US3646315A

US3646315A - Self-regulated heating device

Info

Publication number: US3646315A
Application number: US692004A
Authority: US
Inventors: Leo Marcoux
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1967-12-20
Filing date: 1967-12-20
Publication date: 1972-02-29
Anticipated expiration: 1989-02-28
Also published as: NL6817981A; BR6804793D0; DE1814682A1; FR1595972A; ES361379A1; GB1237717A

Abstract

Apparatus for providing a relatively constant ambient for temperature sensitive devices in which an inner oven, adapted to hold such devices, containing a heating element formed of material having a steep-sloped positive temperature coefficient of resistance (hereinafter referred to as PTC) above a threshold or anomaly temperature is placed within an outer oven containing at least one heating element formed of PTC material having a lower threshold or anomaly temperature than that of the inner oven''s heating element. These heating elements also act as their own temperature regulators. The preferred embodiment employs a single toroidal PTC element located about a component mounting can. Leads connected to this heating element and the component leads are guided along the sidewalls of the electrically insulating housing of the inner oven. The entire inner oven is mounted in a heat conductive shell in which are fitted in recesses at two opposite ends toroidal PTC elements. Leads connected to the outer oven heating elements and the other leads are threaded through guide members placed in aligned apertures in the outer oven shell and are connected to terminals mounted in a base. The heating elements are locked in place by electrically insulating potting material. The heating elements are electrically connected in parallel.

Description

United States Patent Marcoux 1 Feb. 29, 1972 [54] SELF-REGULATED HEATING DEVICE [57] ABSTRACT [72] lnventor: Leo Marcoux, Pawtucket, RI. Apparatus for providing a relatively constant ambient for temperature sensitive devices in which an inner oven, ada ted to [73] Asslgnee Texas instruments lnwrporated Dallas hold such devices, containing a heating element forrEred of material having a steep-sloped positive temperature coeffi- 22] Fil d; D 20, 1967 cient of resistance (hereinafter referred to as PTC) above a threshold or anomaly temperature is placed within an outer [21] Appl' 692,004 oven containing at least one heating element formed of PTC material having a lower threshold or anomaly temperature 52 US. Cl.... ..219/210, 219/200, 219/504 than that Of the inner heating element heating [51] lnt.Cl...; ..H05b 1/00 elements also act as their own temperawl'e regulamm The 58 Field of Search ..219/2|0, s 10, 504; 3 10/89 P'tfctred embtJdiment employs a single ttidttl FTC elemem located about a component mounting can. Leads connected to [56] References Cited I this heating element and the component leads are guided along the sidewalls of the electrically insulating housing of the UNITED STATES PATENTS inner oven. The entire inner oven is mounted in a heat conductive shell in which are fitted in recesses at two opposite 2,86l,l63 H958 Asakawa ..20l/72 ends to'roidal PTC elements Leads connected to the ouer 3907923 10/1961 Johnston et al "219/210 oven heating elements and the other leads are threaded 394L548 6/1962 eta] ""219/210 through guide members placed in aligned apertures in the 3,243,753 3/1966 Kohler ..338/31 Primary ExaminerC. L. Albritton Attorneyl-larold Levine, Edward J. Connors, Jr., John A. Haug and James P. McAndrews outer oven shell and are connected to terminals mounted in a base. The heating elements are locked in place by electrically insulating potting material. The heating elements are electrically connected in parallel.

PATENTEBFEB 2 9 I972 Leo Marco ux,

SELF-REGULATED HEATING DEVICE The outer oven is mounted on seating portions provided in the base and is resiliently held thereagainst by means of a spring member placed between the outer oven shell and a cover secured to the base. Means are provided to insure alignment of the outer oven with respect to the terminals in the base.

To obtain even greater temperature regulation, auxiliary heaters can be employed to distribute the heat more evenly. Further, external voltage regulation can be provided if desired.

This invention relates to apparatus for providing a relatively constant temperature for temperature sensitive devices located therein.

It is known to provide ovens for this purpose which utilize a heater and a thermostat. The thermostat keeps the inside oven temperature within a certain range by turning on and off the heater current by use of movable contacts. This type of oven has certain inherent disadvantages, viz, the temperature varies due to the characteristics of the thermostat--from a maximum to a minimum back to a maximum and so on. Also, since there is a mechanical movement, the longevity of the device is limited.

Another approach has been to employ some type of proportional control wherein relatively complex electrical circuits serve to limit the power input to the heater to equal the heat loss from the oven. This is done, for example, by providing a bridge containing a temperature-sensing device which is used to balance a circuit containing the heater. This type of control eliminates the on/off moving contacts and therefore provides more precise temperature control with no overshoot or thermal cycling, more constant power requirements and no noise due to mechanical operation. However, many ovens of this description do emit electrical noise. Also, such devices are relatively complex and expensive. In copending and coassigned applications, Ser. No. 435,166, filed Feb. 25, 1965, abandoned in favor of continuation-in-part application, Ser. No. 508,643 which issued on Dec. 3, 1968 as U.S. Pat. No. 3,414,705, and Ser. No. 435,165, both Feb. 25, 1965, and which issued on Dec. 3, 1968 as U.S. Pat. No. 3,414,704, devices are disclosed and claimed which provide a relatively constant ambient temperature utilizing self-regulating heating elements. In these devices a temperature gain as high as 30:1 is achieved. Temperature gain is defined as a change in ambient temperature divided by a change in control temperature at constant voltage and component dissipation as measured by the temperature dependent characteristic of the component. Even higher temperature gains are achieved in apparatus disclosed and claimed in copending and coassigned application Ser. No. 518,277, filed Jan. 3, 1966 which issued on Dec. 3, 1968 as U.S. Pat. No. 3,414,706. In that application a device having a first steep-sloped PT C element with a first cavity defined therein for reception of temperature sensitive components is received in a second cavity defined in a second steep-sloped PTC element. The present invention relates to an improved self-regulating heating device of the last-mentioned type.

It is an object of the invention to provide an oven which is simple, highly reliable, long-lasting mechanically and electrically silent in operation and one which results in a closely controlled, relatively constant oven temperature. It is a further object to provide an oven having a heating element which has a self-regulated temperature and will maintain a relatively constant ambient temperature for temperature sensitive devices contained therein, regardless of change in heat demand.

The invention accordingly comprises the elements and combination of elements, features of construction and manipulation and arrangements of parts, all of which will be exemplified in the structure hereinafter described, and the scope of the application of which will be indicated in the appended claims. In the accompanying drawings in which one of the various possible embodiments of the invention is illustrated:

FIG. 1 is a vertical cross sectional view through one embodiment of the invention; and

FIG. 2 is a schematic wiring diagram of the device of FIG. 1.

Similar reference characters indicate corresponding parts throughout the views of the drawings.

Dimensions of certain of the parts as shown in the drawings may have been modified and/or exaggerated for the purposes of clarity of illustration.

Referring now to the drawings, device 10 comprises a base 12 of a conventional molded electrical insulating material, such as a phenolic resin, of any convenient configuration, but shown to be generally circular in shape, having an annular seat 14 for a cover member 16 also preferably constructed of electrical insulating material, which closely fits seat 14. A notch 18 is provided in cover 16 and registers with a projection 20 formed on annular seat 14. Base 12 is also provided with a plurality of mounting posts 22 which are used to mount an external oven assembly 30. In the preferred embodiment there are three such posts (only one is shown in the drawing) equally spaced about the periphery of base 12. Standard fastening means such as screws 24 fixedly secured cover 16 to base 12.

External oven assembly 30 is mounted on posts 22 and aligned by means of a pin 26 in one of the posts which is inserted in a mating aperture 28 formed in the outer housing of assembly 30.

Assembly 30 is composed of an upper and a

lower housing

32 and 34 respectively made of a good thermally conductive material, such as copper, and held together by ring 36 which is crimped over flanges 35 of upper and

lower housings

32 and 34 respectively.

Housing members

32 and 34 are generally of the same configuration, each cup shaped and having a closed end, with

recessed portions

31 and 33 respectively and which mounts a heating element to be further described below. The sidewalls of the housing members cooperated to define an outer oven chamber or enclosure. Bottom housing member 34 has an annular platform 38 which is provided with a plurality of spaced apertures 40. A support member 42 of a conventional molded electrical insulating material such as Nylon is located on platform 38 and is provided with depending tubular legs or guide members 46, each of which is placed through a respective aperture 40 in platform 38. Inner oven assembly 50 is mounted on a plurality of seating members 44, preferably three, which are conveniently formed as a part of support member 42. (Only one seating member 44 is shown in the drawing).

Inner oven assembly 50 contains the component package 52 and the components therein whose temperatures are to be regulated. In the device shown, see FIG. 2, two diodes are illustrated but it will be understood that either one or more than two and other types of devices could be contained in package 52. These diodes DI and D2 have a common lead L1 and separate leads L2 and L3 respectively. The component package 52 is telescopically received in mounting can 54. A heat transfer material may be used to optimize heat transfer between the can 54 and package 52. Can 54 is formed of a good heat conductive material, such as aluminum and provided with an annular flange 56.

An electrically insulating layer is provided on the surfaces of can 54. Although this may be done in various ways, such as by coating with a thermoplastic resin, heat conduction should be kept at an optimum level. This can be accomplished by employing anodized aluminum. Flange 56 of can 54 abuts shoulder 58 formed in tubular casing member 60 of a thermally and electrically insulating material, such as a thermoplastic (e.g., Nylon). Distal end portion 62 of casing 60 has an annular flange 64 in which are formed slots 66 in which are threaded leads L1, L2, and L3. A PTC heater-regulator element 70 of a toroidal or ring-shape closely fits about can 54. Layers 72 and 74 of silver or other conductive material serve as terminal surfaces on opposite faces of element 70. Conventional leads L4 and L5 are electrically connected, as by soldering, to layers 72 and 74 respectively. The leads, as shown, con tact the layers throughout approximately 360 thereby enhancing uniform electrical conductivity through element 70. A sleeve of electrically insulating material, such as a heat shrinkable Mylar, may be placed about the periphery of heater 70 to preclude any possibility of short circuiting the heater. Electrically and thermally insulating potting compound 82 of a conventional type is infilled around element 70. Shoulder 84 formed in end portion 62 of casing 60 seats a disc 86, formed of an electrically insulating material such as a resin impregnated fiber board. Disc 86 is provided with slots to provide access for PTC heater leads L4 and L5. Shoulder 84 is positioned to provide a predetermined airgap 88 between disc 86 and can 54 and potting material 82 thereby resulting in a desired thermal insulation. It is obvious that instead of relying on air gap 88, some other insulation could be used if desired.

A cover member 90 is positioned closely around casing 60 and can be formed of the same electrically, insulating material as the casing, i.e., Nylon. Cover 90 is provided with projection 92 which serves as a component package wedge to maintain package 52 tightly in mounting can 54. Annular projection or wedge 94 in cover 90 biases leads L1, L2 and L3 against hub 63 of casing 60, minimizing heat loss through these leads. That is, it decreases the effective size of the heat sink of the component leads. The component package 52 is in close thermal relation to heater 70 resulting in a more efficient control. As stated above, it has been found useful to apply a coating of a heat transfer compound, such as a silicone compound, to the outer periphery of mounting can 54. This facilitates heat conductance from the PTC element 70 through mounting can 54 to component package 52. One such compound available is GE 641 lnsulgrease. A coating of this heat transfer compound may also be applied to the outer periphery of package 52, further facilitating heat conductance from the PTC heater element.

Two PTC

heater regulating elements

100 and 110 are placed respectively in

recesses

31, 33 of housing 30. Electrical conductive

terminal layers

102 and 104 of silver of some other good electrically conductive material are provided on opposite faces of heater 100 and similar layers 112 and 114 are provided on opposite faces of heating element 110. Lead L8 is attached in a conventional manner to layer 112 while lead L9 is similarly attached tolayer 114. It will be seen that insulating sheath 117 is provided for lead L8 and sheath 119 for lead L9. To preclude shorting of the heater current, an insulting sleeve 99 is placed about the periphery of

heaters

100 and 110 which may be of the same general type as sleeve 80 referred to above.

Potting material

120 and 122 is placed around the

heaters

100 and 110 and concomitant leads.

Base 12 is provided with a plurality of apertures 13 through which is telescopically received tubular members 15. Formed integrally therewith or attached thereto are terminals Tl through T5. The number of terminals provided is a manner of choice. It will be seen that L1 is electrically attached to T1, L2 to T2, L3 to T3, L4, L6 and L8 are attached to T4 and L5, L7 and L9 are attached to T5. The three PTC heating elements are electrically connected in parallel. The attachment is by any convenient means such as soldering. Tubular members 15, it will be seen, are provided with collars 17 to fixedly locate the members in the base 12. After attachment of the leads to the terminal members, the bores of the tubular members may be sealed as by soldering.

A conventional spring member 39 is employed to locate inner oven assembly 30 in a resilient manner which can absorb shocks subjected to the device 10. In the embodiment shown the PTC material for the inner oven, that is, element number 70, has a 120 C. anomaly while the anomaly for the outer elements, that is,

elements

100 and 110, is 80 C. The electronic components are placed within the temperature controlled cavity or package 52 of the relatively high-temperature PTC component. This oven is in turn placed within the cavity of the lower temperature PTC component of 80 C. The oven performanceor'gain may be determined as follows: an oven which regulates to 5 C. over an ambient temperature change of 50 C has a gain of 50 over 5 or 10. Using the compound oven approach, it is possible to multiply oven gain. Thus a 120 C. oven with a gain of 30 placed within an 80 C. oven with a gain of 10 will result in an overall oven gain of 30 times 10 or 300. A 50 C. ambient temperature change will thus be reduced to a variation of 50 over 300 or 0. 1 7 C, measured at the component. As shown in the diagram, the component protected is a diode circuit encapsulated and inserted into the cavity of an oven set forth in out copending application, Ser. No. 508,643, filed Oct. 24, 1965, now U.S. Pat. No. 3,414,705, particularly as shown in FIGS. 6 and 7.

It will be seen that the instant invention fulfills the objectives listed supra, including the exceptionally close temperature control, better than those obtainable with electromechanical controls and which could be matched in performance only by using a relatively costly proportional control or hybrid system. Other advantages which the instant design offers include a fast warmup because of the PTC resistance characteristics. The. oven draws a high-power surge on warmup and then stabilizes quickly, less than 3 minutes as compared to 15 to 30 minutes with conventional ovens. The instant invention also offers a long, reliable life. Control characteristics are entirely dependent on the PTC element resistance anomaly which has extreme long term stability. There are no contacts to wear or mechanical parts to shift or wear.

Since there are no moving parts, there are few assembly operations. Basically, the components to be protected are placed within the package 52, the leads are then threaded through the grooves 66 in casing 60 and downwardly through guide members 46 and attached to the appropriate terminals. The top casing 32 is then attached to the bottom casing 34, ring 36 is cinched about the flanges 35 with the leads Ll-L9 passing through the appropriate guide members 46. The shrinkable electrical insulating cover member 96 keeps the diode leads and inner oven heater leads firmly in place and isolates them from the housing 30. The device is small in size, light in weight and highly resistant to shock and vibration.

It is within the purview of the invention to provide one or more auxiliary heaters. The two external oven heating elements are to a degree localized heat sources which the

casing members

32, 34 tend to evenly distribute throughout the outer oven cavity. However, standard resistance or heating elements could be connected in series with one or both of these elements. The PTC elements will regulate the amount of heating current passing through the auxiliary heaters which may be strategically located at critical points in the housing 30 to insure more even temperature distribution. External voltage regulation although minimized in the above-described construction could be almost completely eliminated by the addition of a simple, low-cost external voltage regulator. When the oven 10 is subjected to ambient temperature variation, say a decrease, the oven responds by increasing the power generation. The greatest portion of this increase is assumed by the PTC elements in the external oven. Power generation in the internal oven changes only slightly. Therefore, a simple voltage regulator design for relatively small power variations and connected to drive only the internal oven could virtually eliminate the temperature variation due to changes in voltage. This approach of course would further improve temperature control. If the external voltage regulator used to drive the internal oven were designed, using as a voltage reference a device with a linear negative temperature coefficient, then internal oven voltage could be made to increase linearly with decrease in ambient temperature. Thus as ambient temperature decreased tending to decrease internal oven control temperature, the external voltage regulator would respond by increasing internal oven voltage and also internal oven temperature. Proper matching of the negative temperature coefficient of the external power supply voltage reference to the positive temperature coefficient of the internal oven control characteristic would result in almost perfect temperature control.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

As many changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

1. A device for maintaining a relatively constant ambient temperature in an enclosure comprising:

an inner and outer housing, the inner housing mounted within the outer housing, the outer housing formed of a good thermal conductor and formed of two generally cup-shaped members, each having a closed and an open end, the closed end formed with a recessed area, the open end formed with an annular flange; a heating element located in each recessed area in heat conductive relation with the outer housing and electrically insulating material filling the remainder of the recessed area fixedly mounting the heating element therein, the open ends of the two cup-shaped members are placed so that the annular flanges face one another and a ring clampingly engages the annular flange of each member; at least one heating element mounted in the inner housing, the heating elements in both housings having an anomaly temperature above which the resistivity-temperature curve has a steep-sloped positive temperature coefficient (PTC); the heating elements in the outer housing having an anomaly point lower than the heating element in the inner housing, and the heating elements in the outer housing electrically connected in parallel to the heating element in the inner, thereby effecting improved temperature regulation. 2. A device according to claim 1 in which the heating elements in the outer oven are toroidal.

3. A device according to claim 1 in which the heating elements are composed of Ba La TiO 4. A device according to claim 1 in which the heating elements are composed of carbon black filled polymer. I

5. A device according to claim 1 in which conductive layers are located on spaced portions of the heating elements, leads are electrically connected to the layers and sleeves of electrically insulating material are provided to insure electrical isolation between the spaced layers on each heating element.

6. A device according to claim 5 in which one member of the outer housing is provided with a plurality of guide aper-- in the guides, a plurality of terminal pins are mounted in spaced apart relation in a base of electrical insulating material, the electrical leads are electrically connected to respective terminal members, the base member has a plurality of seating portions upon which is received the outer housing, a cover having a closed end is received on the base enclosing the outer housing and a spring is placed between the closed end of the cover and the outer housing and biases the outer housing toward the seating portion on the base.

7. A device according to claim 6 in which alignment means are provided to align the cover on the base and the outer housing on the seating portions.

8. A device for maintaining a relatively constant ambient temperature in an enclosure comprising:

an inner and outer housing, the inner housing mounted within the outer housing;

the inner housing comprises an electrically and thermally insulating casing having two ends;

a seatin flange formed in one end of said casing;

an elec rical y insulating and thermally con uctmg component seating member located within said casing with a flange complimentary in shape to said seating flange and seated thereagainst;

a heating element placed in the inner housing about the periphery of said component seating member in close thermal relation thereto;

terminals electrically attached to spaced portions of said heater element;

an electrically and thermally insulating member closing the other end of said casing;

an electrically and thermally insulating cover member closing said one end of said casing;

at least one heating element mounted in the outer housing, each heating element having an anomaly temperature above which the resistivity-temperature curve has a steep-sloped positive temperature coefficient (PT C); the heating element in the outer housing having an anomaly point lower than the heating element in the inner housing, and the heating element in the outer housing electrically connected in parallel to the heating element in the inner, thereby effecting improved temperature regulation.

9. A device according to claim 8 in which leads connected to the terminals of the heating element in the inner housing and leads of components placed within the inner housing are bent so that they extend along the outside of the casing between the casing and the cover and pass through apertures provided in the outer housing.

10. A device according to claim 9 in which an electrically insulating sleeve is placed about the inner oven isolating the leads from the outer housing.

Claims

1. A device for maintaining a relatively constant ambient temperature in an enclosure comprising: an inner and outer housing, the inner housing mounted within the outer housing, the outer housing formed of a good thermal conductor and formed of two generally cup-shaped members, each having a closed and an open end, the closed end formed with a recessed area, the open end formed with an annular flange; a heating element located in each recessed area in heat conductive relation with the outer housing and electrically insulating material filling the remainder of the recessed area fixedly mounting the heating element therein, the open ends of the two cup-shaped members are placed so that the annular flanges face one another and a ring clampingly engages the annular flange of each member; at least one heating element mounted in the inner housing, the heating elements in both housings having an anomaly temperature above which the resistivity-temperature curve has a steep-sloped positive temperature coefficient (PTC); the heating elements in the outer housing having an anomaly point lower than the heating element in the inner housing, and the heating elements in the outer housing electrically connected in parallel to the heating element in the inner, thereby effecting improved temperature regulation.

2. A device according to claim 1 in which the heating elements in the outer oven are toroidal.

3. A device according to claim 1 in which the heating elements are composed of Ba997La003TiO3.

4. A device according to claim 1 in which the heating elements are composed of carbon black filled polymer.

6. A device according to claim 5 in which one member of the outer housing is provided with a plurality of guide apertures, an electrically insulating ring with depending tubular guides is mounted in the one member with the guides inserted through the guide apertures, the electrical leads are received in the guides, a plurality of terminal pins are mounted in spaced apart relation in a base of electrical insulating material, the electrical leads are electrically connected to respective terminal members, the base member has a plurality of seating portions upon which is received the outer housing, a cover having a closed end is received on the base enclosing the outer housing and a spring is placed between the closed end of the cover and the outer housing and biases the outer housing toward the seating portion on the base.

8. A device for maintaining a relatively constant ambient temperature in an enclosure comprising: an inner and outer housing, the inner housing mounted within the outer housing; the inner housing comprises an electrically and thermally insulating casing having two ends; a seating flange formed in one end of said casing; an electrically insulating and thermally conducting component seating member located within said casing with a flange complimentary in shape to said seating flange and seated thereagainst; a heating element placed in the inner housing about the periphery of said component seating member in close thermal relation thereto; terminals electrically attached to spaced portions of said heater element; an electrically and thermally insulating member closing the other end of said casing; an electrically and thermally insulating cover member closing said one end of said casing; aT least one heating element mounted in the outer housing, each heating element having an anomaly temperature above which the resistivity-temperature curve has a steep-sloped positive temperature coefficient (PTC); the heating element in the outer housing having an anomaly point lower than the heating element in the inner housing, and the heating element in the outer housing electrically connected in parallel to the heating element in the inner, thereby effecting improved temperature regulation.