US3838285A

US3838285A - X-ray tube anode protective circuit

Info

Publication number: US3838285A
Application number: US00358869A
Authority: US
Inventors: M Siedband; J James; A Patel
Original assignee: CGR Medical Corp
Current assignee: CGR Medical Corp
Priority date: 1973-05-10
Filing date: 1973-05-10
Publication date: 1974-09-24
Anticipated expiration: 1991-09-24

Abstract

An X-ray tube protective circuit wherein the selected exposure time and X-ray tube power are compared prior to operation to determine whether or not a condition exists which exceeds the anode rating of the tube and thereafter prevents operation in the event that such condition does exists while permitting an X-ray exposure for a safe condition. The protective circuit utilizes a single empirically derived anode ''''rating chart'''' curve which is representative of a generalized or standard X-ray tube rating chart curve. This empirical curve is generated by a series connected string of resistors connected to the exposure time select switch. This curve is then tilted and/or offset for selected operating modes in order to substantially conform to the acual rating curve for the respective various focal spot sizes and anode speeds of the particular X-ray tube in use.

Description

- United States Patent i 1 Siedband et al.

X-RAY TUBE ANODE PROTECTIVE CIRCUIT Inventors: Melvin P. Siedband, Madison, Wis;

Jack L. James, Westminister; Ambalal T. Patel, Baltimore, both of Md.

CGR Medical Corporation, Baltimore, Md.

Filed: May 10, 1973 Appl. No.: 358,869

Assignee:

References Cited UNITED STATES PATENTS Ohmart Bougle EXPOSURE TIME SELECT is so ZSEC 3/2 SEC I/60 SEC l/120 SEC Primary Examiner-James W. Lawrence Assistant Examiner-D. C. Nelms Attorney, Agent, or FirmBrady, OBoyle & Gates [57] ABSTRACT An X-ray tube protective circuit wherein the selected exposure time and X-ray tube power are compared prior to operation to determine whether or not a condition exists which exceeds the anode rating of the tube and thereafter prevents operation in the event that such condition does exists while permitting an X-ray exposure for a safe condition. The protective circuit utilizes a single empirically derived anode rating chart curve which is representative of a generalized or standard X-ray tube rating chart curve. This empirical curve is generated by a series connected string of resistors connected to the exposure time select switch. This curve is then tilted and/or offset for selected operating modes in order to substantially conform to the acual rating curve for the respective various focal spot sizes and anode speeds of the particular X-ray tube in use.

10 Claims, 5 Drawing Figures PATENIEDSEPZMHH sum 1 ll 4 L i E \x mm u 5m 0 m 252 n m E- 5 H 5% 52 5: 59:55 m 5 m u n n on w m 3 3 N 3 o 4" 5% 208 33 5 fim TE o 1 X-RAY TUBE ANODE PROTECTIVE CIRCUIT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to X-ray apparatus and more particularly to an electrical system for automatically preventing X-ray exposures under overload conditions.

2. Description of the Prior Art Modern, high power, X-ray tubes normally include a tungsten filament encased in a cathode cup which is mounted a short distance away from a rotating tungsten anode. The anode, in turn, is connected to a motor armature and bearing assembly with the entire structure mounted within the glass envelope of the X-ray tube. The tube is placed such that the motor armature and that portion of the glass envelope surrounding it are within the motor stator winding. When the stator winding is energized, the anode rotates so that during the X-ray exposure, new areas of the anode are brought within the electron beam cross section. The thermal capacity of the tube, i.e., the maximum X-ray exposure possible without damage to the anode, is determined by the energy level per exposure which is a function of the peak power expressed in terms of voltage (kV) and current (mA) and the exposure time in seconds (sec.), the area on the anode subtended by the electron beam as well as the shape, and finally the speed of rotation of the anode.

In an effort to obtain the maximum output per exposure of a particular X-ray tube, the manufacturer attempts to rate the respective tube at the maximum possible value per exposure such that the anode is brought almost to the point of melting during each X-ray exposure. In order to do this, the manufacturer publishes curves called rating charts which describes the maximum exposure for a particular X-ray tube under the various conditions of operation. Such a curve is plotted as a function of time versus the anode current (mA) for different high voltage potentials (kV) applied to the anode of the X-ray tube. For a given anode potential the same curve can represent time versus peak power by simply multiplying the mA coordinate by the potential.

Prior art protective circuits provide a control circuit which is responsive to analog voltages which, in turn, are functions of the desired power level and the exposure time duration, respectively, being then operative to compare the manual settings with predetermined and known maximum safe values allowed by the tube rating chart" curve for the particular X-ray tube employed. These analog voltages are developed by means of suitable power supply voltages feeding two separate voltage dividers, one of which corresponds to X-ray tube power while the other corresponds to exposure time. If the X-ray generator operates with two X-ray tubes and if each X-ray tube has two different focal spot sizes, and further if the X-ray tube anodes are permitted to rotate at two different speeds, separate voltage dividers or switch decks for exposure time (eight in number) would be required to be mounted on the X-ray exposure time switch in order to simulate each rating chart" curve for the particular mode of operation.

In U.S. Pat. No. 3,290,596 issued to J. Bougle, it was recognized that in the case of several X-ray tubes, rating charts relating to each focus may be resolvable in logarithmic coordinates into a number of straight lines and may be deduced from one another by simple translation according to the axis of the coordinates. 5 The present invention also recognizes the similarity in shape of the various rating charts for X-ray tubes but also recognizes the fact that a simple translation i.e., shifting of one simulated curve will not duplicate substantially all of the required curves, due to the fact that in some cases the slopes as well as the offset of the various curves differ widely.

SUMMARY Accordingly, it is an object of the present invention to provide an improved anode protective circuit, which eliminates the need for multiple voltage divider switch decks, ganged on the shaft of the exposure time switch, requiring replacement when one type of X-ray tube is substituted for another. Briefly, the invention comprises a single resistive voltage divider, simulating an empirically derived standard or generalized rating chart curve, fabricated as a single switch deck coupled to the exposure timing switch. An operational amplifier and a plurality of like circuits for both varying the gain and-an input offset bias potential of the operational amplifier for each selected operating mode are individually coupled to the voltage divider switch deck for tilting and shifting the generalized curve in order to correspond to the actual curverequired for the selected mode of operation. The output of the operational amplifier is then fed into a comparator circuit which additionally receives an input corresponding to the selected power level. The comparator determines whether or not a harmful condition exists and in the event such occurs, due to the operator selection of power and time of exposure, it energizes a relay to prevent an X-ray tube exposure. Each of the circuits for varying the gain of the operational amplifier comprises a variable resistance connected in series between the voltage divider and the inverting input of the operational amplifier and a potentiometer coupled across a source of constant potential providing a selected bias to the inverting input of the operational amplifier. Thus the characteristic of a single voltage divider is electrically tilted and/or offset thereby replacing the multiple switch decks heretofore required not only aiding installation but future maintenance procedures as well.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical schematic diagram illustrative of a typical prior art anode protective circuit;

FIG. 2 is an electrical schematic diagram illustrative of the preferred embodiment of the subject invention;

FIG. 3 is a graph illustrative of several selected X-ray DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters refer to like parts throughout, attention is directed to FIG. 1 which is illustrative of a prior art X-ray tube anode protective circuit of which the present invention is an improvement thereon. The circuitry disclosed in FIG. 1 includes means for manually selecting a plurality of X-ray exposure times expressed in seconds and peak power expressed in terms of anode current (mA). The exposure time select means couple to a plurality of

resistive voltage dividers

10, 12, and 14 which are electrical analogs of the respective rating charts" for each operational mode for the one or more X-ray tubes, not shown. Such curves are shown for purposes of illustration in FIG. 3 by reference numerals 15 15 15 and 15,,. For example, where two separate tubes are utilized, one for over-table use and having both a small and a large focal spot operating modes and another tube for under-table use having a selected focal spot size, the three resistive

voltage divider networks

10, 12, and 14 are coupled across a fixed voltage supply (24 volts) and each has a plurality of voltage taps connected to fixed switch contacts of respective rotary switches l6, l8 and with the taps mechanically corresponding to selectable exposure times of 8 seconds, 6 seconds one-sixtieth second, one-twentieth second, etc. The voltage at each tap, however, is representative of the corresponding allowable peak power of the appropriate rating chart curve. The switch 16 permits selection of the exposure time and the corresponding allowed peak power as indicated from the respective rating chart curve for the over-table small focal spot mode of the first tube, while switch 18 is operable for the over-table large focal spot mode of the first tube. The switch 20, on the other hand, is adaptable to select allowed peak power voltage signals corresponding to the "plurality of exposure times forthe under-table focal spot mode of the second tube. The movable contact of the switches l6, l8 and 20 are selectively connected to fixed contacts of one section of a two section ganged manually operable X-ray tube current (mA) select switch 22. More particularly, the movable contact of time select switch 16 is commonly connected to

fixed switch contacts

24, 26 and 28 while the movable contact of switch 18 is connected to

fixed contacts

30 and 32. The movable contact of switch 20, on the other hand, is connected to the single fixed contact 34. The movable contact 36 of switch 22 is coupled to the noninverting input of a voltage comparator circuit 38 by means of a fixed resistor 40. The other input of the comparator 38 is grounded. Thus the three upper positions of the switch 22 are adapted to couple analog voltages of the selected exposure time and the corresponding allowed peak power from the voltage divider 10 to the comparator 38 while two positions of the switch 22 are adapted to select voltages from voltage divider 12. The remaining position is adapted to provide selected voltages from voltage divider 14. As noted, each of the voltage dividers l0, l2 and 14 provide an electrical analog of the operating characteristic of a particular X-ray tube and operating mode. This characteristic, as provided from the tube manufacturers rating charts can be graphed, for example as shown by FIG. 3, and calculations made therefrom to provide the necessary resistance values for the required exposure times for each voltage divider network. Moreover, the movable contact of switches l6, l8 and 20 are ganged together on the same shaft of the X-ray exposure time switch, not shown, and as such the voltage di- The tube rating charts also take into account peak power. An analog circuit for this variable is provided by the single resistive voltage divider network 42 coupled across the X-ray tube high voltage potential (kV) which is held constant. A plurality of voltage taps corresponding to selectable values of anode current (mA) terminate in the

fixed contacts

44, 46, 48, 50, 52 and 54 of the other section of the switch 22 which is defined as the mA select switch. The movable contact 56 thereof is mechanically coupled to movable contact 36 such that when movable contact makes electrical connection with fixed contact 44, movable contact 36 makes electrical contact with fixed contact 24. It can be seen, therefore, that as the mA select switch 22 is moved between the upper three positions, corresponding to 25mA, mA and IOOmA, voltages corresponding to selected exposure time and peak power can only be taken from resistance voltage divider 10 corresponding to the over-table focal spot mode. In the same manner, mA switch positions for 200mA and 300mA select voltage divider network 12, while voltage divider network 14 operates in conjunction with the last position of the switch providing 200mA. It should be pointed out that the voltages appearing at the mA switch contacts 44, 46, etc. correspond to power inasmuch as the kV potential is applied across the resistive voltage divider is constant, taking note of the fact that power is equal to the product of the anode current (mA) and anode voltage (kV).

The movable contact 56 of the mA select switch 22 is coupled to an inverter amplifier 58 whose output is connected to the common connection with resistor 40 by means of fixed resistor 60 which has the same ohmic value as resistor 40. The purpose of the inverter amplifier 58 is to provide a positive polarity voltage to the input of the comparator 38 inasmuch as a negative polarity high voltage (kV) is applied to the voltage divider 42 and since the fixed supply potential coupled across the

voltage dividers

10, 12 and 14 is of a negative polarity thereby providing a negative potential to the resistor 40.

In operation, the circuit values are selected so that the voltages corresponding to the selected exposure time and the anode current or peak power are compared, the one voltage being of a negative polarity, and the other being of positive polarity. Since they are commonly applied through

resistors

40 and 60 to the comparator 38 an algebraic summation takes place yielding a resultant voltage. If this voltage is at some positive value, an output will be provided which would be of proper polarity to cause transistor 62 to become conductive and energize a relay coil 64 which in turn operates a set of relay contacts 66 for opening or deactivating an X-ray exposure control circuit, not shown. In the event that the resultant input to the comparator 38 is of a negative polarity, an output is provided which will not cause transistor 62 to conduct and not energize relay coil 64 to open the relay contact 66.

Thus when the operator selects the tube in the mode of operation desired, the appropriate switch deck including the required voltage divider l0, 12 or 14 is selected by a relay arrangement, not shown. When the exposure time is set, a voltage is selected from that deck and compared to the voltage associated with the selected electron beam current of the X-ray'tube. The voltages are then compared and an X-ray exposure is either permitted or forbidden as a function of whether the selected power exceeds the anode rating of the tube or whether it is within bounds.

As noted above, the switching decks including the voltage dividers must be selected for each X-ray tube focal spot and operating characteristics. In the field, if it is necessary to change from one tube type to another, the appropriate switching decks must also be changed. Now turning attention to the subject invention and more particularly to FIG. 3, there is shown semilog plots of several rating chart curves from which it is observed that most tube characteristics have a substantially straight line characteristic extending from the region of 0.2 seconds to seconds with a bend in the curve in the region of 0.1 seconds. While the Bougle patent US. Pat. No. 3,290,596 recognized that in the case of several different tubes rating charts relating to each focus may be deduced from one another by simple translation; however, FIG. 3 further indicates that the slope of various rating chart curves differ as well as being offset from one another. The present invention recognizes this latter fact and provides a single empirically derived resistive voltage divider network or switch deck 68 activated by the exposure time select mechanism to provide substantially a log function voltage characteristic with respect to time in order to provide an electrical analog of a standard or generalized curve such as shown in FIG. 3 and denoted by reference numeral 70. The electrical voltage divider characteristic of the switch deck 68 is effectively tilted and/or shifted (offset) for each X-ray tube and focal spot as well as any other operating parameter which calls for a separate tube rating chart curve in order to substantially match a curve for the instant operating condition. More particularly, the voltages selected from the fixed

switch contacts

70, 72, 74, 76, 78, 80 and 90 and corresponding to desired exposure time, are fed by means of the movable switch contact 84 to the movable contact 86 of one section of a three section mA select switch 22'. The switch section additionally includes fixed

switch contacts

88, 90, 92, 94, 96 and 98, while the other two sections correspond to the mA select switch configuration shown by reference numeral 22 in FIG. 1. Whereas FIG. 1 disclosed three voltage

divider switch decks

10, 12 and 14 for the three separate modes of operation, the circuit configuration shown in FIG. 2 includes three sets of resistive circuit means 100, 102 and 104 which are individually connected to the input of a feedback voltage amplifier 105 between the comparator circuit 38 and the single standardized voltage divider switch deck 68. Each of the circuit means 100, 102 and 104 is identical and is comprised of a variable resistance or rheostat 106 coupled in series to one input of the amplifier 105 through the switch contact 36 as well as a resistor potentiometer 108. The potentiometer is coupled across a fixed supply potential (+6.8" and 6.8) applied to

circuit terminals

110 and 112 with the slider of the potentiometer 108 being coupled to the same (negative) input of the operational amplifier by means of a fixed resistor 114 through the second section of the mA select switch 22 including the movable contact 36. The circuit 100 is used for the over-table small focal spot mode and is adapted to be selected by the upper three switch positions of switch 22 which also select voltages corresponding to mA, 50mA and IOOmA of the voltage divider network 42 while circuit 102 is utilized for the over-table large focal spot mode associated with the 200mA and 300mA switch positions. And finally, the 5 single under-table focal spot mode operable with the 200mA select position of switch 22 utilizes circuit 104.

Since

circuits

100, 102 and 104 are identical in circuit configuration, the operation of one of the circuits 0 in relation to the feedback amplifier 105 will suffice for the other two. The variable resistor 106 varies the operating characteristic (gain) of the amplifier 106 to effectively tilt the voltage divider characteristic of the switch deck 68 whereas the potentiometer 108 acts to provide 5 a shift or offset of the analog function of the rating chart curve by providing a DC input bias voltage to the amplifier. This can best be understood by referring now to FIG. 4 wherein a hypothetical electrical operational amplifier 116 is disclosed coupled between input and

output terminals

117 and 119, respectively. The operational amplifier 116 includes a feedback resistor 118, an output resistor 120, a variable resistor 122 in series between the input terminal 117 and the amplifier 116 and a resistor 124 connected between the input of amplifier 116 and a terminal 126 which is adapted to receive a bias voltage e Assuming that the operational amplifier is of the inverting type, reference to FIG. 5 illustrates the transfer characteristic of such a circuit as shown in FIG. 4. The voltage gain which comprises the ratio of e le assuming an infinity gain amplifier 116, is determined by the ohmic values of the fixed

resistor

118 and 122, i.e., R /R If for example the ratio of Il /R is l/5, a transfer characteristic such as shown with respect to the linear portion of the curve 128 of FIG. 5 will be provided. If the value of the resistance 122, however, is reduced wherein R /R is equal to l 2, the characteristic becomes steeper such as shown by the curve portion 130 thereby effecting a tilt of the transfer characteristic. If the bias voltage applied to terminal 126 of FIG. 4 is at zero or ground potential, the

curves

128 and 130 shown in FIG. 5 will pass through the origin as shown; however, if a 1.0 volt bias potential is applied to terminal 126, the output voltage e will be shifted or offset accordingly, such as shown with respect to lines 132 and 134. Thus, by varying the ohmic value of resistance 122, a tilt of the transfer characteristic of the amplifier 116 can be effected whereas the application of a suitable bias potential applied to terminal 126 can effect an offset of the transfer characteristic.

Referring now back to FIG. 2, the variable resistance 106 corresponds to the variable resistance 122 of FIG. 4 while resistor 144 corresponds to the resistor 124. In a like manner, a feedback resistor 136 in FIG. 2, corresponds to resistor 118 in FIG. 4 and resistor 138 corresponds to the output resistor 138. In operation, when an X-ray tube is changed or its mode of operation altered such as changing focal spot sizes, it is not now necessary to change the resistive voltage divider switch deck as required by the prior art but rather it is only necessary to suitably adjust the resistances 106 and/or 108 of

circuits

100, 102 or 104 to effectively change the slope and/or offset of the characteristic of the voltage divider 68 for each new rating chart curve as required. Thus the analog of a single standard rating chart curve is selectively adjusted for each operating mode by means of respective adjustment of the rheostat 106 and potentiometer 108. By a manual preselection of desired anode current, a particular negative potential will appear at the movable contact 56 which is coupled to a summing point 140 by means of resistor 60. The setting of the mA select switch 22' will also cause

movable switch contact

86 and 36 to make a corresponding connection with associated fixed switch contacts. Now a preselection of a desired exposure time will couple a negative voltage from a fixed contact 70 82 to the input of the operational amplifier 105 and modified by the action of the

elements

106 and 108 to provide a positive output potential which is coupled to the summing point 140 by means of fixed resistor 40. The comparator 38 is now operable such that if summation of the voltages at summing point 140 is some negative value, the output thereof will cause transistor 62 to conduct and open relay contact 66. On the other hand, if the voltage at summing point 140 comprises a positive potential greater than zero, the comparator output will not cause transistor 62 to conduct. Relay contacts 66 therefore remain closed.

In summation, therefore, the subject invention is directed to the improvement of providing a voltage amplifier with selective gain and bias adjust means coupled between selective voltage taps of an exposure time select voltage divider deck and a comparator to which is also applied voltage corresponding to the desired anode current. The analog voltage corresponding to the selected exposure time and the anode current are respectively of opposite polarity which are summed and the resultant voltage therefrom being adapted to operate circuit means which either allow or disallow an X-ray exposure.

Having disclosed what is at present considered to be the preferred embodiment of the subject invention,

We claim:

1. X-ray tube anode protective circuitry including I circuit means being operated by the output of a comparator circuit which compares selected values of analog voltages representative of X-ray tube rating chart curves for preventing energization of the X-ray tube in the event preselected operating parameters are outside of the ratings of the X-ray tube, comprising the combination of:

a supply voltage;

a single voltage divider network, for multiple operational modes having respective different rating chart curves for at least one X-ray tube, coupled across said supply voltage and having a currentvoltage characteristic adapted to simulate a generalized rating chart curve which is a function of a plurality of exposure times and providing a plurality of voltage points corresponding to allowable peak power for said exposure times;

first switch means having a movable contact and a plurality of fixed contacts respectively connected to said plurality of voltage points on said voltage divider;

a voltage amplifier having an input and output and including circuit means coupling the output of said amplifier to the input of said comparator circuit;

a plurality of first circuit means, each respectively adapted to vary the signal transfer characteristic of said voltage amplifier, selectively coupled between the input of said amplifier and the movable contact of said first switch means for each of said multiple operational modes whereby the output of said amplifier provides an output simulating the actual rating chart curve for the selected multiple operational mode;

an X-ray anode supply voltage;

a second voltage divider network coupled across said anode supply voltage and providing a plurality of voltage points proportional to the product of X-ray anode voltage and beam current of the X-ray tube; and

second switch means having a movable switch contact and a plurality of fixed contacts selectively connected to said plurality of voltage points of said second voltage divider network and additionally including second circuit means coupling the movable contact of said second switch means to the input of said comparator circuit whereby the resultant voltage at said input of said comparator comprises a signal input which is a function of the preselected operating parameters.

2. The protective circuitry as defined by claim 1 and additionally including switch means operable in conjunction with said second switch means, selectively coupling said plurality of first circuit means to the input of said amplifier.

3. The protective circuit as defined by claim 1 wherein each of said first circuit means includes means for selectively varying at least the gain of said amplifier.

4. The protective circuit as defined by claim 3 wherein said voltage amplifier comprises a feedback amplifier including an impedance coupled between its input and output, and said means for selectively varying the gain of said amplifier comprises a variable impedance coupled in series between the input of said amplifier and said movable switch contact of said first switch means.

5. The protective circuitry as defined by claim 4 and additionally including circuit means in each said first circuit means for providing a bias voltage to the input of said amplifier for further varying the signal transfer characteristic of said amplifier.

6. The protective circuitry as defined by claim 5 wherein said circuit means for providing a bias potential to the input of said amplifier comprises a second supply voltage and a potentiometer coupled across said second supply voltage, said potentiometer including a movable voltage tap coupled to the input of said amplifier.

7. The protective circuitry as defined by claim 5 and additionally including third switch means ganged with said second switch means, having a movable switch contact and a plurality of fixed contacts selectively connected to said plurality of first circuit means for varying the signal transfer characteristic of said amplifier, said movable switch contact of said third switch means being connected to the movable switch contact of said first switch means.

8. The protective circuitry as defined by claim 7 and additionally including fourth switch means ganged with said second and third circuit means, having a movable switch contact and a plurality of fixed contacts selectively coupled to the opposite end of said first circuit means for varying the signal transfer characteristic of said amplifier, said movable switch contact of said fourth switch means beingcoupled to the input of said amplifier.

being coupled at a common junction thereat.

10. The protective circuitry as defined by claim 1 wherein said first and second voltage divider networks 5 are comprised of resistor divider networks.

Claims

1. X-ray tube anode protective circuitry including circuit means being operated by the output of a comparator circuit which compares selected values of analog voltages representative of Xray tube rating chart curves for preventing energization of the X-ray tube in the event preselected operating parameters are outside of the ratings of the X-ray tube, comprising the combination of: a supply voltage; a single voltage divider network, for multiple operational modes having respective different rating chart curves for at least one X-ray tube, coupled across said supply voltage and having a current-voltage characteristic adapted to simulate a generalized rating chart curve which is a function of a plurality of exposure times and providing a plurality of voltage points corresponding to allowable peak power for said exposure times; first switch means having a movable contact and a plurality of fixed contacts respectively connected to said plurality of voltage points on said voltage divider; a voltage amplifier having an input and output and including circuit means coupling the output of said amplifier to the input of said comparator circuit; a plurality of first circuit means, each respectively adapted to vary the signal transfer characteristic of said voltage amplifier, selectively coupled between the input of said amplifier and the movable contact of said first switch means for each of said multiple operational modes whereby the output of said amplifier provides an output simulating the actual rating chart curve for the selected multiple operational mode; an X-ray anode supply voltage; a second voltage divider network coupled across said anode supply voltage and providing a plurality of voltage points proportional to the product of X-ray anode voltage and beam current of the X-ray tube; and second switch means having a movable switch contact and a plurality of fixed contacts selectively connected to said plurality of voltage points of said second voltage divider network and additionally including second circuit means coupling the movable contact of said second switch means to the input of said comparator circuit whereby the resultant voltage at said input of said comparator comprises a signal input which is a function of the preselected operating parameters.

8. The protective circuitry as defined by claim 7 and additionally including fourth switch means ganged with said second and third circuit means, having a movable switch contact and a plurality of fixed contacts selectively coupled to the opposite end of said first circuit means for varying the signal transfer characteristic of said amplifier, said movable switch contact of said fourth switch means being coupled to the input of said amplifier.

9. The protective circuitry as defined by claim 1 and additionally including first resistor means coupling the output of said amplifier to the input of said comparator and second resistor means coupling the movable switch contact of said second switch means to said input of said comparator, said first and second resistor means being coupled at a common junction thereat.

10. The protective circuitry as defined by claim 1 wherein said first and second voltage divider networks are comprised of resistor divider networks.