Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 94119815 215 S 9 61 PCT/GB94100315
IMPRuv~ r-lS RE~ .TING l~O EI~ECq~IC SW1'.
Field of the Invention:
This invention concerns i~L ovements relating to
electric switches and more particularly concerns
thermally responsive electrical switches employing
bimetallic elements as thermal actuators.
Bac~round of the Invention:
Many kinds of electrical switches employing
bimetallic actuators are known and likewise many
different forms of bimetallic switch actuators are
known. Early bimetallic switches simply employed a
plain bimetal blade which moved relatively slowly in
response to temperature changes and gave rise to arcing
problems in the switch, and the development of the snap-
acting bimetallic actuator, constructed as a ~; ch~
bimetallic element capable of movinq between oppositely
curved configurations with a snap-action, provided a
major advance in the art.
Various forms of snap-acting bimetallic actuators
are known, such as those disclosed in GB 600055, GB
657434, GB 1064643, GB 15422S2 and GB 2124429 for
example, and various forms of electric switches
employing such bimetallic actuators are likewise known;
WO94/19815 ~ 3~1 PCTIGB94/00315
GB 2124~29 abovementioned for example discloses the
utilization of a pear-shaped snap-acting bimetallic
actuator in a current-sensitive switch where the heating
of the bimetal by electric current flow therethrough is
designed to trip the switch in a current overload
situation.
It is also known to provide heater elements in
bimetallic switches, either in series with or in
parallel with the switch contacts. With a heater
element provided in series with the switch contacts, it
is possible to adjust the switrh; ~g characteristics of
the switch to acco~ms~te special requirements such as
those of motor protection switches for small electric
motors for example. In GB 2 133 931 there is described
a bimetallic switch having terminal parts formed of a
relatively high resistance material, such as nickel
chromium alloy or stainless steel for example rather
than the more commonly used brass, so that heat is
generated in normal operation of the s~itch and affects
the switch characteristics; the response of the switch
to a high overload current is not significantly affected
by the heat generated in the terminal parts since the
bimetal response to the high current will be more rapid
than the time taken for the heat generated in the
terminal parts to transfer to the bimetal, but the
responsiveness of the switch to a current barely at an
WO94/1981S 215 5 9 ~1 PCTIGB94/00315
overload level will be enhanced. With a heater element
connected in parallel with the switch contacts, the
heater passes a current dependent on the relati~e
resist~ces of the heater and of the bimetal so long as
the switch contacts are closed, and the heater current
in this situation may or may not be negligible, but once
the bimetal operates the heater passes the full load
current and generates heat which is transferred to the
bimetal so as to delay its reset period or prevent it
from resetting altogether.
As well as utilizing switch parts formed of
relati~ely high resistance materials, as in GB 2 133 931
abovementioned, it is also known to form switch heaters
as thin film resistors, from con~lctive inks and as
ceramic PTC materials (that is to say ceramic materials
having a positive temperature coefficient of
resistance). PTC material heaters have the advantage
that, when connected in parallel with switch contacts,
the self heating caused by through current in the PTC
material when the switch contacts open not only provides
heat to the bimetallic element of the switch but also
increases the resistance of the PTC material thereby
effectively reducing the current supplied to the switch
load. Examples of bimetallic swit~hes incorporating PTC
material heaters are described in GB 2 252 674.
PTC material resistances have also been used by
WO94/1g815 21 ~ ~ 9 ~1 PCTtGB94/00315
themselves as overload protection devices for electric
motors and GB l 604 lll, for example, discloses the use
of a PTC material resistance connected in series with
the motor w;~ gs and operative to reduce the current
through the motor to a safe level in the event that a
motor overload causes the resistance of the PTC device
to increase significantly. Such devices are also useful
as thermal relays in starting circuits for electric
motors and GB 2 015 823, for example, discloses the
provision of a PTC resistor in series with start
windings of an electric motor, the PTC resistor
permitting a high initial current to flow for starting
the motor and thereafter increasing its resistance so as
to correspondingly reduce the current in the start
lS w;n~ . Plastics materials having PTC characteristics
are also known and in Wo 9l/07804, for example, such a
PTC thermistor is connected in series with the armature
win~ings of a small DC motor and serves a motor
protection function.
From the foregoing it will be seen that the art is
replete with proposals for bimetallic switches,
bimetallic switches incorporating heating elements and
including PTC material heating elements, and PTC
material protective devices. In motor protection
applications, particularly for the protection of small
DC motors such as for example automotive window lift
WO94/19815 PCT/GB94/00315
21~961
motors which are becoming ever smaller in size and more
powerful, the use of simple bimetallic switches gives
rise to problems in that the cycling of the switch as it
intermittently heats and cools can cause jarring of
gears in sensitive gear trains, sets lifetime limits on
the switch, and must be carefully co"L-olled to ensure
that a stalled motor is held at a protected and limited
temperature. Furthermore, in some situations the
internal resistance of a stalled motor increases rapidly
thereby producing a steep decline in through current
which exacerbates the difficulty of protecting the motor
with a current sensitive bimetallic switch and places
rigorous requirements on the design and production of
the switch. The incorporation of PTC resistor elements
into bimetallic switches has, in some measure, overcome
these problems in that the combination of bimetal
characteristics and PTC device characteristics provides
sufficient additional switch variables to provide for
additional design variations to enable particular
application requirements to be met, but at a cost of
increased switch complexity. PTC devices by th~mc~lves
have the advantage of simplicity, but have to be
individually designed to suit specific applications, are
not particularly time stable, and display operating
characteristics that are both temperature and current
dependent.
wo94ll98ls PCT/GB94/00315
21S5961 6
Obiects and Summa~ of the Invention:
The object of the present invention is to provide
a thermally responsive bimetallic switch capable of
overcoming or at least substantially reducing the
abovementioned problems, the switch additionally
preferably being of inexpensive and uncomplicated
construction.
According to one aspect of the present invention
there is provided a bimetallic switch enclosed within a
moulded plastics switch body and the moulded plastics
switch body is for~ed of a polymeric PTC material.
According to a more particular aspect of the
invention there is provided a switch comprising a
mo~ plastics body portion capturing therein first
and second terminal conductors, and a snap-acting
bimetallic actuator secured to one of said conductors
and carrying a contact which constitutes the moving
contact of the switch, such contact being arranged for
co-operation in switching operations with the other of
the two conductors, and the moulded plastics body
portion being formed of a polymeric PTC material.
Any suitable polymeric PTC material can be utilized
in the practice of the invention, but it is preferred to
utilize a polyolefin material, e.g. polypropylene,
incorporating one or more conductive fillers and
preferably also incorporating a non-conductive filler
wos4/1s81s PCT/GB94/00315
~15~
such as a fibre to provide stability and
reproduceability. Carbon black is the preferred
cnn~nrtive filler and we prefer to utilize a relatively
low proportion (0 to 5%) of a high ~o~-7ctivity carbon
black (such as Ketjenblac~ EC 600 from Akzo) and a
relatively high ~o~ortion (0 to 30%) of a car~on black
such as to make a substantial contribution to the PTC
effect (e.g. Elflex 120 from Cabot Corporation). The
non-conductive filler may for example be from 0 to 40%
glass fibre. The percentages quoted are by weight.
The bimetallic actuator of the switch is
advantageously of such low thic~ness as to be responsive
to through current as low as 2 amps or less, for example
a bimetallic element of the order of 0.076mm (0.003
inch) thickness, and the switch body part preferably
provides physical ~u~oLL for the bimetallic element
during switching operations.
In an exemplary embo~im~nt which will be described
in detail hereinafter the bimetallic actuator is of a
kind having a generally U-shaped cut-out defining a
tongue between spaced apart leg portions which are
bridged adjacent the free end of the tongue. The
terminAl conductors are formed as simple wires and the
tongue of the bimetal is secured to one of the terminal
conductors, for example by welding, and the bridging
portion carries the contact which co-operates with the
WO94/19815 PCTIGB94/00315
other conductor. No discrete contact is provided on the
other co~ ctor which however comprises a silver or
silver alloy coating, for example a silver antimony
coating as described in Wo 92/14282. The moulded
plastics PTC material body portion of the switch defines
an enclosure for the bimetallic actua~or and, as will
hereinafter be described, also incorporates portions
which provide structural ~ o~L for the bimetal. The
resultant switch is of simple and easily manufactured
construction which enables small size to be achieved for
~nh~nr~ sensitivity to very low overload currents.
Furthermore, by variation of the characteristics of the
PTC material the switch can readily be customised to
suit partic~lar applications.
To further enhance the current sensitivity of the
switch, a series-connected heating element could, if
desired, be provided for injecting heat into the
bimetallic actuator when the switch is in closed
condition, and in a particularly convenient arrangement
such a series heating element may be constituted by a
portion of one or the other or both of the two ter~;~l
conductors of the switch which is formed as a resistance
heating element. Such an arrangement could be arranged
to obtain a more rapid switch response to a current
overload situation than would be obtained if the series
heating element were not provided.
WO94/19815 21~ ~ 9 6 1 PCT/GB94/00315
The above and further features of the invention are
set forth with particularity in the appended claims and
will best be appreciated from consideration of the
following detailed description of an exemplary
S emho~ t given with reference to the accompanying
drawings.
~rief Descri~tion of the Drawinqs:
Figure l is a sectional side elevation view of the
subject switch on the line I...I in Figure 2;
Figure 2 is a plan view showing the switch of
Figure l with its top cover removed; and
Figure 3 shows an alternati~e con~-~ctor
construction which could if desired be employed in the
switch of Figure l so as to provide a series co~ne~ted
heater within the switch.
Detailed Descri~tion of the Embodiment:
The switch hereinafter described is in many
respects identical to the switch described in W0
92/20086, but differs principally therefrom in that the
moulded plastics body portion l of the switch is formed
of a polymeric PTC material. The ac~o~p~ying drawings
are identical to those in WO 92/20086.
The views in the drawings show the switch to an
enlarged scale and the dimensions indicated are the
actual dimensions of the switch in millimetres. The
moulded plastics body portion l of the switch is thus
WOg4/19815 PCT/GB94/00315
~1~5-~6~
generally rectangular with ~ n-~ions of 10.5mm x 6.0mm
x 2.7mm and the terminal conductors 2, 3 project r
outwardly by a further 7.Omm. A top cover for the
switch has a thic~ness of o.5mm. The switch thus has
such small overall size that it may conveniently be
supplied in a bandolier suitable for use by automatic
component insertion equipment.
Any suitable plastics material exhibiting PTC
characteristics could be utilized in the practice of the
invention for forming the body portion 1, but it is
preferred to utilize a polyolefin material, e.g.
polypropylene, incorporating one or more ~o~ ctive
fillers and preferably also incorporating a non-
con~rtive filler such as a fibre to provide stability
and repro~lce~hility. Carbon black is the preferred
conductive filler and we prefer to utilize a relatively
low proportion (O to 5%) of a high conductivity carbon
black (such as Ketjenblack EC 600 from Akzo) and a
relatively high proportion (O to 30%) of a carbon black
such as to make a substantial contribution to the PTC
effect (e.g. Elflex 120 from Cabot Corporation). The
non-conductive filler may for example be from O to 40%
glass fibre. The percentages quoted are by weight.
Within the body portion 1 of the switch there is
defined a chamber 4 which has dimensions of the order of
8.0mm x 5.0mm x 1.7mm, and an upstand 5 occupies part of
WO94/lg815 ~1~ 5 9 61 PCT/GB94/00315
this chamber. A simple copper wire conductor 2 has a
square or rectangular section and is moulded into the
body portion 1 at one end thereof with its forward part
received in a recess in the upper surface of the upstand
5, and a simple copper wire conductor 3 has a circular
section, though it too could have a square or
rectangular section, and is moulded into the opposite
end of the body portion 1 so as to be ~Yroo~ at 7
within the chamber 4. A bimetallic actuator 6 is welded
to the forward part of conductor 2. The portion 7 of
conductor 3 constitutes a switching contact of the
subject switch and accordingly conductor 3 is preferably
formed of silver plated copper wire or more preferably
silver-antimony plated copper wire as described in WO
92/14282; conductor 2 may be similarly formed though
this is not essential.
In accordance with the teachings of WO 92/14282
abovementioned, a preferred form of con~rtor wire
providing excellent electrical characteristics in
combination with superior wear characteristics comprises
a copper wire, or a wire formed from a copper alloy
having a thermal conductivity at least 90~ that of
copper, and more preferably 95% to 99% that of 99.95%
pure copper, provided with a thick plating layer of
silver and antimony, and the conductor wire 3 above-
described can advantageously have this construction. By
WOs4/19815 PCT/GB94/0031
~ i55~6~ 12
the use of a thic~ plating (e.g. at least 30 microns and
preferably 40 microns thickness) comprising fine silver
(99.9% purity) with a small amount of antimony,
typically about 1% and particularly between 0.3% and
S 0.7%, on a conductor formed of copper or a high thermal
CQ~ ti~ity copper alloy, the formation of silver
powder during switching operations is inhibited and a
life of about 70,000 switching cycles may be obt~; net~ ~
Bimetallic actuator 6 is of the Otter Controls type
comprising a ~;Ch~ blade of bimetallic material having
a generally U-shaped cut-out 8 which defines a tongue 9
between legs 10 which are bridged by a bridging portion
11. The moving contact of the switch is constituted by
a sil~er contact 12 welded to the underside of bridging
portion 11 as best shown in Figure 1. The actuator 6 is
secured to terminal conductor 2 by virtue of the tongue
9 being welded thereto. The shape of the bimetallic
blade is such as to ~nhAncp its responsiveness to
th~uu~l~ currents by increasing the current density in
the legs l0 and in the contact-carrying forward region
of the blade.
The upstand 5 provides support for the forward
portion of conductor 2 which in turn provides support
for tongue portion 9 of bimetallic actuator 6, whereas
the legs 10 and bridging portion ll of the bimetal 6 are
free to move within the chamber 4. By virtue of this
WO94/19815 215 ~ 9 61 PCT/GB94100315
arrangement, the temperature responsive characteristics
of the switch can better be predetermined since
switch;n~ operations are effected substantially
exclusively by flexure of the legs lO about the stable
position established for tongue 9 by virtue of its
~u~o~l on con~lctor 2. Furthermore, by supporting the
tongue 9 in this way, the risk of stress cracking at the
root of the tongue is reduced and the working stresses
in the bimetal are concentrated towards its elongate
legs 10.
The switch construction as thus described comprises
a simple bimetallic switch, constituted by the
conductors 2 and 3 and the bimetal 6, with a PTC heater,
constituted by the body portion 1 of the switch,
connected in parallel with the bimetallic switch. In
operation of the switch as thus described, contact 12
will move away from portion 7 of conductor 3, w1th a
snap-action, whenever the temperature of the bimetal 6
rises to a certain predeterm; n~ level, either as a
result of thermal conduction from the switch
environment, or as a result of heating of the bimetal by
current flow therethrough, or as a result of heating of
the bimetal by current flow through the PTC material or
as combination of any two or more of these three
effects. When the bimetal cools sufficiently the switch
will remake. The characteristics of the polymeric PTC
WO94/19815 PCT/GB94/00315
' 2 i 5 ~
material from which the switch body portion 1 is formed
do, however, have an effect upon the overall switch
characteristics and may be designed to prevent the
bimetal from cooling to such a temperature that it
resets so as to cause the switch contacts to remake, or
to inject such an amount of heat into the bimetal 6 in
its open-contacts condition as to modulate the on-off
time of the switch cycles to suit a particular
application, for example a motor protection application
where the cycling of the switch must not result in a
stalled motor being subjected to excessive temperatures.
The fact that the PTC characteristics of the switch body
portion 1 combine with the switching characteristics
inherent in the bimetal 6 provides for the ready design
lS of special swi~ches to suit particular applications.
Further design variables which can be manipulated in
order to adapt a switch according to the present
invention to a specific application can be ob~; n~ by
inclusion of a series-connected heater in the switch as
2~ will hereinafter be described.
The closure 15 may conveniently be moulded as an
integral part of the switch body which is hingedly
coupled thereto and is ultrasonically welded shut after
assembly of the bimetal 6 into the chamber 4 and spot
welding of the tongue 9 to the forward part of conductor
2. The closure lS may be formed so as to isolate the
~ ~la~9~1
WO94/19815 PCTIGB94/00315
chamber 4 from the environment of the switch, or may
alternatively be provided with one or more openings 16
as indicated.
The bimetallic material of the actuator 6 has a
thi~k~CC of only 0.076mm (0.003 inch) and had to be
specially manufact~red for us. By use of such a thin
bimetallic material in a switch construction where the
switch body provides physical support to the bimetal and
also, by virtue of being formed of polymeric PTC
material, contributes to the operational characteristics
of the switch, we have been able to obtain repeatable
switch action for low current switchin~ below 2 amps.
Furthermore, by virtue of the provision of the PTC
switch body in parallel with the bimetal switch
elements, the result is obt~ine~ that, in the event of
failure of the bimetallic switch in a contacts-open
condition, overload current protection is maint~; n~ by
the PTC material.
For certain switch applications the provision of a
series-connected heater may be desirable and the switch
described in the foregoing could readily be modified to
incorporate such a heater. Figure 3 shows an
alternative form of conductor which could be used in the
switch of Figures 1 and 2 in place of the conductor 2.
As shown in Figure 3, conductor 20 is formed of a
resistance heating material and has a forward portion 21
WO94/19815 ~ 9 ~ ~ PCT/GB94/00315
which is adapted to be received within the switch body
chamber 4 and is formed generally as a spiral
terminating in a pad 22 to which the tongue 9 of the
bimetal actuator 6 is spot welded. The ~im~n~ions shown
in Figure 3 are in millimetres, Figure 3 being an
enlarged showing of the conductor.
The described switch is well suited to automatic
manufacture and installation, comprises a m;nir~ of
parts and can be relatively inexpensive, and is capable
of miniaturisation for enhanced current sensitivity.
The switch is, however, but an example of what is
achievable by virtue of the invention and modifications
and ~ariations are possible without departure from the
spirit and scope of the invention. For example, the
bimetal could be pear-shaped as described in GB 2124429
aforementioned for ~h~nce~ current sensitivity, or
could take a variety of alternative shapes.
Furthermore, the wire term;n~lc 2 and 3 could be
replaced by appropriate sheet metal parts insert moulded
into the polymeric PTC material body portion 1 and
having terminal pads accessible in the undersurface of
the body portion for surface mounting of the switch to
a printed circuit board for example either by soldering
or, more preferably having regard to the desirability of
avoiding exposure of the switch to extremes of
temperature, by means of a mechanical spring fastening
WO 94/19815 21~ 5 9 6 ~ PCT/GB94/nO315
17
arrangement .
