US3710252A

US3710252A - High voltage divider unit

Info

Publication number: US3710252A
Application number: US00885788A
Authority: US
Inventors: J Till
Original assignee: AMP Inc
Current assignee: TE Connectivity Corp
Priority date: 1969-12-17
Filing date: 1969-12-17
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09

Abstract

A resistive voltage divider instrument is disclosed which permits very accurate measurements to be taken of the potential of high voltage DC sources, with low DC current drain, by means of a sensitive low voltage differential voltmeter. This voltage divider embodies an instrument packaging concept that utilizes a special insulation system to enclose resistor strings formed of precision wire wound high voltage resistors. The resulting insulated resistor modules are interconnected and positioned within a molded plastic case in such a manner as to minimize all voltage gradients between resistor modules and the ground plane. Special provisions are made for preventing the formation of corona and for minimizing the effects of leakage current errors. Each resistor module contains a series connected string of accurately matched wire wound precision resistors and is filled with an insulating material that is highly resistant to the passage of leakage currents through its volume or across its surface. All interconnections between modules and the connections to the input voltage terminals located at the back of the instrument case are made with special connector and lead assemblies that are resistant to high voltage corona and that provide a quick disconnect feature.

Description

United States Patet n 1 Till [54] HIGH VOLTAGE DIVIDER UNIT James Peter Till, Camp Hill, Pa. [73] Assignee: AMP Incorporated, Harrisburg, Pa. 221 Filed: Dec. 17', 1969 21 App1.No.: 885,788

[75] Inventor:

FOREIGN PATENTS OR APPLICATIONS 1,123,566 9/1956 France ..324/l22 OTHER PUBLICATIONS Hague, BL; Instrument Transformers; book pub. by Pitman.& Sons, London; 1936; pg. 366-369.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Ernest F. Karlsen Att0rneyCurtis, Morris and Safford, William J. Keat- COM 5 KV Jan. 9, 1973 ing, RonaldrD. Grefe, William Hintze, Adrian J. La Rue, Frederick W. Rating, Jay L. Seitchik and John P. Vandenburg [57] ABSTRACT A resistive voltage divider instrument is disclosed which permits very accurate measurements to be taken of the potential of high voltage DC sources, with low DC current drain, by means of a sensitive low voltage differential voltmeter. This voltage divider embodies an instrument packaging concept that utilizes a special insulation system to enclose resistor strings formed of precision wire wound high voltage resistors. The resulting insulated resistor modules are intercon-' nected and positioned within a molded plastic case in such a manner as to minimize all voltage gradients between resistor modules and the ground plane. Special provisions are made for preventing the formation of corona and for minimizing the effects of leakage current errors. Each resistor module contains a series connected string of accurately matched wire wound precision resistors and is filled with an insulating material that is highly resistant to the passage of leakage currents through its volume or across its surface. All interconnections between modules and the connections to the input voltage terminals located at the back of the instrument case are made with special connector and lead assemblies that are resistant to high voltage corona and that provide a quick disconnect feature.

4 Claims, 11 Drawing Figures PAIENTEDJAu slsza SHEET 1 0F 6 INVENTOR. JAMES PETER TILL BY I ATENTEDJAI 9 I975 3,710 52 1. RI? l I RI5 o VOLT R I COM I i0) RM 4 85 86] [9O Rn 62 Q R|2 I 40? 8 I 42 66 I 0 v \1 82 METER PATENTED JAN- 9 I973 SHEEI 5 [IF 6 HIGH VOLTAGE DIVIDER UNIT BACKGROUND OF THE INVENTION Accurate measurements of DC voltages above volts are usually made with the aid of a resistive voltage divider circuit which serves to reduce the magnitude of the voltage applied to the measuring device. The voltage divider in question comprises a high resistance R in series with a low resistance R The voltage under test is applied across the series combination with R being connected at the grounded end. The divider ratio, which may be expressed as R R /R is chosen to produce a specific voltage output across R,, which can then be determined accurately with a null type differential voltmeter or a potentiometric device. This technique is the well-known volt-box method which is useful for measuring source voltages up to no more than 1,500 volts. Attempts to extend the volt-box method to still higher voltages have failed due to the difficulties encountered in constructing a high voltage divider which would have a constant effective resistance value; i.e. a resistance value that does not change as the applied voltage is varied.

Variation of the effective resistance value with applied voltage may be caused by any one or a combination of at least three basic factors. The first factor is the heating of the resistor wire due to PR losses. The amount of the resulting change in resistance will be dependent upon the resultant temperature coefficient of the entire resistor module assembly. The second is the leakage current through the volume and/or across the surface of the insulation or insulations used to support and protect the individual resistors. This type of leakage increases with increasing applied voltages and effectively shunts and decreases total resistance. Finally, there are corona discharges which tend to appear at any location along the resistor module assembly having a high gradient; such corona discharges effectively leak a part of the resistor current to ground.

The factor of resistor wire heating was overcome first by selecting basic resistors which have a low temperature coefficient, and then by matching them within individual resistor modules so that half have positive temperature coefficients and the other half have negative coefficients. This arrangement will reduce the overall temperature coefficient to a negligible minimum.

The other two factors involving leakage current and corona discharge cannot be reduced to an acceptably low magnitude in any suitable manner. In fact it is difficult to even determine or measure their magnitude. One solution known to the prior art and widely followed was to provide a very large number of individually shielded precision resistors, for example 100 or 200 one-megohm units. These resistors were connected in series and arranged in a vertical helix array between ground and a high voltage electrode structure. See Special Shielded Resistor for High-Voltage D-C Measurements" by J.H. Parks, dated Sept. 26, 1961, and published in the Journal of Research of the National Bureau of Standards-C. Engineering and Instrumentation, Vol. 66C., No. l, January-March, 1962.

The use by Park of individual shields prevented the formation of corona or leakage currents at the surface of any resistance element no matter how high the potential of the shields above ground. The Park arrangement effectively provided a uniform leakage current path around each resistor to ground; also the vertical helical configuration of the resistor string with a large high voltage electrode at the top served to prevent concentration of electric field and corona potential at the high voltage end of the divider. Tests showed that corona and leakage errors for this resistance divider were less than 10 parts per million at 50 kilovolts.

However, there are a number of serious drawbacks associated with the Park resistance divider. In the first place, the Park divider is of such large physical size that it has proved to be too cumbersome or unwieldy to serve as a portable test instrument for field use. Also its construction, requiring a large number of special milled metal parts and large numbers of individual resistors, is such as to result in an instrument that is very expensive to build. Finally, no safety features were present to protect users of the divider from the dangers of high voltage in conditions of use outside the laboratory environment.

SUMMARY OF THE INVENTION The present invention relates to a novel and improved resistance divider instrument for use in measuring high DC voltages.

It is an object of the invention to provide a high voltage divider instrument for measuring high voltage which is compact in size and light in weight so as to form a truly portable instrument suited to field use. Another object is to provide a resistive voltage divider overcoming the drawbacks of the prior art units specified above. A further object is to provide a high voltage divider unit that greatly reduces the cost of construction. Another object of the invention is to provide a voltage divider arranged to accept a plurality of levels of high voltage input values, and further arranged so that the user may select any one of a plurality of division ratios and thereby obtain a desired level of output voltage. It is another object to provide a high voltage divider that is completely safe for the user, which does not have any exposed high voltage points, and which permits adjustment of the various controls in safety even with the high voltage on.

The foregoing objects are attained by the invention through the suppression of the effects of corona and leakage current by employing insulating materials which exhibit very high surface and volume resistivity. Relatively small numbers of precision wire wound high resistance resistors are packaged into modules of special construction to form the high voltage input section of the divider. A plurality of such resistor modules are geometrically positioned in a particular manner from the high voltage input points to ground to form a divider assembly which minimizes the adverse effects of high voltage gradients between resistor modulesand from any given module to ground or to a high voltage input. Tests made on the divider instrument of the present invention proved corona and leakage current error to be extremely low and in the vicinity of only two parts per million at input voltages up to as high as 20 kilovolts. The present divider also features special high voltage leak and connectors for safety purposes. The end result achieved is a completely enclosed system with no exposed high voltage danger points.

The foregoing and other objects, features and advantages of the invention will be better understood from the following detailed description when considered together with the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of the high voltage di-- vider instrument of the invention in use in conjunction with a differential voltmeter;

FIG. 2a is a side view, partly in section, of the high voltage divider unit;

FIG. 2b is a view, from the rear, of aportion of the unit showing the input voltage terminals;

FIG. 3 is a schematic circuit diagram of the high voltage divider of the present invention;

FIG. 4 is an exploded perspective view depicting the construction of one of the resistor modules employed in the high voltage divider;

FIG. 5 is a perspective view in section of the resistor module of FIG. 4 when assembled;

FIG. 6 is an exploded perspective view, similar to FIG. 4, of another resistor module used in the high voltage divider;

FIG. 7 is a perspective view, comparable to FIG. 5, of the resistor module of FIG. 6, as assembled;

FIG. 8 is a perspective view of a portion of the high voltage divider unit, illustrating the manner in which the resistor modules of FIGS. 4-7 are mounted thereon, together with the circuit connections required; and

FIGS. 9 and 10 are simplified circuit diagrams useful in explaining the operation of the high voltage divider of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description, reference is made to specific materials, dimensions, circuit parameters, components, etc., which are being given only by way of example. The true scope of the invention is defined in 'the appended claims.

Referring now to FIG. 1, the high voltage divider unit of the invention is indicated generally at 30, and is shown suitably connected to a conventional DC differential voltmeter 32, such, for example as a Model No. 891-A solid state DC Differential Voltmeter available from the John Fluke Mfg. C0., P.O. Box 7428, Seattle, Wash. 98133. The combination of the precision voltage divider 30 and the sensitive low voltage voltmeter 32 may be used to obtain an accurate measurement of the unknown potential of a high voltage DC source, not shown. It is the purpose of the present high voltage divider 30 to allow measurement of high voltages, up to or kilovolts or more, with accurate low voltage voltmeters such as the Fluke meter 32, while imposing only minimum loading effects on the voltage source under test. The unit is a resistance divider constructed to obtain accurate voltage division of such high voltages in either positive or negative polarity.

The voltage divider 30 receives the high voltage from the source to be measured via a cable 34 connected to the appropriate one of its plural input terminals (see FIG. 2b); voltage divider 30 in turn has its output connected to the precision voltmeter 32 by a lead 36, with the lead 38 providing a ground connection. A ratio selector switch 40 controls the range or nominalmagnitude of the DC output voltage of divider 30, while an auxiliary voltmeter 42 located within the divider 30 permits an approximate initial determination of the magnitude of the applied inputvoltage so that the correct input terminal (see FIG. 2b) may be employed.

Referring to the schematic circuit diagram shown in FIG. 3, the present high voltage divider unit comprises an input section, seen across the top of the figure, and an output section which encompasses the restof the figure. There are six separate resistor modules, identified as 1A, 13, 2A, 28, 3A and 3B, which are connected together in series to make up the input section of the instrument. The details of the construction of the resistor modules will be considered later. In the illustrated embodiment, the divider is arranged to accept input voltages up to and including 15 kilovolts. Three

input terminals

44, 46, and 48 are provided, with inputs up to 5 kilovolts being imposed on terminal 44. Inputs between 5 and 10 kilovolts will be applied to terminal 46 and those between 10 and 15 kilovolts to terminal 48, the one which is connected to one end of resistor module IA.

Input terminal

46 and 44 are connected, respectively, to the junction of

modules

13 and 2A and the junction of

modules

2B and 3A as shown.

The output section consists of a series connected resistance string including adjustablev potentiometers R- 19, R-l6, and R-l3 together with fixed resistors R-18, R-l5, and R-l2, relatively positioned and interconnected in the manner shown in FIG. 3. A protective resistor R-20 is connected in shunt across potentiometer R-19 to ensure circuit continuity in the event of a fault causing an open circuit in the potentiometer. The other potentiometers R-16 and R-13 are provided with similar protective shunt resistors R-17 and R-l4. The said output resistance string is connected from one end of resistor module 3B to a center terminal 62 of the ratio selector switch 40. The latter switch may comprise a rotary switch having two decks as shown, with two

center terminals

60 and 62, a pair of

rotatable contact arms

64 and 66 ganged together, and corresponding sets of six selectable switch terminals 71-76 and 81-86. The voltage divider instrument 30 has three separate low voltage output taps 52, 54, and 56 in order to provide any one of three different levels or ranges of output voltage, depending on the setting of switch 40. The tap 52 provides for an output voltage level of 100 volts, while

taps

54 and 56 serve as outputs for levels 'of 10 volts and 1 volt, respectively. Output tap 52 is connected to the upper end of potentiometer R-l9, tap 54 is coupled to the junction of R-18 and R-l6, and the remaining tap 56 is connected to the junction of R-15 and R-l3.

The first or upper deck of ratio selector switch 40, see FIG. 3, is employed in a manner to permit the user to selectively connect any one of the

taps

52, 54, 56 to the'final output terminal 80 of the divider unit 30. Thus in the upper deck of rotary switch 40, the switch terminals 76 and are unused; the

switch terminals

74, 73, and 72 are connected, respectively, to taps 52, 54 and 56; and the switch terminal 71 corresponds to the Meter position of switch 40, which is also unused in the upper deck. The center terminal 60 of the said upper deck is, of course, connected directly to the unit output terminal 80.

Referring now to the second or lower deck of switch 40, the center terminal 62 thereof is directly connected to R42 at the lower end of the output resistance string, as has been stated. The switch terminal 86 is connected to the common return conductor 90, as shown, in FIG. 3 and the switch terminal 85 is unused.

Switch terminals

84, 83 and 82 are directly connected together as shown, and a resistor R- is connected between 84 and the common return at 90. The switch terminal 81 in the lower deck serves as the Meter position; it is connected to one side of the auxiliary voltmeter 42, the other side of the meter being connected to the common conductor at 90. Finally, a meter protecting resistor R- 1 l is connected from 62 to 86 and thus is permanently shunted across the voltmeter 42.

In FIG. 9 there is shown the schematic circuit diagram for resistive voltage dividers in general, and in FIG. 10 appears a simplified or equivalent circuit diagram of the present divider 30. Thus FIG. 10 corresponds to FIG. 3 in that resistor R-6 is the equivalent resistance for the combined resistances of modules 1A and 1B; R-5 and R-4 similarly are equivalents for module pairs 2A-2B and 3A-3B, respectively; resistor R-3 is the equivalent resistance for the parallel-series combination of R--R-l9-R-18; resistor R-2 represents elements R-l7-R-16-R-15; and resistor R-l represents R-l4-R-13-R-l2 together with the equivalent of R-l 1-R10 which are connected in parallel at

positions

82, 83, 84 of switch 40. Considering FIG. 9, the division ratio D of the voltage divider is defined by:

Correspondingly, the multiplier M of the circuit is given by:

The multiplier M is the more convenient factor for use when making actual measurements of unknown input voltages, since with the unknown input E applied to one of the

input terminals

44, 46 or 48, the user has only to read E from differential voltmeter 32 and then obtain E from the relation:

The same relationships apply to the present divider 30 as represented in FIG. 10, but now the values of R and R,, are determined by the selection made of a particular input terminal and a particular output terminal in a given case. This is best seen from considering specific examples: (1) if the input is to the 15 kilovolt terminal 48 and the output is taken from the 1 volt tap 56, then R R4 and R R-2 R-3 R4 R-S R-6; (2) if, on the other hand, the input is applied to the 5 kilovolt tap 44 and the output is taken from the 100 volt tap 52, then R,, R-l R-2 R-3 and R R-4, in this case R- 5 and R-6 not being used.

The voltage divider 30 is arranged to provide the following multiplier factors M, as appears in Table 1:

TABLE 1 Max. Input Voltage Multipliers M 15 KV 15,000 1500 150 10 KV 10,000 1000 100 5 KV 5,000 500 50 Max. Output Voltage 1 volt 10 volts 100 volts Therefore, the voltage divider 30 is constructed to provide the following set of resistance values, with respect to the equivalent circuit of FIG. 10, which results in a divider having the multipliers M of Table 1:

TABLE 2 ELEMENT RESISTANCE R-6 50 megohms R-S 50 megohms R-4 49 megohms R-3 900,000 ohms R-2 90,000 ohms R-l 10,000 ohms Where the instrument 30 is intended to be accurate to within i 0.01 percent, the actual resistance parameters required for the circuit, referring to FIG. 3, are given in the following table:

TABLE 3 ELEMENT RESISTANCE Modules 1A 18 49.990 megohm Modules 2A 28 49.990 megohm Modules 3A 3B 49.000 megohm R-20 10,000 ohms R49 10,000 ohms R-l8 897,500 ohms R-17 1,000 R 16 1,000 R-15 89,750 ohms R-l4 1,000 ohms R-13 ohms R-12 8,965 ohms R-l 1 1,000 ohms R-10 100,000 ohms The resistive input impedance of divider 30, input terminal to common terminal, is as follows:

TABLE 4 INPUT TERMINAL INPUT IMPEDANCE l5 KV (48) megohm i 0.01% 10 KV (46) 100 megohm i 0.01% 5 KV (44) 50 megohm i 0.01%

In the operation of the high voltage divider instrument 30, the source of the voltage to be measured should first be de-energized, for safety. The cable 34 from the voltage source is to be initially connected to the 15 KV input terminal 48. This presents a minimum loading effect on the source and minimizes the chances of damaging the divider due to overvoltage. Next the ratio selector switch 40 is set at its Meter position (see FIGS. 3

arms

64 and 66 set to switch terminals 71 and 81) and the voltage source is energized. This places the auxiliary voltmeter 42 in series with the output resistance string and applied a reduced voltage proportional to the input voltage to meter 42. The user will now obtain an approximate initial measurement of the unknown input by the reading taken from meter 42, which will allow him to determine what input range or input terminal to use for the final accurate reading to be obtained with differential voltmeter 32. The auxiliary meter 42 also provides an indication of the polarity of the input voltage.

The voltage source should be de-energized again, and cable 34 connected to the

appropriate input terminal

44, 46 or 48. Of course, for input voltages in the range of 0-5 KVDC, terminal 44 is to be used; for inputs from 5 KVDC to 10 KVDC, terminal 46 is used; and terminal 48 is used for inputs in the range of 10 KVDC I5 KVDC. Next, the desired output voltage range is chosen, and ratio selector switch 40 is moved to the corresponding position. For instance, if it is desired to read the output voltage on a 0-100 volt range on meter 32, then switch 40 is set in the position with

arms

64 and 66 engaging switch terminal 74 and 84. The unknown input source is again energized, a voltage reading is taken from meter 32, and the meter reading is converted to the true value using. the appropriate multiplier M in accordance with Table 1. The contents of Table 1 appear on the front of the divider instrument itself as indicated in FIG. 1 at 92. As a final example, suppose the initial reading on meter 42 indicates that the input source under test is at approximately 8 kilovolts. The input voltage is then applied to the 10 KVDC input terminal 46 via cable 34; assume that the 100 VDC output range at tap 52 is selected and that a reading of 80 volts is obtained from the differential voltmeter 32. From table 1, it is found that the applicable multiplier is M=l00. Therefore, E, M'E (i.e. the reading), and E,,, 80 volts X 100 8,000 VDC.

It should be realized that the DC input impedance of the measuring instrument, such as meter 32, connected to the divider output constitutes a shunting resistance across the lower resistors of the divider. However, the effect thereof is negligible and can be ignored, since potentiometric and differential voltmeters such as meter 32 exhibit essentially infinite input impedance when at null. It is also pointed out that the 100 volt output at 52 is intended primarily for use with a differential voltmeter, whereas the 10 volt output at 54 serves best for interfacing to recording instruments via a unity gain amplifier, and the 1 volt output at 56 serves foruse with a precision potentiometer measuring instrument.

Turning now to a consideration of the resistor modules 1A-3B and their details of construction, it should be realized that these modules comprise one of the most important aspects of the invention. The modules 1A-3B, forming the high voltage input section of the divider, must handle the high voltages involved, and their construction has been found to be critical to realizing the high accuracy (maximum error i 0.01

percent) desired and to solving the problems of corona, leakage current, etc., discussed in the Background and Summary sections, supra.

Reference is now made to FIGS. 4 and showing the construction required for resistor modules 18 and 2B, the module being shown in exploded form before assembly in FIG. 4 and as assembled in FIG. 5. The

module comprises an outer insulating shell 110 in the form of a hollow, relatively thin-walled support tube 1 having a square shaped cross-section, as shown in the drawing. The tube is formed of a laminated glass epoxy, such as G-10 which is a special grade silicone glass epoxy. This is a material having very high volume and surface resistivity values. The shell or tube 110 is provided with a pair of end plates 111 and 112 to cover its opposite ends, the plates being comprised of the same G-l0 glass epoxy. End plate 111 is dimensioned to fit inside tube 110 while plate 112 is flush with the other end of the tubeQThe plates have centrally located tapped holes 113 and 114, and plate 112 further has a large aperture 115 in the upper right quadrant together with a small aperture 116 below it. The other end plate 111 is provided with a large aperture 117 aligned with 115. An insulated rod 118 of 6-10 glass epoxy, threaded at both ends, is adapted to be positioned along the center line of the module and to engage tapped holes 113 and 114.

Each module contains a series connected string of five precision wire wound high resistance resistors 120 each having a small central hole or aperture through which they are mounted on the rod 118 as shown. The radially extending terminal lugs of the resistors are bent over into overlapping positions, as best seen in FIG. 4, to be soldered together to make the series circuit connection. The modules 1B and 2B feature a trimming potentiometer 121 having a slotted adjustment shaft 122 to be mounted in aperture 116 at the front end of the module. The trimming potentiometer is provided so that a pair of modules such as lA-lB can have their total series resistance set at the required circuit value during factory calibration of the assembled divider 30. The modules 1B and 2B are provided with A-MPf LGH* Trademark of AMP Incorporated %L high voltage receptacle-type connectors at each end for safe, positive electrical connection of the internal resistor string to external parts of the circuit. These high voltage connectors are shown at 123 and 124, and they are to be mounted in apertures and 117 of the end plates. The back end plate 11 contains two further apertures through which extend a pair of

tubes

125 and 126 used to introduce a filling and encapsulating material into the module after assembly, as will be described.

The modules 1B and 2B are assembled in the'following manner. The resistors are stacked onthe rod 118 and their terminal lugs are bent over and soldered together. Care is taken here to get a ball-shaped solder joint with no sharp edges or pointed portions, thereby reducing the chances of forming corona inside the module. The receptacle-

type connectors

123 and 124 are bonded in place in the end plates with any suitable epoxy adhesive, and the potentiometer 121 is similarly bonded into aperture 116 of end plate 112. The threaded rod 118 carrying the resistor string is placed in engagement in the end pieces, and the back end of the resistor string is attached and terminated to the conductor 124 by soldered conductor wire. Appropriate connecting wiring is also made to trimming potentiometer 121 to connect it in series to the remaining end of the resistor string and to the other connector 123. Then the outer shell or tube is slid into position over the resistor string assembly and bonded to end pieces 1 11, 112 with epoxy adhesive.

The module is then potted with an encapsulating resin, preferably a Baker System 37 polyurethane filling medium, which consists of a Vorite 63 prepolymer (66 percent by weight) and a Polycin 52 polyol (34 percent by weight). This resin is preheated and vacuum treated to remove all air bubbles and is then pressure injected into filling tube 125, with the other tube 126 serving as an outlet for air displaced by the filling medium. The filled module is heat-treated to cure and set up the encapsulant at elevated temperature, and the ends of

tubes

125, 126 are trimmed flush with the outside of the module. The finished module is painted with an epoxy point, one which is free of carbon black, to maximize surface resistivity. The module is ready for mounting in the divider 30. 7

FIGS. 6 and 7 depict the construction for

modules

2A and 3A, which is basically the same as the other module depicted in FIGS. -4 and 5, except that the trimming potentiometer is omitted and the connector receptacle arrangement is modified. Parts in FIGS. 6 and 7 which are the same as in FIGS. 4 and 5 have like reference numerals. The forward end plate in this second type of module has a pair of

large apertures

127 and 128 positioned as shown in FIG. 6. These apertures receive and mount respective

high voltage connectors

129 and 130, which are an A-MP* LGl-I* Trademark of AMP Incorporated 9&1. connector and an A-MP' LGII Trademark of AMP Incorporated lL connector, respectively. These connectors are fully described in U.S. Pat. No. 2,958,844, issued Nov. 1, 1960, to W. A. Smith et al., and assigned to the present assignee. The module of FIGS. 6 and 7 is otherwise identical to the previous one, and is assembled the same way. In FIGS. 6 and 7 the forward terminal lug of the first resistor is wired, internally of the module, in parallel to the two

connector receptacles

129 and 130.

Referring to FIGS. 1, 2 and 8, the manner of assembling the modules to construct the voltage divider proper will now be explained. As shown in FIGS. 1 and 2, the divider instrument includes an L-shaped main frame or base 140 of a tough, resilient molded plastic, on which the resistor modules of the circuit input section are mounted. A back cover shroud 142 engages the base 140 to enclose the circuitry. The auxiliary meter 42, the ratio selector switch 40, and the output resistance string are disposed and suitably mounted and wired together behind the upstanding face of the base frame 140, in any suitable fashion, not shown. Turning to FIG. 8, this figure depicts the mounting of the resistor modules lA-3B above base 140 and within shroud 142, the shroud being indicated in phantom in the figure. The view is from the back side of the instrument, with the back wall of shroud 142 considered to be removed. There are provided three mounting boards or

plates

144, 146, and 148 on which the modules are fastened. The mounting boards are attached to, but separated from, the main frame and from each other by any suitable insulated spacer means, not shown. The modules are held in the relative staggered positions shown (best seen in FIG. 8) and are fastened simply by screw passing through holes in the mounting boards from below and engaging blind tapped holes in the module shells. The mounting

boards

144, 146, 148 are composed of the same G-10 glass epoxy material as mentioned previously, and are located with a special silicone cone insulating varnish (a General Electric varnish SR-98), a high surface resistivity material. The same varnish coating is applied to the exterior surfaces of the modules. This coating provides a moisture barrier or seal between modules and ground plates to combat high humidity; it also increases the surface resistivity between any module and ground, and also from module to module.

Considering again the relative positioning of modules lA-3B as shown by FIG. 8, these positions are such that, upon assembly, voltage gradients between adjacent modules (both in the same plane and above and below in other planes), and even between distant modules, are minimized. Thus, for example, the junction of modules 23 and 3A and the input SKV conductor are all at 'the same potential, as seen from inspecting FIG. 3. The corresponding ends of

modules

3A and 2B are therefore located close together, but are also separated as far as possible from the back end face of module 3B which is at a quite different potential, and also from the back end face of module 2A for the same reason. The angled positions of 3A and 2B in the bottom module plane and the skewed position of module 2A achieve this result.

The modules are suitably interconnected as shown by insulated wire jumper leads terminated at their ends by A-MP* LGl-I* Trademark of AMP Incorporated AL and 11.. pin type high voltage connectors, as seen in FIG. 8. Thus leads 150, I52, and 154 go to the high voltage input tenninals 44, 46 and 48, respectively. The jumper lead 156 interconnects modules 1B and 2A. The jumper leads 158 and 160 at the opposite side of the assembly similarly interconnect modules IA and IE on one hand, and modules 2A and 213 on the other. In each instance the pin connectors of the jumpers mate directly with the receptacle high voltage connectors disposed in the end walls of the resistor modules.

While wire-wound resistors have been disclosed above, it is contemplated that other types of precision resistors, such as metal film resistors, can equally well be employed. Also, the input section could be extruded to accept higher DC voltages such as 20 KVDC or 30 KVDC, etc. Also, the output voltage ranges made available could be varied.

While various embodiments of the invention have been shown and described, it will be understood that various modifications may be made.

I claim:

1. An instrument for coupling a source of voltage in the kilovolt range to a low voltage measuring device, comprising:

high voltage input means adapted to be coupled to said source of voltage;

impedance module means coupled to said high voltage input means, said impedance module means including means for preventing corona discharge and leakage currents and provided with a plurality of spaced input taps for respective different values of high voltage;

output circuit means coupled to said impedance module means, said output circuit means including a plurality of output voltage taps for providing respective different fractions of the voltage appearing on each input tap;

selectively operable means coupled to said output circuit means for selectively coupling said output voltage taps to said low voltage measuring device, an auxiliary voltmeter, and

means for connecting the impedance module means in series with said auxiliary voltmeter.

2. An instrument for coupling a source of voltage in the kilovolt range to a low voltage measuring device, comprising:

high voltage input means adapted to be coupled to said source of voltage;

impedance module means coupled to said high voltage input means, said impedance module means including means for preventing corona discharge and leakage currents;

output circuit means coupled to said impedance module means, said output circuit means including a plurality of output voltage taps; and selectively operable means coupled to said output circuit means for selectively coupling said output voltage taps to said low voltage measuring device, and auxiliary voltmeter means within said instrument;

said selectively operable means further comprising means for selectively coupling said auxiliary voltmeter means to said output circuit means while simultaneously disconnecting said output circuit means from said low voltage measuring device.

3. A precision circuit for dividing inputs in the 5 kilovolt range, comprising:

including a plurality of series connected precision resistors, and further including means for preventing corona discharge and leakage and provided with a plurality of spaced input taps for respective different values of high voltage;

low voltage output circuit means coupled to said impedance module means, said output circuit means including a plurality of precision low voltage resistors and a plurality of output taps associated therewith for providing respective different fractions of the voltage appearing on each input tap;

output terminal means;

selectively operable means coupled to said low voltage output circuit means for selectively coupling said output voltage taps to said output terminal means, an auxiliary voltmeter, and

4. A precision circuit for dividing inputs in the kilovolt range, comprising:

high voltage input means; 7 impedance module means coupled to said high voltage input means; said impedance module means including a plurality of series connected precision resistors, and further including means for preventing corona discharge and leakage currents,

low voltage output-circuit means coupled to said impedance module means, said output circuit means including a plurality of precision low voltage resistors and a plurality of output taps associated therewith; and

output terminal means;

selectively operable means coupled to said low voltage output circuit means for selectively coupling said output voltage taps to said output terminal means,

and auxiliary volmeter means;

said selectively operable means further comprising means for selectively coupling said auxiliar voltmeter means to said low voltage output circuit means while simultaneously disconnecting said output taps from said output terminal means.

Claims

1. An instrument for coupling a source of voltage in the kilovolt range to a low voltage measuring device, comprising: high voltage input means adapted to be coupled to said source of voltage; impedance module means coupled to said high voltage input means, said impedance module means including means for preventing corona discharge and leakage currents and provided with a plurality of spaced input taps for respective different values of high voltage; output circuit means coupled to said impedance module means, said output circuit means including a plurality of output voltage taps for providing respective different fractions of the voltage appearing on each input tap; selectively operable means coupled to said output circuit means for selectively coupling said output voltage taps to said low voltage measuring device, an auxiliary voltmeter, and means for connecting the impedance module means in series with said auxiliary voltmeter.

2. An instrument for coupling a source of voltage in the kilovolt range to a low voltage measuring device, comprising: high voltage input means adapted to be coupled to said source of voltage; impedance module means coupled to said high voltage input means, said impedance module means including means for preventinG corona discharge and leakage currents; output circuit means coupled to said impedance module means, said output circuit means including a plurality of output voltage taps; and selectively operable means coupled to said output circuit means for selectively coupling said output voltage taps to said low voltage measuring device, and auxiliary voltmeter means within said instrument; said selectively operable means further comprising means for selectively coupling said auxiliary voltmeter means to said output circuit means while simultaneously disconnecting said output circuit means from said low voltage measuring device.

3. A precision circuit for dividing inputs in the kilovolt range, comprising: high voltage input means; impedance module means coupled to said high voltage input means; said impedance module means including a plurality of series connected precision resistors, and further including means for preventing corona discharge and leakage and provided with a plurality of spaced input taps for respective different values of high voltage; low voltage output circuit means coupled to said impedance module means, said output circuit means including a plurality of precision low voltage resistors and a plurality of output taps associated therewith for providing respective different fractions of the voltage appearing on each input tap; output terminal means; selectively operable means coupled to said low voltage output circuit means for selectively coupling said output voltage taps to said output terminal means, an auxiliary voltmeter, and means for connecting the impedance module means in series with said auxiliary voltmeter.

4. A precision circuit for dividing inputs in the kilovolt range, comprising: high voltage input means; impedance module means coupled to said high voltage input means; said impedance module means including a plurality of series connected precision resistors, and further including means for preventing corona discharge and leakage currents, low voltage output-circuit means coupled to said impedance module means, said output circuit means including a plurality of precision low voltage resistors and a plurality of output taps associated therewith; and output terminal means; selectively operable means coupled to said low voltage output circuit means for selectively coupling said output voltage taps to said output terminal means, and auxiliary volmeter means; said selectively operable means further comprising means for selectively coupling said auxiliar voltmeter means to said low voltage output circuit means while simultaneously disconnecting said output taps from said output terminal means.