GB2245440A - Split element electric blanket having increased fault current ratio - Google Patents

Abstract

A heating circuit comprising two resistances R1, R2 in series with a semiconductor device 5, R1 being at least double the value of R2 and shunted by at least one semiconductor device D1, D2 of opposite polarity to device 5. The semiconductor elements 5, D1. D2 ensure that wherever contact between R1, R2 occurs, sufficient fault current is drawn to rupture fuse 4. <IMAGE>

Description

A CIRCUIT HAVING FLEXIBLE HEATING ELEMENTS FOR USE IN ELECTRICAL BLANKETS AND THE LIKE.

This invention relates to acircuit having flexible heating elements, particularly for use in electrical blankets or the like, of the type including a couple of coaxial electrical resistances kept apart by an insulating means.

In the circuit according to the invention, the resistances are connected in series with the interposition of a semiconductor element Snd one of said resistances has a higher value with respect to the other resistance.

The higher value resistance has at least one semiconductor element parallel connected thereto, which is of opposite polarity to the first mentioned semiconductor.

Heating elements for use in electrical blankets are known and comprise a couple of resistances coaxially wound on a filament-shaped core and spaced apart by an intervening insulating material, such as PVC.

This insulating material, when attaining a certain temperature, melts to cause a contact- between the two resistances, whereupon a substantial rise in absorbed current occurs which is sufficient to cause blowing of a safety fuse arranged in series with the circuit.

This system utilizing a fuse as a protective means does, however, not provide sufficiently safe protetion, in that the value of the absorbed current may vary according to where the contact of the two resistances is taking place-.

For this reason, it is often preferred that a thermofuse is employed which, however, is rather high in assembly cost because of this thermofuse being not suitable for automatical soldering by means of usual wave soldering devices as used for printe-d circuits.

As a result, a manual very expensive soldering operation must be effected.

Moreover, the times for a thermofuse to act may substantially vary depending on where the short is occurring.

In other cases, use is made of a normal current fuse but in order to make sure that this latter does melt whatever the point where contact of the two resistances occurs, it is necessary that an insulating material, such as polythene, is employed which permits shorts to be produced in several places thereby causing a further increase in absorbed current, so that interrup ion of the fuse takes place before melting of its outer coating - usually of PVC - which would endanger the primary insulation of the heating element.

However, polythene has a drawback in that it melts at very low temperatures which results ip large quantities of waste product being returned from the market.

Therefore, a need is felt in this industrial field for means which enable use to be made as an insulation member in this type of heating filaments, of a material, such as PVC,to ensure that the protective fuse should melt in case of shortcircuit, whatever the point where this short may occur.

More particularly, this invention provides a circuit in which the heating elements are a couple of coaxial resistances connected in series with the interposition of a semiconductor, one of the resistances being of higher valow as compared with the other resistance, and in which, arranged in parallel with said higher value resistance, is a semiconductor of opposite polarity to the first mentioned semiconductor.

This invention will now be described in further details, by way of example only and with reference to the accompanying drawings wherein - Figure 1 shows a length of a heating element according to the invention; - Figure 2 shows a circuit having the flexible heating elements of the invention; - figure 3 is-a diagram of a circuit according to the invention sho-wing the control devices for the current.

Referring now to figure 1, the flexible heating elements utilized in the circuit of the invention, essentially consist of a couple of coaxial- resistances R1, R2 wound on- a core 1 formed of a filament-shaped element of polyester or Rayon, the two resistances being separated by an internal insulating layer 2. preferably made from PVCA and externally covered by a second layer 3 of insulating material, which is also of PVC.

The terminals of the two resistances R1, R2 are indicated by reference letters A,B and A',B', respectively.

According to the invention, the two resistances are con nected in series, preferably at the terminals thereof- that are at a same end of the heating element and, provided in series with resistances R1, R2 is a protective fuse 4.

In order that a PVC insulating element between resistances R1, R2 can be safely utilized, it is necessary that in case of a short there is sufficient current absorbed to cause blowing of fuse 4,whatever the point where contact of said reistances R1, R2 may occur.

In this connection, it is to be noted that the delayedtype of fuses manufactured according to electrical stan-dards, melt within two minutes when a current equal to 2.1 times the rated current is flowing thereacross.

In view of the fact that the supply voltage may undergo changes of the order of + 10% and the value of the heating element has a manufacturing tolerance of + 5%, it follows that in order to be able to use a fuse without any risk, it is necessary that in case of a short the absorbed current should be at least 2.8 times as high as the rated current.

Then, if we consider that usual, commercially available fuses only cover a certain range of values, it is clear that the greater the shortcircuit current with respect to the rated current, the more simplified the choice of the fuse to be used will be.

The circuit of the invention permits this value to be attained and exceeded.

To this end, interposed between the resistances R1, R2 is a semiconductor element 5, in this case a SCR whose GATE is controlled by a circuit 6 to be described hereinafter and which serves as a control circuit for regulating the supply current and, thus, the temperature of the heating elements.

According to one feature of the invention, the resistances R1, R2 have different values, in particular, R1 has a higher value with rcspect to R2. 2 Connected in parallel to the higher value resistance, in this case the resistance R1, are a couple of semiconductors, such as two diodes D1, D2 that are arranged so as to be reversed in polarity with respect to the SCR 5.

With the above described circuit ar-rangement, for values of R1 equal to twice the value of R2, any contact of the two resistances due to localized overheating capable of melting the insulating element 2, will be sufficient to increase the absorbed current to at least 3.1 times the rated current, with instantaneous blowing of fuse 4 as a result.

EXAMPLE Assuming, for example, R2 = 1 R1.

6 For example, R2=100,) and R1=600# , with supply voltage of 220 V.

When the SCR is in a conducting state, the rated (1 rated) current of the circuit is equal to

It is, in fact known, that SCR is conducting for one half wave only.

Let now assume that a contact of R1 and R2 occurs at the terminal B/-B'. The effective absorbed current will equal

rated.

Assuming now that a contact is produced between P and R2 at the middle point in the path, thereby to cause less shortcircuit current to be absorbed. Due to the existence of diode D1 which is shortcircuiting R1 during the negative half wave, the equivalent resistance of the circuit during this half wave will be equal to 1/4 R1 + 1/2 = 200In,, whereas during the positive half wave, the equivalent resistance will equals 1/2 R1 + 1/2 R2 = 350.sL .

Therefore, the shortcircuit current will be equal to:

rated.

In any other place the contact between R1 and R2 should occur, the shortcircuit current will always be higher than that resulting from a contact at the middle point and, thus, more than sufficient to melt the fuse.

Moreover, this circuit has a further advantage in that R2 is effective to act as a protection against current peaks (that is to act as a current limiter) for both the diode D1 and the SCR which latter, should they be arranged directly across the supply line, could easily be damaged to cause themselves a shortcircuit with the result of the product being endangered.

Furthermore, an optimum safety is achieved in the case when the SCR (due to its being malfunctioning) should itself become a short. In such a case, in fact, there would be an instantaneous rise in current because of D1 bypassing R1.

In case of any malfunctioning of diode D1, the diode D2, which is connected in parallel with, and has the same orientation as D1, wo-ul-d be effective to- act in lieu and place of this latter.

On the other hand, should the contact of resistances occur at points A and A', absorption of current by the circuit will tend to the infinite with instantaneous blowing of fuse 4 being the result.

Figure 3 is a diagram of a circuit according to the invention showing the devices that are capable of driving the GATE of SCR in order to vary the power supplied to resistances R1 and R2.

This control circuit of the GATE is shown by way of example only and any other suitable regulating < levi-ce may be a substitute therefor.

The circuit of the invention ha-s higt safety properties; the only damage capable of endangering its efficiency being the possibility of the diode D1 becoming an open circuit.

In order to overcome this problem, it is sufficient to provide a second diode D2 arranged in parallel with the first mentioned diode D1.

It is to be noted that the series connection of the resistances at the two terminals thereof that are on the same side (A and A' or B and B') is not critical, although this arrangement is to be considered as achieving the best results at present.

By having the resistances R1, R2 connected in a different manner, for example, by having A connected to B1 orA with B, there would happen, as the case may be, either the flowing of the highest shortcircuit current across the diode D1 with the risk that this latter opens before interruption of fuse 4, or a shortcircuit current only limited by R2.

On the contrary, with the circuit as shown figure 2, these risks are prevented and a maximum safety in operation is achieved.

It should be apparent that many changes, easily thought by those having ordinary skill in the art, may be made to the above described embodiment of the invention, without departing from the spirit and scope of this invention.

Claims (8)

1. A circuit having flexible heating elements of the type comprising two coaxial resistances mounted on a central core and kept apart by an intervening layer of insulating material with a protective fuse being arranged in series, characterized in that said resistances are connected in- series with the interposition of a semiconductor element, and one of said resistances has a higher value with respect to the other resistance, at least one further semiconductor element being connected in parallel with said higher value resistance and being of opposite polarity to said first mentioned semiconductor element.

2. The circuit having heating flexible elements according to claim 1, wherein said first semiconductor element interposed between the series connected resistances is a SCR and said semiconductor connected in parallel with said higher value resistance is a diode.

3. The circuit having flexible heating elements according to claim 2, wherein at least two diodes are connected in parallel with rhe resistance having higher value.

4. The circuit having flexible heating elements according to any preceding claims, wherein said resistances are connected in serie at those terminals thereof which are on the same side as the heating element.

5. The circuit having flexible heating elements accord ing to any preceding claim, wherein said resistances are connected in series at those terminals thereof which are not on the same side as the heating element.

6. The circuit having flexible heating elements according to the preceding claims, wherein one of the resistances has a higher value with respect to the other resistance.

7. The circuit according to the preceding claims, wherein the layer of insulating material arranged between said resistances is a layer of PVC material.

8. A circuit substantially as hereinbefore described with reference to the accompanying drawings.