Note: Descriptions are shown in the official language in which they were submitted.
~373~
This invention relates to electrical devices which comprise at
least two electrodes and at least one PTC element, and methods of employing,
especially for heating and temperature sensing, such devices.
The resistivity of many electrically conductive materials changes
w.ith temperature. The terminology which has been used in the past to describe
the dif~erent kinds of resistance/temperature variation is variable and
oEten impreciâe but, broadly speakingJ materials which increase in resistivity
are designated PTC (positive temperature coefficient) materials; those that
decrease in resistivity are designated NTC (negative temperature coefficient)
materials; and those which show no substantial change in resistivity are
designated CW (constant wattage) or ZTC (zero temperature coefficient)
materials. However, some materials show quite different temperature ranges;
for e~ample a material may show CW behaviour at low temperature and PTC
behaviour at higher temperatures and/or may show, in a specific temperature
range, a very much greater rate, or change in the rate, at which resistivity
changes with temperature than outside that ra~ge.
In this specification, the terms "composition exhibiting PTC
behaviour" and "PTC composition" are used to denote a composition having at
least one temperature range (hereinafter called a "critical range") which is
within the limits of -100C and about 350C, at the beginning of which the
composition has a resistivity below 105 ohm. cm.; and in which the composition
has an R14 value of at least 2.5 or an Rloo value of at least 10 (and prefer-
ably both), and preferably has an R30 value of at least 6, where R14 is the
ratio of the resistivities at the end and the beginning of a 14C range, Rloo
is the ratio of the resistivities at the end and the beginning of a 100C range~
and R30 is the ratio of the resistivities at the end and the beginning of a 30c
- 1 - . ~
3~39
range. The term "PTC element" is used herein to denote an element comprising
a PTC composition as defined above. A plot of the log of the resistance of a
PTC element, measured between two electrodes in contact with the element,
against temperature, will often, though by no means invariably, show a sharp
cllln~e in slope over a part of the critical temperature range and, in such
cascs, the term "switching temperature" ~usually abbreviated to T ) is used
herein to denote the temperature at the intersection point of extensions of
the substantially straight portions of such a plot which lie each side of the
portion showing the sharp change in slope. The PTC composition in such a
PTC element is described herein as having "a useful Ts". The term "anomaly
temperature" has also been used in the past to denote the temperature at
which a PTC element shows a sharp increase in the rate atwhich resistance
increases with temperature.
PTC compositions and electrical devices, especially heaters, which
contain PTC elements, have been described in a number of publications. Reference
may be made for example to United States Patents Nos. 2,978,665; 3,243,753;
3,351,882; 3,4~2,358; 3,413,442; 3,591,526; 3,673,121; 3,793,716; 3,823,217;
3,858,144; 3,861,029; 3,914,363; and 4,017,715; British Paten~ No. 1,409,695;
Brit. J. Appl, Phys. Series 2, 2 569-579 (1969, Carley Read and Stow);
Kautschuk und Gummi II WT, 138-148 (l958, de Meij); Polymer Engineering and
Science, Nov. 1973, 13, No. 6, 462-468 (J. Meyer); United States Patent Office
Defensive Publication No. T 905,001; German Offenlegungsschriften Nos.
2,543,314.1, 2,543,338.9, 2,543,346.9, 2,634,931.5, 2,634,932.6, 2,634,999.5,
2,635,000.5, and 2,655,543.1; and German Gebrauchsmuster 7,527,288. Reference
may also be made to British Patent Specification No. 1,521,460 and German
Offenlegungsschriften Nos. 2,746,602, 2,755,076, 2,755,077 and 2,831,799.
3~
As discussed in German Offenlegungsschrift No. 2,543,31~.1, current
inrush is an important problem which can arise in the use of electrical
devices containing PTC elements, especially heaters. Such devices are
usually used in a way such that the YTC element is initially at room temperature
when current first passes through it, but subsequently operates at an elevated
working temperature ~hereinafter referred to as the "operating temperature")
at which its resistance is substantially higher than at room tempera~ure.
As a result, when current is passed through the PTC element, the size of the
initial current in the circuit containing the PTC element can be very much
greater than it is at a later stage when the device is at its operating
temperature. If, as in the case of heaters, a substantial current is required
for effective operation at the oper~ing temperature, the size and duration of
the initial current can be such that the device itself or other components of
the circuit can be permanently damaged, unless precautions are taken to prevent
this initial current inrush. ~nother important problem which can arise in the
use of electrical devices containing PTC elements, especially heaters, is the
formation of "hot~lines" in the PTC element. As discussed in German Offen-
legungsschrift No. 2,543,314.1, and also in British P~tent Specification No.
1,521~460, if the preferred current path through a relatively thin PTC element
is transverse to the thickness of theelement then, as the temperature of the
element increases, there is a tendency for a part of the element, extending
across the thickness of the element, to~le heated much more rapidly than the
remainder, thus giving rise to a socalled "hot-line". The presence of a hot-
line seriously reduces the heat output of a PTC element, because relatively
little heat is generated outside the hot-line; in addition the presence of a
hot-line renders the heat output non-uniform and can damage the PTC element.
-- 3 --
3~
German Offenlegungsschrift No. 2,543,314.1 describes inter alia
electrical devices which comprise at least two electrodes, at least one
first electrically resistive layer and at least one second electrically
resistive layer; at least a part of the sur-face of the first layer being
contiguous with at lea.st a part of the surface of the second layer; the first
:layeriexhibiting a positive temperature coefficient of resistance and having
an anomaly temperature; the second layer having a substantially constant
resistance (as defined in said application and Offenlegungsschrift) below
the anomaly temperature of the first layer; and the electrodes and the resis-
tive layers being such that, at the higher of (a) the anomaly temperature of
the first layer, and (b) ~he temperature at which the resistance of the first
layer exceeds the resistance of the second layer, current flowing between the
electrodes predominantly follows the directionally shortest path through the
first layer. As described in detail in said application and Offenlegung-
sschrift, in such devices the formation of "hot-lines" is substantially avoided.
In addition, the said application and Offenlegungsschrift describe how the
problem of current inrush can be mitigated by appropriate choice of the posi-
tioning of the electrodes and the relative resistivities of the resistive layers
in such devices, the problem of current inrush can be substantially reduced.
While the invention described in said application and Offenlegungsschrift is
extremely valuable, the restrictions on the choices mean that it does not
provide a solution to the problem of current inrush which is satisfactory in
all cases.
The present invention provides a novel electrical device which
comprise at least two electrodes and at least one PTC element and which, when
used in applications in which current inrush can cause problems, can be
_ 4 _
operated (or inherently operate) in a way which substantially mitigates those
problems. It is to be noted that the problems associated with current
arise in applications in which the utility of the device depends not only on
the way in which the current passing through the device varies with temperature
but also on the current having a sufficiently high absolute value at operating
temperatures to produce a desired result, or example, in the case of a heating
device, an adequate generation of heat. The novel devices of the invention can
of course be used in such applications, but they can also be used in other
applications in which a lower current passes through the device at its operating
temperatures and in which the utility of the device depends primarily upon the
way in which the current passing through the device varies with temperature,
for example when the device is used for temperature sensing. The invention,
therefore, includes the use of the novel devices in such other applications
as well as in the applications in which current inrush causes problems. It is
also to be noted that although some of the novel devices, when used in applica-
tions in which current inrush can cause problems, inherently operate in a way
which reduces those problems, others of the novel devices must be operated
in particular ways if they are to reduce those problems. The invention includes
the use of such other devices in such applications even when they are not
operated in those particular ways, other means then preferably being used to
overcome the current inrush problems.
In one aspect, the present invention is based on the discovery that
in an elactrical device which comprises at least *wo electrodes which are
connectable to a source of electrical power, at least one PTC element and at
least one relatively CW element (as hereinafter defined), the problems assoc-
iated with current inrush can be substantially reduced by including in the
-G-
device at least one current-directiny element such that, when -the
electrodes are connected to a source oE electrical power while the
device is at a temperature (generally room temperature) below its
opera-ting temperature or subs-tan-tially immediately (as hereinafter
defined) after such connection, the current path between the
electrodes passes through at least one PTC element and at least one
relatively CW element, with th.e resistance of that current path
being greater than the resistance of the current pa-th which would
be adopted if the current-direc-ting element ~as replaced by an
element of the same shape (`a term used herein to include
dimensions~ but composed of the same composition as that relatively
CW element.
In accordance with this invention there is provided
an electrical device whi.ch comprises
(a~ at least two electrodes whi.ch are connectable to a
source of electrical power;
(b~ at least one PTC element which is composed o:E a
PTC composition havi.ng a useful Ts of to 280C;
(c~ at least one relatively constant wattage (RCW)
element which is in physical contact with sai.d PTC element but does
not surround said PTC element;
(d) at least one current-directing (CD) element which
is composed of a relatively insulating composition, and which lies
between the electrodes so that, at least when the electrodes are
first connected to a source of electrical power, the current path
passes through the PTC and CW elemen-ts and the resistance of that
73~3~3
--7--
current path is greater than the resistance of the current path
whi.ch would be adopted i:E the CD element was replaced by an
el.ement of the same dimensions as the CD element but of the same
composltion as the RCW element; each of the electrodes being in
physical contact with a PTC element or an RCW element throughout
its length.
The term "CD element" is used herein to denote such a
current-directing element. The presence of the CD element(s)
increases -the initial resistance (or the effective initial
resistance as explained hereinafter) of the device, but preferably
has comparatively little or no eEfect on the resistance of the
device at elevated ope.rating temperatures, and thus reduces the
ratio of the effective initial current to the current at elevated
operating temperatures. The initial resistance (.or the effective
initial resistance) of the device is preferably more than 50%,
especially more -th.an 80%, of the resistance of the device when it
is being used at elevated operating temperatures to supply
substantial thermal output, especially when it is being operated
at a temperature around the effective Ts of a PTC elemen-t therein.
~o The terms "relatively CW element" and "RCW element"
are used in this specificat;.on to denote an element ~hose resist-
ance is less than the resistance of the PTC element or elements
over at least a part o~ the temperature range in which the device
can be operated, or, if the.re is more than one RCW element, each
element of a combination of elements whose combined resistance is
less than the combined resistance of the PTC element or elements
over at least a part of the temperature range in which the device
3~
-7a-
can be operated.
~ s will be further elucida-ted belo~, current can flow
between the
~73~
electrodes of a device constructed according to the invention along a
plurality of different paths, but will predominantly flow along the path
or paths of least electrical resistance. It is, therefore, to be understood
that reEerences in this specification to the current path (and similar terms)
mean the preferred current path oE least electrical resistance (i.e., that
carrying the greatest current 'flux'). The resistivity of any segment of the
PTC element o-~ elements (and in many cases, the resistivity of any segment
of the RC~ element or elements and, in some cases, the resistivity of any
segment of the CD element or at least one of the CD elements) is dependent on
the temperature of that segment. In consequence, the preferred current path
between the electrodes, the total resistance between the electrodes and the
individual contributions to that total resistance from the PTC element or
elements and the RCW element or elementsl will generally all be influenced by
the absolute and relative values of the temperature in the different parts of
the device; furthermore, all of them will generally bc changing from the time
that the electrodes are first connected to a source of electrical power to
the time an equilibrium temperature has been reached.
In the devices according to the invention, the CD element may be
composed of a relatively insulating composition, i.e., a composition which has
a resistivity sufficiently high to ensure that, if the ~or each) CD element
is composed of such a composition then, as soon as the electrodes are connected
to a source of electrical power, the CD element will cause the current to take
a path which passes through at least one PTC element and at least one RCW
element and whose resistance is greater than the resistance of the current path
which would be adopted if the CD element was replaced by an element of the same
shape but composed of the same composition as the RCW element. Su~h a composi-
~t73~
tion is referred to herein as an "RI composition".
Alternatively the CD element can be composed of a composit~on which
can be converted into a rclatively insulating composition by passing electric
cllrrent therethrough. Such a composition is referred to herein as a "poten-
tiaLly relatively insulating composition" or a "PRI composition". In this
case it is essential that the initial current path between the electrodes
should pass through the CD element and substantially immediately create
therein a relatively insulating zone ~which may be part or all of the element),
such that the subsequent current path between the electrodes passes through
at least one PTC element and at least one RCW element, with the resistance of
that current path being greater than the resistance of the current path which
would be adopted if the RI zone was replaced by a zone of the same shape but of
the same composition as th~t RCW element.
The term "substantially immediately" is used herein to mean that the
defined current path is established sufficiently rapidly that the duration
of the initial current surge is insufficient to damage any o-f the components
of the circuit, for example generally less than 5 seconds, preferably less than
2 se~onds, especially less than 1 second. For example, when the CD element
is composed of a PRI composition, providing the RI zone is created sufficiently
~0 rapidIy, the effective initial resistance of the device will be its resistance
after the RI zone has been created, and although there may be a very high
initial current while the RI zone is being created, that highinitial current
will be so transient th~t it will not have an adverse effect. It should be
noted that the term "subsequent current path" is used herein merely to mean
the current path for an appreciable period after the RI zone has been created,
since there are embodiments of the invention in which the CD element, at some
73~
later stage after the electrodes have been connected to a source of
electrical power, ceases to direct current in the way initially required.
Particularly important PRI compositions are PTC compositions. When
the CD element is composed of a PTC composition, it can be an integral part
of the PTC element, the clevice being so construct~d thatthere is a highly
favoured current path through that part of the PTC element when the electrodes
are first connected to a source of electrical power. Alternatively the CD
element can be a separate component which is of a PTC composition which is the
the same as or different from the PTC composition in the PTC element. Since
the speed with which an RI zone will be created in such a CD element is
dependent inter alia on the thermal mass of the element and the rate at which
heat is removed from it, it is generally desirable that when the CD element is
a separate component it should be re~atively thin and thermally insulated.
The electrical devices of the invention can contain two or more CD
elements, for example one or more elements of an RI composition a.nd another
of a PRI composition; in this case, the element or elements composed of an RI
composition does or do not necessarily direct the current as soon as the device
is connected to a source of electrical power, but must do so as soon as the
relatively insulating zone has been created in the CD element of a PRI composi-
tion.
The CD element must be of a composition which initially is, or at
least part of which substantially immediately becomes, a relatively insulating
composition. However, it is to be understood that at some later stage in the
operation of the device, after the CD element itself and the other parts of
the device have been
- 10 -
~73~
heated by passage of current therethrough, the resistivity of the CD element
may be the same as or lower than the resistivity of other parts of the device.
The PTC elements of the devices of the present invention may be of
any PTC composition. IloweverJ for many uses ceramic PTC compositions, e.g.
doped barium titanateJ are undesirably rigid. It isJ thereforeJ preferred to
use a conductive polymer compositionJ i.e. a dispersion of at lea~t one
finely divided conductive fillerJ preferably carbon blackJ in a polymer or
mixture of polymers, for example as described in the patents and patent
applications referred to above. The PTC elements will generally have a
resistivity at 10C of 1 to 2J500 ohm.cm, preferably 2 to 1000 ohm.cm, with
resistivities at the lower end of this range, e.g. 1 to 250 ohm.cm, preferably
5 to 50 ohm.cm, being preferred for devices for use ~ith electrical supplies
of low voltage e.g. DC of 12 to 36 volts, and higher resistivities, usually at
least 80 ohm.cm, e.g. 80 to 500 ohm.cm, being preferred for devices of use
at higher voltages, e.g. AC of 110, 2~0 or ~80 volts. The time taken to
establish the defined current path will usually be shorter, the higher the
voltage. The PTC composition preferably has a useful Ts within the range of
from 0 to 280C~ particularly between 35 and 160C. It is also preferable
that the PTC composition have an R30 value of at least 6.
The RCW elements used in the present invention are preferably also
conductive polymer compositions. The resistivity of the RCW element(s) at 20C
may be greater or less than that of the PTC element~s3 in the same device,
generally in the range 0.1 to 1000 ohm.cm, typically l to 250 ohm.cm. The RCW
composition may exhibit PTC behaviour but, if it does so, it should preferably
not have a critical range below any critical range of the PTC element. It is
often useful to employ a PTC element having a first useful Ts in conjunction
with an RCW element having a second useful T which is higher, preferably at
least 25C higher, than *he first useful Ts.
The CD elements used in the present invention can be composed of
nny RI or PRI composition. Suitable RI compositions include for example air
alld other fluids, and compositions comprising a natural or synthetic organic
polymer. Typically the RI composition will have a resistivity at room
temperature whichis at least 5 times, preferably at least 10 times, the
resistivity at room temperature of any of the other conductive elements in
the device. The resistivity can of course be much higher, e.g. at least 2,500
ohm.cm, but the invention also contemplates the use of RI compositions whose
resistivity at the elevated operating temperature of the device is comparable
to, or lower than, the resistivity of at least one of the other elements, so
that at such operating tempera*ure current can flow through the CD element.
As noted above, particularly important PRI compositions are PTC compositions,
and the device can be so constructed that there is highly favoured initial
current path through a part of the PTC element, so that that part of the PTC
element provides a CD element, or a part thereof. When a CD element is
provided in a PTC element by placing a round electrode adjacent to the PTC
element so that there is a limited area of contact between the electrode and
the PTC element, the device will normally also include at least one other CD
element which is composed of an RI composition, and which is adjacent to the
limited contact area, since the requirement for a highly favoured initial
current path will normally mean that the RI zone created in the PTC element is
relatively small and will not, in itself, redirect the initial current to a
sufficient extent to cause a useful reduction in current inrush. A convenient
way of creating a highly favoured current path is for the PTC element to
- 12 -
~173(~
contact two electrodes of opposite polarity, with the contact area with one
oE the electrodes being limited, for example to less than 20% of the ~otal
swrface area of the electrode, or alternatively with the PTC element
having a thin section at some point between the electrodes.
~ len the CD element is composed of a PTC composition and is not an
integral part oE a PTC element, the PTC composition of the CD element
generally has a useful Ts which is below, preferably at least 25C below,
the useful Ts f the PTC element.
The presence of a CD element in the devices of the invention will
normally (but not necessarily) cause the current to take a geometrically
longer path through the RC~I element. However, it is only necessary that
the resistance of the current path adopted by reason of the presence of CD
element~s) should be greater than it would be in the absence thereof.
The electrodes used in the present invention may have any suitable
configuration and be composed of any suitable material. For most purposes,
and especially when the electrode is long, compared with its other dimensions
and with the electrode spacing, it is preferable to use electrodes of copper,
aluminum or another metal having a suitably low resistivity. ~or example
the electrode may be a solid or stranded wire, e~g. a tin-coated copper wire,
or a solid or perforated metal tape or plate, or a woven wire mesh. However,
for some devices, satisfactory electrodes can be composed of other materials,
e.g. conductive polymers, having a suitably low resisti-vity, preferably a
resistivity which between 20C and the operating temperature of the device,
e.g. 150 C, is not more than 0.1 times the resistivity of any other element of
the device. The term "electrode" is used herein to include electrodes as
described above which have a coating thereon of a ~or another) conductive poly-
- 13 -
73(~
mer composition having a resistivity hhich is higher than that of the metal
(or other) core.
The devices of the invention can be of any configuration which will
Eulfil the requirements set ou~ above. Preferably, the elements and electrodes
are so arranged that, when the device is connected to a source of electrical
p~wer and heat is being removed therefrom at substantially the same rate as it
is being generated by the passage of current through the device, the formation
of hotlines is suhstantially avoided. With this object in view, the devices
preferably comprise at least one PTC element which isat least in part in the
form of a layer, and preferably also at least one RCW element which is at
least in part in the form of a layer, the surfaces of the layers being at
least partially contiguous. Advantageously at least 50%, preferably at least
75%, of the surface of at least one of the electrodes is in contact with a PTC
element, with 100% being particularly preferred, not only for electrical
characteristics but also for ease of manufacture; in such devices at least
part of the PTC element has a generally annular cross-section when it surrounds
a round electrode, and such a cross-section is included in the term "layer"
used above.
It is been found tha~ the devices of the invention are of particular
value when they are in the form of elonga~e devices for use as heaters or
temperature-sensing devices. The devices will normally have an outer layer of
insulating material.
Several forms of device constructed in accordance with the invention
will now be described in greater detail, by way of example, with reference to
the accompanying drawings, in which each of Figures 1, 2 and 4 to 8 show cross-
sections through elongate devices of the invention which have substantially
constant cross-section throughout their length. Figure 3 shows a similar
- 14 -
3~
device which is not,however, included within the claims oE this application
because the PTC element does not surround or physically contact eithero~ the
electrodes. In the Figures, electrodes aredenoted by numerals 1,2 and 3, -the
electrodes being round stranded wire electrodes [e.g. 26 AWG (dlameter 0.01875
inch, 0.0~8 cm) tin-coated copper wire comprising 19 strands] in Figures 1 to
~ and 6, and strip electrodes [e.g. of tin-coated copper 3 x 250 mil ~0.008
x 0.6 cm)] in Figures 5 and 7; PTC elements are denoted by numerals 5 and 6
when they and the electrodes are so arranged that a part of the element
provides a CD e]ement or part of a CD element; and by numerals 8 and 9 when
this is not the case; separate CD elements are denoted by numerals 15 and 16;
insulating coatings are denoted by numeral 25; sources of electrical power,
e.g. batteries, are denote~ by numeral 30; and a switch is denoted by numeral 31.
Referring now to Figure 1, which shows a device which is particularly
useful as a heater, an electrode 2 makes line contact with a PTC element 5,
and CD elements 15 and 16 are composed of air. When the electrodes 1 and 2
are connected to a source of electrical power, the initial current flow is
directly between the electrodes through PTC element 5, but the heating effect
of this current substantially immediately creates an RI zone in the PTC element
and shuts off this current path (the RI zone thus created also acts as a CD
element as described earlier at ~age 12). The current then flows between the
electrodes through RCW element 10 and PTC element 5, the predominant current
path first being the geometrically shortest one available through the elements
5 and 10, and gradually becoming longer as the PTC element is selectively
heated by resistance heating, until at equilibrium substantially all the PTC
element through which current is passing is at a temperature approaching the Ts
of the element. In this equilibrium state, which may be reached, for example,
- 15 -
7~
in 30 to 100 times the time taken to create the RI zone in the PTC element,
some of the current will now pass directly between the electrodes through PTC
element 5, since the zone which was initially relatively insulating now has a
resistivity which is comparable to the resistivity of other parts of the PTC
e]ement 5. It will be seen that if the RCW element 10 extended into the voids
15 and 16~ which are CD elements, this would reduce the length of the initial
current path through the RCW element~ If the RCW element filled the voids 15
and 16, there would no longer be such a highly favoured initial current path
through the PTC element so that creation of the RI zone would take substantially
longer.
Using a device as shown in Figure 1 in which the electrodes are 26
gauge wires and both the RCW and PTC layers are about 10 mil ~0.25 mm) thick
and have a room temperature resistivity o about 5 ohm.cm3 with a l~ volt power
supply, an RI zone will be created in the section of the device closest to the
power supply in a very short time, e.g., of the order of 5 milliseconds, but
the longer the device the longer will be the time taken to create an RI zone
throughout the length of the heater. For example a time of about 5 seconds
might be needed for a 10 foot (about 3 metres) length.
Figure 2 illustrates a heater similar to that shown in Figure 1,
except that electrode 1 is separated from PTC element 8 by solid CD element 15
which also replaces voids 15 and 16 of Figure 1 and which is composed of an RI
or PRI composition. When CD element 15 is composed of a PTC material having a
Ts below the Ts f PTC element, the device operates in substantially the same
way as the device of Figure 1, the CD element being heated substantially
immediately to a temperature at which it directs the current through the RCW
layer. The lower the Ts f the CD element, and the higher its resistivity, the
- 16 -
3~
the shorter will be the time needed to create an RI zone therein.
When the whole length of CD element 15 composed of PTC material is
at a temperature such that it contains an RI zone, and the current path is
tilro1lgll the RCW layer, the resistance of the device can be substantially higher,
~.g. I)y a Eactor of 2 or more, than it is when thecurrent can (at any point
alollg the device) pass directly between the electrodes through PTC element 8
and CD element 15. 7'he device is, therefore, very useful as a temperature
sensor. One way of using the device in this way is to pass a low current~
insufficient to cause substantial resistive heating, through the device and to
monitor the current; a sharp decrease in the current indicates that the whole
length of the device has reached a particular temperature. Thus the
device can be distributed in serpentine fashion throughout a liquid or solid
body to be heated, and used to indicate when the whole of the body has reached
a particular temperature. The body can be heated externally or internally
by a separate heater. Alternatively the device itself can first be used as a
heater using a relatively high current, and then, after switching o-Ef the
relatively high current and allowing the device to reach thermal equilibrium
with the body, the device can be used as a temperature sensor as described above.
Figure 3 shows a laminated heater having planar CW, CD and PTC
elements, the CD elements being composed of PTC material. Initial current flow
is diagonally across the device, but RI zones are created substantially
immediately in CD elements 15 and 16, causing the current to flow in serpentine
fashionJ through significant portions of the width of CW elements 10 and 11 and
PTC element 8.
Figure ~ shows a heater having a PTC element 5 which joins the two
electrodes but has a thin central section flanked by CD elements 15 and 16
a~
which may be composed of an RI composition ~for example a ~oamed RI composition
to provide thermal insulation of the thin section of the PTC element) or may
be composed of a ~RI composition, preferably a PTC composition having a useful
Ts below the useful T of the PTC element. Initial current flow is through
the thin section of the PTC element, creating a RI zone therein, and the, if
CD elements 15 and 16 are PRI elements, through them until RI zones are
created therein. Subsequent current flow is through the parts of the PTC
element which surround the electrodes and through RCW element 10.
Figure 5 shows another laminated heater. Initial current flow is
across the upper section of PTC element 5 which lies between the two planar
electrodes 1 and 2, but this substantially immediately creates an RI zone in
this section of the PTC element, and subsequent current flow is through RCW
element 10 and the lower section of PTC element 5, and the edge portions of
both elements.
Figure 6 shows a device which is useful as a heater and as a
temperature sensor, The device has electrodes 1, 2 and 3 (which may be as
described in Figure 1), elactrodes 1 and 2 being surrounded by PTC elements
5 and 6, with which electrode 3 makes line contacts. PTC elements 5 and 6 can
be the same or different, but element 6 preferably has a lower useful Ts than
element 5. RCW element lO surrounds and contacts electrode 3 and PTC elements
5 and 6, leaving voids 15 and 16 adjacent electrode 3 which are CD elements.
In one method of using this device, electrodes 1 and 3 are connected
to a suitable source of electrical power and the device used as a hea~er in
substantially the way described in Figure 1, electrode 2, having essentially no
active role at this stage. If the heating current is then turned off, and the
device allowed to reach thermal equilibrium, electrode 2 can be used as a
temperature sensor by connecting electrodes 2 and 3 to another source of
-~ electrical power, in substantially the same way as described in Figure 2.
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In another method of using this device, electrodes 1 and 2 are
connected to one pole of a suitable source of electrical power, and electrode
3 is connected to the opposite pole, and the device used as a heater. When
the PTC elements 5 and 6 are identical, the device operates as two heaters
in parallel, each heater operating substantially as described with reference
to l:igure 1. If, however, PTC element 6 has a lower useful T than element
5, the device operates in this way or an initial period, but as the tempera-
ture increases and element 6 approaches and exceeds its T , the thermal output
of the heater drops~ This type of behaviour is useful when a reduction in the
thermal output of the heater over a particular te~perature range is desired.
Figure 7 shows a device which is useful as a heater and as a
temperature sensor. Except that it is a laminar article, it is similar to the
device shown in Figure 6, and can be used in the same ways. In Figure 7 the
device is shown connected to battery 30 for use as a heater.
Figure 8 illustrates a device useful as a heater. In this embodiment
of th0 invention, conductors l and 2 are solid conductors, for example 20 AWG
(0.032 in, 0.8 mm, diameter), which are each surrounded by an annular layer
5,6, of PTC material, e.g., a conductive polymeric composition of thickness
0.02" (about 5.0 mm). Between the two layers 5,6 is positioned a rod 33, of I-
shaped cross-section, of insulating material, e.g., of overall cross-section
0.070 x 0.10 inches ~about 1.8 x ~.5 mm~. The assembly of PTC layers 5 and 6
and the rod 33 is surrounded by a RCW layer 10, which is in turn surrounded by
an insulating layer 25, these layers 10 and 25 being, e.g., of thickness about
0.01 inches, or about 2.5 mm.
The devices illustrated in Figures 1, 2, ~, 5, 6, 7 and 8 are examples
of a preferred class of devices according to the invention, namely those having
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~3730~
a subst~ntially constant cross-section and comprising
(a) at least two elec~rodes which are connectable to a source of
electrical power;
~b) at least one PTC element which is composed of a PTC composition
having a useful Ts of 0 to 280C and which surrounds and
physically contacts substantially the whole of the surface of one
of said electrodes;
(s) at least one relatively constant wattage (RCW) element which
surrounds said electrodes and PTC elements and which makes
physical contact with the or each said PTC element; and
(d) at least one current-directing (CD) element;
said electrodes and said PTC, RCW and CD elements being so arranged that, when
the electrodes are connected to a source of electrical power while the device
is below its operating tempera*ure or substantially immediately after such
connection, the current path passes through at least one PTC element and at
least one RCW element, with the resistance of that current path being greater
than the resistance of the current path which would be adopted if the CD
element was replaced by an element of the same shape but composed of the same
composition as that RCW element~
The devices illustrated in Figures 1 and 6 are examples of a preferred
sub-class of the class defined above, namely those which comprise
(a) at least two round electrodes which are connectable to a source
of electrical power;
(b) at least one PTC element which is composed of a PTC composi~ion
having a useful Ts of 0 to 280C) which surrounds and phys~ally
contasts substantially the whole of the surface of one of said
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73~3~
electrodes, and which makes contact with another of said
electrodes over a limited contac~ area;
(c) a relatively constant wattage ~RCW~ element which surrounds
said electrodes and PTC elements and which makes physical
contact wi.th the or each said PTC element and with at least
one of said electrodes; and
(d) current-directing ~CD) elements composed of a relatively
insulating ~RI) composition and adjacent said limited contact
area.
In these devices, preferably at least 30% of the surface area of the (or each)
said PTC ele~ent is contacted by said RCW element, preferably at least 50%
when the device contains only two electrodes. It is also preferred that the
ratio of the area of the (or each) said PTC element contacted by said CD
element to the area contacted by said CW elemen~s is 0.05:1 to 1.5:1, espec-
ially 0.1:1 to 1.2:1, particularly 0.2:1 to 1:1. In order that these devices
can be operated with maximum efficiency as heaters it is desirable that the
ratio of the external surface area of the CW element to the volume occupied
by and enclosed by the CW element should be high, preferably at least ~:1,
especially at least 20:1, e.g. about 50:1, but generally not more than about
80:1.
The device illustrated in ~igure 2, is an example of a preferred
sub-class of the class defined above, namely those which comprise at least one
CD element having a Ts which is below, preferably at least 25C below, the
useful Ts f any PTC composition, and which device comprises a :Eirst electrode,
a PTC element which surrounds said first electrode and the whole of ~hose
external surface is in contact with said RCW element and said CD element~ and a
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~73(~
second electrode the whole of whose surface is in contact with said RCW
element and said CD element.
The device ;.llustrated in Figure 3 is an example of a second class
oE cle~vices according to the invention, namely those which comprise
(1) a :Eirst generally planar RCW layer ~ving a first electrode in
contact with a portion thereof;
(2) a first generally planar CD layer composed of a PTC composition
and having a Eirst face and a second face;
(3) a generally planar PTC layer having a first face and a second
face;
(~) a generally planar second CD layer composed of a PTC composition
and having a first face and a second face; and
(5) a second generally planar RCW layer having a second electrode
in contact with a portion thereof;
one face of said first RCW layer being partly in contact with the first ace
of said first CD layer and partly in contact with a part of the first face
of said PTC layer; the second face of said PTC layer being partly in contact
with a part of one face of said second RCW layer and partly in contact with the
first face of said second CD layer; and the second face of said second CD layer
being in contac~ with another part of the face of the second RCW layer.
The devices illustrated in Figures 5 and 7 are examples of a third
class of devices according to the inventionJ namely those which comprise
(a) a first planar electrode;
(b) a PTC element which surrounds and physically contacts said
first planar electrode;
(c) a second planar elect~ode which physically contacts said PTC
element whereby the portion of said PTC element which is
sandwiched between the electrodes is a CD element; and
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~373~
(d) a CW element which contacts said second planar electrode and
said PTC element.
In each of the devices constructed, according to the i~vention,
the RCW element advantageously has a resistivity that does not increase by more
than a factor of 6 in any 30C segment below the T of the PTC element.
For a heater constructed according to Figure 8, the PTC layer advan-
tageously has a room temperature resistivity of 3 to 150 ohm.cm, preferably
4 to 15 ohm.cm, while the CW layer advantageously has a room temperature
resistivity of from 2 to 20, preferably 6 to 159 ohm.cm. The PTC layer is
advantageously composed of about 45% medium density polyethylene, about 10%
ethylene/propylene diene rubber, about 44% furnace black and about 1% anti-
oxidant, the percentages being by weight, with a resistivity of from 6 to 10
ohm.cm. at room tempera~ure, with a T at about 112C. The CW layer is advan-
tageously a blend of about 82% ethylene/ethyl acrylate copolymer, with 18%
ethyl acrylate, about 17% carbon black and aboutl% antloxidant, with a resis-
tivity of about 11 ohm.cm. at ~oom temperature. The I-shaped insulating
barrier and the jacket are advantageously highdensity polyethylene. The device
is advantageously cross-linked by irradiation, e.g., to a dose of 10 megarads.
