DE7923665U1 - Thermal switch - Google Patents

Description

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Thermal switch

The invention relates to a thermal switch according to
the preamble of claim 1.

Well-known thermal switches, such as bimetal strips, ϊ

Fuses or control valves with thermocouples, ι,

solve a switching process when a | j is exceeded

specifiable temperature value with a relatively large? ^:

lateral delay off. Usually, thermal |

switch used to interrupt an electrical circuit f.

For the operational safety of electrical devices and Ιέ

Systems is particularly the fault voltage and its continuous |

er, amplitude increase and repetition of importance. About |

the electrical distribution network can generate interference voltages in

Devices and systems to malfunctions or destruction of f

Semiconductors in sensitive electronic circuits, to malfunctions in control systems, to the loss of ' Information or rollovers in the case of poor isolation | tion or creepage or clearance distances chosen that are too small | to lead. Long delay switches are for |

protection of such devices is unsuitable. Bimetal |

Stripes also show weariness after prolonged use symptoms that lead to a change in the response temperature.

Thermal switch with a relatively short switching delay using a switching process off

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The object of the invention is to trigger a Switching process with high selectivity of the switching condition to ensure. ■ ·

The object is achieved by the features of claim 1. Further developments of the invention are described in the subclaims.

The invention is based on the new knowledge of the kinetics of the shape memory effect. One advantage of the invention is in that the switching process is selective with respect to the initial value caused by an overcurrent temporal change of the temperature in the switching element

releasing element made of a shape memory alloy inter alia by DE-OS 2 026 629, 2 139 852 and 2 701 88Ί known. There will be a relatively large change in shape this switching element when its temperature increases above the initial temperature of the phase transition from the Low- to high-temperature phase of the shape memory alloy used to operate a switch. The switching element The current to be switched can flow through it directly and be heated. The warming can also be caused by |

a heater inserted into the circuit take place. By f From DE-OS 2 701 884 it is known to reduce the current in the circuit when a certain maximum current width is exceeded to interrupt. A problem with the Kunatruktion of switches especially for line and over-current protection is to ensure good selectivity in the switching conditions, i.e. the switch should only open after reaching a current tolerance value when exceeding a predeterminable current increase and not with all overflows.

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and is not related to the transmitted energy. The one applied to the switching element by a clamping device The bias voltage can be adjusted so that the switching element cools down after a subcritical overcurrent has passed can be deformed back.

The invention is described below on the basis of exemplary embodiments. Show it:

Fig. 1 shows the principle of the shape memory effect,

a) the one-way effect,
b) the two-way effect,

2 shows the elongation as a function of temperature,

a) for the one-way effect,

b) for the two-purpose,

Fig. 3 is a diagram of a simple thermal switch in one also suitable for testing purposes,

4 shows the chemical energy G of the two phases martensite and austenite of a shape memory alloy as Function of the temperature T (schematic),

5 shows the percentage of martensite M as a function the temperature T,

6 shows the electrical resistance R of a shape memory alloy (a) before and (b) after heating as a function of time t,

7 shows the scheme of a mechanical model from a

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Spring K, a shock absorption device with a damping constant oC and a mass m,

Pig. 8a, 8b, 8c a semi-schematic representation of a thermal switch in a thermoelectric switch with additional locking element and mechanical

Switching gain in different positions and

9 is a functional diagram of the thermal switch.

Fig. La) shows the one-way and Fig. Ib) the two-way shape memory effect. These effects occur with shape memory alloys, ie with thermoelastic martensites, which after a so-called pseudo-plastic deformation at low temperature return to their original shape when they are heated above a critical temperature. These alloys exist in a martensitic low-temperature phase of low, face-centered orthorhombic or monoclinic symmetry and in an austenitic high-temperature phase of high, cubic, space-centered symmetry. In the martensite phase there are no platelets or lamellae with different crystallographic orientations. At a high temperature of 600 C, for example, the alloy is brought into a desired, for example elongated, rod shape and tempered. During cooling, when a critical temperature is exceeded, the austenitic phase changes to the martensitic phase. If now deformed, eg bent, at a low temperature T, a quasi-stable shape is created through stress-induced or reoriented martensite formation, which has the original straight structure. When heating above a temperature T _? , which is above a critical temperature, the one-way effect spontaneously turns the curved structure into a straight structure. at

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a subsequent cooling down no longer takes place any spontaneous change in shape. However, the alloy is deformed again at the temperature-i temperature T, then repeated the described?

K Effect when heating to T. G

In the case of the two-way effect, a deformed

The, for example, curved body made of a shape memory alloy spontaneously only partially when heated to a temperature T " ^: ." reverts to its original shape. In a subsequent one
Cooling down to T, it deforms again spontaneously, however

not up to the degree of deformation applied from the outside.

The two temperature-induced, spontaneous shape changes;. are for successive temperature cycles between |;

T ₁ and T_ repeatable. [

Fig. 2 shows another representation of the memory effects. j

Depending on the alloy composition, f .; for the one-way effect shown in Fig. 2a, expansions or

pseudo-plastic deformations up to about 8 % when opening;.

\\ c>'. ::. Ti r.backr: ov; onnon wonlon, v /.' ihrond for drr. in fin- ? ^ '

shown Zv: eiwegeffekt expansion b "w. deformation

mings up to l, p % are known. While by heating up
the deformation in the one-way effect goes back to zero and
on cooling dior.on retains the shape that has been restored, with the two-way effect there remains a deformation of about
2 * back. When cooling below a certain temperature j, a spontaneous deformation of about 1.5 % occurs,

which spontaneously regresses when the alloy is heated up.

The two-way effect can be achieved by generating dislocations f and lattice defects, e.g. ί

a) by an irreversible plastic deformation over the pseudo %. plastic range of the alloy, ie of more Jj!

ti suffered

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than about 8 %, or

b) be produced by increasing the temperature by about 50 K above the final temperature of the austenite transformation.

The portion of the deformation leading to dislocations is not recovered during heating. When cooling down again the internal stress field of the lattice defects favors the regression of Mart '.- nsitororientierungen, the was generated by the originally applied tension den, and causes a smaller |

Shape change in the direction of the original deformation takes place ^ri determination. A small two-way effect is created on subsequent thermal cycles.

If tension is exerted on a shape memory alloy, the temperature range of the phase transition shifts to higher temperatures, as indicated by dashed lines in Fig. 2a. The conversion of the low to high temperature phase are between about-180 ° C and +250 C, depending on the alloy composition.

In Fig. ¹ J chemical energy G is the martensite and austenite phase as a function of temperature T. At low temperatures, martensite has a lower energy than austenite. Above a temperature T-, at which both phases are in thermodynamic equilibrium and have the same chemical energy G, austenite has a lower energy than martensite.

• β III III

• · · * I I · I I

9S (III I

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If austenite is to be converted into martensite by cooling, so the shape memory alloy must be cooled below T, as shown in Fig. 5, because of the volume Differences in elastic or nucleation energy included must become.

The transformation begins at a martensite transformation start temperature T _M "; at a martensite transformation end temperature T _{MD it} is completed, that is, no austenite remains. Even at temperatures above T, however, an externally applied stress can cause a

chemical energy contribution to the nucleation or / for the martensite, i.e., it can e "generated in stress-induced martensite

will. Will eir. Body made of a memory alloy initially below T .. _o or Τ ,. _Ώ cooled down and then

IHo Wr

is formed, among other things, by a twin shear process a reorientation of the martensite platelets is produced which, when heated, leads to a reversal of the deformation leads.

The conversion from martensite to austenite begins at a so-called critical temperature T above T _n and ends at a temperature T ".

In order to be able to create a large pomegranate effect, the volume change caused by the phase transition must be small, otherwise many dislocations are generated, to adjust the volume differences. The martensite is solidified or hardened by an increased dislocation density. hardened and reorientation is no longer possible due to an applied stress, as is the case with steel, for example. The for an application as the most important property of the switching element of a memory alloy is that in the

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m *

-10 -

Reconversion of the deformation during heating work can be done or a voltage can be generated. For Ni-Ti based alloys its energy density of 2 MJ / m is achievable, which is much higher than that of bimetal. There are tensions due to a hindrance to the change in shape during heating Can be generated from more than 650 MPa. The achievable The force and the path depend on the sample geometry. With a round bar whose length is 10 times as great as its diameter, the bending force is 20 times smaller than the tensile force, but the displacement of the free end in the The case of the bending is 10 times as large as the displacement under tensile stress.

In FIG. 6, curve a shows the resistance R of a wire made of a mechanically loaded or prestressed shape memory alloy before heating as a function of time t. Curve b was generated by sending a short-term current pulse from a capacitor discharge (875 V, 25 μF ) through this wire, thereby causing heating. The resistance kenr.2eichr.et the structure or the phase transformation of the alloy. The ccraccsoncn's resistance oscillation; ■ ^f .preehcnde movement of the wire is explained using a mechanical model shown in Fig. 7, in which K is the spring constant of a spring, m a mass, γ aie damping constant, x the path or a deflection, χ or χ the first or The second derivative of the path in terms of time and g is the acceleration due to gravity. The differential equation applies to this model:

mx '+ A' χ - Kx = -mg. (1)

with the initial condition:

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• ·

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x = x + ² ^ and x = 0 for t = 0, (2)

max iv

where χ means the maximum elongation that can be achieved by Aufmax

heating a spring body as a result of a shape memory effect is attainable. Under these assumptions, the differential equation (1) has the solution:

x ₌

jt.-f

is implemented with an effective grounding force F.

The spring constant K depends on the type of movement, such as tension, Bending, etc., on the type of shape memory alloy and the size of a mechanical preload. Without The preload results in K for tension or compression as:

■ ah ■ ά

X £

Exp (- /) tv t) COS (U / t + V) + -rj? (3) ί

COSJ, K I

with ύ

/ m (1 - λ ² )

tan V »- -2 λ and (5) f?

a = ^ ₇ . (6) I.

^U = I ^{K x} max = / ^{F dx} J

represents the elastic energy of the spring which, when heated up, results in a change in shape within a characteristic | static movement time $.

with 2

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q Cross-sectional area of the porm memory body 1 length

d relative density

C specific heat capacity

L s specific entropy

/ lh specific enthalpy

C specific heat capacity

£ relative change in shape caused by deformation and £ _M maximum relative change in shape due to phase change.

If a body or wire made of a shape memory alloy is heated by an electrical current I ₃ flowing through it, the following applies to temperature changes dT / dt over time in a temperature range in which there is no change in shape caused by a shape memory effect:

dT _ RI (t) ² (10)

dt "C V d

where V is the volume of the body.

Above the critical temperature T. "for the reverse deformation, a mechanical stress ß ~ proportional to T - T. _g arises in the shape memory body:

• ir ⁶ O

6 = --- (T- T) ₅ (11)

constant

where Gl is a material ^. This tension (T is in the thermal switches shown in FIGS. 3 and 8a, reference number 38, are used to trigger a switching process.

The principle of the thermal switch is illustrated in FIG. 9.

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purifies. Any time-dependent physical quantity f (t), which in the event of a predefinable condition, e.g. when a certain limit value is exceeded, a switching process trigger or make a switching signal y, available 5, is in a converter 4l in one of these

Input variable f (t) assigned time-dependent heat or thermal energy Q (t) transformed, if it is not is already given as heat. This heat Q (t) is I in a further converter 42 in an acceleration χ

; "ίο transformed, which a comparator 43 with a loading |, acceleration limit value x '"is compared. For χ> x'_

f is the switching signal at the output y of the comparator 43

I y, an, otherwise the signal y_. Synonymous with

_{p this signal y n which is} dependent on the output of the comparison

h I

I 15 or y ₂ is an “on” or “off” or “toggle” a switch.

In the thermal switches according to FIG. 3 and FIG. 8a, reference number 38, the time-dependent physical quantity f (t) is an electrical current I (t). The converters 4l and 42 are " 20 formed by the switching element 18 from a shape memory alloy. The electric current I (t) is in this Switching element according to

t
Q (t) = ' ^d Rl ^ dt (12)

1 25 in heat, this in accordance with equation (11) in mechanical

I tion and this according to equations (7) and (1) in force F or loading

I acceleration x "is transformed. The comparator 43 is

I represented by an adjustable mass 16, the

I. Inertia of this mass: -: for realizing the acceleration

I 30 limit value x "is used. Only when one of the

I ^G

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I Shape memory effect when the peeling element is heated up

I generated force or acceleration χ this specified

If the limit value is exceeded, the

! Switching signal or a switching process. Slow running p 5 solve current or temperature changes in the switching element 18

I do not cancel any switching operation. Equally dissolving streams

'i rungerij the temperature changes outside the phase

1 transitions between Τ _ησ and T. “or between Τ .. _ο and T _MÜ

ψ. Ao Ar l'lo i'lr

correspond, no switching signals off. ^ s is so-10 'with a selectively acting thermal switch, which is below

- among other things for a quick shutdown of overcurrents

I is suitable. For this, the phase transition from martensite to

Austenite corresponding to the temperature range from T to T. used. This switching range is determined by the dimensioning 15 of the switching element, the composition of the alloy and mechanical Adjustable preload.

i The thermal switch shown schematically in Fig. 3 is

] i stands from the switching element 18 in the form of a round rod

I 2

P or wire of 6.5 now length and 0.33 now cross section

I 20 ^{made of} an alloy of 3 Mj Ni, I3 M % Al and 84 M % Cu, I compare wire number 9 in Tables 1 and 2. A further

I res example of a switching element is there with wire

Number 8 denotes. The upper end of the switching element l8

is held in an upper holder 19 which is immovably attached 25 to an attachment 40. The lower end of the switching element is coupled via a bracket 15 with an adjustable mass 16 of 14.8 g and a tension spring Ik , which can also be omitted and which in the present example is set with the spring constant K = O. The tension spring is held immovable on one side in a fastening I3. Mass 16 and tension spring 14 work together as a tensioning device and generate mechanical tension

Af

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that in the low temperature phase by stretching more pseudo-plastic! " deformed switching elements 18, which below its tear-£ tension lies. With a |

A current I is sent at room temperature of 20 ° C, which is to be monitored for the occurrence of an overcurrent. Exceeds the current-time integral

SZ = f I dt, Ct = t ₂ - t _± (13)

within a time interval with Jt = 2 ms the value of

2
SZ = 1000 AS ₅ so the switching element 18 tears and interrupts the circuit, as shown in Fig. 3b. The switching element acts as an electrical fuse;. ' adjustable sensitivity, if the current rise is faster than a predeterminable Fourth, the mechanical tension T exceeds Zerreisspannung of the switching element ^l before the mass 16 begins to move. The ; The initial shape of the switching element is chosen so that the distance between the fragments after tearing is large enough to ensure separation of the arc. In the event of a slowly increasing overcurrent, the mass 16 is moved and 6 "will never exceed the breaking voltage of the switching element. The values of m and K are expediently selected using a two-way memory alloy for the switching element so that the switching element is deformed back by cooling, compare 2b, after an overcurrent with a subcritical rise has passed. This thermal switch can also be used with a switching element with a one-way memory effect, whereby changes in shape of up to about 8 % can be used when tearing ^

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of the switching element in a known manner as a means for triggering another switching process, if necessary with intermediate amplification of the switching signal, be used. The thermal switch acting as an electrical fuse according to FIG. 3 is instead of a fuse can be used in a conventional thermoelectric switch 50 according to FIG. 8, which is used for securing electrical circuits against slowly or abruptly increasing overcurrents is known.

Fig. 8a shows such a thermoelectric switch 50 in a semi-schematic representation, but with one Thermal switch 38, which is a switching element 18 with a two-way memory effect that does not tear when heated and thus does not interrupt the circuit directly can. In contrast to the thermal switch according to FIG. 3, the thermal switch 38 has the upper holder 19 of the switching element 18 is not mounted immovably, but with a linkage 12 in the catch arm 23 via a mechanical one Coupling 20 connected and movable against a tension spring 11.

The electrical circuit goes from a connection line 7 via a fixed contact 6, a contact end 8 of a contact arm 4, which can be pivoted about a linkage 1, via a connecting line 28, an additional securing element 9, a Connection line 22, the upper bracket 19 through the switching element 18 to the lower bracket 15 to a connection line 17 ·

An overcurrent flows through the switching element, the one Change in shape or lead here to contraction due to the shape memory effect

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1) the linkage 12 is offset against the spring force of the tension spring 11 downwards and

2) the mass 16 and the tension spring 14 coupled to it moved upwards.

The movement time £ t according to equation (8) is characteristic of the second movement. As soon as the movement is completed j the voltage T ₃ disappears, compare equation (11).

The first movement will only trigger switch 50 if 5 "rises above a predeterminable critical value, which for Trsnsla ^ ion of the articulation 12 downwards by e.g. 1 mm is sufficient. Otherwise, the switching element 18 due to the setting of the values of mass 16 and spring force K of the tension spring 14 after the passage of a subcritical overcurrent deformed back by cooling

However, if the linkage 12 is deflected downward, the tentacle arm 23 is moved around a linkage 21 in the Pivoted clockwise, as indicated by dashed lines, and gives an angle arm 37 for pivoting against the Clockwise free. This pivoting is blocked as long as a hook end 25 of the tentacle arm 23 is at one end of the catch 24 of the angle arm 37 is applied. Under the action of a compression spring 2, which is supported on an attachment 3, and seeks to move the contact arm 4 to its articulation 1 clockwise, the angle arm 37 is by means of a push arm 29 pivoted about a linkage 26 counterclockwise, as shown in Fig. 8b. The contact arm 4 lifts from the fixed contact 6 and interrupts the electrical connection between the

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Connection line 7 and the connection line 28. The Push arm 29 is connected to contact arm 4 by a linkage 5. At its lower end it has a linkage 27, in an opening 36 of the angle arm 37 can slide when the angle arm is pivoted. When pivoting, the articulation 27 slides in the opening 36 to the left, whereby the also around the articulation 27 pivotable return fog 32 is moved by means of a push arm into an off position shown in FIG. 8c.

The back lever 32 is subject to the tensile stress of a tension spring ~ 5'4, which is anchored in a fastening 37. It is connected to one end of the push arm 30 via a movable linkage 3I. The other end of the push arm 30 is connected to the push arm 29 by the movable articulation 27. The switch 50 can also be switched on again with the reset lever 32 by pivoting it clockwise.

The thermal switches according to FIG. 3 and FIG. 8a, reference number 38, v / ground expediently in addition to an additional switching element 9 applied, which can be connected to the tentacle arm 23 by means of a mechanical coupling 10 and to others protection-relevant parameters breaks.

Rated values for switching elements 18 which are suitable for use in the thermal switch 38 are given in Tables 1 and 2 under wire numbers 1 to 7. The values in table 1 apply to a room temperature of 20 ° C., a deflection due to one of the shape memory effects of 1 mm and a spring constant K = 0 of the tension spring 1 * 1. The tension spring Ik can be omitted in both of the thermal switches described> however, it is suitable for stabilizing the position of the thermal switches and for application

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ters 50 according to FIG. 8 can be done by pulling or pushing.
The change in position caused by the switching element can
in a known manner, for example by means of optical, electrical
or hydraulic detectors detected, amplified and on
transfer v / earth another switching element.

ay / I

-19 -

a bias on the switching element 18.

For the memory alloy of the switching element 18

are suitable, among other things, from the material properties

and, in terms of price, especially alloys based on Ni Ti, |

Ni Ti Cu according to DE-OS 2 644 041, Cu Zn, Cu Al, Ni Zn | also with ternary and other components such as Cu Al Ni, pj,

Cu Zn Al Ni etc.

It goes without saying that the invention relates to what is presented above

posed is not limited. For example, shaping j

I of the switching element 18 strip-shaped, tubular, spiral |,

be shaped, curved, etc. The switching element |

can be twisted by bending, twisting, compressing or stretching | be shaped. The triggering of a thermoelectric switch- 1

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Wire no.

1
2

5
6th

7th
8th

_ 20 _

Table 1

1 mm

5.2

5.2

78

78

780

3.6

6.5

mm

20th
20th

40
1.5
0.33

38 950 950 3 800 14.8 3 700 14 800

38 14.8

K (Il)

MN / m

0.36

0.36

3.6

3.6

0.3

0.3

SZ A ² S

ι.ιοί. io ^: ι. ιοί, ιοί, ioi. ιοί.ίο ¹

1.10 ^J 1.10-

ms

1 5 5

10 2

10

20th

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_ 21 _

CM

ω ■ π

CO
Egg O S. OO
Ln co
H Alloy
proportion of
M % μ "
co hD
1-3 • Η · Η d · Η Η 3
525 E- <OS «GO
Ln in
• \ * \ O Ι ^ Λ ZT
ZT in η m η oo B. Uh ι in t—
σ σ ι
ri «O
• iH (H
co hO ■ 0.01
0.01 S.
■ Ö O
~ hÖ 0.06
0.04 Wire-
No. 0.0587
0.021 from the t> - VO
ο ο 6.35
7.19 oo σ \
η ft
za- t ~ -
II
in in

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Designation list

1 * Linkage of 4 2 Compression spring J Attachment of 2 It Contact poor 5 Articulation 6th Fixed contact 7th Connecting cable 6th End of contact from 4 Assurance Element 10 mechanical coupling 11th Tension spring 12th Articulation 13th Fortification of 14 14th Tension spring 15th lower bracket from 18 16 Dimensions 17th Connecting cable 18th triggering switching element 19th upper bracket of 18 20th mechanical coupling 21 Linkage of 23 22nd Connecting line 23 Tentacle 24 Catcher from 37 25th Hook end from 23 26th Linkage of 37 27 Articulation 28 Connecting line 29 Push arm 30th Push arm 31 Articulation

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32 • a •
• Reset lever 33 Articulation 34 Tension spring 35 Fortification of 34 36 Breakthrough from 37 37 V / inkelarm 38 Thermal switch 40 Attachment 41 Viand ler 42 Converter 43 Comparator 50 counter A. Austenite C. specific heat capacity d relativ density f (t) time-dependent physical quantity F. force ε Acceleration due to gravity G chemical energy H specific enthalpy I. electric current K Spring rate 1 length m Dimensions M. Martensite q Cross sectional area Q (t) time-dependent energy S. specific entropy t Time t
m mechanical movement time T temperature U elastic energy of the spring • · · Il I t
* * · III ■ 't ItI • · · · I I <ι ι

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X Path, deflection X speed X acceleration " ^X G Acceleration limit X
Max maximum elongation y Switch output, switching signal ^y i Switching signal "On ^{17 y} 2 Signal "off" SZ Current-time integral eC Damping constant C. elastic deformation £ d applied by deformation relative change in shape ^ M maximum relative change in shape through phase transformation λ Damping constant ρ specific electrical resistance was standing (T mechanical tension Material constant > Phase angle UJ Angular frequency

• * I I · 'ti I' • · Il

Claims (1)

175/78 Rz / approx 26.2.79 Protection claims 1. thermal switch, .- with a switching element (l8) of a temperature-dependent variable shape made of a shape memory alloy. - That at a first point in a first holder (15) and - at a second point in a second bracket (19) is attached, characterized in that - This first holder (15) is movably arranged and ■ s - firmly connected to a body (l6) of predeterminable mass the iiät. 2. Thermal switch according to claim I _3, characterized in that - This second holder (19) is immobile. . ··, 3 · Thermal switch according to Claim 1, characterized in that that - this second holder (19) is attached to a movable part of an elastic body (11), - which is immovably arranged in at least one place. 4. Thermal switch according to claim 3, characterized in that that - This elastic body (11) is a spring. - 2 - 175/78 j 5 = thermal switch after at least one of the previous the claims, characterized in that - This first holder (15) is attached to an elastic body (14) which is at least one point (13) is arranged immovably. 6. Thermal switch according to claim 5, characterized in that | i that ! - The elastic body (14) is a spring. 7. Thermal switch according to at least one of the preceding claims, characterized in that - The switching element (18) made of a shape memory alloy with a two-way memory effect. 8. Thermal switch according to at least one of the preceding claims, characterized in that - The switching element (18) is rod-shaped. • · · * ι 1-1 tii * t · • «· · * I I 1 I I · • I * t ■ I I ι I «