Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BackcJround oE the Invention
Electrothermally operated switches, for example,
automotive signal and warning flashers~ may be required
to switch many amperes of electric current. The
electrical switching capacity is limited by the temperature
rise in the switch terminals due both to contact arcing and
~ to heat resistively genera-ted within the material of the
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terminals~
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~` Efficient manufacturing methods of electro~
thermally operated switche~ employ resistance welding of
electrical leads and -the like to the switch terminals.
Switch terminals have conventionally been made of low carbon
steel to enable such welding.
Summary of the Invention
It has now been discovered that the upper
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-- electrical load capacity limit for the entire swi~ch has ~
been provided by the heat generated in the high electrical ~; ;
resistivity of the low carbon steel of the switch terminals.
This heat was poorly carried away by the high termal
resistivi-ty of that material. However, substitution of
a material having low electrical and thermal resistivity,
such as copper or aluminum, would undesirably prevent
~ resistance welding to the terminal during fabrication. ~-
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Broadly speaki.ng, the present inventlon pro-
vides in an elcctrothermall~ operate~ swi.tch o:E the -type
havin~ a member which is held in a f.irst position when
a heating element .is unheated and which i.s moved to a
second position when the heating elem~nt is heated, and
in which the member has at least one electrical contac-t
a:Eixed to it which is caused to move. wi.th-the memher
,
to make ana break electrical connection with at least
. one ~ixed electrical contact, and the makiny and breaking
oE t.he electrical connection directly.or indirectly con-
trols the connection of electrical power to an external
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load, and in which. there i.s a switch terminal carrying
electrical power to the external.load when the electrical
contacts are engaged, the improvement comprising the
- terminal being of clad construction, and having a first :
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layer! whi.ch is of copper or aluminum, clad wlth a
- second layer, which.is of steel. :~
. ThNs the present invention teaches a switch
`. terminal of copper, or other low electrically and ther-
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- 20 mally resistive material, having-àt least one of its
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: faces clad with a weldable metal such. as low carbon steel.
; - The low electrical resistivity of the switch terminal
proportionately reduces the amount of heat electrically.
generated therein. For example., changing the material ~-
~: of the terminal from steel to substantially all copper
: reduces the material resistivity from 17 to 1.77 ohm-
centimeters at 2a degrees Celsius. At the same current,
.: this would reduce the heat generated by a factor of
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approximately nine. In addition, the same substitution
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of materials increases the -thermal conducti.vity of the
matexial Erom 0.~107 to 0.9 calories/second s~uare cm for
a ~rature gradient of 1 degree ~elsius per cen-timeter thickness.
T.he higher thermal conductivity oE the :
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material more rapidly carries off the hea~ generclted both
wi~lin the material and at the switch ~ontacts.
Brief Description_of the Drawings
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Fig. 1 shows a schematic diagram of one type of
electrothermally operated switch employing clad terminals oE
the present invention.
Fig, 2 shows a plan view of an electrothermally operated
switch wherein like numbers refer to like parts.
Fig. 3 illustrates a partial perspective view of the
switch shown in Fig. 2.
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Detaîle Des~ lption oE the Preferred Lmbod;ment
A 1ashe~, as used in this disclosure is deflned as
an electrical switch having a mem~er which is held in a
irst position by a pull elem~nt when the pull element
is unheated and which is moved to a s~cond position
when the pull element is heated.' The member has at
least one electrical contact af~ixed to ~ which is
caused to move with the ~ember to make arld break elec-
trical connection with at least one fi~ed electrica~ co~tact~
. The manner in which the switch terminal is used is
shown in Fig. 1. One type ~ flàsher~ shown at ~0 contains
a heater 12 which is in se~ies with a battery 14g a turn
signal or ~arning switch 16 and a load ~8. The load 18
can consist o~ parallel combinations of a plurality of
: ~ ~ electric lamps 20 to ground. The heater 12, when subjec~ed
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:~- to electric current through closed switch 16 heats and
allows the expansi~n of a mechanical pull wire, not showng
which allows a set of electric contacts 22 to close,
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`~ The electr~cal resistan~e of the heater L2 is very
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much greater than the load 18. Consequentlyg when the
2û elec~ic cont`ac.ts 22 are opened as ~howng the voltage drop
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~ : across the heater 12 ~s great enough to maintain the
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~ ~ . electric lamps 20 ~xtingulsh2d, When the elec~ric contacts
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22 are elosed, the ull battery voltage is connected be- ~
tween an input tenminal 24 and an output terminal 26 o the
flasher. The heater 12, ~eing bypassed by the closed
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electric contacts~ begins to cool. When the heater 12
has cooled suEficiently, the eontrac~ion o the pull wire,
not shown, causes the electrîc contacts 22 ~o open and
begin a new heating sequence. Durîng the period ~hat
the electric contacts 12 are closed, a high electrlc
current flows through input and output term~nals 24,
26. The heat generated in ~he material of the înput
and output terminals 24, 267 combined wi~h the problem
o~ removing the heat from the terminals, places a limit
on the number of lamps 20 which can be employed in~ tbe
load
.
The flasher 10 shown is only one o~ a large number
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of ~xam~es of electrothermal switches in which the input
and~or output terminals o~ t~e present invention may be . .
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~ employed. For example, the load current may flow through- .
;` . a pa~r of auxiliary switch terminals which conform to
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t~is disclosure ~hich are electro-mechani~ally actuated
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by the ~henn~l switc~ without dèparting from the spirit ~ `
and ~cope of theipreserlt invention. Electrotherma~ swltches
:~ 20 employing bimetallic, thermally deformable elements instead
of a mem~e~;moved by a pull wire are also included within
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;: . t~ scope of the present invention.
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. Referring now to Fig.~2, there is shown a plan view of
.. : an illustrative example of a flasher employing ~he present
invention In which 1ike numera1s are used on parts correspondin~
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to those describ~d ln connection wlth Fi~, l, The input
and output tenminals ar~ supported by being molded into a
plastic base ~8~ Th~ e~ternal porkion 30 of the inpu~
terminal and the e~ternal portion 37 of the output ter
minal are conveniently shaped identic:all~ to standard
. sealed beam terminals adapted to automotive use~ The
inner portion 34 of the input termincll 24 ls attached ~o
a ænap blade 36 o~ the type described in Unitad States
Patent 3,037,102. The heater 12 is wrapped about a pul~
~lO ribbon 38 wbich is af~îxed at its ends to the snap blade
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; 36. First end 40 o~ the heater 12 is connected to the
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~; ~ input terminal 24. In some cases th~ connection is made b~
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welding the first end~40 to the snap blade 36 a~d using
the conducti~it~ of the:snap blade 36 for electric :.
~`~``~~~~ connection ~o the input terminal 24. The eo~tacts 22
: consist of a irst contact element 42a9 welded tG the
snap blade 36 and a second contact eleme~t 4Zb~welded ~ ~ :
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to a flange 44 on the upper~ end of the output:terminal
26.~A finger 46 on the output terminal is welded~to a
20~ ~ conductor ~ar 487 The:conductor bar 48 is connected
: ~ at its seco~:1d end to a:second end of the heater 50.
.` The long terminal path from the contacts 22 to the ex-
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ternal portîon 3~ o:E output terminal 26 a~ lows substantial
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; resistance hea~ing of output terminal 26 to occur.
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Re~erring now to Fig. 3, the two points on the out-
put terminal ~6 which require welding, between Einger
46 and conductor bar 48 and between flange 44 and second
con~ac~ element ~2b are advantageously on the same ~ide
o output terminal 26. Thus, for ~his particular ~lasher
it would be satisfactory if only the single sur~ace to
which welding is desired would be of a material which
enables welding. It would occur to one skilled ln the
ar~ that other electrothermal swi~ch designs would require
-~ 10 welding to both suraces of a switch terminal.
The output switch terminal 26 is fabricated of a
high conductivity material 26a such as, but not limited
to, copper or aluminum havin~ clad thereon a weldable ve~eer
26b such as, but no~ limi~ed to, low carbon steel. Thus~
s~ - the low electric and thermal resistance of copper is
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effective to bo~h reduce the amount of hea~ electrically
generated in the terminal and also to more rapidly carry
off the heat.
The input terminal 2h of the 1asher design shown
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~ 20 is less prone to electrical heating due to the shorter
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thermal path between its ends and its external portion
and also the radia~ion assistance it obtains ~rom ~he
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intimately connected snap blade 36. Thus, although the
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input terminal 24 may conveniently b~ fabricated of the
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~ same clad material~as the ou~put ~erminal 26, this:~ :
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particular El~sher design may not require i~. Other
electrothenmally operated switches may require the use
of clad materials ~or both term~nals~
The t`hickness ratio of high conduetL~e material
to weldabl~ material should be ~rom about 2:1 to about
30:1 with best restllts obtainable from about 5:L to
about 15:1 but is pr~erably about 10 1~ The en~ire input and~or
output terminals ~4, 26 may optionally be plated w:ith a pro~
tec~v~ metal~ic coating such as gold or cadmium to reduce
corrosion and possible contac~ resistance.
It will be understood that the cla~ms are intended to
cover all ehanges and modi~ications o~ the pr~erred e~bodi-
ments of t~e invent10n3 herein chosen for the purpose of
illustration which do not constitute departures from the
spirit and scope of the inven~ion.
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