Abstract:
An overvoltage protection circuit for protecting a pass element in a controlled voltage supply circuit electrically connected between a circuit power supply interconnection terminal region suited for electrical connection to a circuit power supply and an output terminal, the pass element being protected from voltage surges that may occur on the circuit power supply interconnection with respect to a voltage reference interconnection. A voltage reference is provided electrically connected in series with a voltage. The voltage divider and the voltage reference are connected in series with one another between the circuit power supply interconnection and the voltage reference interconnection. A threshold switch is electrically connected to a corresponding one of the voltage divider output and one of the voltage divider terminating regions terminating regions, and has an output coupled to the pass element control region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Provisional Patent Application No. 60/921,162 filed Mar. 30, 2007 for “SURGE PROTECTED POWER SUPPLY”. 
    
    
     BACKGROUND 
     The present invention relates to regulated voltage supply circuits and, more particularly, to regulated voltage supply circuits with protection against the effects of voltage magnitude surges. 
     Modern aircraft have many electrical and electronic devices positioned and operating therein. All such devices, unless batteries or generators, require electrical power to be supplied thereto to operate, and usually also require, in at least some portions thereof, that this power be regulated in some sense. Typically, a voltage from a voltage source is supplied to such devices with the magnitude thereof regulated to some extent so as to generally remain at or near some selected value. Often, the supply electrical conductors over which such regulated voltage is supplied from the source to the devices extend for substantial distances through the aircraft and are connected to plural ones of such devices. 
     In operation, such aircraft will on occasion have to fly through or near thunderstorms and, as a result, will encounter lightening strikes thereon. Such strikes often cause short duration voltage magnitude surges on the supply electrical conductors, and such transient voltage excursions from the corresponding value selected therefor typically last somewhere around one to two hundred milliseconds and have peak magnitudes of several hundred Volts or more in waveform having a very rapid rise to such a voltage peak followed by a significantly slower falloff. Many of the devices supplied electrical power by the supply electrical conductors cannot withstand such surges without damage to at least some portions thereof, and so there is a desire to supply voltage of a selected value to such portions of these devices, or the entire device, in a manner protecting them, or it, from such surges. 
     SUMMARY 
     The present invention provides an overvoltage protection circuit for protecting a pass element in a controlled voltage supply circuit electrically connected between a circuit power supply interconnection terminal region suited for electrical connection to a circuit power supply and an output terminal region between which the pass element can be directed at a control region to provide a conductive path of a selected conductivity, the pass element being protected from voltage surges that may occur on the circuit power supply interconnection with respect to a voltage reference interconnection. A voltage reference capable of maintaining a substantially constant voltage between a pair of terminating regions for a range of electrical currents through that pair of terminating regions is provided electrically connected in series with a voltage divider capable of maintaining at an output thereof a selected fraction of the voltage between a pair of terminating regions. The voltage divider and the voltage reference are connected in series with one another between the circuit power supply interconnection and the voltage reference interconnection. A threshold switch having first and second terminating regions and a control region by which that threshold switch is capable of being directed to provide a conductive path between threshold device first and second terminating regions of a selected conductivity has the threshold device first terminating region and control region each being electrically connected to a corresponding one of the voltage divider output and one of the voltage divider terminating regions terminating regions, and has the second terminating region being coupled to the pass element control region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an electronic circuit embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic diagram is provided in  FIG. 1  of a magnitude regulating electronic voltage supply circuit,  10 , for providing a voltage at a selected value to other user circuits (not shown) connected to this voltage supply circuit to allow them to operate based on this provided voltage. An unregulated, or insufficiently regulated, source of voltage (not shown) provides voltage to circuit  10  having a magnitude usually at or near some selected nominal value with respect to a ground voltage reference terminal,  11 , in supply circuit  10 , this voltage supplied through a positive voltage source terminal electrically connected to a supplied voltage terminal,  12 , in supply circuit  10 . A control terminal,  13 , is operated by a voltage supply operation initiation circuit (not shown) that selects between switching voltage supply circuit  10  off so as to not supply voltage to other user circuits at an output terminal thereof,  14 , through placing control terminal  13  at a sufficiently large voltage with respect to ground voltage reference terminal  11 , and switching voltage supply circuit  10  on so as to supply a regulated output voltage to the user circuits at output terminal  14  through placing control terminal  13  at a sufficiently small voltage with respect to terminal  11  while limiting the electrical current therethrough. 
     Control terminal  13  is electrically connected to the otherwise unconnected base of one of a pair of pnp bipolar transistors,  15 , that are electrically connected to one another in a Darlington circuit configuration. The otherwise unconnected emitter of transistor pair  15  is electrically connected to a series connected pair of shorted bridge configuration resistor circuits,  16  and  17 , at the interconnection between shorted bridge configuration resistor circuit  17  and a bidirectional zener diode,  18 , that is also electrically connected in series with circuits  16  and  17 . Bidirectional zener diode  18  has a selected breakdown voltage greater than the nominal value of voltage supplied to voltage terminal  12  typically a quarter to a third greater. The series connected group of shorted bridge configuration resistor circuits  16  and  17  and bidirectional zener diode  18  are electrically connected between supplied voltage terminal  12 , to which shorted bridge configuration resistor circuit  16  has one end connected, and ground voltage reference terminal  11  to which bidirectional zener diode  18  has one end connected. 
     Thus, for voltages on supplied voltage terminal  12  that are less than the breakdown voltage of bidirectional zener diode  18  (and for voltages greater than this breakdown voltage as will be described below), the placing of a sufficiently low voltage on control terminal  13  switches on Darlington connected transistor pair  15  allows drawing electrical current through series connected shorted bridge configuration resistor circuits  16  and  17  without bidirectional zener diode  18  also drawing any current therethrough. This current drawn by Darlington connected transistor pair  15  through series connected shorted bridge configuration resistor circuits  16  and  17  is provided through the common collector interconnection of transistor pair  15  at the resulting voltage there, and this current is provided at the resulting voltage to the interconnection junction of a noise suppression capacitor,  19 , having its opposite end electrically connected to ground voltage reference terminal  11 , and the input of a commercially available integrated circuit chip voltage regulator,  20 . Although a different switching device could be used, the high gain of Darlington connected transistor pair  15  results in very little of the current drawn through series connected shorted bridge configuration resistor circuits  16  and  17  being diverted from voltage regulator  20  out control terminal  13 . Thus, voltage regulator  20  and its output load essentially determine the current drawn through transistor pair  15  and series connected shorted bridge configuration resistor circuits  16  and  17 . 
     Shorted bridge configuration resistor circuit  16  has, at the end thereof connected to supplied voltage terminal  12 , a pair of resistors,  16 ′ and  16 ″, electrically connected in parallel with one another and both connected at one end to supplied voltage terminal  12 . A second pair of resistors,  16 ′″ and  16 ″″, in shorted bridge configuration resistor circuit  16 , are electrically connected in parallel with one another and both are connected at one end thereof to one end of parallelly connected resistors  16 ′ and  16 ″ and at the opposite end thereof to shorted bridge configuration resistor circuit  17 . These in shorted bridge configuration resistor circuit  16  resistors are of a relatively large resistance value and are provided as four resistors rather than one resistor of an equivalent value to be able to dissipate more heat developed therein by electrical currents therethrough. 
     Shorted bridge configuration resistor circuit  17  has, at the end thereof connected to shorted bridge configuration resistor circuit  16 , a pair of resistors,  17 ′ and  17 ″, electrically connected in parallel with one another and both connected at one end to shorted bridge configuration resistor circuit  16 . A second pair of resistors,  17 ′″ and  17 ″″, in shorted bridge configuration resistor circuit  17 , are electrically connected in parallel with one another and both are connected at one end thereof to one end of parallelly connected resistors  17 ′ and  17 ″ and at the opposite end thereof to bidirectional zener diode  18 . These resistors in shorted bridge configuration resistor circuit  17  are of a relatively small resistance values, typically having an equivalent resistance of one hundredth that of the equivalent resistance of the resistors in shorted bridge configuration resistor circuit  16  as an example. They are provided as four resistors rather than one resistor of an equivalent value to allow the ability for making small incremental changes in the equivalent value thereof through being able to independently change the value of each of the four different resistors to thereby provide selected increments in the equivalent value thereof in different implementations of circuit  10 . A noise suppression capacitor,  17   v , has each side thereof electrically connected to a corresponding one of the two ends of shorted bridge configuration resistor circuit  17 . 
     Voltage regulator chip  20  has a resistor,  21 , electrically connected at one end to a regulated voltage output thereof, and the other end of the resistor to an output voltage sensing input of regulator chip  20 . A resistor,  22 , is electrically connected at one end to the output voltage sensing input of regulator chip  20  and at the other end to ground voltage reference terminal  11 . The selection of magnitudes of the resistances of these two resistors allow selecting the magnitude of the output voltage provided by voltage regulator chip  20  which is typically from a third to a half of the nominal voltage supplied to supplied voltage terminal  12 . A further noise suppression capacitor,  23 , has one end electrically connected to the output of voltage regulator chip  20  and its opposite end electrically connected to ground voltage reference terminal  11 . 
     Use of a zener diode here in place of voltage regulator  20  with a breakdown voltage matching the output voltage of that regulator is possible but poses a failure risk upon occurrences of voltage surges on supplied voltage terminal  12 . There would be a corresponding surge of electrical current through that zener which could easily be great enough for it to overheat and fail. Voltage regulator  20  being a series pass element regulator will draw no more current than that which has been needed theretofore at its output by its load with the voltage surge instead being taken up across its series pass element along with shorted bridge configuration resistor circuits  16  and  17 . 
     The output voltage provided at the output of voltage regulator chip  20  is provided through a small value current limiting resistor,  24 , to an astable multivibrator arrangement in a charge pump circuit arrangement that is for providing a voltage between the gates and sources of a plurality of parallelly interconnected, n-channel, metal-oxide-semiconductor field-effect transistors (MOSFETs) operated in a source-follower circuit arrangement. These MOSFETs serve as the output power pass devices in a series type output voltage regulator to provide regulated output voltage at output terminal  14  of supply circuit  10  based on the nominal value voltage provided thereto at supplied voltage terminal  12 . 
     The astable multivibrator arrangement has a pair of voltage divider resistors,  25  and  26 , electrically connected in series with one another and having one end thereof electrically to the end of resistor  24  not connected to the output of voltage regulator chip  20  to be between resistor  24  and ground voltage reference terminal  11  to which the other end of the voltage divider is electrically connected. A noise suppression capacitor,  27 , has each side thereof electrically connected to a corresponding one of the two ends of this voltage divider. In addition, this same end of resistor  24  is connected to a positive voltage supply lead,  28 , and a relatively negative voltage supply lead,  29 , from ground voltage reference terminal  11 , form the voltage supply and ground return interconnections for a commercially available integrated circuit chip comparator,  30 , and are electrically connected thereto at the corresponding comparator interconnection terminals. The interconnection junction of resistors  25  and  26  forming the voltage divider, just described above, provide a comparator reference voltage value for comparator  30  by being electrically connected to the positive input terminal, or positive control input or control region, of that comparator. 
     A resistor,  31 , is electrically connected between the output terminal of comparator  30  and the positive input terminal of that comparator to thereby provide positive feedback to result in a regenerative process following a sufficient voltage “triggering” excursion at the comparator negative input terminal, or negative control input or control region thereof, with a polarity matching that of the current output state and also in a hysteretic switching characteristic. This regenerative process causes the output to rapidly change toward next being in the opposite one of two possible output states alternative to that state which was current at the “triggering”, these states being approximately at the extremes in the comparator output voltage operating range between the two voltage values to which the comparator is connected, the voltage at resistor  24  and ground at ground voltage reference terminal  11 . A further resistor,  32 , is electrically connected between the output terminal of comparator  30  and the negative input terminal of that comparator to thereby provide negative feedback and to charge and discharge a capacitor,  33 , electrically connected between that negative input terminal and ground voltage reference terminal  11 . The charging and discharging of capacitor  33  provides sequential “triggerings” at the negative input terminal of comparator  30  so that neither of the comparator output voltage states is stable over time and so oscillates between those output voltage states. A further resistor,  34 , is electrically connected between the end of resistor  24  and the output of comparator  30  to provide supplementary electrical current at this output when the output is in or near the relatively positive output voltage state, current which is drawn away at the comparator output when in or near the negative output voltage state. 
     This oscillating of the output voltage of comparator  30  between its two voltage extremes serves to charge and discharge a capacitor,  35 , connected on one end thereof to the output of comparator  30  and on the other end to the interconnection junction between two diodes,  36  and  37 , an arrangement which provides isolation between voltage values occurring on either side of capacitor  35  with respect to voltages that consist of sufficiently small frequency components. Capacitor  35  is connected to the anode of diode  36  and to the cathode of diode  37 . The anode of diode  37  is connected through two current limiting resistors,  38  and  39 , electrically connected in series with one another, to output terminal  14  of supply circuit  10 . The cathode of diode  36  is electrically connected to several circuit components including the cathode of a zener diode,  40 , which has its anode electrically connected to ground voltage reference terminal  11 . The breakdown voltage selected for zener diode  40  sets the output voltage provided by supply circuit  10  on output terminal  14  thereof at the value of that breakdown voltage less the gate to source voltage of the supply circuit  10  pass MOSFETS to be described below. 
     In addition, the cathode of diode  36  is electrically connected to the cathode of a further zener diode,  41 , having its anode electrically connected to the junction between resistors  38  and  39 , and to one side of each of a capacitor,  42 , and a resistor,  43 , both of which have their opposite sides also electrically connected to the junction between resistors  38  and  39 . The breakdown voltage for zener diode  41  is on the order of eight tenths that of the output voltage of voltage regulator  20  and limits the gate to source voltage of the supply circuit  10  pass MOSFETS to be described below. The capacitance of capacitor  42  is much larger than that of capacitor  35 , typically on the order of twenty times as large for example. 
     Finally, the cathode of diode  36  is electrically connected to one end of each of a plurality of resistors,  44 ,  44 ′,  44 ″ and  44 ′″. The opposite ends of each of these resistors are each electrically connected to the gate of a corresponding one of a plurality of parallelly interconnected, n-channel, metal-oxide-semiconductor field-effect transistors (MOSFETs),  45 ,  45 ′,  45 ″ and  45 ′″, which are the output power transistors serving as the pass elements of this series regulator formed by supply circuit  10 . The sources of each of these MOSFETs are electrically connected to output terminal  14  of supply circuit  10 , and the drains of each of these MOSFETs are electrically connected to supplied voltage terminal  12 . An interrupted supply voltage maintenance capacitor,  46 , has one side thereof electrically connected to output terminal  14  of supply circuit  10  and the other side electrically connected to ground voltage reference terminal  11 . This capacitor has a relatively large capacitance chosen to be large enough to maintain the voltage across an output load connected between output terminal  14  and ground terminal  11  for times exceeding those occurring for circuit protection interruptions in the operation of supply circuit  10  as a result of encountering voltage surges on supplied voltage terminal  12 . 
     When the output of comparator  30  is in its low state, capacitor  35  is charged (discharged from previous charge) from the voltage at output terminal  14  (set by the load and capacitor  46 ) through resistors  38  and  39  and diode  37  with diode  36  being reversed biased to prevent discharging capacitor  42 . The oscillatory change in comparator  30  to the opposite output voltage high state leads to the output of comparator  30 , resistor  34  and charged capacitor  35  together charging relatively larger capacitors  42  and  46  with the charging current limited by resistor  39 . The oscillation of output states of comparator  30  causes this charging of capacitors  42  and  46  to occur repeatedly so that the voltage across them increases (which charging is countered in capacitor  46  by its discharging in the other half the cycle to a net charge change of zero, but there is in any event little of either charging or discharging of capacitor  46  because of the large capacitance value of that capacitor). 
     The voltage across capacitors  42  and  46  is limited in two respects, first, the voltage across capacitor  42 , which sets and maintains the gate to source voltage of each of pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″, is limited by the breakdown voltage of zener diode  41  to protect those gates. Further, the voltage drop across capacitors  42  and  46  connected in series with one another between the cathode of diode  36  and ground voltage reference terminal  11  is limited by the breakdown voltage of zener diode  40 . This last breakdown voltage less the gate to source voltage of each of pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ is the maximum output voltage supplied at output terminal  14  of supply circuit  10 . That maximum voltage and the alternative lesser voltages typically provided there, depending on the voltage occurring on supplied voltage terminal  12 , are maintained on terminal  14  by the repeated charging of capacitor  42  during continual circuit operation (and of capacitor  46  but which charging, as indicated above, is countered in capacitor  46  by its discharging in the other half the cycle to net to net the very small changes therein to zero) with the capacitor  42  voltage remaining independent of the voltage on capacitor  46  so long as the voltage drop across capacitors  42  and  46  is less than the breakdown voltage of zener diode  40 . 
     That is, the voltage maintained on capacitor  42  will keep pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ switched on in saturation, and so very near to the voltage on supplied voltage terminal  12 , so long as that voltage remains less than the breakdown voltage of zener diode  40 . Thus, in this circumstance, pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ will keep capacitor  46  charged to near that voltage occurring on supplied voltage terminal  12 . As a result, the output voltage at output terminal  14  of supply circuit  10  in this circumstance, being close to the nominal value of the voltage provided to supply circuit  10  on supplied voltage terminal  12 , will in this circuit arrangement result in pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ being operated in or near saturation to keep the power dissipated in them relatively small. 
     Any charging attempts through the oscillation of the output voltage of comparator  30  of capacitors  42  and  46  to a sum thereacross beyond the breakdown voltage of zener diode  40  are returned to ground through that diode. Hence, the relatively small increases in the voltage provided on supplied voltage terminal  12  in the last circumstance that keep the voltage potential less than the breakdown voltage of zener diode  40  are dropped first across capacitor  46  until a voltage is reached causing zener  40  to conduct. However, larger increases in the voltage provided on supplied voltage terminal  12 , large enough to place the voltage potential to being greater than the breakdown voltage of zener diode  40 , are dropped across the drains of pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ resulting in the output voltage on output terminal  14  being limited, in the presence of such larger voltages on supplied voltage terminal  12 , to a maximum value equal to the breakdown voltage of zener diode  40  less the gate to source voltage of those pass elements. The voltage on terminal  14  remains at this maximum in the presence of these larger voltages on supplied voltage terminal  12  with the gate to source voltage of pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ decreasing due to the current drained through now conducting zener  40  sufficiently to force them into their pinch-off or linear operating region. 
     But, as the magnitudes of these increases in voltage provided on supplied voltage terminal  12  get even larger, so does the power that must be dissipated in those MOSFETs as their drain voltages correspondingly increase. Surges of voltage in the range resulting from lightning strikes can cause dissipation in these MOSFETs that exceed their safe operating areas, and so protection of them leads to the need to cause them to be switched off at least for a time sufficient for such surges to have fallen in magnitude sufficiently for the danger to the pass MOSFETs to have passed. During such times those MOSFETs are switched off, the stored charge on interrupted supply voltage maintenance capacitor  46  will supply the voltage needed by electrical loads connected between output terminal  14  of supply circuit  10  and ground voltage reference terminal  11 . 
     The need for such switching off of the pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ in times of extreme voltage surge occurrences on supplied voltage terminal  12  must first be sensed and the sensing portion of the sensing and switching circuitry for this switching off of them is provided by a pnp bipolar transistor,  50 . Transistor  50  has its emitter electrically connected to the junction of shorted bridge configuration resistor circuits  16  and  17  and has its base electrically connected through a current limiting resistor,  51 , to the junction of shorted bridge configuration resistor circuit  17  and bidirectional zener diode  18 . The current drawn through transistor pair  15  for voltage regulator  20  and the circuitry supplied thereby, and the current drawn through zener diode  18  if the voltage surge on supplied voltage terminal  12  exceeds the breakdown voltage of that zener diode only some relatively small value, causes only an insufficient voltage drop across shorted bridge configuration resistor circuit  17  to switch on transistor  50 . This is because of the small equivalent resistance value of shorted bridge configuration resistor circuit  17  relative to that of shorted bridge configuration resistor circuit  16 . 
     During sufficiently large voltage surges on supplied voltage terminal  12 , however, as set by the resistor values used in shorted bridge configuration resistor circuits  16  and  17  and the breakdown voltage of bidirectional zener diode  18 , transistor  50  will be switched on as substantial additional electrical current flows in shorted bridge configuration resistor circuit  17  and through bidirectional zener diode  18  as a result of the voltage surge on supplied voltage terminal  12  exceeding the breakdown voltage of that zener diode. That breakdown voltage maintains the voltage value at resistor  51  connected to the base of transistor  50 , and also across transistor pair  15  and the input of voltage regulator  20 . 
     The switching on of transistor  50  through its emitter voltage being increased with respect to its base voltage by the voltage drop occurring across shorted bridge configuration resistor circuit  17  causes an electrical current to flow from its collector through a zener diode,  52 , from its cathode to its anode and through a resistor,  53 . Part of this current sets the voltage at the junction of resistor  53  and the anode of a diode,  54 , by flowing in that diode and a resistor,  55 , electrically connecting the cathode of that diode to ground voltage reference terminal  11 . This voltage value at the junction of resistor  53  and the anode of diode  54  is set by the resistor values and the breakdown voltage of zener  52  to be more that that supplied at the output of voltage regulator  20  by at least the emitter to base voltage of a further pnp bipolar transistor,  56 , having its emitter electrically connected to this junction. The base of transistor  56  is connected to the end of resistor  24  opposite the end thereof connected to the regulated output of voltage regulator  20 , and the collector of transistor  56  is connected to the positive input terminal of comparator  30 . As a result, the switching on of transistor  50  increases the voltage on the emitter of transistor  56  sufficiently to switch on this latter transistor into saturation to thereby increase the voltage on the positive input terminal of comparator  30  above the voltage supplied to its positive voltage terminal through interconnection  28  thereby ceasing its output voltage oscillation. The voltage on capacitor  42  setting the voltage between the gate and source of each of pass MOSFETS  45 ,  45 ′,  45 ″ and  45 ′″ is quickly dissipated through resistor  43  thereby switching off those MOSFETs. 
     The switching on of transistor  50  can be caused to occur at different selected values of sufficient surge voltage increases between supplied voltage terminal  12  and ground voltage reference terminal  11  by adjusting the value of the equivalent resistance in shorted bridge configuration resistor circuit  17 , through suitably selecting the resistance values of the resistors therein, relative to the equivalent resistance in shorted bridge configuration resistor circuit  16 . If, for example, the switching on of transistor  50  is set to occur at a relatively smaller surge voltage increase value (even though the surge voltage reaches a much greater value peak) by such resistor value selections, fewer members may be needed in the plurality of parallelly interconnected, n-channel, MOSFETs  45 ,  45 ′,  45 ″ and  45 ′″ as there will be less voltage dropped across them thus allowing larger currents through each so that the total current can be split between fewer of them while still leaving each in its safe operating area. 
     A table of typical active component selections, and typical passive component parameter values, for the circuit of  FIG. 1  is the following: 
     
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 V 12  = 28 V 
               
               
                   
                 R 16′  = 1 kΩ 
               
               
                   
                 R 16″  = 1 kΩ 
               
               
                   
                 R 16′″  = 1 kΩ 
               
               
                   
                 R 16″″  = 1 kΩ 
               
               
                   
                 R 17′  = 10 Ω 
               
               
                   
                 R 17″  = 10 Ω 
               
               
                   
                 R 17′″  = 10 Ω 
               
               
                   
                 R 17″″  = 10 Ω 
               
               
                   
                 C 17   v  = 1000 pF 
               
               
                   
                 V Z18  = 36 V 
               
               
                   
                 C 19  = 0.1 μF 
               
               
                   
                 VR 20 : LM117H 
               
               
                   
                 R 21  = 249 Ω 
               
               
                   
                 R 22  = 2 kΩ 
               
               
                   
                 C 23  = 1.0 μF 
               
               
                   
                 R 24  = 20 Ω 
               
               
                   
                 R 25  = 10 kΩ 
               
               
                   
                 R 26  = 10 kΩ 
               
               
                   
                 C 27  = 0.47 μF 
               
               
                   
                 Comp 30 : LM211 
               
               
                   
                 R 31  = 10 kΩ 
               
               
                   
                 R 32  = 10 kΩ 
               
               
                   
                 C 33  = 1000 pF 
               
               
                   
                 R 34  = 4.99 kΩ 
               
               
                   
                 C 35  = 1000 pF 
               
               
                   
                 R 38  = 1 kΩ 
               
               
                   
                 R 39  = 1 kΩ 
               
               
                   
                 V Z40  = 43 V 
               
               
                   
                 V z41  = 10 V 
               
               
                   
                 C 42  = 0.01 μF 
               
               
                   
                 R 43  = 100 kΩ 
               
               
                   
                 R 44  = 10 Ω 
               
               
                   
                 R 44′  = 10 Ω 
               
               
                   
                 R 44″  = 10 Ω 
               
               
                   
                 R 44′″  = 10 Ω 
               
               
                   
                 MOS 45 : STB7NK00Z 
               
               
                   
                 MOS 45′ : STB7NK00Z 
               
               
                   
                 MOS 45″ : STB7NK00Z 
               
               
                   
                 MOS 45′″ : STB7NK00Z 
               
               
                   
                 C 46  = 5600 μF 
               
               
                   
                 V Z52  = 10 V 
               
               
                   
                 R 51  = 20 Ω 
               
               
                   
                 R 53  = 2 kΩ 
               
               
                   
                 R 55  = 100 kΩ 
               
               
                   
                   
               
             
          
         
       
     
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.