Positive temperature coefficient resistor heater

Abstract

A temperature regulating heating element comprises a PTCR body having a surface with a pair of adjacent electrodes oppositely connected to a voltage source by two lead wires, respectively. Thus in the PTCR body the electrical current and the source of heat is concentrated near the electroded surface. An electrically insulating layer may cover the electroded surface of the body to prevent shorting of the electrodes by a conducting object that is to be heated by thermal connection to the electroded surface.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrically powered heating elements, and more particularly to positive temperature coefficient resistors (PTCRs), commonly made of doped barium titanate, used as temperature regulating heating elements.

It is known to use a PTCR as a heating element whereby electrical energy is provided from a voltage supply to an electroded PTCR body and whereby the object to be heated and temperature regulated is placed adjacent to or surrounds the PTCR heating element. It is known to control the temperature of temperature sensitive electronic components in this way. Small ovens are also constructed employing these principles. It is also known to control and change the impedance of a second PTCR by placing it in thermal contact with a PTCR heating element.

The most commonly known PTCR element is characterized as having a sharp transition at the so-called anomaly temperature, below which the electrical resistance of the PTCR is low and above which it is several orders of magnitude larger. Thus when connected to a voltage source, the current is initially high, thereby causing the PTCR element to rapidly heat until the anomaly temperature is reached, at which point the current drops to a low value and subsequently little additional heat is generated in the heater.

Such PTCR heating elements are typically formed from a PTCR body having two opposite flat surfaces on which are deposited metal film electrodes which serve as the electrical terminals. Electrical current normally flows between these oppositely disposed electrodes through the PTCR body. The load or object to be heated is thermally coupled to the body at one or both electrodes. As a consequence at least one of the electroded faces transmits thermal energy from the PTCR body to the load, and the regions of the body adjacent to the thermally coupling electrodes are most readily cooled thereby. For this reason the PTCR body tends to switch or change to a high impedance at a plane within the body that is generally parallel to the electroded faces. The aforementioned outer regions tend to remain below the anomaly temperature. When both surfaces are thermally coupled to the load, the temperature profile within the body from electrode to opposite electrode tends to have a maxima near the center of the body and decreasing therefrom to each electroded face. When only one electroded face is coupled to the load, the maxima is shifted toward the opposite electrode.

Thus most of the thermal energy generated must pass through outer regions of the body in order to reach the load. Such bodies have relatively high thermal resistances. Therefore, the conventional PTCR heating element tends to be sluggish in responding to a drop in the load temperature. Even more importantly, the maximum rate at which heat can be transmitted to a load from a conventional PTCR heater is limited by the thermal resistance of the outer regions of the PTCR body.

Finally the conventional PTCR heater, when initially connected to a voltage source draws a high inrush current, typically an order of magnitude higher than the steady state heating current that follows. For many well known reasons, relating to similar characteristics possessed by other devices, such high inrush currents represent an undesirable feature.

It is therefore an object of the present invention to provide a PTCR heating element that is capable of greater rates of heat output.

It is a further object of the present invention to provide a temperature regulating PTCR heating element that is highly responsive to changes in load temperature.

It is a further object of the present invention to provide a PTCR heating element that is capable of more precisely regulating the temperature of a load at or near the PTCR anomaly temperature.

It is yet a further object of this invention to provide a PTCR heating element having a low value of inrush current.

SUMMARY OF THE INVENTION

A temperature regulating heating element comprises a PTCR body, with two sets of alternately disposed electrodes bonded thereto. Each electrode is strip-shaped and adjacent electrodes are members of opposite sets, such that when a voltage source is connected between the two electrode sets, electrical currents in the body tend to flow predominantly near the body surface between adjacent electrodes. Thus only a thin region of the PTCR body is interposed between a load, that is thermally connected to the electroded surface, and the dominant source of the heat that is located near the surface in the body. An electrically insulating layer may be disposed over the electroded surface such that an electrically conductive object to be heated may be thermally coupled thereby to the electroded surface without shorting the electrodes to one another.

BRIEF DESCRIPTION OF THE DRAWINGS

In FIG. 1 is shown a top face view of a PTCR heating element according to a first preferred embodiment of the present invention.

In FIG. 2 is shown a side view of the heating element of FIG. 1.

In FIG. 3 is shown the bottom view of the heating element of FIG. 1.

In FIG. 4 is shown in perspective view the PTCR heating element of FIGS. 1, 2 and 3, having a voltage supply connected thereto.

In FIG. 5 is shown a top face view of a PTCR heating element according to a second preferred embodiment.

In FIG. 6 is shown a side view of the heating element of FIG. 5.

In FIG. 7 is shown a bottom view of the heating element of FIG. 5.

In FIG. 8 is shown in cross section a third preferred embodiment of a heating element of the present invention, the heating element of FIG. 5 having an insulative layer over the electrodes and being mounted in an insulative base, an object to be heated being mounted on the insulative layer.

In FIG. 9 is shown a portion of FIG. 8 in magnified detail.

In the FIGS. 1, 2 and 3 is shown a first preferred embodiment of the heating element of this invention, wherein the same parts in the different views are designated by the same numbers. The PTCR body 10 has a disc shape having two opposite flat and mutually parallel faces and a perimeter face as seen in FIG. 2. On the top face as seen in FIG. 1 are bonded two metal film electrodes 12 and 13, each having a long strip or more particularly a finger-like shape and being separated from each other by about the same distance along their respective lengths. On the bottom or opposite face of body 10 as seen in FIG. 3 there are two metal film electrodes 14 and 15 each having a long finger-like shape and forming a pattern similar to that of electrodes 12 and 13 on the top face.

The preferred means for connecting the electrodes of the PTCR heater of FIGS. 1, 2 and 3 to a source of voltage is shown in FIG. 4. Electrodes 12 and 14 are connected to one terminal 27 of the supply 29 via lead wires 25 and 24, respectively, and electrodes 13 and 15 are connected to the other terminal 28 of the supply 29 via lead wires 23 and 22, respectively. Thus a voltage exists between interdigitated electrodes 12 and 13 on the top face, between electrodes 14 and 15 on the bottom face, between electrodes 12 and 15 over a portion of the perimeter face, and between electrodes 13 and 14 over another portion of the perimeter face. Since the thickness of the body 10 is made to be about equal to the aforementioned distance between electrodes on the same face (e.g., 12 and 13 or 14 and 15), there is created a PTCR body 10, having on its surface a first set of electrodes (12 and 14) and interdigitated therewith a second set of electrodes (13 and 15), each electrode being disposed everywhere adjacent to and about equally distant from electrodes of the other set. Therefore adjacent electrodes are oppositely polarized and heating currents are caused to flow predominantly near the surface in the body 10.

The temperature regulating heating element of this first preferred embodiment is especially well suited for use as a liquid submersed heating element with substantially all body surface areas being uniformly capable of transmitting heat to the surrounding liquid.

In a modification of the heater of the first preferred embodiment, the electrodes on the bottom face, namely 14 and 15, are removed or omitted.

Here, only two electrodes are employed, namely 12 and 13, and the object to be heated would be placed adjacent to the top surface.

In FIGS. 5, 6 and 7 are shown a top, edge, and bottom view, respectively, of a second preferred embodiment of the present invention. A plurality of metal film electrodes are bonded to the top face of the PTCR body 30. Each electrode is comprised predominantly of long strip or finger-shaped portions, the distance between adjacent electrodes being approximately constant. One set of electrodes 32 is interdigitated with respect to another set of electrodes 33. A metal film 34 serves as a connective means tying the set of electrodes 32 together electrically, film 34 having been formed simultaneously with the electrode pattern and being contiguous with electrodes 32. In a similar manner, metal film 35 interconnects with the set of electrodes 33. The two metal films 34 and 35 have extensions 36 and 37, respectively, that extend beyond the top face on to the peripheral face (see FIG. 6) and further onto the bottom face (see FIG. 7).

Two self-supporting leads (not shown) may be connected to any convenient point on the two metal film extensions 36 and 37, respectively; however extensions 36 and 37 themselves serve as film leads to electrode finger sets 32 and 33, respectively. When a voltage source, either a.c. or d.c. is connected to the two leads, the PTCR heating element will heat and regulate the temperature of an object that is placed in thermal contact with the top electroded face.

In FIG. 8 is shown in cross section the PTCR heating element of FIG. 5, having a thin electrically insulative layer 39 bonded to and covering the top electroded face. The body 30 is held by bonding or by mechanical fasteners (not shown) in a base 50 made of an insulative material such as polysulfone, and is seated at mating surfaces 51 therein. A cavity 52 is provided under the body 30 wherein a leaf spring 53, made for example of steel or of a beryllium-copper alloy, is compressed between the floor of the cavity and metal film extension 36 of the PTCR body. Another leaf spring 54 (not shown) is similarly compressed between base 50 and the other metal film extension 37. Leads 55 and 57 are connected to the leaf springs 53 and 54, respectively. The leads 55 and 57 have insulating jackets 56 and 58, and they exit the base 50 through a hole provided for this purpose. Thus when a voltage source is connected externally to leads 55 and 57, the electrode sets 32 and 33 are oppositely polarized.

A metal cup 60 is shown resting in contact with the insulative layer 39. The cup 60 contains a liquid 61 to be heated and temperature regulated. The insulative layer 39 provides electrical insulation between the metal cup 60 and the sets of electrodes 32 and 33. The layer 39 is made only as thick as is necessary to provide this insulating function and may therefore be only a few thousandths of an inch thick over the electrodes when the voltage source is a 110 V.a.c. line and the insulative layer material is a fluorocarbon resin such as Teflon (a Tradename of Dupont) or other high temperature insulating materials such as polysulfone, polyimide, or Parylene (a Tradename of Union Carbide). A number of well known methods of applying a uniform insulating coating are suitable, such as depositing the resin in liquid form over the electroded face, curing at an elevated temperature and bonding the layer to the face. Also vapor deposition of Parylene provides an exceptionally uniform layer and requires no subsequent curing.

The thin layer 39 additionally serves to provide the thermal coupling between the body 30 and the cup 60. It represents a small thermal resistance when its thickness to area of thermal contact is as small as 0.001 inch per square inches. However, about one order of magnitude smaller thermal resistance can be realized by making the layer 39 of a ceramic, or glass, or a mixture thereof as described in U.S. Pat. No. 3,619,220 by Maher filed Sept. 26, 1968.

In FIG. 9 is shown a detail of FIG. 8 drawn to a larger scale, wherein a dotted line 30c represents an isothermal surface being at the PTCR anomaly temperature in the PTCR body 30. It is believed that such an isothermal surface exists very close to the electroded face when heat is being taken from that surface by an adjacent object such as the metal cup 60, and electric currents flow between adjacent electrodes 32 and 33 in the body 30. The body region 30a is at a temperature below the PTCR anomaly temperature and the body region 30b is at a temperature above the anomaly temperature. Thus the electric heating currents are predominantly flowing in the surface region 30a, and the thermal resistance of the bulk of the body 30 is of no consequence to the dynamics of heating.

The conventional PTCR heating element employing oppositely polarized electrodes on opposite faces of a PTCR body operates in a manner analogous to an electrical series resistor circuit. That is, the relative power dissipated in each element is proportional to its resistance. When the surface of a conventionally electroded PTCR is coupled to a thermal load its temperature will drop. This causes the incremental resistance to drop which lessens the power being dissipated at the surface, and consequently the most power is generated in the warmer higher resistance portions of the PTCR element. Thus the power must be transferred from the higher resistance area deeper within the ceramic through the thermal resistance of the cooler surface layer.

In the present invention, electrical heating currents are forced to flow predominantly near the PTCR body surface between adjacent and oppositely polarized electrodes that are disposed thereon, thus decreasing the distance and therefore the thermal resistance between the major source of heat and the object being heated. Most applications for a PTCR heater, whether conventional or of this invention, require an electrical insulating layer, such as a layer 39 in FIG. 8 herein, to be provided between the one or more load adjacent electrodes and the load to be heated.

Prototypes were made according to the third preferred embodiment as shown in FIGS. 5, 6 and 7. The body is comprised of a standard barium titanate doped with niobium as is described in the paper, PTCR Substrate Heaters for Planar Silicon Devices by F. Kahn, presented at the Electronics Components Conference in Washington, D.C., in May 1972. In principle any PTCR material will be appropriate. The PTCR disc shaped body had a diameter of 1.2 inches and a thickness of 0.1 inch. The electrodes were formed by a standard silver paste screening and firing process. Conventional PTCR heaters were also made using PTCR bodies as above described but having solid film electrodes oppositely disposed on the two large surfaces. The conventional units served as controls in subsequent testing.

In one test, the prototypes and the controls were thermally coupled to an aluminum block (weighing 1,050 grams) via an interspersed 0.001 inch thick sheet of MYLAR (a Dupont Tradename for polyethyleneterephthalate) and connected to the 120 V.a.c. line. In Table I are shown the best results, where t is the time to raise the temperature of the aluminum block by 30.degree.C, I.sub.av is the average current drawn, and Ip is the peak inrush current.

TABLE I ______________________________________ Prototypes Controls ______________________________________ t 327. seconds 460. seconds I.sub.av 0.76 amps 0.57 amps Ip 2.3 amps 7.0 amps Ip/I.sub.av 3 12.3 ______________________________________

It can be seen that the prototypes made according to this invention raised the temperature of the block by 30.degree.C in about 30 percent less time than the time required for the controls. In addition the inrush to average current of the prototypes was less than a quarter of that in the controls.

Claims

What is claimed is:

1. A PTCR heater comprising a disc of PTCR material, said disc having two opposed flat surfaces and a perimeter face, a first pair of arcuate interdigitated metal film electrodes bonded to one of said surfaces, said electrodes each having a finger-like shape, the separation between adjacent portions of said electrodes being substantially constant, an electrical connective means for connecting the alternative of said electrodes to one lead and the remained of said electrodes to another lead, said leads being capable of connection to an electrical energy source such that said adjacent electrodes are oppositively polarized and the heating electric currents in said disc flow predominantly near said one of said surfaces.

2. The PTCR heater of claim 1 wherein said means include metal films on said one surface contacting said electrodes, said films extending over said perimeter face onto the other of said surfaces.

3. The PTCR heater of claim 1 wherein a second pair of arcuate interdigitated electrodes is bonded to the other of said surfaces, said second pair of electrodes having a pattern similar to the pattern of said first pair of electrodes, said disc having a thickness substantially equal to said separation.

4. The PTCR heater of claim 3 wherein said means include connections such that an electrode of each of said pairs of electrodes is at one polarity and the other electrode of each of said pairs of electrodes is at the opposite polarity, whereupon heating currents in said disc also flow near said perimeter face.

5. The PTCR heater of claim 1 additionally comprising an electrically insulating layer disposed over said electrode pattern and over the intervening areas of said surface.

6. The PTCR heater of claim 5 wherein said layer has been deposited over said pattern and said intervening areas of said surface in liquid form, cured and bonded thereto.

7. PTCR heater of claim 5 wherein said insulating layer consists of glass.

8. The PTCR heater of claim 5 wherein said insulating layer consists of ceramic.

9. The PTCR heater of claim 5 wherein said insulating layer consists of a glass ceramic composite.