Патент USA US2135587код для вставки
‘Nov. 8, 1938. M.- J. LOVELADY 2,135,587 VARIABLE RATIO ARM BRIDGE Filed Sept. 28, 1935' 2 Sheets-Sheet 2 _._____._____.__‘ ’ 1 | I l “5.23 Patented Nov. 8, 1938 2,135,587 UNITED STATES PATENT OFFICE I 2.135.587 ~ VARIABLE RATIO ARM BRIDGE Maurice J. Lovelady, Gollinnwood, vN. 1., assign or to Radio Corporation of America, a cor poration of Delaware Application September 28, 1935, Serial No. 42,644 2 Claims. (Cl. 175-183) ' This invention relates broadly to an instru ment for measuring electrical properties. More the slider S and the junction oi’ the known and unknown resistances. An alternating current E speci?cally my invention is a direct reading in strument for measuring a relatively wide range of values of resistance, capacitance, or induct- the bridge is unbalanced, the alternations of the current will be audible in the telephone receiver. 5 ance. The slider is moved back and forth until a null ‘ The art of measuring electrical properties is is applied across the adjustable ratio arms. If point is observed in the response of the tele-, an old one. Perhaps the most widely known in- phone receiver. strument for measuring resistance is the Wheat- I After the balance has been obtained, as indi 10 stone bridge; Modi?cations of the Wheatstone bridge have been made to adapt it ‘to the measurement of capacitance and inductance. Most instruments of the latter type are cumbersome, expensive and complicated. 15 These instruments are mainly used by skilled technicians. I have cated by the null point, the unknown value may 10 be determined by multiplying the known or standard value by the ratio of the bridge arms A to B. This may be expressed as follows:— ' - overcome these di?iculties by making a light, portable, inexpensive bridge which may be used to measure a wide range of electrical constants. one of the objects of mymvention is to make 20 a small light weight. portable instrument for measuring resistance, capacitance, and inductance_ - : Unknown rwstanccAxvalue of standard ,csistance l5 B ' It Wm be Observed that the scale for the slide wire ratio is not linear. That is for small movements where A is nearly equal to B the 20 scale Wm be approximately “near but as A be‘ comes much smaller or much larger than B, the Another object is to make a direct reading in- scale Wm become very crowded or Very open- In an open linear scale is shown on the left Fig Another object is to make a direct reading in- - and m contrast 9' non'nnear scale 0? the type 25 25 strument for measuring a very wide range 01 described above is shown on the right. The electrical properties_ crowded scale is very undesirable and 18 over A further object is to design a measuring in- come in the ease of an ordinary Wheetstene strument for measuring electrical constants. strument employing relatively few standards 30 which may be selected by simple switching means. . ‘ Additional objects will appear from the accompanying speci?cation and appended claims. My invention may best be understood by ref_ 35 erence to the accompanying drawings in which Figure I is ‘a diagram of a conventional wheatstone bridge, Fig 11 is a schematic diagram of one embodiment of my invention, ‘ 40 Fig. 111 is a schematic diagram of one of the switches used in combination with the circuit of my invention, ' Fig. IV illustrates a linear and a non-linear scam, 45 Fig. V illustrates one form of panel which may be used, and . Fig. VI is a wiring diagram of my bridge arrangement. ' A conventional Wheatstone bridge as repre50 sented in Fig. I is comprised of variable ratio arms A and B which are serially connected to a standard and an unknown resistance’ or other electrical unit whose resistance is to be determined. An indicating device D, such as a tele55 phone receiver or the like, is connected between bridge by empleymg a relatively large number of standards so the ratio arms are generally at 30 a balance between the ratios of 4/6 or 6/4. In Fig- H e Scheme-tie dlagmmendleates One form of bridge arrangement whleh overcomes the dif?culties just described. An adjustable re sistance M is used for one of the ratio arms. The 35 other arm is composed of a ?xed resistance N whose ohmic value is equal to one tenth the ohmic value of the adjustable arm M at its max imum_ setting. A single pole double throw switch P may ‘be used to connect either’the ?xed 40 resistance M10 arm N or another M10 arm Q whose resistance is ?xed at an ohmic value equal t0 the Ohmic resistance of the maximum Setting of the adjustable arm M. If the switch P is connected to the fixed resistance arm Q of the 45 larger ohmic value, the ratio of M to Q may be varied from .1 to 1. 11? the switch P is connected to the ?xed resistance arm N of the smaller ohmic value the ratio of M to N may be varied from 1 to 10. Thus the use of either of two 50 ?xed arms and one variable arm will not only give a total ratio 0!..1 to 10 or 1 to 100 but also the ratio may be read directly on a linear scale, as shown in Figure V. Although the arrange ment‘just described may be used in the measure- 55 - 2 2,185,587 ment of resistance, modi?cations are required in the measurement .of inductance or capaci tance. The reactance of a capacity, assuming a con stant frequency is impressed, will vary inversely transformer supplies the alternating current for the bridge, which will be described below. The output of the bridge is represented by a ground connection ‘I and a capacitor 43 which connects the output of the bridge to the input with the size‘ of the capacity. That is, at a ?xed of a two stage audio frequency ampli?er. The frequency, the larger 9. capacity the smaller will _ ampli?er is represented by the double purpose be its reactance. If the straight direct reading thermionic tube 45 and the connections within bridge arrangement is employed, as just de the broken lines 41. The grid, 49 which connects 10 scribed, the balance might be made to a null to the coupling capacitor 03, is returned to 10, point but the scale reading would be erroneous. ground through a grid resistor 5|. The true reading would be the reciprocal of the 03 is connected through an iron core choke '55 to the positive terminal of the power supply. The grid 51 of the second ampli?er is coupled to the iron core choke by a capacitor 59. The grid is 15 returned to ground through a grid resistor GI. The output of this ampli?er comprises its anode scale reading. The di?iculty may be overcome by reversing the relative position of the known 15 and unknown capacitance. One simple method is to use a double pole double throw switch T as reversing means as shown in Figure 111. a 1 A capacitor or an inductor ordinarily does not offer pure reactance but has as ‘well a ?nite 20 resistance. The resistance of a capacity or in ductor ordinarily broadens the balance or null point. If the resistance of the standard capaci tor or standard inductor is balanced against the resistance of the unknown capacitor or unknown 25 inductor, the effect of the two resistances will be eliminated and the capacitance or inductance may be determined by exactly balancing the bridge. Since the resistance of the standard may be greater, equal to, or less than the un 30 known, it is necessary to put a variable phase balancing resistor U in series with either the known or unknown. This may be accomplished with a single pole double throw switch V, as shown in Figure III. The complete wiring dia 35 gram also shows means for short circuiting the phasing resistor when the bridge is arranged for resistance measurements.’ ;~ ' The vbridge circuits described above are sche matic in form. A practical alternating cur 40 rent bridge requires an ampli?er to bring up the output to a point where telephone receivers may be used. It is customary to'employ audio currents of a frequency of one thousand cycles; The anode ‘63, the primary 65 of transformer 61, a resistor 69 and a return through the potentiometer of the power supply. The secondary H of the out v20 put transformer is connected to a Jack 13 into which a pair of telephone receivers (not shown) may be plugged. The bridge circuit is represented by a number of standards and the required switching means. 25. The inductance standards consist of three in ductors 15, 11, 19 which are connected to the ?rst three contacts of the two nine position switches 8|, 03. _ The three resistance standards comprise re 30, sistors, 85, 81, 89 which are connected to the fourth, ?fth and sixth contacts 61 the multi-po- . sition switches 3|, 33. The three capacitance standards 95, 91, 99 are connected to the sev- \ enth, eighth and ninth contacts of the multi-po 35 sition switches. The movable contact arms IOI, I03 of these switches are moved with a single control I05, as are the movable arms I01, I09 of the double throw reversing switch III. The double throw switch is'used to reverse the_stand ard and unknown connections for capacitance measurements as previously described. Two of the ?xed contacts H3, N5 of the reversing therefore to be complete the bridgeishould in- . switch I II are connected to one of the termi 45 elude an ampli?er and a source of one thousand nals II1 to which the element of uknown value 45 cycle alternating current. The complete ,circuit is to be connected. The other of the terminals arrangement is shown in Figure V-I. " . ~ 7 - H1, is connected to the variable phasing resistor A source of alternating current, such as. 110 ' H9 and to the ?xed contact I2I of the single volts 60 cycles, is connected to the input‘ I, 3 , pole double throw switch I23. The second vari of the recti?er system which is included-within; able phasing resistor I25, is a vemier, which is the broken lines 5. The recti?er is of the well‘. used for ?ne adjustment and is serially connect known voltage doubler type. A thermionic rec ed to the ?rst variable resistor I I9. The remain ti?er 1 with two cathodes and two anodes-is connected to a ?lter network 9. Across the ?lter 55 network a potentiometer II may be connected. The negative terminal I3 of the potentiometer is grounded. This, and all grounds illustrated, are . to a common chassis terminal and not to. actual earth. An actual earth connection might short 60 circuit the alternating current input line. Within the broken line IS a thermionic tube I1 is connected to generate oscillations of a fre quency of 1000 cycles; although other frequen cies may be used. The grid I9 is connected 65 through a grid leak grid condenser combination 21 to a circuit 23 tuned to 1000 cycles. The an ode 25 is coupled'through the transformer ‘21 to , the‘ grid circuit. The resistor 29 reduces the anode voltage supplied by the recti?er to an ap 70 propriate value. The resistor 29 may be by 50' ing ?xed contact I21 of the single pole double throw switch is connected to the lower terminal of the Vernier phasing resistor I25 and the mov 55' able contact arm IOI of the multi-position switch 8|. The movable contact I29 of the single pole double throw switch is connected to ground. The ratio arms of the bridge are-composed of either of two ?xed resistors I3l, I33, which are 60 similar to N and Q in Figures If and III. These resistors are connected together and to the slider I35 ‘of the variable resistor arm I31. The lunc tion of the ?xed resistors and the slider is con nected to the‘ coupling capacitor 43 which is 65 connected to the input of the audio ampli?er. The voltage input to the bridge is represented by the secondary 39 of the balanced transformer 31. The secondary is connected to the movable con tact I20 of the single pole double throw switch 70 passed by a capacitor 3I. A tertiary winding I30 and to the ?xed terminal of the variable re 33 on the transformer 21 is connected to the sistor I31. The secondary is also connected to primary 35 of a transformer 31, Whose, center the movable contact arms I01, I09 of the double tap is grounded to balance the transformer with throw double pole switch III. The remaining 75 respect to ground. The secondary 39 of the ?xed contacts of the double pole double throw 75 3 9,185,587 "switch are connected together. The movable contact I03 of the lower multi-position switch binding posts which are the terminals 2|! for the unit of unknown value. The power supply is 83 is connected to the ?xed contact I39 of the ' represented by a cord and plug 22!. double pole double throw switch. The method of using the bridge is as follows: It should be noted that the movable contacts of the double throw reversing switch and the movable contacts of the multi-position switches are operated with a uni-control. To avoid di?i culties the double throw reversing switch has 10 contact arrangements which automatically oper ate when the switch is ‘positioned for capacitance readings. A second switch “5 is operated in con junction with the single pole double throw ratio arm switch I30. This switch “I connects a small’ variable capacitor between the high potential ter minal of thehlghest valued standard resistor and ground. The small variable capacitor is only used when resistances of very high value are being measured.‘ A high resistance in the unknown 20 arm may have a substantial capacity reactance to ground which would unduly broaden the null point. This may be balanced out by adjusting the small variable capacitor. By the way of ex ample, I shall give a table of values of the sev 25 eral elements of the bridge. It should be under stood that other constants may be used, but I have found the following useful over a vary wid range of values: ' ~ 10 millihenries 1000 microhenries 1 microfarad .01 microfarad .0001 microfarad ' 35 Standards of resistance ____ _. 10 ohms > scale the ratio selector switch is turned to high 15 and the variable ratio arm adjusted for a balance. If a balance is found on this range the value of the unknown resistance will be from 10 to 100 ohms and will be ten times the scale reading at the balance point. If the unknown resistor has 20 an ohmic value greater than 100, the above de scribed operation is repeated on the higher re sistance standards until a balance is found. The measurement of inductance is performed ' in substantially the same manner. The only dif 25 ference is that the resistance component of the inductor must be balanced out by adjusting the phasing resistors. The usual method is to move 10000 ohms ance position and then move the coarse phasing 30 adjustment to balance out the residual 1000 cycle note. The ?nal adjustment results from succes sive adjustments of the vernier phase control and the variable ratiov arm. At the exact balance point the residual 1000 cycle note will disappear procedure for measuring inductance. However, Fixed ratio arms _________ __ 1000 ohms . Variable ratio arm _______ __ 1000-10000 ohms Phasing resistors coarse_____ 10000 ohms Phasing resistors vernier“..- 55 ohms Balancing capacity _______ __ 10-285 micromicro farads * Partly inherent capacity. pear or become a null at the balance point and will increase in intensity on either side of the balance point. If no balance is found on this Capacity is measured in accordance with the 100000 ohms 45 switch is placedon the lowest value of resistance (1-10), and the variable ratio arm is moved slow ly from 1-10. The response in the telephone re 10 ceivers will be a 1000 cycle note which will disap and a true zero response will be found. 1000 ohms 40 unknown terminals, the ratio selector switch is placed on “Low”, the nine position selector the variable resistor arm to an approximate bal- - 30 Standards of inductance___ 1 henry Standards of capacitance" An unknown resistance is connected across the ' With the above standards and variable ratios, resistances from 1 to 1,000,000 ohms, inductances from 100 microhenries to 10 henries, and capaci ties from 1 micromicrofarad to 10 microfarads may be measured and their values determined directly from the variable resistor arm scale. The entire device ‘may be assembled within a 55 small portable case 9% inches long, 61/2 inches high, and 41/2 inches deep. The total ‘weight is about 51/2 pounds. The front panel of the case is shown in Figure V. The adjustable ratio arm pointer 2M with its linear scale 203 is shown in 60 the central portion of the illustration. The con trol knob 205 connects to the nine position switch. The double throw switch 201 in the up per right,hand corner is the ratio selector switch. The small knob in the lower right is the control 65 for the variable capacitor which is used to bal ance out the capacity reactance of high resist ances. To the left of this small control knob and in the following order are the off-on switch 209, the phone jack 2| I, the double throw switch ‘M3 for placing the phasing resistors in the standard or unknown arms, and the knob 2|5 for the ver nier phasing resistor. Immediately above the vernier phasing adjustment knob, is the control knob 2i‘! for the coarse phasing adjustment. In 75 the upper left hand corner of the panel are two one additional step is required._ There is a small amount of inherent capacity. in the terminals 40 and leads. This value is usually 25 to 30 micro microfarads. The unknown terminals are left open and the bridge is carefully balanced. The reading for the balance is the inherent capacity which must be subtracted from the ?nal reading 46 which is obtained after the unknown capacitor has been connected and balanced. I have described a compact, self-contained portable bridge. Its accuracy will be determined largely by the precision of its standards and the 50 care with which the variable ratio arms are chosen. Accuracy within about 2% is well within practical design. The bridge lends itself to ready measurements within a wide range of values of resistance, inductance or capacitance. The fore 55 going description, constants and circuits are mere ly by way of example. Those skilled in the art will ?nd numerous modi?cations within the scope of my invention which is only limited by the priorv 60 art and the appended claims. I claim as my invention: 1. In a portable measuring device of the Wheat stone bridge type, the combination which includes a continuously variable resistor operable in co operation with a linear scale, a ?rst and a second 65 ?xed resistance, a ?rst switch for selectively con necting one of said ?xed resistors to said variable resistor whereby said variable and said selectable ?xed resistors form the conjugate variable arm and ratio arm of said bridge respectively; a plu rality of standards of known values of resistance, capacitance, and inductance, a second switch to select one of said standards, terminal connections for an unknown capacitor, resistor, or inductor which is to be measured, connections including 8- 75 \4- I " ’ - ; amass? ' vreversing switch wherebysaid unknown and’ said I and connections whereby'said fourth switch cona standard are‘ serially connected across’ said vari- "' nects said variable capacitor across at least one, able arm and said ratio arm to form the remain 'ing' twoconjugate arms of said bridge, said re versing switch being operably connected to ‘said > second switch whereby the selection of a capaci tive standard automatically reverses the relative of said vresistance standards to thereby. permit the inherent shunt capacitance across high r'e-' sistances which are to be measured to ‘be bal anced out; means for energizing said bridge, and means for indicating a balance. position of said standard and said unknown; 9. - 2. A device of the character described in claim _ ~ second’ variable resistor, and a third switch 1 which is further characterized in that said whereby said second variable, resistorvmay be _-connected in series with said standard or said 10 means for energizing said bridge is an audio ire-'- '10 ._ quency‘ vacuum tube oscillator. _. ‘unknown; a variable capacitor, a fourth switch, - MAURICE‘J. LOVELADY._ 1.