GB2259186A

GB2259186A - A ptc device having three terminals

Info

Publication number: GB2259186A
Application number: GB9118423A
Authority: GB
Inventors: David Andrew Smith
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1991-08-28
Filing date: 1991-08-28
Publication date: 1993-03-03
Anticipated expiration: 2011-08-28
Also published as: GB2259186B; GB9118423D0; WO1993005523A1; JPH06510166A; AU2483392A

Abstract

A device having a positive temperature coefficient of resistance is formed of a ceramic or polymeric material and has first and second terminals 1, 2 for current flow through the device and a third terminal 3 which can be utilized either to sense the condition of the device or to control the condition of the device. The device is suitable for use as a voltage-controlled thermostat or heater. <IMAGE>

Description

IMPROVEIIENTS RELATING TO EERMAIIgY RESPONSIVE DEVICES This invention concerns improvements relating to thermally responsive devices and more particularly concerns thermally responsive devices comprising PTC elements, that is to say elements formed of electrically conductive materials which exhibit a sharp increase in resistivity over a defined temperature range.

Doped ceramic PTC materials have long been known and are advantageous in that their electrical properties are stable, but disadvantageous in that their electrical properties are not always well suited to their desired applications and also as regards their mechanical limitations. Conductive polymeric PTC materials, i.e. polymers having a particulate conductive filler dispersed therein, have recently been the subject of research and have been proposed to be used in heaters and in electrical circuit protection devices. However, insofar as we are aware such proposals for use of polymeric PTC materials have concentrated upon their self regulating effect in response to through currents and other applications have been neglected.

According to the present invention there is provided a PTC device, preferably but not essentially a polymeric PTC device, having at least three terminals, namely first and second terminals designated as current flow terminals and a third terminal designated as a sensor or control terminal.

Thus in accordance with a first embodiment of the invention, a PTC device can be configured such that the voltage at the third terminal is dependent upon the temperature of the device. In accordance with a second embodiment of the invention, the device can be configured such that the third terminal determines the current flow between the first and second terminals by virtue of enabling the injection of a current into the device to determine the temperature of the device.

Any suitable PTC material can be utilized in the practice of the invention, but it is preferred to utilize a polymeric PTC and particularly, though not exclusively a polyolefin, e.g. polypropylene, incorporating one or more conductive fillers and preferably also incorporating a non-conductive filler such as a fibre to provide stability and reproduceability. Carbon black is the preferred conductive filler and we prefer to utilize a relatively low proportion (0 to 5%) of a high conductivity carbon black (such as Ketjenblack EC 600 from Akzo) and a relatively high proportion (0 to 30%) of a carbon black such as to make a substantial contribution to the PTC effect (e.g. Elflex 120 from Cabot Corporation). The non-conductive filler may for example be from 0 to 40t glass fibre. The percentages quoted are by weight.

The invention will best be appreciated from consideration of the following detailed description given with reference to the accompanying drawing wherein the single figure is a schematic showing of a PTC device configured for use as a current or temperature sensor.

Referring to the drawing, schematically illustrated therein is a PTC device, which may be of a ceramic or polymeric material as aforesaid, connected in series with a load and a voltage source. The PTC device has electrodes 1 and 2 designated as current terminals and has a third electrode 3 designated as a sensor terminal and shown coupled to a digital volt meter. In normal operation of the circuit thus shown, a current IN can flow through the device between terminals 1 and 2 at a range of temperatures to TTRIp-DX (where DX is a low value > 100C) and under these conditions the voltage at the third terminal 3 will be close to the circuit supply voltage Vs.If the state of the circuit changes by reason of the current rising above IN to ITRIp thereby causing the device temperature to rise to TRIP, or alternatively if a combination of the self-heating of the device and the ambient temperature causes the device temperature to rise to TTRIP, then the resistance between the two current carrying terminals 1 and 2 will change from a low resistance typically less than 10 ohms to a high resistance state where its resistance is many times (typically 102 to 1010 times) greater than its low resistance state.In this high resistance state of the device, the voltage at the third terminal 3 will change from its initial state close to V8 to a new voltage VNEW = VS I1-3 x R1-3 where R1-3 is the resistance between terminals 1 and 3 and I1-3 is the current flowing between terminals 1 and 3.

The PTC device is preferably planar with the current carrying. terminals 1 and 2 of maximized areas and with minimized curreiit path lengths through the device, so as to minimize the device resistance so that the device has no significant effect upon the functioning of the load with which it is connected in series. The terminals may be formed as metallized areas on the device, particularly in the case of a PTC device formed of a ceramic material such as doped barium titanate for example, or may be embedded into the device in the case of a PTC device formed of a polymeric material.

Currently available PTC devices utilized for their current protection facilities do not (so far as we are aware) incorporate any facility for enabling the state of the device to be monitored, and the present invention as embodied in the device described hereinabove provides this facility. As motors and other automatic applications of PTC devices become more complex, so there is a requirement for built-in diagnostic facilities and the present invention is seen as satisfying this need, both on a real time basis and on a basis of monitored information logged over a relatively long time period. Not only does the third terminal provide a voltage sensing facility, but also it enables the current flow in the device to be monitored (since the voltage is proportional to current flow) which correspondingly provides an indication of the current flow in the series-connected load.In the case of a load constituted by an electric motor for example, information regarding the running condition of the motor could thus be obtained and could be utilized in a feedback control arrangement. This could be most useful, for example, in window lift motors and like applications for determining friction settings. A feedback arrangement could also be useful in providing information to the driver of a vehicle for example regarding the fact that a motor (such as a window lift motor) of the vehicle was overheating.

In an alternative application of a three terminal PTC device as above described, the injection of a current flow between terminals 3 and 2 of the device can be used to control current flow between terminals 1 and 2. The current between terminals 1 and 2 will normally be below the level necessary to cause selfheating of the device to such an extent as to cause it to become non-ohmic and, with the device in this condition, it will readily be appreciated that the passage of an additional current through the portion of the device between terminals 3 and 2 can be such as to cause that portion of the device to be heated above the temperature at which it becomes non-ohmic. Since only a relatively small current is required to be injected in order to hpld the device in a state wherein the portion between terminals 3 and 2 has a relatively high resistance, this injected current being smaller than the normal current flow between terminals 1 and 2, it can be seen that the device has current gain. On removal of the injected current, the portion of the device between terminals 3 and 2 will cool below its critical temperature at which it becomes non-ohmic and the initial normal current flow will be restored. The device thus exhibits bistable characteristics.

The current which flows between terminals 1 and 2 after removal of the current injected at terminal 3 is dependent upon a number of factors, including, the voltage between terminals 1 and 2, the ambient temperature, the thermodynamic equilibrium of the device, the heat sinking of the device, the voltage applied to the third terminal 3, and the material composition of the PTC device.

A PTC current control device as hereinabove described would be relatively inexpensive to manufacture, capable of handling relatively high currents and would work equally well on alternating and direct current. It could be utilized as an adjustable (voltage controlled) thermostat or as a voltage controlled heater. The cut-off characteristic of the device is capable of being adjusted by selection of the composition f the PTC material, and also can be influenced by the electrode geometry; for example, by arranging electrode 3 so that it extends non-parallel to electrodes 1 and 2 a more gradual cutoff characteristic will be achieved.

Claims

CLAIMS:

1. A PTC device having first and second terminals for current flow through the device and having a third terminal arranged for utilization as a sensory and/or control terminal.

2. A PTC device as claimed in claim 1 wherein the first and second terminals are spaced apart from each other and the third terminal is between the first and second terminals.

3. A PTC device as claimed in claim 1 or 2 wherein the device is formed of a ceramic material.

4. A PTC device as claimed in claim 1 or 2 wherein the device is formed of a polymeric material.

5. A PTC device as claimed in claim 4 wherein the polymeric material comprises a polyolefin containing an electrically conductive particulate material and an electrically non-conductive fibre material.

6. A PTC device as claimed in claim 5 wherein said conductive material comprises carbon black.

7. A PTC device as claimed in claim 5 or 6 wherein said non-conductive material comprises glass fibres.

8. a PTC device as claimed in claim 5 or 6 or 7 wherein said polyolefin comprises polypropylene.

9. A PTC device as claimed in any of the preceding claims connected in an electrical circuit wherein the third terminal of the PTC device provides a sensory function indicating the status of the device.

10. A PTC device as claimed in any of claims 1 to 8 connected in an electrical circuit wherein the third terminal of the PTC device provides a control function determining the status of the device.