Патент USA US2403521код для вставки
July 9, 1946. 2,403,521 R. W. GILBERT ‘ ELECTRON I C MI CROAMMETER Filed Aug. 20, 1943 a “A Patented July‘9, 1946 . 2,403,521 _ (“UNITED STATES PATENT OFFICE ' ELECTRONIC mcnoamunrnn . Roswell W. Gilbert, Montclair, N. J., assig-nor to _ Weston Electrical Instrument Corporation, _ Newark, N. J., a corporation of New Jersey _ Application August 20, '1943, Serial 1%.499311 6 Claims. "(01. 171-95) 1 This invention relates to ‘an electronic am meter for the measurement of direct currents of, an order substantially below the minimum values that can be measured directly by sensi-' tive permanent magnet-movable coil instru lishes through a resistive load Rs may be'meas ured by, connecting the source and load across the terminals T, T’ of a, measuring circuit com prising an ammeter A having a resistance Rm. ments, and more particularly to electronic micro- ' A positive potential e is established at the ter minal T by the flow of current i through the ammeters of small size that are fully portable measuring circuit, and this potential varies with and therefore adapted for usein the ?eld. and is a measure of the magnitude of the cur Various types of electronic or “vacuum tube rent. The source Es and load Rs may be dis 'voltmeters” have been proposed, and ‘these in 10 connected from the terminals T, T’ and replaced struments have been employed in the measure by a circuit network, see Fig. 2, that is adjustable ment of small current values by passing the cur rent through a resistor, and impressing the re sultant voltage drop upon the grid of the volt meterinput tube. The range of current meas to establish a current of the same value i, but in the reverse sense, through the measuring cir cuit A, Rm. The network includes a resistor R 15 in series with a direct current source of adjust urements was limited, however, and the present able voltage value e that is shown schematically invention extends the measuring range into new regions of minute currents of the order of small as a potentiometer resistance RI connected across a battery E. The positive terminal of bat fractions of a microam-pere. . tery E\is connected to the terminal T’ of the Objects of the invention are to provide novel 20 measuring circuit, and the resistor R is con > electronic ammeters for the measurement of ex v nected between the terminal T and the potenti; tremely small currents of the order ofa micro ometer tap. A voltmeter V is ‘connected between Y ampere and less. An object is to provide an'elec the potentiometer tap and the positive terminal tronic microammeter tha. functions on voltage of the 'battery E to register the particular volt variations, the microamm ter including a cur 25 age value c which esablishes a reverse current ‘ rent-balancing network for degeneratively com of the desired value 1' inthe measuring circuit. pensating the ampli?er gain in such manner that The current sources Es and E of the networks the voltage output is a measure of the current of Figs. 1 and 2 thus establish equal currents z‘, I input. Other objects are to provide electronic in opposite sense, through the measuring circuit microammeters that may be energized from the 30 and, if both networks are connected across the conventional light and. power circuits, usually a measuring circuit as shown in Fig. 3, the cur ?ll cycles per second circuit, and that have an rent in the measuring circuit is balanced out, accuracy and a stability of operation that are i, e. is reduced to zero. The measuring circuit‘ not substantially affected by ?uctuations in the ‘ A, Rm may therefore be opened or removed with; voltage of the energizing source. 35 out a?ecting voltage or current conditions at These and other objects, and the advantages‘ any point. in the connected current source net of the invention, will be apparent from the I01 .work and current balancing network. The source lowing speci?cation when taken with the accom- ‘ pa'nying drawing in which: current i is absorbed in the resistor R of the cur rent balancing network when the measuring cir Figs. 1, 2 and 3' are diagrams of circuits that 40 cuit is opened or is of substantially in?nite re are the'electrical equivalents ofanelectronic sistance, and the magnitude of the source cur current measuring circuit, or portions thereof, rent 11 may be indicated by an appropriate grad in, '1‘ u r ’ ' 4 a. the invention; uation of the scale of the voltmeter V since the Fig. i is a schematic circuit‘ diagram of an “balancing” voltage e varies with'the current i 45 for any predetermined value of the resistor R. embodiment of the invention; and Big. 5 is a circuitdiagram of a multirange elec A manual adjustment of the potentiometer tronic microammeter embodying the invention. along resistance Rl "to establish a current balance Themethod of operation of microammeters condition is obviously undesirable, and the basic vii-y- -~ --; the invention may be most readily circuit arrangement for obtaining an automatic‘ understood by considering ?rst the character so current balance is illustrated schematically in Fig. .istics oi-siinpli?ed circuits that are the electri 4. The source circuit, in which the current is to, cal equivalents of portions of, and of, the elec be. measured may comprise a direct current source tronic current measuringv circuit. > its and, a variable resistance element that, as As indicated by the Fig. 1 circuit diagram, the current i that a direct'current source Es estab showmis a phototube P. The source circuit is con- I 55 nected across the microammeter input terminals 2,403,521 . v 3 . p ter-tapped secondary windings III, II supplying low voltage alternating current ‘to the cathode T, T’ that are joined to the grid G and cathode K, respectively, of a modulator tube I. An alter nating current potential is establishing on a sec ond grid G’ by a source 2 that may be, and pref erably is, a conventional 60 cycles light or power circuit. The modulated output of tube I, which ' is proportional to the grid input, is ampli?ed by the alternating current ampli?er 3 and impressed . upon a recti?er’ that is biased, by a battery or other direct current source 5, to block all direct current output in the absence of ampli?er volt age output peaks less than some predetermined heater circuits, n 1; shown, and a secondary wind ing I2 supplying igh voltage current to a full wave recti?er tube I3- The recti?ed output of v tube I3 ?ows through the'?lter I4, and the voltage divider I5 to which the plate and screen grid ele ments of the modulator tube I and ampli?er tubes 3a, 3b are connected. The input terminals T, T’ of the microammeter , 10 are connected to the control grid G of tube I and v to ground, 1. e. to the chassis of the apparatus, , ‘level. This insures the return of the pointer of the voltmeter V to zero position in spite of hum respectively. The cathode K of tube I is connected except for the blocking bias, would establish a ‘eliminate spurious output voltages that may ap to the chassis through the resistance I6 of the and spurious ampli?er output components which, 15 voltage divider, the resistance being adjustable to recti?er output. A condenser _C may be, and‘pref- ' pear as a result of leakage. This shielding against ‘ leakage effects is essential in the accurate meas erably is,v shunted across the instrument V to urement of currents of the order of microamperes smooth out ripple components in the output of and less, and it is attained by terminating all in the recti?er. The voltmeter V is connected across 20 sulation at approximately the same potential as the recti?er to measure the output voltage e, and the grid G of modulator tube I. All insulation is the resistor R is connected between the negative tied to the chassis, and the circuit is checked for potential terminal of recti?er 4 and the control correct operating conditions by short-circuiting grid G of the modulator tube I. The operation of the current-measuring circuit 25 the input terminalsT, T’ and adjusting the resist ance IE to obtain a zero reading at the output of Fig. 4 may be considered, for simplicity of eXpla~ voltmeter. V. The modulator grid G‘ and the chas nation without resort to a mathematical analy sis are at the same potential, since they are di $15, as analogous to that of the Fig. 3 circuit. The rectly connected, and leakage effects are cancelled intensity of the light reaching the phototube, and 30 ‘out by adjusting the potential of cathode K to such positive value that meter V reads zero when there this current may be considered as ?owing through is no current between the input‘terminals. Hav the grid-cathode impedance of tube I to establish ing balanced out all spurious potentials, the short a correspondingly varying positive potential e; on circuit is removed from the terminals T, T’ and the control grid G. This grid bias modulates the tube output which, after ampli?cation, is recti?ed 35 the meter V will then respond only to changes of input current. . ' to develop an output voltage e that varies with the An alternating current voltage'is applied‘ to grid bias er and therewith with the input current grid G’ of tube I through; condenser I'I, lead I8, .1‘. The condition of zero current in the measur current 1 through the phototube P varies with the ~ and voltage divider resistances I9 that are con ing circuit, as described above with reference to nected across a section of the winding I0 ofithe 40 _ the Fig. 3 circuit,is established when the ampli?er power transformer. Resistance couplings are 3 is adjusted to provide a recti?ed output‘ volt provided to the ampli?er tubes 3a, 3b,‘ and the age 6 that, for any grid bias 81 developed by'the secondary winding of the input transformer 20 input current 1', establishes a reverse current i in the balancing network. Actually, however, the input modulator tube I functions on voltage vari 45 ation and draws no appreciable current, so the entire input current is absorbed in the degenera tive resistor R. A current balance is thus estab of recti?er 4 is returned to the voltage divider 2| to impress a bias on recti?er 4 to suppress hum and spurious ampli?er output components. A plurality of degenerating resistors RI-RA of different values are preferably included for a control of the measuring range of the appa ratus, the desired range being selected by ad tains a de?nite relation between'the grid bias el 60 justment of switch 22 to connect the appropriate and the recti?ed output voltage e, and the output degenerating resistor. between the recti?er-out voltage e therefore ?uctuates with and is a meas put circuit and the grid G of modulator tube I. ure of the input current i. The scale of the volt The resistors RI—R4 have distributed capacities meter V may therefore be calibrated directly in. to ground that must be charged before a cur- » values of the input current i. The current bal 55 rent that is applied to the right hand terminal ance condition depends upon the magnitude of - lished at any given input current i that main the degenerative resistor R, and a plurality of cur- ' ' of a degenerating resistor appears at the oppo site end. Expressed di?‘erently, an appreciable rent ranges may be incorporated in the microam-r time-lag is encountered before the’ voltage at meter by providing a plurality of resistors of dif ferent values and a range-change switch for con 60 the modulator grid G assumes a balanced con dition in response to the output current ?owing ' necting the desired‘ resistor into the measuring through the degenerating resistor. In practice. system. v this time lag sets up a “hunting” condition in The complete circuit diagram of a multirange the circuit that causes the pointer of the output microammeter embodying the invention/is shown _ in Fig. 5. The general design and'the' physical 65 meter V to oscillate continuously. I The time lag is eliminated by connecting a construction of the electron tube circuits conform to good practice in the radio field, and various conventional circuitelements of the illustrated ' bank of small degenerating condensers 23 across the degenerating resistances, whereby any change in the direct current output voltage is separately identi?ed by reference numerals. The 70 transmitted immediately by the small condenser several tubes are of the indirectly-heated ‘cathode and appears at the modulator grid independ type, and the usual "power supply unit" is em; ently of distributed capacity'to ground along ployed for energizing the microammeter from a the degenerating resistors. A rapid circuit bal 110-120 volts, 60 cycles alternating current light ‘ ance is thus obtained, and the pointer of the or power circuit. The power transformer has cen '75 modulator-ampli?er-recti?er system will ‘not be 2,403,021 output meter V moves to a de?nite position that is a measure of the input current. , The invention is not restricted to any particu lar range of current measurements and the fol lowing data as to one practical embodiment of the invention should not be viewed as a limita tion to the speci?ed values of circuit elements or to the sta'tedcurrent ranges. The voltmeter V was a conventional direct current instrument rectifying the output of said modulator, said recti ?er having an output circuit‘ including a volt, meter for measuring the .voltage developed by the modulated output of said modulator, and means including a degenerating resistor connected be tween said recti?er output circuit and said modu lator input circuit for establishing a condition of equality between the current to be measured and the degenerative current through said re‘ having a full vscale de?ection ‘at 50 volts,‘ 10,000 10 sistor, whereby the recti?er output voltage is a measure of the current input to said modulator. ohms per volt, and the modulator-ampli?er cir 2. In an electronic ammeter, the invention as cuits were designed to develop a recti?er output recited in claim 1 wherein said degenerating re . of 50 volts at a modulator grid potential‘ of sistor hasan unavoidable distributed capacity to +0.5 volt. The degenerating resistors Rl-Rl had values of 50,000; 5,000; 500 and 50 meghoms 15 ground, and a small condenser is shunted across‘ said resistor to establish a potential balance prior respectively, and the corresponding top values of to thecompletion of the charging of said dis the several measuring ranges‘ were 0.001, 0.01, 0.1 tributed capacity by the recti?er output. and 1.0 microampere.“ The calibration of the apparatus was substantially independent or all > 3. In an electronic ammeter, the invention as factors except the values of the degenerative 20 recited in claim 1; in combination with means resistors Rl-Rl, and was particularly insensi for terminating all’ supporting insulation in. a - . leakage shield adjustable in potential to minimize tive to changes in the voltage of the 60 cycles insulation leakage. power supply. ‘The error produced by plus or 4. In an electronic ammeter, the invention as minus 20% voltage variation from the design center 117 volts was approximately 0.5%. 25 recited in claim 1 wherein there are a plurality of degenerating resistors of different magnitudes, While I have illustrated and speci?cally de and switch means is provided for selectively con scribed an electronic circuit employing a vacuum necting the desired one of said resistors in circuit tube modulator, it is apparent that the other to determine the measuring range of the am types of non-linear modulating devices‘ may be ' employed such as, for example, recti?er ring 80 meter. , 5. In an' electronic apparatus for measuring’ modulator, or vibrating contact modulator. The small direct currents, a modulator tube and means vacuum tube modulator is preferred because of impressing an alternating current voltage upon its sensitivity which makes it particularly adapt an element thereof, input terminals connected . able to the production of a practical device of extremely low current measuring range. Elec 85 to the cathode and a grid of the modulator tube, whereby the modulated output is proportional to tronic ammeter circuits in which the input cur ' the grid input, means for amplifying the modu rent is absorbed or dissipated in a degenerating resistor are believed to be broadly new, and it is ~ lated output, a recti?er tube working out of said ampli?er means, a voltmeter in the output cir to be understood that the invention is not limited '. to the particular circuit arrangement herein 40 cuit of said recti?er, and a degenerative resistor connected between the recti?er output circuit and shown and ‘described. Various modi?cations the grid of said modulator tube. that may occur to those familiar with the design 6. In an electronic apparatus for measuring ' and construction of electronic apparatus fall small direct currents, the invention asrecited in within the spirit of my‘invention as set forth in 45 claim 5 wherein said degenerating resistor has the following claims.‘ ' an unavoidable distributed capacity to around, I claim: ' ,1. In a direct current electronic ammeter, a modulator having an input circuit to be connected and a small condenser'is shunted across said re sistor to establish the potential balance prior to ‘ the charging of said distributed‘capacity-by the in series with the circuit traversed by the current \ _ 1 to be measured, a ‘source of alternating current 50 recti?er output. for excitation of said modulator, a recti?er for ' ROSWELL W. GERBER-T.