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Патент USA US2406974

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Sept# 3, 1945 '
A. w. vANcE
2,406,974
POWER SUPPLY FOR ELECTRON MICROSCOPES
Original Filed Nov. 15, 1940
3 Sheets-Sheet 1_
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Sept» 3» 1946'
A. w. vANcE Y
2,406,974
POWER SUPPLY FOR ELECTRON MICROSCOPES
Original Filed Nov. 15, 1940
f
3 Sheets-Sheet 2
@Mraz
I... ____________ __..
:inventor ‘
Bg @MMA
attorney
Sept.- 3, 194e.
2,406,974
A. w. vANcE
POWER SUPPLY FOR ELECTRON MICROSGOPES
original Filed Nbv. 15, 1940
3 Sheets-Sheet 5
Zmventor
Bu
2,465,974
Patented Sept. 3, 1946
UNITED .STATES
[OFFICE
2,406,975;
POWER SUPPLY FOR ELEcfrRoN
`N1IGROSCOPES
`
Arthur W. Vance, Granbury, iN. J., assigner to
Radio Corporaties pf America, a sommation 9i
Delaware
pplication April 28, 1942, Serial No. 440,721,
"which is" a division of application Serial No.
365,750, Novembei` 15, 1940,\now Patent No.
2,302,900, dated November`- 24, 1942. >Iüyidßíl
and this application April 17.1943, ?Serial N0,
483,488
1 Claim. (Cl. 171.7312),
l
ated at a high negative potential, with the anode
The subject invention relates to DOWcI' Sui),
ply systems for electronic devices and has for its
principal object the provision of an improved
method and means for producing a highly s_ta
grounded, .the filament supply is necessariiy at
a high voltage with `respect to ground. It lis a
further object of this invention _to _eliminate the
necessity iof supplying _direct ,current for the elec
tron gun `>by `energizing the liilament with radio
frequency currents. A _st-,ill further object or this
invention is tp provide an improved high Voltage
rectiiier circuit having a feedback stabilizer which
bilized source of voltage for use with the elec,
tron _microscope whereby greatly improved Der».
formance may be obtained.
This application is a `division ,of applicant@
copending U. S. application, Serial Number
440,771, filed April 28, 1942, which is a Ydivision
of U. S. application, Serial Number~365,750, filed
November 15, 1.94.0, upon which U. S. Patent
permits the required high ycltage to be obtained
with less equipmentand fewer parts than here
tofore been required, thus greatly reducing the
cost V_and size of the power supply system. Still
2,302,900 was grantedto applicant November 2%,
further objects nof the Apresent invention are to
provide an improved automatic relay and pro
19.42.
The development of _the electron microscope
tectivren
has presented new problems in the `design of high
voltage power supplies and in the design .of cur
' Y
l
rent regulating systems for the Various electro.
to
magnetic coils which constitute 4the vmagnetic
lens system of the microscope. In order to pro
duce a beam of -h-igh _velocity electrons, an elec
tron gun is utilized which is energized with a
direct current voltage which may be as high as
100 kilovolts. The current load of the high volt
age source is of the order of 1 milliampere, and
the problem, therefore, is one of voltage stabil.
ity. The magnetic lenses require currents of the
order of 400 milliamperes which likewise must be
maintained constant over short periods.
.energizing thehigh voltage rectifier; and
de improved current regulators for use
in conjunction .with the microscope lenses.
Al'o‘rieiiy, these _objects .are accomplished hy em
ploying ‘a source lof .radio frequency currents cou
pled »to a voltage doubling rectiñer circuit in
which the ~ñ-lanfrent of :oneof the rectiñer tubes
is4 enengizÍ _Y _with radio frequency currents, thus
The stability of the voltage supply system for 30
an electron microscope is measured over the
u1,t,`¿to provide an improved oscillator
ply’lngä the «radio frequency currents util
facilitating fil-tering problems, 4deriving a poten
tial »corresponding >in amplitude to the rectified
outp -Í A, _and utilizing this potential to control the
output of Y»the oscillator supplying the high fre*
quency currents to the >rectifying system.
nThis invention nwill be vbetter understood from
the following ‘descigîption when considered in
period of time required to properly expose a pho
connection with the accompanying drawings in
tographic plate used to record the images. In
which Figure '1 is a `block diagram yof the essen
practice, the longest .exposure is approximately
tial
_elements of a compiete power supply system
30 seconds. Consequently, the voltage supply 35
for anfelectron microscope.; Figure 2 is an equiv
system must not vary appreciably within this
alent lcircuit diagram illustrating the operation
period, since slight variations in the .electron
ofthe high voltage rectifier and filter system;
speed or in the focus of the magnetic `ñelds will
Figure 3 is asimpliñed circuit diagram of the
defocus the electron image and distort .the pho
system including the high voltage rectiiier; Fi.,
tograph.
It is also necessary to consider the short-time
constancy of the power ¿supply system since this
limits the resolution of the microscope. `Theo
retical calculations based on a resolving limit
of 10 angstrom units indicate that the most crit
ical current must not vary more than .004 of one
percent and that the high voltage should not
vary more than .005 >of one percent.
1t is well known that the electron beam must
be thoroughly shieldedfrom all .external fields.
The difficulty of shielding magnetic fields pro
duced `by A60 cycle currents has heretofore ne
cessitated the use of direct >current to supply
power to ¿the ñlarnent of the high velocity elec
tron gun. Since the cathode is normally ,opere
ure fi is the circuit diagram .of the driver oscil~
latorwhich supplies lhigh yoltage radio frequency
currents to .the high voltage rectifier; Figure 5
isthe circuit diagram ora D.-C. modulating am
445 plifier; Figure 64 is .a circuit diagram of a pro
tectiye _circuit `including an overload relay; vFig
ure f7 Ais the .circuit ,diagram of a .regulated cur
rent supply. suitablefor use in_conjunction with
,the electromagnets .ofthe lens system; and Fig
5.0 ure 8~gis Lanalternative current regulating system.
`vThe power >supply system
Referring to Fig. 1, a complete power supply
system for an electron microscope ,is illustrated
in ¿blocktdiagram form. -Power isderived from a
3
andere
60 cycle power line 9 and applied to a number
of regulated power units which energize various
components of the system and which are designed
to have output voltages which are determined by
their intended use. The regulated power supply
Il energizes a pair of low frequency oscillators .
I3 and l5 which are utilized, respectively, to
energize the filament of the electron microscope
and the filament of one of the high voltage recti
4
inductor` Lo through the coupling capacitor CI
and also the cathode of the second rectifier V2.
The'anode of the second rectifier is connected
to the cathode of the high velocity electron gun
of the microscope and is the source of high volt
age direct potential whose polarity is negative
with respect to ground.
Theríilament of the ñrst voltage rectiíier Vl is
supplied from a conventional 60 cycle line, while
l’iers. The high frequency-high voltage rectifier 10 _the filament of the high voltage rectifier V2 is
and filter circuits are represented by the block
obtained from thev filament oscillator E3 which is
l'l and will be described in detail hereinafter.
connected to input terminals E3, the circuit in
Another power unit i9, which may be unregu-v
cluding a coupling transformer T2 having an
lated, energizes a driver oscillator 2l which `gen
untuned secondary and a tuned primary.
erates high voltage radio frequency currents for
»Radio frequency actuation of the high voltage
the high voltage rectifier. A modulating ampli- ‘
rectifier has a number of advantages over the
fier 23 is energized through a connection 22 by
use of Ya conventional 60 cycle alternating power
the unregulated power unit i9 and is also sup
, source for the production of high voltages of
plied with a voltage over a lead 25 which is pro'-' " great constancy at low current drains. For ex
portional to the rectiñed output voltage. This 20 ample, much smaller iilter condensers are re
voltage is compared to that supplied to the mod#
quired for a given ripple output. This means that
ulated ampliñer by a standard battery 21, in
a circuit which will be described in detail here
inafter, to produce an outputcontrol voltage
which is utilized to control the vamplitude of
oscillation of the driver oscillator 2|. This con
trol connection is illustrated by _lead 29.
The
ampliiier is also connected by a lead 20 to a reg
ulated power source 3l which supplies a negative
the stored energy inthe ñlter circuit is vvery
much less than that of the usual 59 cycle “brute
force” filter system. The importance of this is
that in case of a, flash-over in the high voltage
circuit, due to a loss of vacuum in the micro
scope, for example, the stored energy is not sufri
cient to burn up the equipment, as was vthe case
in the earlier microscopes. Since Aresonant cir
voltage of the order of _350 volts, the purpose 30 cuits are used, the ripple fed through the rectiiier
of which is to be described later.
capacitor is sinusoidal and consequently can be
Another regulated power unit 33 supplies a
resonated out to a large degree. Furthermore,
direct voltage of the order of +400 volts to the
by using low loss coils in the resonantl circuit, an
driver oscillator 2! and to the modulating am
extremely high impedance may be realized which
pliiier 23 through a lead 35. In order to achieve 35 occupies but a small space and is extremely light
the highest degree of accuracy, the modulating
ampliñer 23 and, in addition, a current regulator
¿ii for the objective coil are Supplied with a reg
uìated current through connection _24 from a
source 39 which is used to energize the filaments '
of certain of the tubes of these devices. Reg
ulation in the current regulator 39 is achieved
by utilizing the _350 volts obtained from the
regulated power supply 3l by a connection 32 as
in weight as compared to an equivalent imped
ance'at the conventional power frequency. Also
the exciting power required by such a coil is
greatly reduced.
`
The use of radio frequency high voltage power
supply also improves the operation of the out
put control system.. When the control is oper
ated on the low voltage input side of the rec
tiiier,~as is desirable, the speed of control is lim
a standard reference voltage, in a manner which 45 ited -by the frequency of the supply. Conse
will be described hereinafter. The projection coil
quently, this limitation is negligible Vwhere the
and the condenser coil lll and d3 are energized
supply voltage is a radio frequency voltage.
by current regulators ¿i5 and 61, which derive
The use of radio frequency to energize the
power from power supplies 42, 44, respectively,
cathode 0r lilament of the high velocity electron
both regulators having a connection over con
gun has two important advantages. In the 'first
ductor 32 to the regulated power supply 3l. A
place, the microscope may be readily shielded
particular advantage of this system of intercon
from the stray high frequency ñelds which are
nection is that one power supply whichis care
produced, and this result is aided by the fact
fully regulated controls the output voltages of
that no 60 cycle high voltage transformers are re
the other power supply units, and control by a
quired which have very large external iields. In
regulated or standard voltage is eñectuated in
the second place, the use of radio frequency
a much more economical manner than by any
other method.
The high voltage rectifier
Referring now to Fig. 2, a circuit diagram of
the high voltage rectiiier system is shown. Since
an extremely high voltage is produced, the high
voltage equipment has been mounted in an oil
tank 5l which not only protects the user from
harm, but also decreases the danger of accidental
eliminates the necessity for employing bulky stor
age batteries or their equivalent.
The operating frequency for the rectiiier s-ys
tem is not critical. The problem is essentially
that of obtaining the required output voltage
with a minimum exciting power. This, of course,
requires obtaining the maximum resonant imped
ance of the high voltage coil. The resonant im
pedance is given by the formula:
flashover or short circuits. The driver oscillator
2| is connected to a series resonant circuit com
prising an inductance Lo and a capacitance Co,
the latter representing the effective lumped ca
pacity between the point 54 and the ground due
to the primary of transformer T2, and the rec
tiiier tubes Vl and V2 in series with the capac
itor CI. The cathode of the first rectifier VI
where C is the distributed capacity of the coil
plus the rectiñer interelectrode capacities plus
other stray capacities such as capacity from the
primary to the secondary of the iilament trans
former T2. It is obvious that C should be kept
is grounded, while its anode is connected to the 75 to a minimum,` and with C at a minimum, Z may
2,406,974'
s.
6
resistor 61, 10 meg., serially connected between
the high voltage source and the regulator. The
lower end of resistor 61 is bypassed to ground
only be raised by decreasing f or by increasing
Q, where Q is the well known efficiency factor:
and connected through an isolating resistor to
' an adjustable tap on a standard battery 69, the
and R1. and Rc are the effective series A.-C. re
negative terminal of which is grounded.
sistances of L and C, respectively. In the fre
quency region from approximately 20 kc. up to
several kundred kc., the maximum Q possible
standard voltage produced by the battery 69 is
in a coil of given volume is more or less independ
ent of frequency; thus, the coil should, in gen
The
therefore compared to the divided voltage appear
ing across resistor 6l, so that no input voltage is
10 applied to the amplifier 23 when the divided
wherein the inverse voltage of the half wave recti
Voltage obtained from the high voltage output is
exactly equal to that of the standard battery.
By varying the taps on the standard battery 62,
the output voltage may be controlled, for example.
fier VI is rectified by V2, thereby charging the out
put capacitor C2 to a voltage nearly equal to twice
in 5 kv. steps from 30 to 60 kvs.
The details of the driver oscillator which sup
the peak voltage which appears across the induct- Y,
ance Lo. It is to be noted that one side of the
plies high voltage for the rectifier tubes VI and V2
eral, operate below 50 kc.
The rectifier circuit is seen to be of the type
is shown in Fig. 4. Since the resonant frequency
of the series resonant inductor Lo and Co included
input circuit is grounded and that no primary
winding is required on the high voltage coil.
20 Within the oil tank 5I varies considerably with
temperature, a master oscillator-power amplifier
The filament transformer T2 has a low inter»
is impractical without automatic frequency con
winding capacity which is preferably of low power
trol to keep it at resonance, It is therefore pro
factor. The primary and secondary windings
posed to utilize a self-oscillating circuit-l whose
must be spaced sufficiently to withstand the peak
frequency is determined wholly by the resonant
voltage output, and, as a result, a large amount
frequency of the load Lo, Co. This oscillator
of energy must be stored in the tuned primary
must also be capable of modulation over a con
in order that the secondary may absorb the power
siderable range in order to provide control over
necessary to excite the filament. Preferably, the
the output voltage. The circuit utilized comprises
tuned primary of transformer T2 constitutes the
tank circuit of the driving filament oscillator. 30 a two-stage oscillator including tubes V3 and Vfl,
the output of the latter being coupled to the input
The size of the secondary is selected to match theI
of the former by a conventional impedance cou-4
impedance of the filament load and to provide
pling system, tube V4 being shunt fed through
the desired voltage. The filament current may
inductor L4 and coupled to the resonant circuit
be controlled conveniently by varying the fre
quency of the oscillator I5.
35 load through a small series resistance Pt. In
ductor L4 is resonated at the operating frequency
Fig. 3 shows the rectifier circuits connected to
by a capacitor C5. A feedback voltage is obtained
the other elements of the power supply system.
by the drop across resistor R., the terininais of
It will be noted that the negative voltage for the
which are connected to the primary of a shielded
electron gun cathode obtained from the output of
rectifier V2 is applied to a center tapped resistor 40 transformer 'I I, the secondary of which is coupled
to the input of the first oscillator tube V3. The
51 connected across the secondary of the trans
transformer 'il preferably has a broad frequency
former T3 which supplies radio frequency current
response, this being accomplished, for example,
to the filament of the electron gun 59. The filan
by a damping resistor and capacitor '3.3 and l5.
ment, as a whole, is 60 kilovolts below ground
The feedback gain is sufficient only to sustain
potential and must therefore be carefully insu~
oscillation at or very near the resonant frequency
lated. The radio frequency current supplied by
of the load Lo, Co.
the filament oscillator I3 is applied to the fila
The amplitude of oscillation is controlled by
ment through a coaxial cable 6 I.
varying the screen grid potential of> the output
‘ It will be noted that the filter capacitor C2;
.005 microfarad, for example, is connected to 50 tube V4. This is illustrated in Fig. 4 by the potenY
tiometer connection shown in dotted lines, the
ground through a parallel connected coil L5 and
potentiometer representing the control bias de
condenser C5. The values of these reactors are
rived from the D.--C. modulating amplifier 23.
such that at the operating radio frequency, effec
The details of this connection are illustrated in
tive inductive reactance of L5 and C5 resonates
with capacitor C2 and forms an effective ground 55 Fig. 5. The tank circuit L4, C5 has a high L/C
ratio and it therefore exerts only slight control
for ripple voltages appearing in the output cir
over the frequency of oscillation. Its principal
cuit. At a higher frequency, for instance, midway
function is to maintain reasonably sinusoidal
between the fundamental and second harmonic
voltage conditions in the plate of V4.
L5 and C5 become parallel resonant, presenting a
Referring to Fig. 5, a unique D.-C. amplifier
highimpedance, but as there is no ripple at this
frequency anyway, no harm results.
At higher
frequencies including the vsecond harmonic, the
shunt capacitor C6 in series with the output
capacitor C2 provides an effective ground. A
spark gap 63 is connected across the shunt reac
tors L5 and CE in order to discharge voltage surges
Without causing a breakdown of the elements.
High voltage regulator
The amplitude of the rectiñed direct current is
controlled by a circuit including the D.-C. modu
lating amplifier 23 which controls the driver
oscillator 2|, a voltage divider comprising a high
resistance capacity-compensated high voltage 're
sistor E5, 1100 meg., and a relatively low resistance
is illustrated suitable for use in applying the
small variable D.-C. voltage of the control sys»
tem to the driver oscillator to effectuate control
of the amplitude of oscillation. Resistor 51 cor
65 responds to the similarly numbered resistor in
the voltage divider circuit of Fig. 3. At any in
stant, the potential of the lead 22 connecting
this resistor to the control grid of the first arn
pliiier tube 'll is equal to the sum of the nega
tive divided voltage and the voltage due to the
standard battery 69. The latter battery is pref
erably tapped in steps and constitutes the main
control of the high potential output.
The filament of the ñrst amplifier tube 'l1 is
_ connected to a regulated current source, which
2,406,974
may be of the type illustrated in Fig. 8 and here
inafter described, in order to assure constant
electron emission. Screen grid potential is ob
tained from a voltage divider 19-8l connected
to a suitable source of positive potential which
voltage rectifier tube VI. The ground return is
made through resistors ||1, H9, |2|, meter A,
resistor |23, the energizing coil of an overload
relay |25 and a resistor |21. A neon regulator
tube |29 and a pair of condensers |3| and |33
is applied to terminal 83. Plate voltage is ob
tained from a resistor |05 connected between the
plate and the terminal 83. The plate is connected
to the control grid ofV the second amplifier tube
85 through a parallel connected resistor 81 and 10
sistors ||1, ||9 and` |2I, respectively. The output
shown in Fig. 4.
voltage divider |5| across a portion of which a
are connected between theoutput terminals of
the meter A and the junction points of the re
terminal of the meter A is. also connected'to
ground throughy a limiting neon tube |35.
capacitor 89. A phase control network compris
The overload relay circuit includes a switch
ing vresistor 9| and capacitor 93 is connected in
|39 which, in its normal position, makes contact
shunt with the plate of the amplifier tube 11.
to a grounded terminal |4I. When actuated by
Screen grid potential for the second amplifier
an overload cur-rent, the relay armature makes
» tube 85 is derived through a dropping resistor 15 contact to a terminal |43 which is connected to
95 from a suitable source of positive potential
a source of negative voltage, for example, the
which is connected to a terminal 91. A gas ñlled
-350 volts provided by the power supply 3|. The
regulator tube 95 is utilized to control the screen
armature is connected to the grid return of the
grid potential. The anode of the output tube
oscillator tubes of the driver oscillator as illus
85 is connected through an anode load resistor
trated in detail in Fig. 4. The armature is also
|0| to the positive supply terminal 83 and also
connected through a push button |41 to the
through an isolating resistor |03 to the screen
relay coil through a current limiting resistor
grid of the second oscillator tube V4, which is
|49. The same armature is also connected to a
`
It will be noted that the plate voltage of _the 25 neon indicator tube |53 is connected.
first ampliñer tube 11 is impressed on the grid
A current surge, caused by an arc or break
of the second amplifier tube 85 through the cou
down of a high voltage termi-nal, flows through
pling resistor 81. Since the cathodes of the two
the limiting resistors H1, ||9 and |2| and
ampliiier tubes are both operated at ground po
charges capacitors |3| and |33. In addition,
tential, it is necessary to overcome the effect of
the plate voltage on the grid of the second am
the voltage across the output meter A is limited
by the neon tube |29. The meter is therefore
protected from damage. The sustained over
pliiier. This is accomplished by connecting a
source of negative voltage, for example, -350
load flows through the actuating coil of the re
volts derived from the voltage supply 3| illus
lay |25 and connects the armature to the nega
trated in Fig. 1, through an isolating resistor 35 tive potential source, thus Vremoving the normal
|91 to the control grid of the output tube. This
ground connection and applying a high negative
has an effectl similar to that of the usual series
potential to the driver oscillator sufñcient to
bucking battery in D.-C. ampliñers, but has the
advantage that one terminal of the source of neg
stop its oscillation. Since the driver oscillator
provides the high voltage forvthe highV voltage
ative voltage is operated at ground potential, 4.0 rectifier system, it will be apparent that the re
lay immediately shuts off the high voltage sup-`
which is'not true in the conventional case.
It has also been found that it may be desirable
to drive the screen grid of the oscillator tube
more negative than is possible by direct control
ply. At the same time, the relay connects a
holding circuit through the push button |41 sov
that current flows through a resistor |55 and
from the amplifier tube 85. This is accomplished 45 through the relay to hold it in its closed posi
by connecting the ~350 volt source to the lead
non. It will remain m this position untu the>
|09 through a resistor |||, thus reducing the
circuit is broken by operati-ng the push button
average potential of the screen and making it
|41. If the fault has cleared, the overload relay
possible to drive the screen more negative when’
will open, restoring the high voltage to the rec
the output -tube 85 is drawing maximum plate
tiñers. VIf the fault has not cleared, the >high
current.
voltage will not remain on,- indicating that thev
circuit must be checked.' When the overload
Protective circuit
relay is actuated, the neon output indicator |53
It has been found that, due to the high voltage
will light, thus providing a visual indication to4
used, there is some'danger of arcing at the high
the operator that the high voltage has been cut
olf.
voltage terminals. This may occur within the
vacuum chamber of the microscope, due to a
Low voltage or current regulators
failure of the vacuum, for example, or it may
occur by reason of a failure of the insulation of
A current regulator of the type preferred forthe high voltage condensers.
60 use with the projection and objective coils of the
Referring to Fig. 3, it will be noted that the
microscope is illustrated in Fig. '7. A conven
ground return of the high voltage supply flows
tional rectifier |51 and iilter |59 supply a high
through the output current meter A. In order
voltage to the plate of a triode IGI, the cathodeto protect the meter from danger, a protective
of which is serially connected to ground through
circuit |52 has been provided which absorbs sud
the projection or objective coil |53 and current
den current surges and limits the current to sub
control resistorsY |65 and |61, the former being
stantially the normal value. In order to pro-v
adjustable. >The voltage drop across the latter
tect the apparatus from sustained overloads,` a
resistors is compared to the voltage of a stand
cutout or overload relay 69 has’been provided.
ard battery |69 and applied to the grid of a
The details of the protective circuit and over 70 regulating tube |1I, the plate of which is ener
load relay are illustrated in Fig. 6, to which ref
gized by a suitable source of positive potential
erence is now made.
Y,
,
Yconnected to an input terminal |13 through a
Transformer ||5 corresponds to the'similarly
plate resistor |15. The plate of this control
numbered transformer of Fig. 3 which supplies
tube is connected to the grid'of the triode IGI
60 cycle alternating current to the first high 75 through acoupling resistor |11.
2,406,974
It will be observed that the cathode of the tri
ode ISI is positive with respect to ground.
However, the plate of tube ITI cannot become
suiiiciently less positive than the cathode of
10
ITI to vary the amplitude of the current in a
direction tending to compensate, for the change.
The measured stability of the power supply
system herein described has been found to be
well in excess of that required for stability and
definition of an electron microscope. Several
hundred photomicrographs have been success
rent. In order to supply the proper voltage to
fully taken with no indication that the results
the grid of the tube IBI, it is connected through
have been limited by variations of the power
an isolating resistor I 'I9 to the source lof regu
lated negative voltage, as in the case of the 10 supply. This is in distinct contrast with micro
scopes of the prior art in which a large percent
D.-C. amplifier described above.
age of the photomicrographs is spoiled by reason
A current regulator of somewhat simpler form
of changes in the power supply voltage. The
which does not require a standard battery is
system is so stable that the microscope may
illustrated in Fig. 8. This amplifier is prefer
ably used for the condenser coil of the micro 15 be reset to previous conditions without observing
the electron image or the focusing. Exposures
scope and may also be used to regulate the fila
thus made have good resolution. In addition, it
ment current of the D.-C. ampliñer and the con
is possible to make wide variations of the image
trol tube I'II ofthe current regulator illustrated
intensity Without changing the focus.
above in Fig. '7. The unit may also be con
A particularly severe test which was success
sidered as a voltage regulator since the voltage 20
tube IGI to provide the necessary bias for the
latter tube under conditions of low output cur
across a constant impedance is constant when
fully passed by the electron microscope operated
in conjunction with the power supply and con
the current through it is constant. C‘onse
trol system of the present invention is that of
quently, the same control circuit is employed,
reducing the vacuum during operation until an
for example, in regulating the output of the
internal
arc takes place, causing the overload re
regulated 800 volt power supply Il.
25
lay to actuate, then restoring the vacuum, and
As before, the conventional rectiñer and ñlter
applying the high voltage to obtain the original
I5'I and I 59 are connected to the plate of a
picture exactly in focus without readjustment.
limiting triode IBI, the cathode of which is con
Previously known microscopes could not be
nected to ground through the electron micro
scope condenser coils or the regulated filament, 30 treated in this manner and generally had to be
taken apart and repaired after an internal flash
as the case may be, and through the control and
over due to the severity of the discharge from
adjusting resistors |65 and |61. The voltage
the filter.
drop across the latter resistors is compared to
I `claim as my invention:
a voltage derived from the regulated -350 volts
In a stabilized power supply system including
obtained from the power supply 3|, thus elimi 35
a plurality of separate direct current power sup
nating the standard batteries |69 utilized in the
ply units energized by a common power line, one
preceding regulator. A particular advantage of
of said units having its positive terminal
thismethod is that the standard voltage source
grounded and the other units having their nega
is operated with one terminal grounded, which is
not possible where standard batteries are used in 40 tive terminals grounded, regulating means for
stabilizing the negative voltage output of said
a series circuit. Since the cathode of the triode
one unit, means for applying said negative volt
IGI is positive with respect to ground, as noted
age to other units of said system, means for com
above, it may not be necessary to apply the auxil
paring the positive output voltages of said other
iary negative potential to the grid of this tube so
that in this case the plate of the control tube I'II 45 units with said negative voltage to obtain a
diñerence voltage of substantially zero ampli
is connected directly to the grid of the triode I6I.
tude, and means responsive to changes in the
It will be apparent that in both cases, changes
amplitude of said diñ'erence voltage for control
in the current through the coil and control re
ling the output voltages of one of said other units.
sistors produces a variable voltage drop across
"
ARTHUR W. VANCE.
the resistors which is applied to the control tube 50
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