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Патент USA US2403521

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July 9, 1946.
Filed Aug. 20, 1943
Patented July‘9, 1946 .
2,403,521 _
' 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)
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
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
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 ,
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
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
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
across a section of the winding I0 ofithe
_ the Fig. 3 circuit,is established when the ampli?er
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.
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
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
to be measured, a ‘source of alternating current 50 recti?er output.
for excitation of said modulator, a recti?er for
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