be powered in such a way that there is not a conductive path bridging the break in the circuit provided by the circuit breaker when the circuit breaker is in the off position or has been tripped. Presumably, this means powering the electronics from one or the other side of the physical separation provided by the circuit breaker. Generally, the side connected to the power source is referred to as the “line” side, while the other side is referred to as the “load” side. When the circuit breaker is turned “on” or “reset,” the line side is electrically connected to the load side, forming the circuit that the circuit breaker protects. When the circuit is complete or “closed” (the circuit breaker is on), whether the electronics are powered from the line side or the load side may make little difference. However, when the circuit is incomplete or “open” (the circuit breaker is off or tripped), the side from which the electronics are powered determines whether the electronics actually receive power while the circuit breaker is in the open condition. This has practical consequences, for instance, if the electronics are powered from the load side, there is a latency in protection by the electronics from the time that the circuit breaker is turned on until the electronics “power up” to their useful state. This is particularly important in a “reset” situation, where the circuit breaker may have previously detected a fault that caused the circuit breaker to trip. Insuring that the electronics are powered from the line side (or the load side, as the application may dictate), is often a design and implementation requirement for a circuit breaker. 
     In the airplane industry, circuit breaker mounting locations are keyed to accept thermal circuit breakers in one specific orientation. Once the circuit breakers have been mounted in a panel, one electrical terminal is bolted to a solid line bus bar while the other terminal receives a terminal lug crimped onto a load wire. Prior to the introduction of electronic components into circuit breakers, the electromechanical basis of a thermal circuit breaker made the orientation of the line and load connections with respect to the terminals of the circuit breaker irrelevant. This led panel designers to route line buses and load wires in the most convenient configuration possible, many times interchanging the orientation of the line and load terminals with respect to the keyed mounting locations of the circuit breakers. 
     Densely packed circuit breaker panels, keyed mounting locations, solid line bus bars, and tightly secured wire bundles make retrofitting these panels to accept circuit breakers that require a specific orientation a difficult and expensive proposition. Instead of reconfiguring a circuit breaker panel, one could provide a different circuit breaker for each orientation and current rating, e.g., one part for line bus configuration “A” and another separate part for line bus configuration “B”. This at least doubles the part number quantities, causing additional expense for manufacturing, ordering and inventory, among other things. More importantly, the improper installation of circuit breakers can defeat the added safety afforded by the fault detection circuitry in the improved circuit breakers. 
     There exists a need for a reversible circuit breaker that includes fault detection electronics. The reversible circuit breaker preferably should include an automatic voltage source selector to set a power source path to its electronics package. Preferably, the automatic voltage source selector automatically detects a power source path from among a plurality of power source paths and then either (or both) selects the power path to be connected to the electronics package or severs the connections to the alternative paths. The present invention provides the solution to these needs. 
     SUMMARY OF THE INVENTION 
     The invention provides systems and methods for connecting an electronics package to an electrical path, while maintaining and fixing appropriate electrical isolation from other electrical paths. Among the many uses of the invention, the automatic power source selector is advantageously used in reversible circuit breakers having electronic fault detection components that require a connection to a power source. A circuit breaker that includes the invention may be installed in a plurality of orientations and will automatically connect the fault detection electronics to the appropriate power supply path. 
     The present invention comprises a system for selecting a power path from among a plurality of available power paths, connecting power paths and disconnecting unselected power paths, as required. A power source selection circuit is included in a reversible circuit breaker. The power source selection circuit detects the presence of a voltage on a one side of the reversible circuit breaker and disables the power path from a second side of the reversible circuit breaker to an electronics package included in the reversible circuit. 
     An inexpensive analog embodiment of the power source selection circuit is provided. The inexpensive analog embodiment uses power available on a first path to automatically detect and permanently interrupt a second power path. Power available from both power paths is used to remove the power source selection circuit from their connection to the power paths after the second power path is permanently interrupted. 
     The invention also provides a method to select a power path from a plurality of power paths and to automatically connect a selected path and/or disconnect an unselected path from an electronics package. The method tests the available power paths for a desired potential and then electrically isolates some or all of the remaining power paths. A method for installing a reversible circuit breaker that includes the present invention is also provided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
     FIG. 1 is an isometric view of an exemplary reversible circuit breaker that includes the present invention; 
     FIG. 2A is a block diagram of the reversible circuit breaker coupled in a circuit in a first orientation; 
     FIG. 2B is a block diagram of the reversible circuit breaker coupled in a circuit in a second orientation; 
     FIG. 3 is a block diagram of the reversible circuit breaker; 
     FIG. 4 is a block diagram of the power source selection circuit; 
     FIG. 5 is a functional flow diagram illustrating a power source path selection method, in accordance with the present invention; and 
     FIG. 6 is schematic diagram of an actual embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary reversible circuit breaker  110  is illustrated in FIG.  1 . The reversible circuit breaker  110  includes a housing  112 , a reset button or lever  114  and terminals  116  and  118 , and a ground connection (not shown). Solely for convenience in the following discussion, terminal  116  will be referred to as A-side terminal  116  and terminal  118  will be referred to as B-side terminal  118 . The present invention makes the A-side terminal  116  and B-side terminal  118  interchangeable as is illustrated in FIGS. 2A and 2B. The reversible circuit breaker  110  is shown in block format in FIG. 2A in a first orientation  120 . In the first orientation  120  A-side terminal  116  is connected to a line  122  and the B-side terminal  118  is coupled to a load  124 . The line  122  may be any AC or DC power source. The load  124  is generally any device or component requiring power to operate. The reversible circuit breaker  110  protects the circuit comprising the line  122 , the line-side path  126 , the reversible circuit breaker  110 , the load-side path  128 , the load  124  and the neutral or common path  130 . The reversible circuit breaker  110  has a ground path  132  that may be shared by the neutral or ground path  130 , depending upon the application. 
     FIG. 2B shows the reversible circuit breaker  110  in a second orientation  134 . In the second orientation  134 , the reversible circuit breaker  110  is “reversed” in circuit  136  so that the B-side terminal  118  is coupled to the line  122  through line path  126  and A-side terminal  116  is coupled to the load  124  through load-side path  128 . As will become apparent in the following discussion the present invention is not limited to two terminals or two orientations. For example, a circuit breaker with a circular housing and a plurality of terminals could be inserted into a circuit such as circuit  136  in a plurality of orientations while retaining the benefit of the present invention. 
     The reversible circuit breaker  110  of the present invention is shown in block diagram format in FIG.  3 . When closed, the reversible circuit breaker  110  conducts electricity along a circuit path comprising an A-side circuit path  312  and a B-side circuit path  314 . A circuit break assembly  316  is interposed between the A-side circuit path  312  and the B-side circuit path  314  such that the circuit break assembly  316  can interrupt the electrical connection between the A-side circuit path  312  and the B-side circuit path  314 . The circuit break assembly is generally an electromechanical mechanism that either completes the circuit path by engaging a first contact that is coupled to the A-side circuit path  312  with a second contact that is coupled to the B-side circuit path  314 . Conventional circuit break assemblies utilize a bimetallic conductor that deforms in response to heat induced in the circuit by excess current flow. The bimetallic conductor causes a mechanical action that separates the first contact from the second contact, thereby interrupting the circuit path. 
     The reversible circuit breaker  110  also includes an electronics package  318  that detects various fault conditions in the circuit path  136  being protected by the reversible circuit breaker  110 . For example, the electronics package  318  may include electronics that detect a ground fault or an arc fault. When the fault is detected, the electronics package  318  activates the circuit break assembly  316  via connection  320 . The electronics package can detect, monitor or control other aspects of the circuit and is not limited to fault detection. 
     The electronics package  318  has a power connection  322  and a ground connection  132 . A power source selection circuit  326  supplies power to the electronics package  318  via the power source connection  322 . The power source selection circuit  326  is coupled to the A-side terminal  116  via an A-side power path  328 . Similarly, the power source selection circuit  326  is coupled to the B-side terminal  118  through a B-side power path  330 . The circuit break assembly  316 , the power source selection circuit  326 , the electronics package  318 , and the reversible circuit breaker  110  may have a ground connection  132  as required and as would be recognized by one skilled in the art. 
     When the circuit break assembly  316  is closed (the first contact is engaged with the second contact), the A-side power path  328  and the B-side power path  330  will share a common potential. However, when the circuit break assembly is open (the first contact is separated from the second contact), one of the power paths  328  or  330  will be connected to the line-side path  126  while the other will be connected to the load-side path  128 . The power source selection circuit  326  selects either A-side power path  328  or B-side power path  330  according to the requirements of the application, coupling the selected path to the electronics package power path  322  and severing the connection with the unselected path. The power source selection circuit may be a manual switch or an automatic selection circuit as is described in detail below. A-side power path  328  and B-side power path  330  should be electrically isolated so that an alternate circuit path is not formed when the circuit break assembly  316  is open. 
     The power source selection circuit  326  is shown in more detail in FIG.  4 . As indicated above, the power source selection circuit  326  is coupled to the A-side power path  328  and the B-side power path  330 . The A-side power path  328  is coupled to a first side of a test block circuit  410 . A second side of the test block circuit  410  is coupled to a first side of a path interrupter  412 . A second side of the path interrupter  412  is coupled to a first side of a termination block circuit  414 . A second side of the termination block circuit  414  is coupled to the power supply junction  322 . The B-side power path  330  is symmetric to the A-side power path  328  just described, i.e., the B-side power path  330  is coupled to a first side of a test block circuit  416 . A second side of the test block circuit  416  is coupled to a first side of a path interrupter  418 . A second side of the path interrupter  418  is coupled to a first side of a termination block circuit  420 . A second side of the termination block circuit  420  is coupled to the power supply junction  322 . In other words, power paths  328  and  330  are respectively controlled by test block circuit  410 ,  416 , path interrupter  412 ,  418 , and termination block circuit  414 ,  420 , any of which may block the power through their respective power paths. 
     As just described, A-side power path  328  and B-side power path  330  join at power junction  322 . In order to select one of the power paths  328  or  330  and to disable the other, an automatic selection circuit  430  is provided. The automatic selection circuit  430  is coupled to each of the alternative power paths  328  and  330 . A-side test path  432  is coupled to A-side power path  328  and B-side test path  434  is coupled to B-side power path  330 . The A-side and B-side test paths  432  and  434  are coupled to each of the removal circuits  436  and  438 . In some embodiments, the removal circuits may be combined (e.g., under microprocessor control) and in others it is preferable to supply separate removal circuits (e.g., in the analog embodiment shown below in FIG.  6 ). 
     Interruption selector circuit  440  is coupled to both the A-side test path  432  and the B-side test path  434  through removal circuit  436 , while removal circuit  438  couples both the A-side and B-side test paths  432  and  434  to interruption selector circuit  442 . Interruption selector circuit  440  is coupled to the path interrupter  412  and may also be coupled to test block circuit  410  and termination block circuit  414 . Similarly, interruption selector circuit  442  is coupled to path interrupter  418  and may also be connected to test block circuit  416  and termination block circuit  420 . The automatic selector circuit  430  may be implemented using a microprocessor, digital electronics, or analog components. In a microprocessor embodiment, an analog to digital converter can sample the A-side test path  432  and B-side test path  434  and cause interrupt selector circuit  440  to activate path interrupter  412  while instructing interruption selector circuit  442  to disable path interrupter  418  or to activate path interrupter  418  while instructing interruption selector circuit  440  to disable path interrupter  412 . 
     A method  510  for implementing the present invention is illustrated in FIG.  5 . In a block  512 , an electrical path is selected for testing. This electrical path under test is electrically isolated from the other available paths in a block  514 . The electrical path under test is then tested for a desired potential in a block  516 . If a desired potential is not found on the electrical path under test, a decision  518  directs execution to block  512 , where another electrical path is selected for testing. If the desired potential is found, the method  510  connects the path under test  520  to the electronics package  318  and/or interrupts the alternative paths. The logic circuitry used for testing may then be electrically isolated from the plurality of paths in a block  522 . The method  510  ends in a block  524 . 
     While the testing of the electrical paths is illustrated in FIG. 5 as sequential, all paths may be tested at once in block  516  and the paths connected or disconnected from the electronics package  318 , as required by the particular application (block  520 ). In a digital embodiment, the electrical paths may be tied to the inputs of logic gates that test for the presence or absence of a desired potential on the paths. Based on the presence or absence of the desired potential, the logic can be configured to cause the output of the logic gates to activate an electronic or electromechanical switch that either closes or opens the electrical path  322  to the electronics package  318 . For example, in a reversible circuit breaker, the digital logic could detect the presence of a voltage on the A-side test path  432  (and optionally the absence of a voltage on the B-side test path  434 ), which would cause the output of the digital logic to open an electronic or electromechanical switch (e.g., path interrupter  418 ) in the B-side power path  330  (and optionally close an electronic or electromechanical switch, e.g., path interrupter  412 , in the A-side power path  328 ). 
     During testing in block  514 , the digital logic can control an electronic or electromechanical switch (e.g., test block circuit  416 ) to open and isolate the A-side power path  432  from the B-side power path  434 . Similarly, the digital logic can control an electronic or electromechanical switch (e.g., termination block circuit  420 ) to open and isolate the A-side power path  432  from the B-side power path  434  while the B-side power path is permanently interrupted. For instance, if the path interrupter  418  is a fuse or circuit breaker, opening the termination block circuit  420  protects the A-side power path  328  while an electric current of sufficient amperage is directed through the path interrupter  418  until the B-side power path  330  is interrupted (e.g., the fuse blows or circuit breaker trips). The digital logic can then optionally direct a removal circuit (e.g.,  436  and  438 ) to remove the digital logic from the circuit, as shown in block  522 . A symmetrical design allows for the selection of the B-side power path in the same way discussed above with regard to the A-side power path. In fact, any number of electrical paths can be tested in this way. Of course, instead of discrete logic, a programmable logic device, microprocessor, micro-controller, or the like, could be used. 
     The method  510  may also be implemented using inexpensive analog components, as is illustrated in FIG. 6. A power source selection circuit  610  utilizes inexpensive analog components to select either A-side power path  328  or B-side power path  330 . A-side power path  328  is connected to a first side of a resistor  612 , a second side of resistor  612  is connected to a first end of a fuse  614 . A second end of fuse  614  is connected to the anode of a silicone-controlled rectifier (SCR)  616  and a first end of a resistor  618 . A second end of resistor  618  is coupled to the gates of both SCR  616  and SCR  624 . The cathode of SCR  616  is coupled to a first side of a resistor  620 . A second side of resistor  620  is coupled to a first end of a fuse  622 . A second end of fuse  622  is coupled to the anode of an SCR  624 . The cathode of SCR  624  is connected to a ground  626 . SCRs  616  and  624  and resistors  618  and  620  comprise an analog embodiment of the interruption selector circuit  442 . The fuse  622  acts as the path interrupter  418 . 
     The B-side test path  434  is symmetrical to the A-side test path  432 . B-side power path  330  is coupled to a first end of a resistor  628 . A second side of resistor  628  is coupled to a first end of a fuse  630 . A second end of fuse  630  is coupled to the anode of a SCR  631  and a first end of a resistor  632 . The cathode of SCR  631  is coupled to a first end of a resistor  634 , the second end of the resistor  634  is coupled to a first end of a fuse  636 . The second end of fuse  636  is coupled to the anode of a SCR  638 . The cathode of SCR  638  is coupled to ground  626 . A second end of resistor  632  is coupled to the gates of both SCR  631  and SCR  638 . The SCRs  631  and  638  and resistors  632  and  634  comprise the interruption selector circuit  440  and fuse  636  acts as a path interrupter  412 . 
     The second end of resistor  612  is also coupled to a first end of a resistor  640 , the second end of resistor  640  is coupled to a control input of an opto-isolator  642 . The second end of fuse  630  is coupled to an input of opto-isolator  642 . The output of opto-isolator  642  is connected to ground. The opto-isolator  642  can be modeled as a light emitting diode (LED) that activates a transistor circuit. The second end of resistor  640 , therefore, drives the LED portion of the opto-isolator  642  to activate the transistor portion of opto-isolator  642  to close. The use of an opto-isolator  642  is preferred because of the superior electrical isolation that it affords between the A-side test path  432  which controls the activation of the opto-isolator  642  and the B-side test path  434  which is connected to ground by the opto-isolator when activated. Resistor  640 , opto-isolator  642 , and fuse  630  comprise an analog embodiment of the removal circuit  436 . 
     An analog embodiment of the B-side removal circuit  438  includes a resistor  644  that has a first side coupled to resistor  628  and a second side coupled to a control input of an opto-isolator  646 . The control input of the opto-isolator  646  is modeled as an LED connected to ground. When voltage is applied to this LED, light activates a transistor portion of the opto-isolator  646  whose input is connected to the second end of fuse  614 . The output of opto-isolator  646  is connected to ground. 
     A-side power path  328  is coupled to the electronics package  318  through an analog embodiment of the test block circuit  410 , the path interrupter  412  and the termination block circuit  414 . The analog embodiment of the test block circuit  410  has a TRIAC  647  with a first end coupled to the A-side power path  328  and second side coupled to the first end of resistor  634  and the first end of fuse  636 . A first end of a resistor  649  is coupled to the first end of the TRIAC  647  and the A-side power path  328 . A second end of resistor  649  is coupled to the gate of TRIAC  647 . 
     An analog embodiment of the termination block circuit  414  includes a TRIAC  648  with a first end connected to the second side of fuse  636  and the anode of SCR  638 . A second end of TRIAC  648  is coupled via the power path  322  to the electronics package  318 . A first end of a resistor  650  is coupled to the anode of SCR  638  and the second of fuse  636 . A second end of resistor  650  is coupled to a gate of TRIAC  648 . 
     Keeping with the symmetrical nature of the circuit  610 , an analog embodiment of the test block circuit  416  has a TRIAC  652  with a first end coupled to the B-side power path  330 . A second of the TRIAC  652  is coupled to the first end of resistor  620  and the first side of fuse  622 . A first end of a resistor  653  is coupled to the first end of TRIAC  652  and the B-side power path  330 . A second end of resistor  653  is coupled to the gate of TRIAC  652 . An analog embodiment of the B-side termination block circuit  420  includes a TRIAC  654  with a first end coupled to the second end of fuse  622  and the anode of SCR  624 . A second end of TRIAC  654  is coupled via the power path  322  to the electronics package  318 . A first end of a resistor  656  is coupled to the first end of TRIAC  654 , the second end of fuse  622 , and the anode of SCR  624 . A second end of resistor  656  is coupled to the gate of a TRIAC  654 . 
     Circuit Operation 
     The operation of circuit  610  illustrated in FIG. 6 will now be explained assuming that the reversible circuit breaker  110  is oriented in the circuit as illustrated in FIG.  2 A. The A-side power path  328  is coupled to the line-side path  126  and the B-side power path  330  is coupled to the load-side path  128  or floating (e.g., because of an intervening switch). It is assumed that circuit breaker  110  is open when inserted into the circuit, so that A-side power path  328  and B-side test path  330  are not at the same potential. This initial condition may be ensured, for example, by shipping the reversible circuit breaker  110  with a protective collar (not shown) around reset button or lever  114  that maintains the reversible circuit breaker in the open condition until it is installed. When the line  122  is energized, a voltage will appear on A-side test path  432 . This voltage will appear on the second side of resistor  618 , which will activate the gates of SCRs  616  and  624  causing them to begin conducting current. With SCRs  616  and  624  closed, the current will flow through resistor  612 , fuse  614 , through SCR  616 , resistor  620 , fuse  622 , and SCR  624  to ground  626 . The cumulative resistances of resistor  612  and  620  will limit the current that flows through fuse  622 , but should be of a sufficiently low value to allow current in excess of the capacity of fuse  622  such that the current path to ground blows fuse  622 . On the other hand, fuse  614  should have a fuse capacity that exceeds the current permitted by the cumulative values of resistors  612  and  620 . In this way, the voltage appearing on A-side power path  328  will blow fuse  622 , permanently interrupting the B-side power path  330  from reaching the electronics package  318  through power path  322 . While the interruption selector circuit  442  is tending to the interruption of the B-side power path, the test block circuit  416  blocks an electrical path from the first side of fuse  622  to the B-side power path because resistor  653  currently has no voltage to activate TRIAC  652 . 
     With fuse  622  blown and fuse  614  intact, the transistor input of opto-isolator  646  has voltage available but no current path to ground because the control LED is connected to the B-side test path  434  and is not currently activated. Similarly, the LED control input of opto-isolator  642 , which is connected to the A-side test path  432 , has activated the input of opto-isolator  642 , but the transistor input currently has no voltage available because of its connection to the B-side test path  434 . The termination block circuit  420  blocks an electrical path between the A-side power path  328  (now available at the power path  322 ) from reaching the B-side power path  330  because the voltage available on resistor  618  continues to be applied to the gate of SCR  624 , which provides a path to ground  626  that pulls resistor  656  low, turning TRIAC  654  off. 
     Once fuse  622  is blown, permanently interrupting B-side power path  330  from reaching electronics package  318  via the power path  322 , the reversible circuit breaker  110  can be closed, for example, by removing the protective collar and pushing button  114 . This causes a voltage to appear on the B-side power path  330  as well as the A-side power path  328 . The voltage appearing on the B-side power path  330  puts a voltage on resistor  644  and the LED control input to opto-isolator  646 . This activates the LED portion of the opto-isolator which, in turn, closes the transistor portion of the opto-isolator  646  creating a path to ground. This path to ground enables a current to flow on A-side power path  328  through resistor  612  and fuse  614  to ground. The resistor  612  is sized to permit a current greater than the capacity rating of fuse  614 , allowing a current flowing to ground through opto-isolator  646  to blow fuse  614 . This permanently removes removal circuit  438  and interruption selection  442  from circuit  610 . Similarly, since the voltage on the A-side test path  432  on resistor  640  and the LED control input of opto-isolator  642  closes the transistor portion of the opto-isolator  642 , power is available on B-side test path  434  and current will flow through resistor  628 , fuse  630 , and opto-isolator  642  to ground. Resistor  628  is sized such that the current in B-side test path  434  exceeds the rated current capacity of fuse  630 , thereby blowing fuse  630  when the path to ground  626  is provided by opto-isolator  642 . 
     The availability of the path to ground  626  through opto-isolator  642  pulls resistor  632  to ground potential along with the gates of SCRs  631  and  638 . This opens SCRs  631  and  638 , protecting fuse  636  during the operation of removal circuits  436  and  438 . 
     The electronics package  318  is now coupled to the A-side power path  328  via TRIAC  647 , fuse  636 , and TRIAC  648 . TRIACs  647  and  648  will remain closed by virtue of the voltage available on the A-side power path  328  supplied via resistors  649  and  650 , respectively. The opening of fuses  614  and  622  maintain the permanent isolation between the A-side power path  328  and the B-side power path  330 . Due to the symmetrical nature of the circuit shown in FIG. 6, the circuit operation is the same (using corresponding elements) as that just described if the reversible circuit breaker  110  was oriented in the circuit as illustrated in FIG.  2 B. 
     Many substitutions are possible in the embodiment shown in FIG.  6 . For example, TRIACs  647 ,  648 ,  652 , and  654  are incorporated to pass AC signals. If a DC signal is all that is required by a particular application, the TRIACs could be easily replaced with SCRs or any other type of switching device such as FETs, electronic switches, or electro mechanical switches. For that matter, all SCRs, TRIACs, and opto-isolators in circuit  610  could be replaced by electronic switches such as transistors and electro mechanical switches. For instance, if electrical isolation is not a concern, each opto-isolator  642  and  646  could be replaced by FETs with the gate connected to the second end of resistor  640  or  644 , the collector connected to the second end of fuse  630  or  614 , and the emitter connected to ground  626 . 
     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.