Abstract:
A circuit for protecting an electronic device from excessive voltages applied to an input or output terminal includes a solid state relay coupling the electronic device to the terminal. The solid state relay may include an opto-transistor coupled between the electronic device and the terminal and a light-emitting diode optically coupled to the opto-transistor. The solid state relay is coupled in series with a current limiting device, such as one or more enhancement mode field effect transistors, and an electromechanical relay. A voltage detector coupled to the terminal detects a voltage larger than a specific value and causes current to flow thorough the light-emitting diode, thereby interrupting the coupling between the terminal and the electronic device. The voltage detector may be coupled between two spaced-apart connections to the coupling path between the terminal to the electronic device so that the voltage detector avoids diverting current from the coupling path.

Description:
BACKGROUND 
     Electronic devices often have input terminals that receive signals of various types or output terminals from which output signals of various types are provided. In fact, many electronic devices have both such input terminals and output terminals. Such electronic devices include internal circuitry or components that may be damaged if a voltage having a large magnitude is applied to an input terminal or an output terminal. For example, electronic test equipment may have output terminals from which precision output voltages or currents are provided. The magnitude of these output voltages or currents may be relatively low, and the circuits or components that provide these output voltages or currents may be damaged if a relatively large voltage or current, such as an AC supply voltage, is applied to the output terminals. 
     It may seem to be a relatively simple matter to protect these circuits or components using, for example, a low current fuse. However, the circuits or components coupled to the terminals may be damaged before a fuse could reach melting temperature. Also, the impedance between the terminals may be too high to allow enough current to flow through the fuse responsive to a high voltage, so that the fuse would not open to protect the internal circuit or component. Fast-acting current sensing components might also be placed in parallel with the terminals. However, it may be important for all of the current supplied by an internal circuit or component to flow from the output terminal, thus precluding the use of a current sensing component in parallel with the output terminals which might draw current from the internal circuit or component that would otherwise flow from the output terminals. For example, the resistance of a circuit component connected between the terminals may be measured by coupling a specific current between the terminals and then measuring the voltage between the terminals. If current from a circuit supplying current to the terminals is diverted to a current sensing component coupled between the terminals, the resistance measurement may be in error. 
     It may therefore be important to be able to quickly decouple the external input or output terminals of an electrical device in a manner that does not draw current from or change the voltage between the input or output terminals. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     In one aspect of the present invention, a protection system for an electronic device can include a current limiting device and a solid state relay coupled in series between an input or output terminal and the electronic device. The solid state relay may include an opto-transistor coupled between the terminal and the electronic device in series with the current limiting device and a light-emitting diode optically coupled to the opto-transistor. The current limiting device may be implemented with one or more enhancement mode field effect transistors. In addition to the solid state relay, an electromechanical relay has a relay switch in series with the current limiting device. A voltage detection circuit detects a voltage applied to the terminal having a magnitude that is greater than a specific value. The detection circuit then immediately causes the solid state relay to close. For example, the voltage detection circuit may apply a voltage to a light-emitting diode used in a solid state relay thereby causing current to flow through the first light-emitting diode. The output of the detection circuit also is applied to a relay coil of the electromechanical relay, causing the electromechanical relay switch to open at a time slightly delayed due to the inherent delay of the electromechanical relay as compared to the solid state relay. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of an embodiment of a prior art electronic device protection circuit. 
         FIG. 2  is a schematic diagram of an electronic device protection circuit according to one embodiment of the invention. 
         FIG. 3  is a schematic diagram of an electronic device protection circuit according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of a prior art protection circuit  10  for an electronic device is shown in  FIG. 1 . The protection circuit  10  includes a pair of output terminals  14 ,  16 , one of which is coupled to the drain of a first n-channel depletion mode field effect transistor (“FET”)  18  through a contact switch  20   a  of an electromechanical (“EM”) relay. A coil that controls the conductive state of the contact switch  20   a  will be described below. A source of the FET  18  is coupled to a source of a second n-channel depletion mode FET  24 . Respective gates of the FETs  18 ,  24  are both coupled to the sources of the FETs  18 ,  22 . A drain of the FET  24  is coupled to both a terminal  28  that may be coupled to an output terminal of an electronic device (not shown) and a voltage detection circuit  30 . The voltage detection circuit  30  detects a voltage that is larger than a predetermined voltage, and then applies a signal to a coil  20   b  of the EM relay, which results in opening the contact switch  20   a . An electronic device coupled to the terminal  28  may then be protected from a voltage that is larger than the specific voltage that may be applied between the terminals  14 ,  16 . 
     In operation, the FETs  18 ,  24 , being depletion mode FETs, are conductive until the source-to-gate voltage reaches a threshold voltage for the FETs. However, since the respective sources and gates of the FETs  18 ,  24  are coupled to each other, the FETs  18 ,  24  never become non-conductive. Instead, the FETs  18 ,  24  initially act as resistors when the applied voltage is increased so that the current through the FETs  18 ,  24  increases accordingly. However, when the FET reaches saturation, the current through the FETs  18 ,  24  remains constant. The FETs  18 ,  24  thus act as current limiters until the voltage detection circuit  30  resets the EM relay to open the contact switch  20   a . In theory, the current limiting effect of the FETs  18 ,  24  protects an internal circuit or component connected to the terminal  28  until the EM relay contact switch  20   a  is opened. However, in practice, since the power dissipated by the FETs  18 ,  24  continues to increase with the applied voltage, the FETs  18 ,  24  may become damaged or destroyed before the EM relay contact switch  20   a  can be opened. For example, although the voltage detector  30  may be able to very quickly detect a significant voltage applied to the terminals  14 ,  16 , one EM relay used in the prior art protection circuit  10  may require about 7 ms for the contact switch  20   a  to be opened. The FETs  18 ,  24  may be damaged or destroyed if the applied voltage switch increases sufficiently during these 7 ms. The prior art protection circuit  10  may be inadequate in many instances. 
     A protection circuit  100  according to one embodiment of the invention is shown in  FIG. 2 . The protection circuit  100  uses many of the same components that are used in the protection circuit  10  of  FIG. 1 . Therefore, in the interest of brevity and clarity, the same reference numerals will be used, and an explanation of the function and operation of these components will not be repeated. The protection circuit  100  may differ from the protection circuit  10  by including a solid state relay  120  to interrupt the flow of current through the output terminal  14  before the FETs  18 ,  24  can become damaged. The solid state relay  120  may include a light-emitting diode (“LED”)  124  coupled between a pair of control terminals  126  one of which is coupled to the output of the voltage detection circuit  30  through a resistor  128 . The solid state relay  120  may also include a depletion mode n-channel opto-FET  130  that is controlled by light from the LED  124 . The opto-FET  130  is coupled between a pair of switch terminals  132 , which are coupled to the sources of respective ones of the FETs  18 ,  24 . The solid state relay  120  may have a response time that is significantly faster than the response time of the EM relay having the switch  20   a  and coil  20   b , and is sufficiently fast to terminate the flow of current through the FETs  18 ,  24  before the FETs become damaged. However, the magnitude of the voltage that the solid state relay  120  can handle may be less than the magnitude of the voltage that the FETs  18 ,  24  can handle. For example, in one embodiment, the FETs  18 ,  24  may have a maximum operating voltage of about 650 volts while the opto-FET  130  may have an operating voltage of only about 60 volts. 
     In operation, if a relatively high voltage is applied between the terminals  14 ,  16 , the current may initially flow through the FETs  18 ,  24  and the solid state relay  120  (i.e., between the terminals  132 ). However, the FETs  18 ,  24  will then limit the current flow to a level that prevents the voltage drop across the solid state relay  120  from exceeding its maximum operating voltage. The voltage detector  30  will quickly sense the relatively high voltage and will automatically apply a signal to the LED  124 . The LED  124  in the solid state relay  120  couples light to the opto-FET  130 , which causes it to turn OFF. Significantly, the solid state relay  120  may terminate the flow of current through the FETs  18 ,  24  before the FETs can be damaged because it may respond much more quickly than the EM relay, which is controlled by the signal though the coil  20   b . Additionally, the EM relay will respond before an excessive voltage is placed across the solid state relay  120 . 
     Although the detection voltage at which the voltage detector  30  applies a signal to the relay coil  20   b  may be fixed in some embodiments, in other embodiments the detection voltage may be dynamic. More specifically, the voltage detector may be programmed with a detection voltage that varies as a function of variations in the voltage that is applied between the terminals  14 ,  16  during normal operation so that the detection voltage is always greater than the normal operating voltage. 
     Another embodiment of a protection circuit  150  in accordance with the present invention is shown in  FIG. 3 . The protection circuit  150  may use many of the same components that are used in the embodiment of the protection circuit  100  shown in  FIG. 2 . Therefore, in the interest of brevity and clarity, the same reference numerals will again be used, and an explanation of the function and operation of these components will not be repeated. The protection circuit  150  may differ from the protection circuit  100  by avoiding the use of any circuit component that may divert current from the current path between an electronic device  160  to be protected and the terminal  14 . The electronic device  160  may be, for example, an electrical testing device that provides a current having a specific magnitude to the terminal  14 . The protection circuit  150  includes a voltage detector circuit  170  that includes a resistor  172  coupling the terminal  14  to a second solid state relay  176  and a third solid state relay  180 . A control terminal  173  of the second solid state relay  176  is connected to the anode of an LED  174 , and the control terminal  173  of the third solid state relay  180  is connected to the cathode of an LED  178 . Each solid state relay  176 ,  180  may include an opto-transistor, such as an opto NPN transistor  184 ,  186 , respectively, that is optically coupled to the respective LED  176 ,  178 . The cathode of the LED  174  and the anode of the LED  178  are connected to respective control terminals  173  of the first and second solid state relays  176 ,  178 , respectively, which are in turn connected back to the conductive path extending between the terminal  14  and the electronic device  160 . The respective collectors of the transistors  184 ,  186  may be coupled through a switch terminal  188  to a supply voltage, such as 5 volts, and the respective emitters of the transistors  184 ,  186  are coupled through another switch terminal  188  to an input of a comparator  190  and a resistor  192  which is coupled to ground. An output of the comparator  190  is coupled to a set (“S”) input of a latch  194  which has an output coupled to an input of a first inverter  196  and an input of a second inverter  198 . An output of the second inverter  198  is coupled to the EM relay coil  20   b , and an output of the first inverter  196  is coupled to the LED  124  of the solid state relay  120 . 
     In operation, if a relatively high voltage is applied between the terminals  14 ,  16 , the current may initially flow through the FETs  18 ,  24  and the solid state relay  120 . However, the FETs  18 ,  24  will then limit the current flow to a level that prevents the voltage drop across the solid state relay  120  from exceeding its maximum operating voltage. Depending on the polarity of the applied voltage, one of the LED&#39;s  174 ,  178  in the solid state relays  176 ,  180 , respectively, will be conductive to turn ON its respective transistor  184 ,  186  when the current through the LED reaches a specific level. The magnitude of the voltage applied to the terminals  14 ,  16  at which the transistors  184 ,  186  turn ON may be set by the selection of the value of the resistor  172 . Thus, when the applied voltage reaches a predetermined specific level, the latch  194  will be set by the comparator  190  to drive the output of the inverter  196  low and cause current to flow through the LED  124  in the solid state relay  120 . At the same time, a low at the output of the inverter  198  may reset the EM relay via the coil  20   b  to open the EM relay contact switch  20   a  after the inherent delay of the EM relay. The current flowing through the LED  124  will then illuminate the LED, thereby turning OFF the LED  124  in the solid state relay  120 . Significantly, the solid state relay  120  may terminate the flow of current through the FETs  18 ,  24  before the FETs can be damaged because it can respond much more quickly than the EM relay. Additionally, the EM relay will respond before an excessive voltage is placed across the solid state relay  120 . It should be noted that any current flowing through the resistor  172  and one of the LEDs  174 ,  176  is returned to the current path between the terminal  14  and the electronic device  160  so that the detection circuit  170  does not divert current flowing from the electronic device  160  to the terminal  14  or vice versa. 
     Although the present invention has been described with reference to the disclosed embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the invention. For example, although the terminal  14  has been described in various places as an output terminal, it may alternatively be in other embodiments an input terminal or an input/output terminal. Such modifications are well within the skill of those ordinarily skilled in the art. Accordingly, the invention is not limited except as by the appended claims.