Abstract:
A secondary protection device for protecting a circuit. The device includes a first positive temperature coefficient (PTC) resistor, a bidirectional zener diode and a second PTC resistor connected in series across input terminals. Output terminals are connected across the bidirectional zener diode for providing a protected output to the circuit. A second zener diode is coupled between one end of the bidirectional zener diode and a ground potential for limiting an output voltage to the circuit.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to surge protection devices and, more specifically to secondary protection devices for protecting circuitry operating with low level voltages. 
     BACKGROUND OF THE INVENTION 
     Bipolar circuitry may be exposed to damaging surges from the outside environment. Typically, the circuitry may be protected with a primary protector and a secondary protector. The primary protector is generally used to protect circuits operating with high level voltages, and the secondary protector is used to protect circuits operating with low level voltages, such as TTL logic level voltages. 
     Secondary protection devices must be fast acting to protect circuits against surges due to lightning or cross voltage appearing at the input terminals to the circuits. Some circuits, such as telephone communication (T1/E1) circuits, are also exposed to potentially destructive DC bias voltages and need isolation from the DC bias to prevent damage. 
     As new communication systems are introduced, secondary protection is provided by designing a protector circuit custom made for the system to be protected. Typically, the protector circuit may be part of other functional circuits in the system. Because the protector circuit is typically placed on the same chip as other functional circuits, precious space on the chip is allocated to the protector circuit. This approach is expensive and time consuming. 
     A need exists, therefore, for a secondary protection device that may be separate from the circuits to be protected. A need also exists for a secondary protection device that may be used for protecting various low level voltage circuits without having to custom design the protector circuit for each low level voltage circuit. Furthermore, a need exists for a protection device that may protect a circuit quickly and reliably. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a secondary protection device for protecting a circuit, including a first positive temperature coefficient (PTC) resistor, a bidirectional zener diode and a second PTC resistor connected in series across input terminals. The output terminals are connected across the bidirectional zener diode for providing a protected output to the circuit. A second zener diode is coupled between one end of the bidirectional zener diode and a ground potential for limiting an output voltage to the circuit. The diode power capabilities are based upon the protection requirements of the circuit. The PTC resistors each have a value of 4-6 ohms nominally. The secondary protection device is modularized and individually inserted in a punch down, 110-type connecting block for protecting the circuit. 
     In another embodiment, the secondary protection device includes a first positive temperature coefficient (PTC) resistor, a fast switching diode bridge and a second PTC resistor connected in series across input terminals. A first zener diode is connected between first and second nodes of the fast switching diode bridge. Second and third zener diodes are each connected between the first and second nodes respectively and a ground potential. Output terminals are connected between a third node of the diode bridge and the ground potential for providing a protected output to the circuit. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
     FIG. 1 is a circuit diagram of a secondary protection device in accordance with one embodiment of the present invention; 
     FIG. 2 is a circuit diagram of a secondary protection device, with isolation, in accordance with another embodiment of the present invention; 
     FIG. 3 is a circuit diagram of a secondary protection device in accordance with yet another embodiment of the present invention; and 
     FIG. 4 is a circuit diagram of a secondary protection device, with isolation, in accordance with another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a secondary protection device, generally designated  10 . Input terminals  11  and  12  provide a connection for receiving an unprotected signal, and output terminals  22  and  24  may provide a connection to a circuit being protected. For example, input terminals  11  and  12  may be connected to a TTL receiver that places a 5 volt level digital signal onto terminals  11  and  12 . Connected in parallel with terminals  11  and  12  are positive temperature coefficient (PTC) resistors  13  and  14  and bidirectional zener diode  17 . Bidirectional zener diode  17  is connected to PTC  13  and  14  at nodes  15  and  16 , respectively. 
     For the embodiment shown in FIG. 1, PTC  13  and  14  are each 4-6 ohm PTC resistors. Due to PTC  13  and  14  being able to change resistance as a function of temperature, PTC  13  and  14  each absorb high current for durations less than 1 millisecond and protect against AC line voltages at terminals  11  and  12  as high as 600 VAC, 47 Hz to 63 Hz. Bidirectional zener diode  17 , which is also a 5 volt component, conducts high current differentially. In operation, when the voltage across nodes  15  and  16  exceeds 5 volts, zener diode  17  begins to conduct. When the voltage across nodes  15  and  16  exceeds 7 volts, zener diode  17  is fully “on”. In this manner, bidirectional zener diode  17  clamps the differential voltage across nodes  15  and  16  to less than 7 volts. It will be appreciated that zener diode  17  turns “on” in less than 50 nanoseconds. 
     Connected between node  15  and ground potential are diode  18  and zener diode  19 . Similarly, connected between node  16  and ground potential are diode  21  and zener diode  20 . Zener diodes  19  and  20  are each 5 volt zener diodes in the example shown in FIG.  1 . Diodes  18  and  21  may be omitted, if switching times of 50 nanoseconds are sufficient for protection. Diodes  18  and  21  may be added, as shown, to decrease the switching times. For example, diodes  18  and  21  each have a capacitance of 10 picofarads or less and turn on in less than 5 nanoseconds. Thus, diodes  18  and  19  clamp the voltage at output terminal  22  to less than 7 volts with respect to ground potential at node  23  and, similarly, diodes  20  and  21  clamp the voltage at output terminal  24  to less than 7 volts with respect to ground. Diodes 25-28 provide a function similar to diodes 18-21, except they clamp negative voltages; diodes 18-21 clamp positive voltages. 
     In the embodiment shown in FIG. 1, the line-to-line voltage between output terminals  22  and  24  is protected to less than 7 volts, and the line-to-ground voltage between respective terminals  22 - 23  and terminals  23 - 24  is protected to less than 7 volts. 
     Another embodiment of a secondary protection device is shown in FIG.  2  and is generally designated as  30 . In this embodiment, the circuit (not shown) connected at output terminals  46  and  48 , for example, is provided with a T1/E1 signal from input terminals  31  and  32 . As known to those skilled in the art, various T1/E1 systems may be used in communications to remove a DC bias of −48 volts or −130 volts, for example, from the T1/E1 signal. In the example shown in FIG. 2, T1/E1 transformer  38  may be used to remove the DC bias between the input terminals and output terminals. As known to those skilled in the art, a T1 refers to a transmission link having a capacity of 1.544 megabits per second and an E1 refers to a transmission link having a capacity of 2.048 megabits per second. 
     Shown added on the primary side of T1/E1 transformer  38  are PTC resistors  33  and  34  and bidirectional zener diode  37 . Bidirectional zener diode  37  is connected in parallel to the primary side of T1/E1 transformer  38  at nodes  35  and  36 . A second bidirectional zener diode  41  is connected to the secondary side of T1/E1 transformer  38  at nodes  39  and  40 . The T1/E1 transformer has a 1:1 (one to one) turns ratio but other transformer ratios may be used depending upon the application. 
     In operation, PTC  33  and PTC  34  absorb any high current present at the input terminals for durations of 1 millisecond or less and protect against continuous AC line voltages of 600 VAC, 47 Hz to 63 Hz. Bidirectional zener diode  37  conducts the high current differentially across the primary side of the transformer and thus protects the input side to less than 7 volts. The second bidirectional zener diode  41  operates in a manner similar to bidirectional zener diode  37  and protects the secondary side of the transformer, as it conducts current differentially between nodes  39  and  40 . Both diodes  37  and  41  turn on completely with a differential voltage that is greater than 7 volts. The turn on speed is less than 50 nanoseconds. Diode  41  is optional and may be omitted. 
     Similar to the embodiment described before, secondary protection device  30  includes diode  42  and zener diode  43  connected between output terminal  46  and node  47 . Node  47  is at ground potential. In addition, diode  45  and zener diode  44  are connected between output terminal  48  and node  47 . Zener diodes  43  and  44  are 5 volt zeners and conduct current to ground, thereby protecting output terminals  46  and  48 . Diodes  42  and  45  may be omitted from secondary protection device  30 . 
     Diodes  42  and  45  may be added, if zeners  43  and  44  have large capacitances (greater than 7200 picofarads). By including diodes  42  and  45  in the device, the overall capacitance between each output terminal ( 46  or  48 ) and ground node  47  is lowered to a value under 10 picofarads. This provides a switching time that is less than 50 nanoseconds. Diodes  49 - 52  provide a function similar to diodes  42 - 45 , except they clamp negative voltages; diodes  4245  clamp positive voltages. 
     In the embodiment shown in FIG. 2, the line-to-line voltage between output terminals  46  and  48  is protected to less than 7 volts and the line-to-ground voltage from terminal  46  and terminal  48 , respectively, to ground node  47  is protected to less than 7 volts. 
     Still another embodiment of a secondary protection device is shown in FIG.  3  and is generally designated as  60 . The circuit to be protected is connected at terminals  77  and  79  for receiving TTL data from input terminals  61  and  62 . Connected across the input terminals are PTC resistors  63  and  64  and diode bridge  67 . Diode bridge  67 , which includes four fast switching diodes  71 ,  72 ,  73  and  74 , is shown connected across nodes  65  and  66 . Completing FIG. 3 are zener diodes  68 ,  75  and  76  connected respectively to nodes  69  and  70 . 
     It will be appreciated that PTC  63  and PTC  64  may be 4-6 ohm resistors. The zener diodes may each be 5 volt zeners. Thus, secondary protecting device  60  effectively conducts high current and voltage on input terminals  61  and  62  away from the circuit being protected (not shown). Zener diode  75  conducts differentially across diode bridge  67 . Zener diodes  68  and  76  conduct current to ground at node  78 . Diode  68  conducts negative current to ground and diode  76  conducts positive current to ground. Diode  75  is optional and may be omitted. 
     Still another embodiment of a secondary protection device is shown in FIG.  4  and is generally designated as  90 . The circuit to be protected (not shown) is connected between terminals  114  and  116  for receiving T1/E1 signals from input terminals  91  and  92 . As previously described, T1/E1 transformer  98  removes the DC bias between the input terminals and output terminals. PTC resistors  93  and  94  and bidirectional zener diode  97  provide similar functions as those having been described for PTC resistors  33  and  34  and zener diode  37 , shown in FIG.  2 . Zener diode  101  operates similarly to zener diode  41  of FIG. 2; it conducts current differentially between nodes  99  and  100 . 
     Diode bridge  104 , which contains fast switching diodes  108 ,  109 ,  110 , and  111 , is shown connected across nodes  105  and  106 . Bridge  104  provides a function similar to bridge  67 , shown in FIG.  3 . Zener diodes  102  and  113 , which are respectively connected between bridge  104  and ground node  115 , provide functions similar to the functions of diodes  68  and  76 , respectively, of FIG.  3 . Diodes  101  and  112  are optional and may be omitted. 
     In each embodiment described above, it has been assumed that the circuit being protected is a 5 volt operating circuit, for example a circuit operating with 5 volt TTL data. For circuits operating at other voltages, for example 12 volt circuits, then 12 volt zener diodes should be used. If another circuit operates with 20 volts, then 20 volt zener diodes should be used. In this manner, secondary protection devices may be made to protect different families of circuits. 
     Each secondary protection device may be individually packaged in a small module. For example, using solid state construction techniques, an individual secondary protection device may be packaged in a module with the following outline dimensions: 4 cm (1.57″)×2 cm (0.79″)×1.3 mm (0.5″) or any industry standard 5-pin protection block. Furthermore, the individual protection devices may plug into a standard protection punch down block or protection panel. Thus, a system designer may incorporate as many individual secondary protection devices as required. One would simply select appropriate secondary protectors and place them in a punch down block.