Thermal trip arrangements

Details Thermal trip arrangements Thermal trip arrangements

1999-07-09

2001-01-16

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Circuit interruption by thermal sensing

C361S761000, C174S260000

Reexamination Certificate

active

06175480

ABSTRACT:

The present invention concerns improvements in or relating to thermal trip arrangements, that is arrangements in electrical circuits which trip or cause a disconnection in the circuit when a predetermined temperature is exceeded.
Some types of electronic components are over-stretched or age and become irreversibly damaged and may, if connected to a power source, get very hot and even catch fire. In these circumstances use of some form of thermal trip or enclosure in a heat resistant housing becomes essential. Thermal trips which are currently employed include commercially available trip assemblies wired in series with the component which is liable to over-heat, or specifically designed circuitry including an external contact, springs and materials that react in predetermined ways at a specific temperature. The addition of external components of this nature not only increases the cost of the circuitry but adds to the thermal mass, making the device slower to operate.
Attempts have been made to incorporate thermal trip arrangements within the circuit including the component which is liable to overheat, utilising the overheat to trip the circuit. U.S. Pat. Nos. 5,311,164 and 4,486,804 are examples of such prior arrangements but in themselves, they suffer from disadvantages, for example neither are especially suitable for use when the component which is liable to overheat is a varistor. Additionally, these prior arrangements have incorporated components which are surface mounted to the circuit boards or other electronic components supporting elements. U.S. Pat. No. 5,311,164 shows a circuit absorbing element in the form an elongate cylinder which is secured on the surface of a base member and is effectively a separate member of the type described in the preceding paragraphs. The arrangement shown in U.S. Pat. No. 4,486,804 involving the use of coil springs on a component which is provided with standard leads is difficult and time consuming to mount on a printed circuit board.
It is the object of the present invention to obviate or mitigate these and other disadvantages.
According to the present invention there is provided a thermal trip arrangement including a thermal trip arrangement including a varistor mounted to a printed circuit board characterised in that the varistor comprises a voltage dependent resistor disc sandwiched between a pair of metallic electrically conducting discs and in that the printed circuit board provides mounting means for the varistor comprising an aperture through the printed circuit board for slidingly receiving the varistor with the central axis of its discs generally parallel to the plain of the circuit board, conducting tracks of the circuit board leading to each side of the aperture adjacent the conducting disc of a varistor mounted therein and heat sensitive fixing means providing electrical contact between said discs and tracks, the heat sensitive fixing means being adapted to hold the varistor in the aperture and release the varistor therefrom when the fixing means reach a predetermined temperature, the mounting means including also biasing means for urging the varistor out of the aperture.
Preferably the biasing means is resilient means. Alternatively the biasing means is gravity.
Preferably the metallic discs are of a diameter less than that of the metal oxide disc.
Preferably the fixing means is solder having a predetermined melting point.
Preferably an independent track of the printed circuit board connects with at least one side of the disc for use in an indicator circuit.
Preferably the resilient biasing means may be part of a switch/contact used to indicate normal operation of the circuit.
Alternatively, the resilient biasing means may operate a mechanical arrangement to give a visual indication of the operating condition of the circuit.
Further alternatively, movement of the disc under the action of the biasing means is used to operate a switch with a contact or mechanical indicator to indicate the operating condition of the circuit.
Preferably, where a plurality of thermal trip arrangements are provided in a circuit the indicator and/or resilient biasing means are coupled together.
Means may be provided to receive a displaced component.
Those parts of the component not in contact with mounting means may be coated to protect against environmental factors.
The conducting tracks on the printed circuit board may be of predetermined geometry to optimise the amount of solder present and its melting characteristics.
Further according to the present invention there is provided an electrical component for use in an arrangement as defined in the preceding fifteen paragraphs.


REFERENCES:
patent: 2777039 (1957-01-01), Thias
patent: 2869041 (1959-01-01), Cola
patent: 3049647 (1962-08-01), Lincoln
patent: 4359701 (1982-11-01), Aoki
patent: 5124876 (1992-06-01), Misencik et al.
patent: 5311164 (1994-05-01), Ikeda et al.
patent: 5366935 (1994-11-01), Alim et al.
patent: 5420755 (1995-05-01), Hiller
patent: 5999412 (1999-12-01), Busse et al.

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