GB2168540A

GB2168540A - Resistors capable of withstanding power surges

Info

Publication number: GB2168540A
Application number: GB08431263A
Authority: GB
Inventors: George France
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-12-12
Filing date: 1984-12-12
Publication date: 1986-06-18
Also published as: GB8431263D0

Abstract

This invention relates to resistors which are capable of withstanding intermittent surges of high power and which remain within their initial tolerances after being subjected to a number of power surges. The resistors are of the thick film or cermet type. They comprise tracks of resistive material, means 3 to interconnect said tracks and means 2 to electrically connect said tracks to an electrical circuit. Each track is placed parallel with and close to another but spaced therefrom so as to distribute the hot and cold zones over the available surface of the substrate to evenly distribute heat over the substrate when subject to intermittent surges of high power. The resistor tracks are each trimmed by the same amount when the resistor is adjusted to final value during manufacture. <IMAGE>

Description

SPECIFICATION Resistors capable of withstanding power surges This invention relates to resitors which are capable of withstanding intermittent surges of high power.

In some applications of electrical resistors there can occur fault or overload conditions which, for a short time, cause a much higher power to be dissipated in the resistors than is the case under normal operating conditions. Because the circuit is subject to these overloads very infrequently it is desirable that the resistor be capable of withstanding the power surges without significantly affecting its properties after the surge has disappeared. It would be very expensive to design the resistor so that the expected power surge is within its normal rating. Furthermore such a resistor would be very much larger than can usually be accommodated within the space available. Under normal operating conditions the resistor is usually required to have characteristics equivalent to a good quality resistor of the cermet (thick-film) type.

An example of an application where a resistor is required that is capable of handling intermittent surges is the line feed circuit connecting a telephone line to an electronic telephone exchange. Normally resistors are used in pairs on one substrate (one for each line) and are a few hundred ohms in value; a selection tolerance of + - 1% is often required. The resistor power dissipation is typically 250 milliwatts but the specification requires that the resistors shall still be operational and within tolerance after both being subjected simultaneously to an a.c.

overload of 500 vts r.m.s. for approximately 200 milliseconds, repeated three times at one minute interval.

In order to produce resistors which are capable of satisfying this requirement it is necessary to design them so that the temperature gradient across the film is as small as possible also so that the heat dissipating film of the resistor covers as much of the substrate surface as possible. In this way the danger of the substrate cracking under overload conditions can be reduced. If there are large areas of the substrate not dissipating power then there are large temperature gradients when a power surge occurs and the thermal shock resulting from the differential expansion of the hot and cold zones can cause the substrate to fracture. Another mechanism which can result in failure during the overload surface is arcing along the surface of the resistor which causes local heating and, ultimately, cracking of the substrate.

During the manufacture of cermet (thick film) resistors it is usual for the resistors to have a spread in value, one substrate to another, of the order of ± 10% after the firing stage. The finished resistor is usually required to have a design tolerance of + 1% or better. It is the normal practice, therefore, to manufacture the resistors so that their mean value after firing is 90% of the design value and then to adjust the fired resistors to their design value.

At present products for this application are manufactured by printing a single track of ink for each of the pair of resistors along the substrate overlapping conductor bars at the ends of the resistors, the conductor bars connecting with terminal pads on one edge of the substrate. It is evident that those regions of the substrate covered by resistor film carrying current will heat up quickly during an overload surge and those regions of bare substrate together with regions covered by conductor bars and terminal pads can only heat up by receiving heat as it flows from the power dissipating zones.On the application of an overload surge, and for a short time thereafter until thermal equilibrium is established, the large thermal gradient between the hot and cold zones causes a large differential expansion of the substrate and, consequently, large mechanical stresses between the hot and cold zones. During the time before thermal equilibrium is established the substrate will break unless it is strong enough to withstand the mechanical stresses that occur.

Mechanical failure due to thermal shock is very dependent upon the relative size and position of the hot and cold zones and for predictable performance it is necessary that once a satisfactory design is established the relative size of these zones does not vary significantly between one resistor and another. Because conventional trimming to value, either by airbraisive or laser, reduces the track width, both the size of the cold zones and the voltage stress per unit length changes; in the case of resistors with an as-fired spread of ± 10% the track width must vary by as much as 19% of the printed width if all resistors in the distribution are to be trimmed to the design value. The resultant wide variation in hot and cold zone size between different resistors in the population causes a wide range of performance under surge conditions.

In order to reduce these variations trimming to value by adjusting film thickness has been used instead of trimming by adjusting the resistor width. In this method the resistor is sand-blasted over its whole surface until the required value is obtained. This process inhibits the use of overglaze as a protection and also requires resistor inks to have a lower than normal glass content if the severe mechanical abraision is not to give rise to micro-cracking of the film which causes the resistance value to drift after trimming has ceased. Such low glass content resistor films are sensitive to subsequent handling and require to have a protective varnish coat applied so that they do not change in value during subsequent assembly operations.

The present design is also vulnerable to voltage breakdown if the length of the resistor track is not adequate to withstand the voltage gradient that arises when the a.c. surge is applied.

With a surge rating of 500 volts r.m.s. the peak voltage is 707 volts and care must be taken to ensure that the resistor track is long enough to produce a voltage gradient less than the breakdown voltage of the resistor film and any protective coatings which may be applied on top of it. With a single track resistor it is not always possible to meet this requirement within the limitations imposed by the physical size that is required for the substrate.

According to the present invention there is provided a resistor with a multiplicity of tracks connected in series by conductor bars, each track being placed close to and parallel with adjacent tracks so that there are very small cold zones between tracks and as much of the substrate as possible is covered by the resistor film. In this way small cold and hot zones are distributed over the available surface of the substrate and large hot and cold regions are avoided. In addition the long total resistor track length produces a comparatively low voltage gradient along the resistor track, greatly reducing the likelihood of arcing during the voltage surge. The resistors are trimmed to value by reducing by similar amounts the width of each track of the resistor along the full length of the track, thus distributing the trimmed zones evenly adjacent to each track of the resistor.Either airbraisive cutting or laser cutting, using a box cut, can be used. If the resistor has N tracks connected in series then each track is trimmed up in value by, Final Value-Untrimmed Value ohms N In this way the total power is dissipated in equal proportions in each of the N tracks and each track has a similar voltage gradient. Because the cold zones are kept small and are distributed among a number (N) of hot zones the cold zones heat up very quickly by virtue of the short distance to the adjacent hot zones thus reducing the time when severe mechanical stress arises because of the thermal gradient.

A single substrate may have more than one surge handling resistor on one or both faces.

A specific example of the invention will now be described by way of example by reference to the accompanying drawing in which: Figure 1 shows a substrate with two resistors after printing and firing of the conductor and resistor films but before trimming to value.

Figure 2 shows the same substrate as in Fig. 1 but with each limb of both resistors now trimmed to value by a box laser cut.

Fig. 3 shows the same substrate as in Fig. 2 with the terminals attached to the terminal pads.

Referring to Fig. 1 of the drawing the pair of resistors comprises a substrate 1 of alumina ceramic on to which are printed and fired solderable terminal pads 2, interconnecting conductor bars 3 and, for each resistor of the pair six tracks 4, arranged so that the tracks of each resistor are connected in series by the conductor bars.

The same resistor pair, after adjustment to final value, is shown in Fig. 2, where each track 4 of the resistors has been cut 5, by a laser so as to increase its value by one sixth of the difference between the fired value of the resistor and the desired final value. During the laser cutting operation the resistance value is continuously monitored by a suitable measuring instrument connected to the terminal pads 2 and the trim of the final track of the resistor is automatically terminated when the resistance value reaches the desired final value.

Fig. 3 shows the same resistor pair after terminals 6 have been attached by soldering to the terminal pads 2.

The resistor tracks 4 of Figs. 1, 2 and 3 may be covered by a fired overglaze protection which is applied before trimming and is not shown on the drawings fior clarity.

Although the example shows a substrate with two resistors on one face similar techniques are applicable to products with one resistor on one face or one resistor on opposite faces or to products with either one or two resistors on one face and anoither thick film circuit on the opposite face.

Claims

1. A resistor for use in an electronic circuit comprising: (a) a substrate, (b) tracks of resistive material, (c) means to electrically interconnect said tracks, (d) means to electrically connect said tracks to said electronic circuit.

2. A resistor as claimed in claim 1, wherein the tracks are connected in series.

3. A resistor as claimed in claim 1, wherein each track is placed close to another but is spaced therefrom so as to distribute small cold zones evenly over the surface of the substrate to minimise thermal gradients over the substrate and resultant mechanical stress on the substrate during power surges.

4. A resistor as claimed in claims 1, 2 or 3, wherein said tracks are interconnected by conductor bars.

5. A resistor as claimed in claim 3, wherein said tracks are interconnected in series by conductor bars.

6. A resistor as claimed in clam 3, wherein said tracks are parallel to each other.

7. A resistor as claimed in claims 1, 2 or 3, wherein the resistor is of the thick film or cermet type.

8. A resistor as claimed in claim 5, wherein said tracks are parallel to each other and are of the thick film or cermet type.

9. A resistor as claimed in claims 1, 2 or 3, wherein the substrate is an alumina ceramic.

10. A resistor as claimed in claims 1, 2 or 3, wherein the tracks are covered by a fired overglaze protective coating.

11. A resistor as claimed in claims 1, 2 or 3, wherein the tracks are trimmed equally until the resistance value monitored continuously by suitable measuring equipment reaches the desired final value.

12. A resistor as claimed in claims 5, 6 or 8, wherein the tracks are trimmed equally until the resistance value monitored continuously by suitable measuring equipment reaches the desired final value.

13. A resistor as claimed in claims 1, 2 or 3, wherein a plurality of seperate resistor paths are placed on the same substrate.

14. A resistor as claimed in claims 5, 6 or 8, wherein a plurality of separate resistor paths are placed on the same substrate.

15. A resistor as claimed in claims 1, 2 or 3, wherein the substrate has two faces and wherein some of the tracks are placed on one face of the substrate while others are placed on the other face of the substrate.

16. A resistor as claimed in claims 5, 6 or 8, wherein the substrate has two faces and wherein some of the tracks are placed on one face of the substrate while others are placed on the other face of the substrate.

17. A resistor as claimed in claims 1, 2 or 3, wherein the substrate has two faces, some tracks are placed on one face while the others are placed on the other face and a plurality of seperate resistive paths are formed on at least one such face.

18. A resistor as claimed in claims 5, 6 or 8, wherein the substrate has two faces, some tracks are placed on one face while the others are placed on the other face and a plurality of seperate resistive paths are formed on at least one such face.

19. A resistor as claimed in claim 1, 2 or 3, wherein the substrate has two faces and wherein the tracks are placed on one face while any other thick film circuit is placed on the other face of the substrate.

20. A resistor as claimed in claims 5, 6 or 8, wherein the substrate has two faces and wherein the tracks are placed on one face while any thick film circuit is placed on the other face of the substrate.