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Патент USA US2406804

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Sept- 3, 1946\
Filed Nov. 6, ‘1943
Patented Sept. 3, 1946
Prafulla Kuniar Chatterjea and Charles Thomas
Scully, London, England, assignors, bymesnc
assignments, to International Standard Electric
Corporation; New York, N. Y.', a corporation oi‘
Application November 6, 1943, Serial ‘No. 509,188
in Great ‘Britain .ianuarygS, 1943..
1 Claim: (Cl. 250—36)
The "present invention relates. to. clectricres
onan'ti circuits ‘particularly those used in oscil
lation generators and employs (the special. prop—
ei'tiesv of thextherinally sensitive'resistance ele
thereforemay. be used‘to generatexoscillations in‘
a‘ resonant: circuit in; a: particularly simple way;
By‘ suitablexacljustment of the'constants' of the.
circuit; oscillations ' may just :be' prevented. from‘
occurring;- and: in"v that casea resonant circuit,
ments known as thermistors.
Thermistors are. thermosensitive resistance ele_
which may have a very low resonance frequency
mentshavinga temperature coefficient of resist
ancevvhich may be either positive or negative-and
and a very highvalueof Q", .may be easily "ob‘_
tained foruse in?lter and. other like circuits.
which is moreover many times the corresponding
According. toone aspect'of the invention; there
coef?cient ‘for a pure metal such as copper; This
fore, theretisnprovided 'an electric resonant cir.-'
cuit comprising an'inductance, a thermistor hav
ing .a: negative temperature coefficient of ‘resist;
ance, and‘ a source of direct current; coupled;
propertyrenders thermistors particularly suitable
for a variety of special applications in electric
together in such a manner ‘that the. ratio of "re-1
Various different- materials are available for
the resistance element of a thermistor, these vari 15 actanceito resistance of the inductance. isiefliec'~
tively raised ‘ to a; predetermined . value.
ous materials having different ‘properties in other
Accordingto another aspect; thezinvention'pro
respects; as: one example, a-resistance material
vides anelectric: resonant circuit comprising- an'
having a high negative temperature coef?cient of
inductance at least part of which is; connected"
resistance comprises a» mixture of manganese
in series with the resistanceielement of fav thermis
oxide,land nickel oxide'w-ith or'without'the-addi
tor‘ having a negative temperature coei?cient‘ of
tion of certain‘ othermetallic oxides, the mix
ture being suitably heat treated.
Thermistors have been employedinxtwo dif
resistance, to'avsource of direct current, theth'ere
mistorvcircuit resistancebeing adjustedso that
the voltage-current characteristic curve‘ of; the
ferent forms: (a) - known‘asral-directlyrheated
thermistor andfcomprisingr a resistance element 25 thermistor circuit'h'as a portion; whose slope is
slightly’- positive,. and. vso "that the current through
of the thermally sensitive resistance material ‘pro
the-resistance element is represented-byapoint
vided with'suitable lead-out conductors or ter
on the said portion of slightly positive slope;
minals, and (b) known as'an'iindir'ectly» heated
Accordingto the invention, also; the thermis
thermistor comprising 'tlie'ele'ment. (a) provided
in addition with a heating’. coil electrically-‘ma 30 toreircuit resistance‘in either of the abovemem.
tioned aspects. may be alternatively adjusted =50
sulatedi from the element. Al- directly 1 heated
that oscillations take place in thereson-antcir
thermistor is primarily intended to b'elcontrolled
cuit. .
by the currentwhich flows through it and which
The invention awill be described with reference
variesthe temperature and also the resistance
to the accompanying drawing in which»:
accordingly. Such a. thermistor willll'also b'e af
Fig; 1 shows'an example‘of one circuit-accord
fected bythe temperature of its surroundings-and
ingv to" the invention; and ‘
maytherefore be used for‘ thermostatic control
Fig; 2fshows ' characteristic curves‘ used to -ex-~
and‘likekpurposes-With or Without direct'h'eating»
plain the operation'of'Fig. 1.
by the current ?owing through it. An indirectly
It will be understood that any'resistances ‘other:
heated thermistor is chie?y designed to be heated 40
than thermistors‘emp-loyed in'this inventionare
by- a controlling current which flowsthrough the
of the ordinary kind in which the resistance value
heating coil and'which will usually, but not nec
is substantially:-independent of the current.
essarily, be different from the current which
Fig. 1 shows an example of a circuit accord
flows through the resistance element, but this
ing to the invention. An indirectly heated ther
type of thermistor may also be subjected to either
mistor T having a negative temperature coeffi
or both of the types of control applicable to a
cient of resistance has its resistance element
directly heated thermistor.
R connected in series with part of an inductance
More detailed information on the properties of
L and a tapped battery or other direct current
thermistors will be found in an article by G. L.
Pearson in the Bell Laboratories’ Record Dec. 50 source E‘. The heating coil 1' of the thermistor
is connected to the battery in series with an ad
1940, page 106.
justable resistance R1. A parallel resonant cir
In the present invention, thermistors having a
cuit is produced by connecting a condenser C
negative temperature coeiiicient of resistance are
employed, since under appropriate conditions
across the inductance L.
In Fig. 2 are shown characteristic curves for the
they may behave like negative resistances and 55
circuit of Fig. 1. The abscissae represent the cur
rents and the ordinates the corresponding volt
Curve A shows the curve for the thermistor T
alone, with no current in the heating coil. As the
current through the resistance element R is in
creased, the voltage across R ?rst increases to a
maximum and then decreases continuously. The
part of the curve beyond the maximum has a
negative slope, and this means that the effective
resistance to current changes is negative.
The straight line B represents the characteris
tic for the resistance of that part of the induct
ance L which is in series with the thermistor (to
gether with any other resistance there may be
in the circuit) and this resistance is assumed to
be substantially constant. Curve C has been ob—
tained by adding the ordinates of curves A and
B, and represents the resistance characteristic
it does not contain any valves which would re—
quire a high tension or cathode heating source.
It would be useful, for example, as a low fre
quency generator; as a vibrator for power packs,
or to generate ringing frequencies for telephone
Referring again to Fig. 2, if the resistance con
nected in series with the resistance element R be
increased, the slope of the curve B will be also
increased and the horizontal part of the curve
C will move to the left until it reaches the volt
age maXimum of the curve. At, or a little after,
this stage, the curve C will be found to have no
horizontal portion, and no portion of negative
slope, but there will be a short region in the
neighbourhood of the maximum of the curve
where the slope is only very slightly positive.
Under these conditions oscillations will not oc~
our, but the ratio Q of reactance to resistance of
of the whole of the circuit. It will be seen that
curve C is horizontal in the neighbourhood of the
point P signifying that the effective resistance
to changing currents is zero. According to
the inductance L may effectively be very high,
and can be given any desired value by suitably
termined substantially by L and C.
The effect of passing a current through the
cult at low frequencies, requiring very bulky coils
or complicated amplifying arrangements by
heating coil 1 will be generally to lower the curves
A and C and to render the voltage maximum less
sharp. Curve D shows the manner in which the
curve A would thus be changed. This provides
a convenient means of adjusting the character
istic of the thermistor to produce the desired os
cillation condition. The operation of the circuit
known methods.
adjusting the circuit. In this way a resonant
circuit L, C tuned to a very low frequency, and
known principles, the circuit will oscillate in the
having a very high value of Q, is obtained in a
neighbourhood of the point P at a frequency de- 7,- very simple way; this is well known to be dini
may also be controlled by including a suitable re~ -
It may be added that another method of ob
taining the desired slightly positive slope is to
adjust the resistance R1 in Fig. 1 instead of ad
justing the resistance in series with R. By in
creasing the current through the heating coil 1
the horizontal portion of the curve C will also
move to the left until it disappears in the neigh
sistance (not shown) in series with the thermis
bourhood of the maximum of the curve. It is
tor element R. This will change the slope of the
further to be noted that in Fig. 1, the current for
line B in Fig. 2.
the heating coil r could, if desired, be obtained
The oscillation output may be taken from the
from a direct or alternating current source (not
tuned circuit L, C in any suitable way (not 40 shown) entirely separate from the direct current
shown). Although adjustable connections to
source E.
the battery and to the inductance L are indicated
What is claimed is:
for convenience, these could of course be ?xed;
Electrical circuit including, connected in se
and if adjustment of the characteristics of the
ries, an indirectly heated, negative characteris
thermistor are not required, one of the directly
tic thermistor, a source of direct current, and at
heated type could be used, the resistance R1 and
least part of an inductance, also including a ca
its connections being omitted.
pacity shunting said inductance and forming
The tuned circuit could furthermore be cou
therewith a resonant circuit, said series connect
pled to the thermistor circuit by means of a
ed circuit also including a variable resistance
separate winding (not shown) coupled to the in 60 connected in series with the heater of said ther
ductance L, and connected in series with the
mistor, whereby the characteristic voltage-cur
thermistor and source E, and various other mod
rent curve of said thermistor may be altered in
i?cations will occur to those skilled in the art.
shape and position, so that a given current ?ow
It should be added that owing to the fact that
therethrough may be made to lie at a predeter
the response of the thermistor to changes in the 55 mined point of said curve, said point being lo
current is not instantaneous, the frequencies to
cated at a portion of said curve having suflicient
which the circuit is applicable will generally be
negative slope to cause said resonant circuit to
low. Thermistors having very rapid response
are however known, and frequencies of several
hundred periods per second may be possible.
‘The circuit of Fig. 1 is particularly simple since
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