US 3805022 A
A heat responsive switch for controlling current flow through a zinc oxide varistor heater. The switch in its broadest form can take the form of a thermostatic device or a PTC device positioned electrically in series with the varistor circuit and physically heat coupled to the varistor whereby the current conducting properties of the PTC device are determined by the temperature of the varistor.
Description (OCR text may contain errors)
nited States Patent [191 lfiulwicki et a1.
SEMICONDUCTING THRESHOLD HEATERS Inventors: Bernard M. Kulwicki, Foxboro,
Mass.; George Trenkler, East Providence, R.I.
Assignee: Texas Instruments Incorporated,
Filed: Oct. 10, 1972 Appl. No.: 296,397
U.S. Cl 219/505, 219/504 Int. Cl. H05b 1/02 Field of Search 219/504, 505, 523; 338/20,
References Cited UNITED STATES PATENTS 12/1963 Boort et a1 337/23 9/1972 Matsuoka et al. 338/20 8/1967 Marcoux 219/505 X 12/1967 Marcoux 219/504 X [451 Apr. 16, 1974 3,476,293 11/1969 Marcoux 219/504 2,248,623 7/1941 Hand 3,586,939 6/1971 Buiting 337/107 X FOREIGN PATENTS OR APPLICATIONS 892,318 3/1962 Great Britain 327/23 Primary Examiner-Bernard A. Gilheany Assistant Examiner-F. E. Bell Attorney, Agent, or Firm-John A. Haug; James P. McAndrews; Harold Levine  ABSTRACT A heat responsive switch for controlling current flow through a zinc oxide varistor heater. The switch in its broadest form can take the form of a thermostatic device or a PTC device positioned electrically in series with the varistor circuit and physically heat coupled to the varistor whereby the current conducting properties of the PTC device are determined by the temperature of the varistor.
5 Claims, 4 Drawing Figures SEMICONDUCTING THRESHOLD HEATERS The disclosure relates to a varistor heater with current limiting capability provided when the threshold of the varistor is exceeded and, more specifically, to a heat responsive switch placed in electrical series with a varistor and heat coupled thereto for controlling the current flow through the varistor heater.
The use of semiconductors such as zinc oxide as heaters is well known in the art. Heaters of this type are nor mally placed in good thermal contact with the load to be heated and are capable of transferring a great deal of heat to the load due to their good thermal conductivity characteristics. However, such materials possess a linear current-voltage relationship. Zinc oxide varistors display slow current rise with increase in voltage thereacross up to a threshold voltage V whereupon the current flow therethrough increases very sharply. For this reason, zinc oxide varistor heaters, upon reaching this threshold voltage, can provide a large power output, mainly in the form of heat, since the voltage times current product, caused mainly by a substantially exponential increase in current flow, increases rapidly. It is clear that a heater of this type can quickly run away and burn itself out. For this reason, the use of the varistor as a heater has not been practical.
In accordance with the present invention, there is provided a switching element which is placed electri cally in series with the varistor heater and is thermally coupled thereto. The switch is responsive to a predetermined temperature of the varistor to quickly lower or completely remove the current flow through the varistor and thereby prevent failure thereof from thermal runaway. In accordance with one embodiment of the invention, the switching elements can be a bimetallic device of well known type. In accordance with a second embodiment of the invention, the switching element can be a PTC device of the typehaving very little change in resistance thereof due to changes in tempera ture until the Curie point has been reached. At this point, the PTC device displays a very rapid rise in resistance. It can be seen that by placing such a PTC device in electrical series with a varistor, the beneficial properties of the varistor can be utilized with heat runaway completely eliminated. Zinc oxide varistors are particularly advantageous because of their very sharp threshold from low to high electrical conductance.
It is therefore an object of this invention to provide a semiconducting threshold heater of the varistor type having a current limiting means responsive to the temperature of the heater.
It is a further object of this invention to provide a varistor heater having a PTC device in electrical series therewith and thermally coupled thereto for limiting the current passing through the varistor.
It is yet a further object of this invention to provide a thermal runaway preventing device for a varistor heater.
The above objects and still further objects of the invention will immediately become apparent to those skilled in the art after consideration of the following preferred embodiments thereof, which are provided by way of example and not by way of limitation, wherein:
FIG. 1 is a circuit diagram of a varistor in electrical series with a heat responsive switch for limiting current thereto;
FIG. 2 is a circuit diagram of a varistor heater in electrical series with a PTC device and thermally coupling therewith;
FIG. 3 is a graph showing the current-voltage properties of the varistor of FIGS. l and 2; and
FIG. 4 is a graph showing the resistance-temperature properties of the PTC device of FIG. 2.
Referring first to FIG. 1 there is shown a load Q to which heat is to be applied. A zinc oxide varistor Z is thermally coupled thereto and has a contact 1 thereon for making contact with a contact 2 of a bimetallic element 3. A voltage V is applied between the bimetallic element 3 and the varistor Z. The contact 2 of the bimetallic element 3 is designed to break contact with the contact 1 when the varistor Z reaches a predetermined temperature due to bending of the bimetallic element away from contact 1. Since the bimetallic element 3 is in heat couplingrelationship with the varistor Z, it will bend toward the right as shown in FIG. I and remove or break current flow to the varistor. It can be seen that a switch of this type will cause intermittent current flow to the varistor Z and prevent overheating and thermal runaway thereof.
Referring now ,to FIG. 2, there is shown a second embodiment of the invention wherein a load Q is to be heated. A zinc oxide varistor Z, the same as in FIG. l, is placed in heat or thermal coupling relationship with the load and also in thermal coupling relation with a PTC device P. The varistor Z and the PTC device are placed in electrical series relationship with a voltage V thereacross. The embodiment of FIG. 2 provides several advantages. To begin with, the thermal conductivity of a zinc oxide varistor is approximately 10 times greater than the thermal conductivity of a PTC device. Therefore, by utilizing the two elements in combination as described in FIG. 2, the beneficial properties of the varistor as to heating properties are utilized whereas the detrimental properties thereof with regard to power runaway are overcome by use of the PTC device. Alternatively, though the PTC device provides excellent results with regard to power usage, the thermal conductivity thereof is poor. It can be seen that the combination of the two elements therefore provides the best of both and eliminates the detrimental properties of each. Furthermore, the embodiment of FIG. 2 has definite advantages over the embodiment of FIG. 1. It is readily apparent that the constant opening and closing of the switch of the type shown in FIG. 1 will eventually cause deterioration of the contacts and ultimate failure of the device. The embodiment of FIG. 2 does not require constant opening and closing of switches and, in fact, acts more as a current control rather than an on and off type of control.
Referring now to FIG. 3, the current versus voltage curve for the varistor is set forth. The term V therein is the threshold point. Below the threshold voltage V the varistor does not conduct much current. Above the threshold the current rises sharply.
Referring now to FIG. 41, there is shown a graph of the resistance temperature characteristics of the PTC device. The term T is the threshold or Curie point. Below the Curie point T the resistance is reasonably constant. Above the Curie point the resistance rises exponentially with temperature.
The varistor threshold is chosen to be slightly less than the applied voltage. The initial resistance of the PTC device is chosen to be low so that most of the power is generated within the varistor. It is desirable that the power generated in the varistor be much greater than the power generated in the PTC element so that the PTC element will be heated primarily by thermal transfer from the varistor and will limit the power to the varistor only when the power transfer to the load becomes low. The PTC element is chosen so that its Curie point T is reached at a temperature that will not allow overheating and damage to the varistor.
Threshold heaters of the type described in FIGS. 1 and 2 possess several unique properties and advantages. One of these is that they operate only when the voltage threshold of the varistor is exceeded. A second advantage is that very large power outputs, compared with a PTC element alone, are possible since the thermal conductivity of zinc oxide varistors as mentioned supra is approximately ten times higher than that of doped barium titanate PTC devices. A further advantage is that the large power outputs can be controlled with a low resistivity PTC element which has minimal voltage blocking capability. Low resistivity PTC elements are much easier to produce to close tolerance and with high yield compared against the high resistivity elements with good voltage blocking capability, which are needed for line voltage applications using PTC elements alone. It is possible to use PTC elements with minimal voltage blocking capability since only a small fraction of the applied voltage is applied across the PTC element.
Devices of the type described above can be applied in temperature-limited heaters in general as well as in devices wherein the threshold properties of the varistor are used to advantage. One example of the latter category is a low voltage switch wherein the heater is coupled to a bimetallic element (the load) which would open a pair of contacts when the line voltage decreases, this protecting electric motors from brownouts. A second example might be a similarly designed motor starting relay which cuts out the start winding of an electric motor when the inductive voltage (back EMF) builds up so that the threshold of the varistor is exceeded.
Although the invention has been described with respect to specific preferred embodiments thereof, many variations and modifications thereof will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.
1. A heating device which comprises, in combination:
a. a heating element which has slow current rise with increase in voltage thereacross up to a threshold whereupon current flow through said element increases rapidly with increase in voltage, and
b. PTC means thermally coupled to said heating element and in electrical series connection therewith and responsive to a predetermined temperature of said heating element to substantially increase the electrical resistance in series with said heating element when said pre-determined temperature has been reached.
2. A heating device as set forth in claim 1 wherein said heating element is a zinc oxide varistor.
3. A heating device as set forth in claim 1 wherein said PTC means is a barium titanate PTC device.
4. A heating device as set forth in claim 1 further including a voltage source in series with said heating element and said PTC means for providing a predetermined voltage thereacross, said threshold of said heating element being lower than said predetermined voltage.
5. A heating device as set forth in claim 4 wherein said predetermined temperature at which said PTC means substantially increases its electrical resistance is a temperature below that which would cause damage and overheating to said heating element.