US2847538A - Thermally controlled switch - Google Patents

Description

2, 1958 w. K. BOUWMAN 2,847,538

'THERMALLY CONTROLLED SWITCH Filed Feb. 18, 1957 INVENTOR WlCHER KORNELIS BOUWMAN AGENT United THERMALLY CONTROLLED SWITCH Application February 18, 1957, Serial No. 640,921

Clains priority, application Netherlands' September 25, 1956 3 Claims. (Cl. 200-137) This invention relates to thermally controlled switches intended for controlling the temperature of electrical apparatus such as, for example, smoothing irons. These switches comprise two contacts which are controlled by a thermo-responsive Operating element which tends to open the contacts with an increase in temperature, and also by a current-dependent Operating element which urges the Contacts in the direction of closure during the passage of current through the switch, the time constant of the therrno-responsive Operating element exceeding that of the current-dependent element. It is pointed out that this does not preclude that the thermo-responsive element is also slightly current-dependent if current were to pass through it and the current-dependent element be slghtly temperature-dependent.

In conventional devices, the thermo-responsive operating element is a bimetal strip but, fundamentally, any thermo-responsive element may be used, the time constant of which exceeds that of the current-dependent element.

It is pointed out that, if this greater time constant is also caused by parts of the apparatus, the temperature of which is controlled, these parts are associated with the switch.

The current-dependent operating element should be fast-acting. In known switches, a hot wire is employed.

The switches referred to operate as follows:

If the temperature of the apparatus to be controlled exceeds a given limit value, the opening of the Contacts is slowly initiated by the action of the thermo-responsive Operating element having the higher time constant. However, the second element responds rapidly to any decrease in current and further opens the Contacts so as to reduce penetration. If the temperature drops below another limit, the element having the higher time constant will slowly close the Contacts so that the contact pressure is intially low. The element having the lower time constant will, however, instantly respond to the first passage of current and cause an increase in contact-pressure.

In known switches of this type, the thermo-responsive element with the higher time constant was also connected in the electric circuit, hence this element required to be isolated from the apparatus to be controlled. On the other hand, this passage of current was useful and' desrable in order to make up for thermal lag on account of the isolated arrangement.

The present invention is based on the realization that in this manner complete compensation is not feasible and, moreover, this manner of compensation makes it necessary to impose conditions on the element having the higher time constant with regard to specific resistance, length and cross-section, also in accordance with the value of the current to be interrupted. Hence, a compromise is ditficult to make. For example, a smoothing iron rated for 110 volts would require a bimetal strip -tates Parent O different from that for use in a smoothing iron of 220 volts.

In accordance with the invention, the Operating element with the higher time constant is insulated from the electrcal circuit and electrically and mechanically connected to the apparatus, the temperature of which is to be controlled, since it consists of a tautened wire which is Secured at both ends to a base, the coefficients of expansion of the wire and the base being different and the excursion of the centre of the wire being transmitted to the Contacts by, means of an insulator. This base may, for example, be the sole of a smoothing iron. lt will, however, be appreciated that the invention is not to be taken as being liimted to the precise constructional details referred to in the foregoing, particular description.

In these constructions, consequently, the current does not at all influence the element with the higher time constant, and the latter is able to follow only with a slight thermal lag the temperature of the apparatus to be controlled, since it is firmly Secured tothis apparatus without any insulation. The lag may practically completely be avoided if the wire is made from a material, for example that commercially sold as lnvarj the coefficient of expansion of which is substantially equal to Zero, since in this case the therrno-responsive Operating element responds to the thermal expansion of a part of the apparatus itself to be controlled, for example to expension of the sole of the smoothing iron. otherwise, such thermo-responsive elements are known per se. Furthermore, it is pointed out that in the present case the term "wire" is to be put broadly and also includes tape or rods.

In order that the invention may be readily carried into eect three examples will now be described in detail with reference to the accompanying diagrammatic drawing, in which Fig. l is a side elevation View of the switch,

Fig. 2 is a detail View of another form, and

Fig. 3 shows a third construction in side elevation.

The reference numerals 1 designate two parts of an apparatus, for example the sole of a smoothing iron, the temperature of which is to be controlled within two predetermined limits. Stretched between these parts 1 is an Invar strip 2 carrying an insulator 3 at the middle. In this case, the sole of the smoothing iron jointly with the "Invar strip constitutes the thermo-responsive element having the higher time constant.

Secured to the insulator 3 are a hot wire 4 and a fiexible conductor 5 leading to an insulated junction contact 6. The other end of the hot wire 4 is Secured to a spring 7 by which the wire 4 and the Invar' strip 2 are tautened. The strip 7 furthermore carres a contact 8. A second contact 9 is secured to a spring 10, one end of which is connected to a junction contact 11 and the other end of which is provided with an adjustment screw 12 co-acting with an eccentric control knob 13.

The operation of the hot wire constituting the thermoresponsive element with the smaller time constant, and the operation of the Contacts have already been set out in the foregoing.

It will be evident that the thermo-responsive element constituted by the parts 1 and 2 will react without any lag to the temperature of the apparatus and is entirely independent of the value of the current consumed by the apparatus. This permits, for example, this thermostat to be used in smoothing irons rated for widely varying voltages, which consequently consume widely varying Currents, without the thermo-responsive element itself needing to be modified.

It will be evident that the invention is not limited to the use of thermo-responsive elements of the aforesaid type. The thermostat constituted by the parts 1 and 2 may, for example, be replaced by a bimetal strip which is directly connected to the apparatus to be controlled, as shown in Fig. 2. In this case, a bimetal strip 20 is Secured by means ofscrews 21 to the sole 22 of a srnoothing iron. The end of the strip carries the insulator 3 and the'remainder-of the switch may be made up similarly to Fig. 1.

In the switch shown in Fig. 3, the thermo-responsive Operating element is an Invar' rod 30 which is Secured to a component part of the apparatus to be controlled, for example .the aluminium-cast sole plate 31 of a smoothing iron. In front of the end of the rod 30 is situated a lug 32 which is .also secured to the sole, while the end 'of a bent lever 33 carrying an insulator 3 at its free end is .clamped between .the lug 32 and the rod 30. This insulator is engaged by a plate spring 34 acting in the direction of the arrow 35. The spring 34 furthermore car'ies a contact36 and amagnet armature 37. A second spring 38 carries a contact 39. The positon of this spring is adjustable by means of an eccentric 40. The armature 37 `can be attrac'ted by an electro-magnet 41.

The operationof this switch is as follows: If the sole 31 of the smoothing iron cools down, the space between the end of the rod 30 and the lug 32 is reduced, and the insulator 3 lifts the spring 34, -so that the contacts 36 and 39 approach each other slowly. Upon contact being established, a circuit is completed from 'a terminal 42 via spring 34, contacts 36 and 39, spring 38, conductor 43,rnagnet coil 41 to a terminal 44. The armature 37 and the coil 41 constitute the current-dependent operating element by which the contacts are rmly closed so that the contact 39 Will betslightly tilted.` When ;reaching the upper temperature `limit the operation reversed: as :soon :as the contacts are slowly opened'the magnet be- 4 comes 'less energized so that the contacts -are instantly completely separated.

What is claimed is:

l. A thermally controlled electric switch for electrical apparatus having a base member and an insulator com prising a thermo-responsive Operating element which opens said contacts upon the reaching of a predetermined temperature level, a current-dependent Operating element which urges the contacts in the direction of closure during the passage of current through said switch, the time constant of the thermo-responsive Operating element exceeding the time constant of said current-dependent operating element, said thermo-responsive Operating element being insulated from said electric circuit and being electrically and mechanically connected to said apparatus, said the'rmo-responsive Operating element including a tautened wire Secured at each end to said base member, the coefficients of expansion of said base member and said wire being different and the movement of a portion of said wire being operatively transferred to said contacts by means of said insulator.

2. A thermally controlled electric switch as claimed in claim l wheren said base is a sole of a smoothing iron.

3. A thermally controlled electric switch as claimed in claim l wheren said current-dependent Operating element is an electro-magnet.

References Cted in the file of this patent UNITED STATES PATENTS 2,584,9,24 Reingruber et al. Feb. 5, 19.52 i 2,7 l6172 Franklin Aug. 23, 1955 2,7l6,173 Russell Aug. 23, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CFDRRECTION Patent No 2,84'7538 August la 1958 Wieher Kornelis Bouwman It is hereby Certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as COT reeted below In the heading to the prnted speoifieation line 9, for the priority date, "September 25, 1956" read February 25, l956 Signed and sealed this 9th day of December 195&

Attest:

H AXL KARL INE ROBERT C WATSON Attesting officer Corrmissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CRRECTION Patent No., 2,847,538 August 12 l958 Wieher Kornelis Bouwmar It is hereby Certified that error appears in the above numbered patent requrng correction and that the said Letters Patent should read as cor rected belowo In the heading to the printed specifcaton, line 9, for the priority date, "September 25, 1956" read February 25, 1956 Signed and sealed this 9th day of December 195&

Attest:

KARL Ho AXLINE ROBERT C. WATSON Attesting officer Commissioner of Patents

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