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Патент USA US2119372

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A May 31, 1938.
K_ R wEND-r
2,119,372
PROTECTIVE CIRCUIT
,
Filed Dec. ‘29, 19:56
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Karl
3nnentor
R.Wendz€
Patented May 31, 1938
2,119,372
‘UNITED STATES PATENT OFFICE
2,119,372
PROTECTIVE CIRCUIT
Karl E. Wendt, Audubon, N. J., assignor to Radio
Corporation of America, a corporation of Del
aware
Application December 29, 1936, Serial No. 118,054
7 Claims. (Cl. 250—2'7)
My invention relates to protective circuits and control grid 3, a ?rst anode 4, a second anode 6
‘particularly to protective circuits for electronic and a ?uorescent screen ‘I, the cathode ray being
tubes such as cathode ray tubes.
electrostatically focused. Suitable de?ecting de
In electrical circuits it is often desirable either
5 to delay the application of a high positive volt
age to certain electrodes of a cathode ray tube
until after the cathodes of certain vacuum tubes
have become heated to an electron emissive state
or to maintain the control grid of a cathode'ray
10 tube so highly negative that the tube is blocked
until the cathode becomes electron emissive. The
reason for such precautions is to prevent injury
to the cathode ray tube ?uorescent screen caused
by the cathode ray remaining stationary on the
15 screen before the de?ecting circuits become op
erative.
An object of my invention is to provide an im
proved protective circuit of the general character
described above.
A further object of my invention is to provide
20 an improved circuit for maintaining the control
grid of a cathode ray tube highly negative until
a de?ecting ?eld is applied to the cathode ray.
A further object of my invention is to provide
an improved protective circuit in which a protec
25
vices such as coils 5 and Ill are provided.
The second anode 6 is supplied with a high 01
voltage from a suitable recti?er-?lter unit 8.
The ?rst anode 4 is supplied with a suitable
positive voltage from another recti?er-?lter unit
9 having resistors H and I2 connected there
across which may have the resistance values in- 10
dicated on the drawing or some other suitable
values. A voltage divider resistor l3 may be pro
vided, if desired, as sometimes is desirable for
supplying voltages to other tube electrodes such
as a screen grid of a cathode ray tube when a 15
tube of this type is employed.
It will be noted that the cathode 2 is connected
to the junction point of the two resistors II and
I2 whereby, in the speci?c circuit shown, four
?fths of the voltage supplied from the recti?er
?lter unit 9 is applied to the ?rst anode 4 when
the resistor I2 is included in a circuit. The re
sistor I2 has a pair of relay contact points I
shunted thereacross, whereby it is shorted out
age supply unit.
when the contacts I are closed. The contact
points I may be shunted by a resistor I 4 and
condenser Hi to reduce sparking. As will be ex
In a preferred embodiment of my invention
the control grid of a cathode ray tube is biased
plained later, the voltage drop across the resistor
I2 is applied to the control grid 3 to bias it highly
tive tube functions also as the recti?er of a volt
30 negatively a large amount as soon as power is ap
plied to the cathode ray tube and associated cir
cuit. The protective circuit includes a recti?er
tube having a thermionic cathode which heats
more slowly than the cathodes of the tubes in the
de?ecting circuits. This recti?er tube, which
supplies biasing voltages to various tube elec
trodes, has a relay winding in series therewith
which operates certain contacts as soon as the
cathode of the protective recti?er tube becomes
40 electron emissive.
This causes one pair of con
tacts to short out a resistor applying the high
negative bias to the cathode ray tube grid and
causes another pair of contacts to close a circuit
which supplies normal bias to the cathode ray
45 tube grid.
Other objects, features and advantages of my
invention will appear from the following descrip
tion, taken in connection with the accompanying
drawing in which the single ?gure is a circuit dia
50 gram of one embodiment of my invention.
Referring to the drawing, my invention is
shown applied to a television receiver compris
ing a cathode ray tube l and associated circuits.
The cathode ray tube may be of conventional de
55 sign including an indirectly heated cathode 2, a
negative until the contact points I are closed.
Video signals, which include picture signals and
synchronizing impulses, are applied to the con
trol grid 3 of the cathode ray tube I through am~
pli?er tubes I5 and I1, which may be of the
screen grid type. The tube H has a cathode iii, a
control grid IS, a screen grid 2| and a plate 22.
A suitable potential is applied to the plate 22
through a plate resistor 23 and the plate 22 is
connected directly through a conductor 24 to the
control grid 3 of the cathode ray tube I.
40
The grid circuit of the ampli?er tube I‘! in
cludes a grid condenser 26 and a grid resistor 21
having such capacity and resistance values that
the incoming positive synchronizing impulses,
which are greater in amplitude than the‘ picture 45
signals, cause the control grid l9 to be suitably
biased to make the ampli?er tube I1 function as
an undistorting ampli?er for the picture signals.
Also, since the bias on the grid l9 varies with the
amplitude of the synchronizing impulses, the 50
voltage drop across the plate resistor 23, due to
the direct current component, varies accordingly,
whereby the bias on the control grid 3 of the cath
ode ray tube is varied. This variation in the
cathode ray tube grid bias is such as to give au- ‘55
2
tomatic background control, as described and
claimed in application Serial No. r718,192, ?led
March 30, 1934, in the name of Waldemar J. Pooh,
and assigned to the Radio Corporation of Amer
ica.
In the above description, it has been assumed
that there is a connection from the lower end
of the plate resistor 23 to the cathode 2 of the
cathode ray tube. This connection will now be
10 described. It includes the voltage divider 28 of
a recti?er-?lter unit 29, this unit including COD-9 _ .
tact points II in series with the recti?er 3| of the
unit whereby voltage appears across the voltage
divider 28 only after the contact points II have
15 been closed. A resistor 32 and acondenser 33
may be connected across the contact points II
to reduce sparking. The connection between the
cathode 2 and the lower end of the plate resistor
23 is completed by a connection fromthe nega
20 tive end of the resistor |2 to- a variable tap 34'
on the voltage divider 28. It will be‘ seen that
the variable tap 34 may be adjusted to include
in the grid cathode circuit of the cathode ray
tube | a voltage which is in opposition to the
25 voltage drop across the plate resistor 23, whereby
the bias on the control grid 3 may be adjusted
to any desired value for a given incoming signal.
This adjustment is made when the contact points
I are closed, this being their position during op
30 eration of the receiver. When the contact points
I are open an additional negative voltage is ap
plied to the control grid, this additional voltage
being the voltage drop across the resistor‘ |2.
A relay coil 36 for actuating the contact points
35 I and II is included in the circuit of a recti?er
tube 31 having an anode 38, a cathode 39, and a
tungsten ?lament heater 4|. The ?lament 4| is
connected to a suitable current source through
a. resistor 42. Since the ?lament 4| has a low
40 cold resistance, a comparatively small voltage is
come electron emissive.
Thus, it will be seen that after power has been
applied to the receiver and before current begins
to ?ow through the relay coil 36, there is no cur
rent ?owing through the voltage divider 28, as
the contact points 11 are open, and, ‘since the 10
contact points I are also open, the voltage drop
across the resistor I2 is applied to the control
grid 3; of the cathode ray tube | to bias the tube
| considerably beyond its cut-off point.
As soon as the cathode of the protective tube 15
31 becomes heated to an electron emissive state,
current ?ows through the relay coil 36. By this
time the cathodes of the various receiver tubes,
including those in the de?ecting circuits, have
become heated to their proper operating temper
ature and de?ecting current has been supplied to
coils 5 and H). V The current ?owing through the
relay coil 36 closes vthe contact points I and II.
The closing‘of, contact points I removes the pro
tective biasfrom the‘ grid 3 of the cathode ray
tube and applies’ full voltage to the ?rst anode 4.
The simultaneous closing of the contact points II
causes current to ?ow through the voltage di
vider 28 and applies a potential to the plate 22
of the ampli?er tube |'|.
Thus, the protective bias is removed and nor
mal bias is applied only after a deflecting ?eld
is supplied. The change from protective'bia's to
normal bias either takes place simultaneously or
the contact points are so adjusted that the pro
tective bias is removed an instant after contact
points II have closed to apply normal bias to the
control grid.
'
It will be seen that the protective recti?er tube
31 has a double function, since it acts both as a 40
applied across it initially, this voltage increasing
delay tube for operating the relay contact points
in value as the ?lament heatsv up and as its re
I and II and as a recti?er tube for supplying bias
sistance becomes greater. In this way, the oath
ode 39 of the recti?er tube 31 is caused to be
ing voltages to the various receiver tubes.
It may be pointed out that the tubes in the
de?ecting circuits (not shown) may be supplied
with voltage from unit 29, in which case the de
?ecting current is supplied to the coils 5 and III
the instant contact points II close.
In addition to protecting the cathode ray tube,
my circuit protects the mercury recti?er tube‘ 3|
and electrolytic condensers such as the ?ltercon
denser 43 in the plate circuit of tube I5, since it
gives the‘ cathodes of tubes 3| and I5 time to be
45 come electron emissive at a later time than the
cathodes of the other tubes in the receiver. It is
assumed that power is supplied to the cathodes of
?laments of all tubes simultaneously.
The cathode 39 and the plate 38 of the recti
50
tron emissive at almost the instant the power is
supplied to the receiver and considerably before
the cathodes 2 and I8 and the cathodes of the
tubes in the deflecting circuits (not shown) be
?er tube 31 are connected across a suitable volt
age source and in such direction as to provide
biasing voltages for the various tubes in the re-
ceiver. In the speci?c circuit illustrated, the
cathode 39 is connected to a plate terminal of
55 the high-voltage winding 44 in the recti?er-?lter
unit 29. The plate 38 is connected through a re
sistor 46, the relay coil 36, a second resistor 41
and a voltage divider 48 to the center point on
the high-voltage winding 44. In order to main
60 tain the voltage across the voltage divider 48
substantially constant, ?lter condensers 49 and
a voltage regulating glow tube 5| are included
in the recti?er circuit in shunt to the voltage
divider.
come electron emissive before voltages are ap
plied to their plates.
,
55
In the drawing, certain voltage and resistance
values and oer-tain tube types have been indi
cated merely by way of example.
I claim as my invention:—
1. In combination, a cathode ray tube having
a cathode, a control grid and an anode, a voltage
supply unit having two resistor elements con—
nected thereacross, said anode being connected
to the positive terminal of said unit and said
The operation oi‘- the circuit is as follows: With ' cathode being connected to the junction point of 65
65
no power applied to the receiver, there is no cur
said resistor elements whereby one of said ele
rent ?owing through the relay coil 36 and the ments is connected between said cathode and
contact points I and II are open. As soon as
said anode, an ampli?er tube having a plate re
power is applied to the receiver, the cathodes of sistor‘, a direct current connection between the
70 all the tubes begin to heat up and, in‘ a short plate end of said resistor and said control grid, 70
a second voltage supply unit having a voltage
time, all these cathodes are electron emissive ex
cept the cathode of the protective recti?er tube divider connected thereacross, said plate resistor
31. Also it should be noted that the ?laments being connected to the positive end of said volt
age divider to supply positive potential to the
of the recti?er tubes in the anode supply units 8
plate of said ampli?er tube, av direct current con 75
v76 and 9 are directly heated and will become elec
3
2,119,372
nection between the negative end of the other of
said resistor elements and a point on said voltage
divider whereby the voltage drop across said other
resistor element is applied to said control grid,
and means for short-circuiting said other resistor
element.
2. The invention according to claim 1 charac
terized in that said last means includes a relay
comprising a relay coil and contact points, said
contact points being connected across said other
resistor element, said relay coil being connected
in series with a thermionic device which passes
current to actuate said relay only after a prede
termined time.
3. Electrical apparatus comprising a cathode
ray tube and a plurality of electric discharge
tubes, said cathode ray tube having a control
grid, means for applying a high negative bias to
said control grid when power is ?rst supplied to
20 said apparatus, and means for removing said bias
after the power has been supplied to said appara
tus for a predetermined time, said last means in
cluding a recti?er tube with a slow heating cath
ode and having a relay coil in series therewith, a
25 voltage divider connected to be supplied from said
recti?er tube, and ?lter elements connected in
shunt to said voltage divider whereby a substan
15
tially steady voltage may be obtained therefrom.
4. Electrical apparatus comprising an ampli?er
30 tube having a plate, a cathode ray tube having
a cathode and a control grid, a power supply unit
having a recti?er and a voltage divider, said plate
being connected to said voltage divider through
including said voltage divider and said plate re
sistor, and means for connecting said voltage di
vider in series with said recti?er in response to
power being supplied to said receiver for a prede
termined time.
5. The invention according to claim 4 charac
terized in that said last means includes the rec
ti?er tube of a power supply unit and a relay coil
connected in series with said recti?er tube.
6. Electrical apparatus comprising an ampli?er 1O
tube having a plate, a cathode ray tube having
a cathode, a control grid and an anode, a power
supply unit having two resistor sections connected
in series with each other and across said unit,
one of said sections being connected between said 15
cathode and said anode, a second power supply
unit having a recti?er and a voltage divider, said
plate being connected to said voltage divider
through a plate resistor, a direct current connec
tion from said cathode to said control grid, said 20
connection including the other of said resistor
sections, said voltage divider, and said plate re
sistor in series, means for short-circuiting said
other resistor section in response to power being
supplied to said apparatus for a predetermined 25
time and means also for connecting said voltage
divider in series with said recti?er in response
to power being supplied to said apparatus for a
predetermined time.
'7. The invention according to claim 6 charac
terized in that said last two means include a relay
coil connected in series with a recti?er tube hav
ing a slow heating cathode.
a plate resistor, a direct current connection from
said cathode to said control grid, said connection
KARL R. WENDT.
35
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