GB2186134A - Heating circuits with protective arrangements - Google Patents

Abstract

A heating circuit for an electric blanket comprises two heating elements 1 and 3. One element (1) has a coaxial overheat sensing winding 2 such that the insulation between the windings will drop significantly if the element overheats. This element 1 is bypassed by diode 8. The protection winding 2 is connected via a diode 4, a resistor 5, and thermal fuse 6, to the pole of the supply opposite to that which feeds the protected element 1. In the event of overheat in the protected element, current will flow through resistor 5. The resultant heating of the resistor is used to trip the thermal fuse 6. The time for fusing is virtually independent of the position of the overheat in the protected element. The basic circuit can be adapted to use two protected elements and also to give three heat settings by the use of additional components. <IMAGE>

Description

SPECIFICATION Heating circuits Overheat protection of heating elements such as are used in electric blankets is now common. The heating element is commonly constructed from two helically wound concentric coaxial conductors. The inner element is usually the heating element and is wound with a suitable type of resistance wire on an insulating core. The second helically wound element is wound over the top of the first element but separated from it by a concentric layer of insulating material. The whole is then sheathed overall with an insulating coaxial layer of material such as PVC. The insulating medium between the two conducting elements is electrically insulating at room temperature, but as the temperature increases the insulation value falls. This fall of electrical insulation can be either abrupt-as caused by the insulating material melting-or gradual.Materials such as polythene exhibit an abrupt melting point and so give a short-circuit between the two elements before the element has reached an unsafe temperature. Other materials such as "doped" PVC show a rapidly reducing electrical resistance as the temperature rises. It is this fall of resistance between the two elements that can be used to limit the temperature of the heating element.

The most common method of sensing overheat of such a heating element is to use the reduction of electrical resistance between the two elements to heat a resistor. The heat generated by this resistor is then in turn used to heat a fusible link ("thermal-fuse") which will rupture and then interrupt the electrical supply to the blanket. The main problem with such systems is that the heat generated in the resistor can vary considerabiy depending on where the overheating has taken place along the length of the heating wire. This is clearly mentioned in British Patents 1588783, 1599709, 20286088, and in particular 2154817A.

There is also an additional problem that can arise in overheat protection of heating elements, when used in electric blankets. Often the elements are arranged, with suitable switching, to give a plurality of heating values.

These heat settings are normally derived by suitable switching of two heating elements so as to give three heating values. It is obviously necessary that the protection circuit should operate, if possible, on all heat settings. Also if possible, under overheat conditions, the heating power in the sensing resistor should be independant of the position of the point of overheat along the length of the heating element. This will give a constant time for the thermal fuse to blow thus obviating either excessive heating of the resistor or excessively long fusing times.

In the invention to be described, the foregoing considerations have been taken into account. The invention is therefore concerned primarily, but not exclusively, with multi-heat heating systems such as are used in electric blankets.

The theoretical circuit of the basic invention is shown in Fig. 1. A heating element 1 of the dual-wound type described in the introduction has one end of its heating element connected to the a.c. mains supply 7. Bypassing the current flow through element 1 in one direction only is effected by the presence of diode 8. A second heating element 3, shown without a heat sensing winding but this is not exclusively the case, is connected to the commoned junction of element 1 and diode 8 remote from the mains supply 7. The other end of this element 3 is then returned via a thermally fusible fuse link 6 to the other pole of the a.c. supply 7. The two elements 1 and 2 are separated from each other by the temperature-sensitive insulating layer 17.The overheat sensing winding 2 of heating element 1 is connected, preferably from both its ends shorted together as shown, but it could be from either end alone, to one end of diode 4.

The other end of diode 4 is connected to a heating resistor 5 whose other end is connected to the junction point of the fuse 6 and the second heating element 3. This resistor 5 is thermally coupled to the fusible link 6, and under fault conditions it is the current flow through this resistor, and its consequent heating effect, that heats the thermal fuse causing it to rupture and thus disconnect the power from the heating elements. If the cathode end of diode 8 is connected to the mains 7 as shown, then the cathode end of diode 4 will be connected to the sensing element 2 as shown. If one diode has reversed connections then the connections to the other diode will need to be reversed. The circuit will operate with either connection.

The action of the invention is as follows.

Using the diodes connected as shown in Fig. 1 and referring to Fig. 1, when the supply 7 feeding the heating element 1 is positive, the diode 8 will conduct and bypass element 1 so that the full mains voltage is applied to element 3. Thus the whole length of the heating element 1 will be at full mains potential during the whole of this period of positive supply. During the other half-cycle of the mains input it will be the supply pole connected to fuse 6 that will be positive and thus diode 8 will be reverse biassed and non-conducting. In this half-cycle both heating elements will be in operation, connected in series. If a fault develops along the length of element 1 due to overheating, the insulating layer 17 will cease to electrically insulate the heating winding 1 from the sensor winding 2.

The presence of diode 4 ensures that it is only on the positive half-cycle of the supply (for the diode polarities shown) that current can flow in the sensing circuit feeding resistor 5. As the heating element is bypassed by diode 8 during the whole of the positive half cycle, the heating voltage applied to resistor 5 will be virtually unaffected by the position of the fault along the element 1 as all this element is at the same potential. Diode 4 is essential in this circuit as otherwise there would be current flow in the resistor 5 on the other half-cycle. The value of this current-flow would depend on the position of the fault along element 1 and thus the total heating of resistor 5 (and thus the fusing time) would depend on the position of the fault.

The basic invention can be in effect duplicated so as to give the same degree of protection to both heating elements provided that both heating elements are fitted with doublewound elements of the type previously described. This can be seen by referring to Fig.

2.

Fig. 2 shows the basic overheat protection circuit as shown in Fig. 1 with the addition of four extra components. The heating element 3 now is of the double-wound type with an overheat detection winding 9 and a temperature-sensitive insulating layer 18. The element is bypassed by means of diode 10 to current flows in the opposite direction to that which causes diode 8 to conduct. Diode 11 and resistor 12 duplicate the functions of diode 4 and resistor 5 for the opposite half-cycle of the mains operation. Resistors 5 and 15 are both thermally coupled to the thermally fusible fuse link 6. When the upper pole of the supply 7 is positive the circuit operates as previously described for Fig. 1 if element 1 has overheated. If however the overheating was in element 3 then it is on the other half-cycle of the mains that current will flow through resistor 12 via diode 11.The circuit operation is therefore the same for each heating element, the circuit being effectively symmetrical. Resistor 5 detects overheat in element 1 on alternate half-cycles of the supply and resistor 12 detects overheating of element 3 on the other alternate half-cycles. For both elements the mechanical position of the overheat in the element does not materially affect the time for the thermal fuse to rupture after the overheat is detected.

It will now be shown, by way of example only, how a protected three-heat system can be developed, using the invention, such that the fault fusing time is constant for all three heat settings and is virtually unaffected by the position of the overheating in the protected heating element.

Fig. 3 shows one particular embodiment of the invention which makes use of the series connection of the two heating elements to derive the three heat settings. Referring to Fig. 3 it will be seen that the configuration of the heating elements and overheat protection components remain unchanged. The components common to the two diagrams, Figs. 1 and 3, have been given the same numbering for easy reference. Switches 12 and 9 are mechanically linked and are shown in the OFF position. Switch 12 is only needed for safety reasons to give a double-pole OFF state.

There are three switch positions marked "A" "B" and "C" and as the sliding contact 9s is progressively moved so as to contact these points, the voltage from the a.c. supply 11 is fed first to contact "A" then contacts "A" and "B" and then finally to all three contacts.

The circuit operation will now be described.

In position "A" which is the low heat position, power is only fed to the heating elements 1 and 3 on one half-cycle due to the action of the diode 10. The power over this half-cycle is therefore V2 R1+R3 where V is the rms value of the supply voltage 11 and R1 and R3 are the electrical resistances of the elements 1 and 3 respectively. The mean power over the whole cycle is therefore V2 2(R1+R3) On the opposite half-cycle of the mains the overheat protection circuit is in the sensing state, operating as has been described earlier.

In position "B" power is fed to both A and B contacts. On the half-cycle where the upper line from the supply is positive, the protection circuit is active but again there is no power heating flow, as any possible conduction is blocked by the diodes 10 and 11. On the opposite half-cycle when the lower supply line is positive diode 11 will conduct, thus effectively shorting out heating resistor 3. The supply voltage is therefore applied only to element 1 and the resulting heating power will be V2 R1 over this half-cycle of the supply. The mean power will therefore be V2 2R1 over the whole cycie. Comparing this with the previous value, it will be seen that if the electrical resistances of R1 and R3 are equal, then the heat output in this second case will be just twice that of the first instance.

In position "C" power is fed to all three contacts of switch 9. In this case the circuit behaviour on the half-cycle when the lower supply pole is positive will be identical to the previous case; all the power will be generated in the double-wound element. On the other half-cycle of the supply, element 1 is shorted out by diode 8, and because there is now a current return for element 3 via contact 9C, power will be produced in element 3. Over the half-cycle this will be V2 R3 R3 so the total power will be V2 V2 2R1 2R3 Again if 1 and 3 have the same electrical resistance, this power will be twice the previous value. If this is called "full heat" for the system, then assuming 1 and 3 have equal electrical resistance values, then position 8 will be half of full heat and position A will be one quarter of full heat.This is the same as for conventionally switched non-protected blankets having equal resistance values for the two heating elements.

It is also possible to modify the previous circuit to cover the case where both heating elements have overheat sensing elements.

This is shown in Fig. 4. This utilises the basic three-heat system as described in the previous section and also utilises the embodiment of the invention shown in Fig. 2. The sensing element 13, diode 14 and resistor 15 protect the element 3 against overheating in the full heat condition, as described in the description referring to Fig. 2. The low heat condition however does present a difficulty in that overheating in element 3 close to the junction of elements 1 and 3 will give only one quarter of the heating power to the resistor 15 due to the diode 11 being disconnected in the low heat position. To overcome this, an extra diode 16 can be connected between the two sensing windings 2 and 13. In the event of overheating of element 3, this diode ensures that on the half-cycle when the upper pole of the supply 11 is positive, current will flow into resistor 5 to activate the protection circuit. This current will have its full value as all element 3 will be at supply potential due to diode 10 being in the non-conducting direction. In this case full protection will be available for both elements in all three heat settings and irrespective of the physical location of the overheat in either element.

Claims (4)

1. An electrical heating circuit arrangement comprising an electrical heating conductor separated from a separate electrical sensing conductor by an insulating material whose resistance falls substantially when the temperature of heating conductor rises appreciably above its normal operating temperature, this heating conductor having a rectifier diode connected between its end connections and having also a second heating conductor connected in series with it, the whole being connected across a.c. mains supply via a thermally sensitive fuse, the sensing conductor being connected via a diode and resistor to the supply pole remote from that feeding into the heating conductor with the sensing conductor, the resistor being thermally coupled to the aforementioned thermally sensitive fuse so that the heat generated by the resistor caused by current flow resulting from the overheating of the element with the sensing conductor will cause the thermal fuse to heat up sufficiently to rupture and disconnect the heating conductors and the elements forming the protection circuit from the mains supply, the diode in series with the resistor being so polarised that it conducts on the half-cycles of the mains supply when the diode connected across the heating conductor is also conducting.

2. A circuit arrangement according to claim 1, where the second heating conductor is also provided with an overheat sensing conductor, this conductor being connected via a diode and current sensing resistor to the supply pole opposite to that supply pole connected to this heating conductor, this second resistor also being thermally coupled to the thermally sensitive fuse, this second heating conductor also having a diode connected across its end connections, the diode polarity being such that each heating conductor will conduct on alternate half-cycles of the a.c. supply, the polarity of the diode in series with each heating resistor being such that under overheat conditions of either heating conductor, the diode connected to the sensing winding of the overheating conductor will conduct on the alternate half-cycles of the supply when the diode connected across the corresponding heating conductor is conducting.

3. A heating circuit according to claims 1 or 2 where the two end connections of individual sensing conductors are shorted together, this common point being then connected to the respective diodes and overheat sensing resistors as in the previous claims.

4. A circuit according to any one or more of the preceding claims where switches and diodes are utilised to enable a plurality of heating powers to be obtained from a single supply voltage, such circuits being substantially as shown in Fig. 3 or Fig. 4 depending on whether one or both heating conductors are to be provided with overheat protection conductors.