GB2433847A

GB2433847A - Heat operated electrical isolator

Info

Publication number: GB2433847A
Application number: GB0526551A
Authority: GB
Inventors: James Lee; Jonathan Catchpole
Original assignee: TYCO ELECTRONICS
Current assignee: TYCO ELECTRONICS
Priority date: 2005-12-29
Filing date: 2005-12-29
Publication date: 2007-07-04
Also published as: GB0526551D0

Abstract

An electrical isolator comprises a heat activated isolator 2 such as a thermal fuse in proximity and thermal communication with a heating assembly 3. The isolator 2 interrupts the flow of current in response to heat generated by the heater assembly 3. Heating assembly 3 may be disposed around the outer periphery of isolator 2 and may comprise an array of resistive elements that may be connected to a common circuit board (fig 2A, 5a,5b). Heating assembly 3 may be thermally enclosed in a housing (fig 3, 7) and may comprise a thermally conducting medium and a brace to hold it in close thermal contact with the isolator 2. The isolator may feed a safety critical avionics load such as one comprising a MOSFET (fig 4, 10a, 10b) and a drive circuit (fig 4, 11). Heating device 3 may be activated by a drive circuit (fig 4, 9) in response to a signal from a digital signal processor.

Description

ELECTRICAL ISOLATION DEVICE

The present invention relates to electrical circuit protection and current safety devices, and in particular relates to an electrical isolation device.

There are many types of current interruption and electrical isolation devices known in the art. Each device is required to break an electrical connection to thereby prevent the passage of current through the device. The connection may he broken, for example, by mechanical means so as to physically separate electrical contacts or by increasing a device's resistance by internal heating effects to thereby impede the flow of current.

Conventional circuit breaker technology is routinely used in domestic and industrial applications, and provides a fast and effective method of interrupting current in response to power surges or short circuits. I-iowever, circuit breakers typically contain several moving parts, which act to separate electrical contacts, thereby shortening the longevity of the device and lowering the device's resistance to vibration and other enviromnental effects.

Other current interruption techniques make use of thermal expansion effects in bi-metallic (strip) switches, which can automatically change state (i.e. turn on or off) in response to internal heating of the strip, as the different coefficients of thermal expansion of the metals cause the electrical contacts to part and close depending on the degree of internal heating. Most thermostatic controls are based on this type of switch.

Although the foregoing devices provide useful solutions to the problem of current interruption and electrical isolation. these devices are generally not suitable for more specialised applications, such as in avionic or aerospace circuitry or applications where the device is eposed to significant vibration, as the moving parts may be unreliable due to vibrational exposure and reduced longevity as a result of constant wear and tear.

Moreover, a key requirement of avionic or aerospace components is size and weight. which may not always he optimum for the example devices.

1-lence, there is a need fbr a reliable, space and weight efficient, electrical isolation s device which can withstand the rigors and demands of avionic or aerospace circuitry, having a high degree of redundancy and yet is simple to manufacture and cheap to produce.

An object of the present invention is to provide an electrical isolation device that can rapidly and reliably isolate an electrical current, in which the current remains isolated once the device is activated.

Another object of the present invention is to provide an electrical isolation device that is heat activated, so that electrical current is isolated in response to a generated ambient heat.

Another object of the present invention is to provide an electrical safety circuit that can receive error signals and isolate an electrical current in response to the error signals.

According to an aspect of the present invention there is provided an electrical isolation device, comprising: a heat activated isolator for isolating a flow of electrical current passing through the isolator; and a heating assembly disposed proximate to the isolator and in thermal communication therewith; wherein in response to heat generated by the heating assembly, the isolator acts to isolate the flow of electrical current passing therethrough.

Embodiments of the present invention will now be described in detail by way of example and with reference to the accompanying drawings in which: Figure 1 is a perspective view of a particularly preferred arrangement of an electrical isolation device according to the present invention; Ficure 2(a) is a side elevation view of the electrical isolation device of Figure 1 showing a preferred arrangement of electrical connections: Figure 2(b) is an end view of the electrical isolation device as shown in Figure 2(a); Figure 3 is a preferred arrangement of the electrical isolation device as shown in Figure 2(a), showing the device within a cut-away housing; Figure 4 is a schematic circuit diagram showing the electrical safety circuit of the present invention in a particularly preferred arrangement.

With reference to figure 1 there is shown a particularly preferred arrangement of an electrical isolation device I according to the present invention (hereinafter referred to as the "isolation device"). The isolation device I comprises a heat activated isolator 2 and a heating assembly 3.

is The heat activated isolator 2 is a current carrying element which is operable to isolate a flow of electrical current passing through the isolator 2, in response to a generated ambient heat. The isolator 2 is provided with electrical connectors (e.g. meta1 pins) 2a which pemit the isolator 2 to be connected in series with a circuit in which culTent isolation is required in order to provide circuit protection and electrical safety, e.g. as arising from component failure and/or short-circuits etc. The heating assembly 3 is operable to generate heat so as to cause the isolator 2 to isolate the flow of electrical current passing through it. The heating assembly 3 is disposed proximate to the isolator 2, so as to maximise the heat transfer efficiency between the assembly 3 and the isolator 2. It is found that the most effective heat transfer occurs when the heating assembly 3 is disposed around the outer periphery of the isolator 2, so as to thereby substantially enclose the isolator 2.

The heating assembly 3 may be arranged so as to he in direct physical contact with an outer surface of the isolator 2, so that heat may be transrnitied conductiveiy via the contacting surfaces. To hold the isolator 2 and heating assembly 3 in close contact, the components may be "braced" together using a metallic belt or strap, to exert a squeezing force on the components. However, heat may alternatively, or additionally, he transmitted conductively by an intervening layer of air. where the heating assembly 3, or some part thereof. is not in direct physical contact with the isolator 2.

The isolator 2 and the heating assembly 3 may also he connected by way of a thermally conductive medium, such as a thermal grease or heat transfer compound. which further increases the heat transfer efficiency between the two components, so as to thereby minimise heat transfer losses.

In particularly preferred arrangements, the isolator 2 is a thermal fuse which is responsive to generated ambient heat, whereupon the fuse acts to isolate electrical current passing through the fuse and into the circuit, thereby creating an "open circuit" at the location of tile isolator 2.

Any suitable conventional thermal fuse may be used with the electrical isolation device 1 of the present invention, depending on the required activation time of the device 1 and the amount of heat required to operate (i.e. "trip") the fuse.

Depending upon the particular application, the thermal fuse trip temperature can be matched to the maximum temperature expected to be attained in the vicinity of the fuse by way of operation of the heating assembly 3 (e.g. the temperature of the air surrounding the fuse and/or temperature of the thermal grease in contact with the fuse, that have been heated by the heating assembly 3). Therefore, the activation time can he determined by calculating the period of time over which the generated heat will cause the temperature of the air and/or thermal grease to attain the maximum temperature corresponding to the trip temperature of the fuse.

In preferred arrangements, the isolation device I is configured so that the thermal fuse trips in about 30 seconds, following the initial rise in temperature of the surrounding air and/or thermal grease. However, the activation time may he shorter or longer than this period, depending on the particular application and desired rapidity of response of the isolation device I A typical thermal fuse for use with the isolation device I has an internal resistance of less than about 1 0 milli-ohms (generally about 2 milli-ohms) and a current handling ability of about 5-25 amps. Therefore, during use, the thermal fuse dissipates relatively little power itself and hence generates negligible heat. Thus s the use of a thermal fuse as the isolator 2 is particularly advantageous, while it is also small in size and weight, and permits bi-directional current flow, thereby avoiding the need for a prescribed polarity of connection. Moreover, since the thermal fuse has no moving parts, unlike conventional circuit breakers and bi-metallic strip switches, the isolation device I has increased reliability, longevity of' use and enhanced resistance to vibration, which is particularly useful for avionics or aerospace applications.

In accordance with the invention one possible thermal fuse rating is 1 6 amps, although other ratings are possible.

In preferred arrangements, the heating assembly 3 is a current canying component having an internal resistance, giving rise to heat generation via "ohmic" (or resistive) heating when current is passed therethrough. The current supply to the heating assembly 3 is separate and distinct to that passing through the isolator 2, and therefore the heating assembly 3 is provided with its own electrical connectors 4a and 4b (e.g. metal pins) for connection to a current supply (see figure 2(a)).

Preferably, the heating assembly 3 comprises an array of' electrically conductive resistive elements, such as a plurality of' metal film resistors. Metal film resistors are found to have suitable power and voltage handling capabilities, and are able to withstand the driving currents supplied by the heating assembly drive circuit. In practice, the resistance value of the metal film resistor is selected based on the available power, current or voltage that is to be supplied to the heating assembly 3, SO as to thereby optimise the effect of the ohmic heating. It has been found that resistors of this type may he operalud at appi oximately 10 times their rated power levels, without component failure.

However it is to he appreciated that a wide range of resistor types may he used in the heating assembly 3, provided they have a requisite power handling capability and are able to generate sufficient heat to contribute to the heat required to operate the isolator 2. Examples include, but are not limited to: carbon composition, s ceramic composition, metal oxide, metal glazed, carbon film and wirewound resistors.

Referring to figures 1 and 2, as shown the resistors are clustered around the ouler periphery of the isolator 2 and are connected at their respective ends to a corresponding common circuit board 5a and Sb (e.g. printed circuit board "PCB") in parallel, as illustrated in figures 2(a) & (b). The resistors may be conventionally soldered into position, as shown by the soldered joints 6.

To optimise the heat transfer efficiency, the resistors are preferably arranged to completely enclose the isolator 2, which in the exemplary arrangement of six 0.6 watts metal film resistors, requires the resistors to he spaced angularly by about 60 degrees, as shown in the end view of figure 2(h). However, it is to he appreciated that any number of resistors, having any suitable power rating. may be used in the isolation device 1, depending on the required activation time and the amount of heat required to operate the isolator 2.

Moreover, in some arrangements two or more separate "annular" clusters of resistors may be provided, to further increase the amount of heat generated to operate the isolator 2 and/or decrease the activation time of the device 1.

It should be noted that the heating assembly electrical connectors 4a and 4b arc also respectively soldered to the common circuit boards 5a and Sb. and therefore merging of one or more soldered joints may occur, as illustrated by the top three soldered joints in figure 2(b), the two outermost corresponding to adjacent 3( resistors. Of course, any suitable soldered joint arrangement may be employed to facilitate connecti on of the resistors.

The use of multiple resistors. and hence multiple heat sources. connected in parallel provides an in-built redundancy to a failure of one or more of the resistors, such that the isolation device I remains operable even in the event of such a component failure. The inherent redundancy is advantageous in situations where s the criticality of circuit protection and electrical safety is extremely important.

such as in avionics applications, where circuit failure may have potentially catastrophic consequences.

in preferred arrangements, the isolator 2 and the heating assembly 3 are enclosed within a housing 7 to provide environmental protection for the components and to prevent unwanted cooling effects (e.g. external convective air flows) around the isolator 2 (sec figure 3). Hence, the components are thermally enclosed within the housing 7, which serves to maintain a substantially fixed body of air around the components to thereby optimise the heat transfer efficiency between the isolator 2 and the heating assembly 3.

The housing 7 may have any suitable shape or form, and he made from a polymenc material, such as plastic having a melting point greater than the maximum temperature attained within the housing 7. A plastic housing is advantageous, as it is cheap, easily fabricated and light-weight. However, other materials may be used, such as ceramics, hut these tend to he hea'vier and more expensive to manufacture.

The housing 7 includes a number of apertures which permit the electrical connectors of the isolator 2 and the heating assembly 3 to pass therethrough, permitting connection to the circuit being protected and the heating assembly drive circuit, respectively.

As shown in the example of figure 3, cut-away for clarity, the electrical connectors 2a of the isolator 2 pass [hiough the circuit boards 5a, 5h and may be bent into any suitable shape permitting connection to the relevant circuit.

Typically, the connectors 2a will be soldered to a circuit board supporting the circuil to he protected.

The electrical connectors 4a. 4h of the heating assembly 3 may also be bent into any suitable shape, and can act as convenient mounting points to secure the isolation device 1 to an external circuit hoard. Additional insulated mounting pins 8, may also he provided on the housing 7, so as to secure it to a sparc hole in the external circuit hoard or other convenient anchor point.

Referring to figure 4, there is shown a particularly preferred application in which the isolation device 1 is implemented. The isolation device 1 forms part of an electrical safety circuit 25, comprising a heating assembly drive circuit 9, which is operable to receive a circuit error signal and to provide electrical current to the heating assembly 3 of the isolation device I. The overall circuit in figure 4 represents part of a safety critical avionics circuit 30 Is designed to provideelectrical power to an associated load. The example circuit comprises 2 MOSFETs lOa, lOb (metal oxide semi-conductor field effect transistors) connected in parallel and driven by a MOSFET drive circuit 11. This arrangement provides a convenient power switching device which allows electrical power (235V a.c. input) to he provided to the load (shown as Output to load" in figure 4).

if one of the MOSFETs fails during use, creating an open circuit at the location of that component, the power switching device continues to operate via the remaining MOSFET (with increased power) until ground crew maintenance is available. However, if the failure of one of the MOSFETs is due to a short circuit, the remaining MOSFET may also fail, such as may typically occur if one of the MOSFET gates and neighbouring pins are shorted together. In this scenario, both MOSFETs lOa, I Oh may fail, along with the MOSFET drive circuit II.

Should one of the MOSFETs I Oa, I Oh suffer a short circuit failure, the load Will receive power continuously, leading to possible malfunction of the load and/or potential failure of an associated circuit.

To prevent such risks from occurring, the electrical safety circuit 25 of the present invention is connected between the power switching device and the load of the avionics circuit 30. Thc drive circuit 9 includes an input which may he connected to a digital signal processor (DSP). or other signal monitonng circuit, in or attached to, the avionics circuit 30. The DSP monitors the MOSFET drive and/or other signals, for instance, within the avionics circuit 30, to determine whether any anomalistic currents or voltages are present, so as to thereby generate an associated error signal.

The input of the drive circuit 9 waits to receive any error signals (e.g. a change in input voltage etc.) and in response thereto, will connect the heating assembly 3 of the isolation device I to an associated power supply (e.g. 30 watts at 42V d.c.).

The heating assembly 3 will thereby begin to heat up via ohmic heating processes and following a period of time equivalent to the activation time of the device 1, the isolator 2 will act to isolate the current to the load. In this way, malfunction of the load and/or associated circuit failure may be prevented, and no further current is provided to the load while the isolation device I is activated.

it is to be appreciated that the drive circuit 9 of the electrical safety circuit 25, may be any suitable circuit arrangement that is capable of receiving error signals and connecting the heating assembly 3 to a power supply.

In alternative arrangements, the isolator 2 may be block of semi-conductor material, or other variable impedance material, which is responsive to heat such that the impedance of the isolator 2 increases significantly with increasing temperature. The impedance may be selected so that virtually no current flow is allowed to pass through the isolator 2 when the maximum temperature is attained, thereby achieving the same effect as a thermal fuse.

3(1 The heatmg assembly 3 may alternatively be a coiled airailgement of resistive wire, helically disposed about the outer periphery of the isolator and along its length. The resistance and length of the coiied wire would be selected to provide the requisite heating to the isolator 2. and a sheath could be provided to hold the

S

wire against the surface of the isolator 2. However, use of a single coiled wire provides no inherent redundancy, as a failure of the wire would prevent heating, and therefore multiple concentric coiled wire arrangements would be necessary for reliability.

Although the electrical isolation device of the present invention is idea] for avionics and aaospace applications, it will be recognised that one or more of the principles of the invention could be used in other electrical protection and safety circuits, where rapid and reliable electrical current isolation is required.

Other embodiments are taken to be within the scope of the accompanying claims.

Claims

CLAIMS

1. An electrical isolation device, comprising: a heat activated isolator for isolating a flow of electrical current passing through the isolator; and a heating assembly disposed proximate to the isolator and in thermal comm uni cation therewith; wherein in response to heat generated by the heating assembly, the isolator acts to isolate the flow of electrical current passing thcrethrough.

2. The device of claim I, wherein the heating assembly is disposed around the outer periphery of the isolator, SC) as to substantially enclose the isolator.

3. The device of claim I or claim 2, wherein the heating assembly is arranged to carry an electrical current and to generate heat via ohmic heating.

4. The device of any preceding claim, wherein the heating assembly comprises an array of electrically conductive resistive elements.

5. The device of' claim 4. wherein the array includes a plurality of electrical resistors.

6. The device of claim 5, wherein the respective ends of the electrical resistors are connected to a respective common circuit hoard.

7. The device of any preceding claim, wherein the heat activated isolator is a thermal fuse.

8. The device of claim 7, wherein the thermal fuse is selected to isolate the flow of electrical curren( iii less than about 30 seconds of boat being generated by the heating assembly.

II

9. The device of any preceding claim, further comprising a thermally conductive medium in contact with the isolator and heating assembly.

1 0. The device of any preceding claim, further comprising a brace to hold the isolator and heating assembly in close thermal contact.

11. The device of any preceding claim, further comprising a housing in which the isolator and heating assembly are thermally enclosed.

i o 12. The device of claim 11, wherein the housing is made from a polymeric material.

13. The device of any preceding claim when operatively connected to an safety critical avionics load circuit.

14. The device of claim 13. wherein the load circuit includes a pair of parallel MOSFETs and a MOSFET drive circuit.

1 5. An electrical safety circuit, comprising: an electrical isolation device according to any of claims I to 12; and a means for receiving an error signal and for providing electrical current to the heating assembly of the electrical isolation device, in response to the error signal.

1 6. The circuit of claim 1 5. wherein the means includes an input arranged to receive the error signal from a digital signal processor.

1 7. The circuit of claim 1 5 or claim 1 6, when operatively connected to a safety critical avionics load circuit.

I 8. An electrical isolation device and an electrical satCty circuit substantially as described herein with reference to the accompanying drawings.