GB2310090A

GB2310090A - Safety circuit for an electric blanket

Info

Publication number: GB2310090A
Application number: GB9602603A
Authority: GB
Inventors: Martin Keith Armstrong
Original assignee: DREAMLAND APPLIANCES Ltd
Current assignee: DREAMLAND APPLIANCES Ltd
Priority date: 1996-02-07
Filing date: 1996-02-07
Publication date: 1997-08-13
Anticipated expiration: 2016-02-07
Also published as: GB2310090B; GB9602603D0; HK1001443A1

Abstract

An electric blanket operable in a low power setting for overnight use and a high power setting has a safety circuit (2) operable only in the low power setting that is responsive to the power level supplied to a heating element (4). If this power level exceeds a threshold level, which is below the high power setting, then the power to the electric blanket is stopped. The power level supplied to the heating element is controlled by circuit (6) which turns on a thyristor (T1) for selected cycles of a half wave rectified mains voltage. The voltage on the thyristor anode when it is off is averaged and compared to a reference voltage at comparator (U1c). If it is less than the reference then triac (T2) is turned on allowing resistors (R1, R2) to heat up and blow a thermal fuse.

Description

ELECTRIC BLANKET This invention relates to the field of electric blankets (i.e. electrically heated blankets and pads). More particularly, this invention relates to electric blanket incorporating safety circuits.

It is known to provide electric blankets capable of use throughout a whole night. For safety reasons, such electric blankets must have an overnight setting at which the electric blanket temperature cannot exceed a temperature considered to be potentially hazardous for overnight exposure, e.g. 37"C, or 60"C in the case of a fault, as set out in European Union Harmonised Standard EN 60967. One way of achieving this would be to use a heating element of an appropriate resistance such that the maximum power would be unlikely to result in a hazardous temperature, e.g. the temperature could not exceed 60"C in any mode of operation, including any fault.

Whilst such an approach of inherently limiting the maximum temperature that can be produced by an electric blanket is safe, it suffers from the disadvantage that it limits the maximum power level at which the electric blanket may be operated and so slows the initial warming produced by the electric blanket. This approach also limits the maximum temperature that may be achieved to below that which some users may wish for pre-warming before entering the bed.

According to the present invention there is provided an electric blanket comprising: a heating element; a power control circuit for controlling supply of electric power to said heating element to be one of a plurality of power levels, each corresponding to one of a plurality of power settings, said plurality of power levels and said plurality of power settings including an overnight power setting and overnight power level that is nonzero and less than a threshold power level and a high power setting and high power level that is greater than said threshold level; an overnight safety circuit operable in said overnight power setting and not operable in said high power setting and responsive to electric signals that are indicative of a power level being supplied to said heating element that is greater than said threshold level to stop electric power supply to said heating element.

The invention provides an electric blanket that is capable of producing both high power operation for rapid warming and a high temperature when desired and yet when operating in an overnight power setting has a safety circuit that limits the power such that a dangerous temperature is unlikely to be reached according to standard tests. This overnight safety circuit achieves this end by only being operable in the overnight power setting and in this mode monitoring electrical signals indicative of the power level supplied to the heating element to determine whether or not it exceeds a threshold power level and, if they do, stopping the electrical power supply.

In addition to the overnight safety circuit, preferred embodiments of the invention comprise a full time safety circuit operable in both said overnight power setting and said high power setting and responsive to electric signals supplied to said heating element that are indicative of a fault in said heating element to stop electric power supply to said heating element.

The overnight safety circuit and the full time safety circuit complement each other in that the overnight safety circuit provides a more sensitive safety mechanism appropriate to the situation in which the bed is continuously occupied whereas the full time safety circuit provides a more coarse level of protection suitable for detecting gross errors, such as fault in the heating element.

Preferred embodiments of the invention incorporate a thermal fuse in a power supply path to said electric blanket.

A thermal fuse is a effective and reliable mechanism for isolating the electric power supply to electric blankets in that it generally is sufficiently slow to blow that it is not adversely effected by power spikes and the like and yet is sufficiently rapid to avoid dangerous overheating of the electric blanket.

Whilst the overnight safety circuit could act to stop the electric power in many different ways, it is preferred that said overnight safety circuit is connect to said thermal fuse to blow said thermal fuse to stop electric power supply to said heating element.

It is more usual for a thermal fuse to be associated with the safety mechanisms for detecting a catastrophic failure within an electric blanket, but a thermal fuse may also be advantageously used as the electric power supply stopping mechanism by the more sensitive overnight protection circuit.

It is also preferred that said full time safety circuit is connect to said thermal fuse to blow said thermal fuse to stop electric power supply to said heating element.

The thermal fuse may be efficiently used for two purposes, namely to stop the electric power supply in response to the overnight safety circuit and to stop the electric power supply in response to the full time safety circuit.

Depending upon how the heating element is driven, the overnight safety circuit could sense the power level supplied to the heating element in various ways, e.g.

phase/peak voltage level monitoring, current monitoring and the like including combinations of the preceding parameters. However, in preferred embodiments of the invention said power control circuit includes a power switching element and said overnight safety circuit is responsive to a mean voltage across across said power switching element and stops electric power supply to said heating element if said mean voltage is less than a predetermined threshold voltage.

Sensing the mean voltage makes the overnight safety circuit less prone to transient effects and mains voltage surges.

It is preferred that said overnight safety circuit includes a comparator for comparing said mean voltage with a reference voltage. The reference voltage may be derived from a Zener diode.

The use of a comparator in conjunction with a Zener diode provides a stable reference base against which the power level supplied to the heating element may be judged.

A particularly effective way of controlling the power level supplied to the heating element may be provided by embodiments of the invention comprising a half wave rectifier for half wave rectifying mains electric power supplied to said electric blanket and wherein said power control circuit selectively blocks different proportions of half wave pulses applied to said heating element to produce different power levels in response to different power settings.

With such half wave rectified pulses, it is highly advantageous that said overnight safety circuit derives clipped pulses from said selectively blocked half wave pulses, said clipped pulses having a predetermined maximum voltage level and a mean value of said clipped pulses being indicative of said power level being supplied to said heating element.

Clipping the pulses sensed from the heating element before they are judged by the overnight safety circuit has the advantage that variations in supply voltage are removed since the supply voltage will be well above the clipped voltage and accordingly the mean value of the clipped pulses will be largely independent of the supply voltage and yet remain strongly dependent upon the correct selective blocking of pulses by the power control circuit.

In order to save power within the control circuits of the electric blanket and accordingly facilitate more economical and reliable construction it is preferred that said comparator is prevented from developing an output during half wave cycles that are blocked by said half wave rectifier.

It will be appreciated that the electric blanket may typically have a plurality of said high power levels and said high power settings.

Whilst the power settings could in some circumstances be automatically controlled to follow a predetermined cycle, it is preferred that said plurality of power settings corresponding to respective positions of a user operable power control.

Within an electric blanket intended for continuous use throughout the night a problem that may arise is from the electromagnetic fields emitted by the electric blanket. These electromagnetic fields may be reduced in embodiments in which said heating element comprises an inner element carrying current in one direction and coaxial with a helical outer element carrying current in an opposite direction.

The combination of the overnight safety circuit of the present invention and the low electromagnetic radiation from appropriate heating elements synergistically combine to provide a product highly suitable for overnight use.

Whilst the invention may be used for both electric over blankets and electric under blankets, it is most highly desirable in electric under blankets where the problems of overheating can be potentially more dangerous.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates the control and safety circuits of an electric blanket; and Figure 2 illustrates various signal waveforms within the circuit of Figure 1.

The power control circuit 6 operates to turn on (conductive) the thyristor Tl during selected mains half-cycles. The user's set heat control VRl allows the selection of different power levels corresponding to different proportions of these half-cycles being selected. The highest power level will correspond to every halfcycle being selected whereas the lowest overnight setting may correspond to perhaps one in ten of these half-cycles being selected.

Considering the normal operation of the electric blanket control circuit illustrated in Figure 1, when the thyristor T1 is off (non-conductive), its anode voltage follows the positive half-wave rectified mains voltage supplied to the blanket via diodes D1 and D2. When thyristor T1 is on, its anode has no appreciable voltage on it (signal A in Figure 2). During positive mains half-cycles, when thyristor T1 is on, the heating element in the blanket is supplied with pulses of electric current and heats up.

The ratio of the times during which the thyristor T1 is turned on compared to turned off governs the average power level supplied to the heating element 4. The power level supplied to the heating element 4 is inversely related to the anode voltage of the thyristor T1, since a voltage only appears at this thyristor T1 anode when T1 is off and, as a consequence there is no instantaneous power dissipated in the heating element 4. (In a given period, the higher the number of positive half mains cycles that are seen at the anode of the thyristor T1, the lower is the average electrical power level being supplied to the heating element 4.) This relationship is used by the comparator Ulc within the overnight safety circuit 2 to determine whether the power level being supplied to the heating element is too high.

The function of the resistors R5 and R6 and the Zener diode ZD2 is to clip (square off) the positive T1 anode half cycle voltages before presenting them to the averaging circuit comprising resistors R18 and R19 and capacitor C7. This clipping of the positive half mains cycle voltages to produce clipped pulses (see signal B in Figure 2) makes the operation of the comparator substantially independent of the actual mains voltage. This improves the reliability of the overnight safety circuit which might otherwise mistake a low mains voltage or a "brown-out" for an incorrect on/off ratio of T1.

The comparator U ic compares the average voltage derived from the anode voltage of the thyristor T1, which appears on capacitor C7, with a reference voltage derived from in between the resistors R16 and R17 that are in turn supplied with a reference voltage from the DC power supply circuit that includes a Zener diode ZD1.

When the voltage capacitor C7 is higher than the reference voltage (i.e. the average blanket electrical power level is lower than the threshold), then the output of the comparator Ulc is held to a low voltage (nearly OV). When the voltage on the capacitor C7 is lower than the reference voltage (the average blanket power supply level is higher than the threshold), then the output of the comparator Ulc goes high (about 13V).

A triac T2 is controlled by the output of the comparator Ulc via a switch S2 and a resistor R21. The switch S2 is only closed when the blanket is in an overnight power setting mode set by a manually operable overall switch (not illustrated) that is the single power control knob for the whole circuit. The triac T2 is normally in an off condition. When the switch S2 is closed, a high output from the comparator U Ic will turn the triac T2 on, but when the switch S2 is open, then the output of the comparator U ic cannot influence the triac T2.

The triac T2 is connected to the full time safety circuit (blanket short-circuit protection circuit) consisting of resistors R1 and R2, diodes D1, D2, D3 and D4 and the thermal fuse to which the resistors R1 and R2 are thermally coupled, for example by close proximity. If the triac T2 turns on, then it causes the resistors R1 and R2 to heat up, eventually rupturing the thermal fuse and permanently stopping the electric power supply to the controller and heating element 4.

The switch S2 is actuated by an overall blanket power control knob (not illustrated). The potentiometer VR1 converts the angle for rotation of the blanket power control knob into an appropriate on/off ratio for the thyristor T1 via the power control circuit 6. A cam on the power control knob ensures that the switch S2 is only closed in the overnight power setting. This ensures that the overnight safety circuit 2 is only capable of operating when the control is set to the overnight power setting and so does not allow it to operate the thermal fuse during normal use when other (high) power settings are being used intentionally.

In summary, when the overnight setting on the controller is selected, the switch S2 is closed and the triac T2 is controlled by the comparator Ulc. If the comparator Ule then detects an on/off ratio of T1 that indicates that the blanket power has exceeded a certain threshold, its output will switch on the triac T2. If this condition last long enough (about 1 minute) the resistors R1 and R2 will heat up the thermal fuse sufficiently for it to open, thereby protecting the user from whatever fault has caused on/off ratio (the blanket power) to be too high.

If the triac T2 conducted during positive mains half-cycles, then it would instantly blow the mains fuse FS1. The heating of the resistors R1 and R2 in response to triac T2 being turned on only requires negative half-cycles of the mains supply. Diode D5 allows triac T2 to conduct during negative half-cycles, but disconnects triac T2 during positive mains half-cycles. This ensures that transient conditions (e.g. at initial switch-on, or due to mains born interference) which may turn triac T2 on momentarily do not blow the mains fuse FS1 and so yield an unreliable product.

The resistor R21 limits the drive current to the triac T2 to a value that ensures that it will always turn on when required (allowing for tolerances and ageing in components and temperature variations), without requiring more DC power than is necessary. This helps to save upon the cost and size of the overall circuit.

The resistor R20 and the diode D11 inhibit the operation of the comparator Ule for half of the time in order to save DC power and reduce the cost of the DC power circuit resistor R7, resistor R8, capacitor C2, Zener diode ZD1, diode D8 and capacitor C3. The triac T2 takes a substantial gate current to ensure it is switched on and this need not be provided during positive mains half cycles with the result that turning off the triac T2 drive during such times helps to save upon the cost and size of the overall circuit.

Claims

1. An electric blanket comprising: a heating element; a power control circuit for controlling supply of electric power to said heating element to be one of a plurality of power levels, each corresponding to one of a plurality of power settings, said plurality of power levels and said plurality of power settings including an overnight power setting and overnight power level that is nonzero and less than a threshold power level and a high power setting and high power level that is greater than said threshold level; an overnight safety circuit operable in said overnight power setting and not operable in said high power setting and responsive to electric signals that are indicative of a power level being supplied to said heating element that is greater than said threshold level to stop electric power supply to said heating element.

2. An electric blanket as claimed in claim 1, comprising a full time safety circuit operable in both said overnight power setting and said high power setting and responsive to electric signals supplied to said heating element that are indicative of a fault in said heating element to stop electric power supply to said heating element.

3. An electric blanket as claimed in any one of claims 1 and 2, comprising a thermal fuse in a power supply path to said electric blanket.

4. An electric blanket as claimed in claims 1 and 3, wherein said overnight safety circuit is connect to said thermal fuse to rupture said thermal fuse to stop electric power supply to said heating element.

5. An electric blanket as claimed in claims 2 and 3 or claims 2, 3 and 4, wherein said full time safety circuit is connect to said thermal fuse to rupture said thermal fuse to stop electric power supply to said heating element.

6. An electric blanket as claimed in any one of the preceding claims, wherein said power control circuit includes a power switching element and said overnight safety circuit is responsive to a mean voltage across across said power switching element and stops electric power supply to said heating element if said mean voltage is less than a predetermined threshold voltage.

7. An electric blanket as claimed in claim 6, wherein said overnight safety circuit includes a comparator for comparing said mean voltage with a reference voltage.

8. An electric blanket as claimed in any one of the preceding claims, comprising a half wave rectifier for half wave rectifying mains electric power supplied to said electric blanket and wherein said power control circuit selectively blocks different proportions of half wave pulses applied to said heating element to produce different power levels in response to different power settings.

9. An electric blanket as claimed in claim 8, wherein said overnight safety circuit derives clipped pulses from said selectively blocked half wave pulses, said clipped pulses having a predetermined maximum voltage level and a mean value of said clipped pulses being indicative of said power level being supplied to said heating element.

10. An electric blanket as claimed in claim 7 and 8, wherein said comparator is prevented from developing an output during half wave cycles that are blocked by said half wave rectifier.

11. An electric blanket as claimed in any one of the preceding claims having a plurality of said high power levels and said high power settings.

12. An electric blanket as claimed in any one of the preceding claims, wherein said plurality of power settings corresponding to respective positions of a user operable power control.

13. An electric blanket as claimed in any one of the preceding claims, wherein said heating element comprises an inner element carrying current in one direction and coaxial with a helical outer element carrying current in an opposite direction.

14. An electric blanket as claimed in any one of the preceding claims, wherein said overnight power level produces a temperature below 37"C in accordance with blanket safety standard European Union Harmonised Standard EN 60967.

15. An electric blanket as claimed in any one of the preceding claims, comprising an electric under blanket.

16. An electric blanket substantially as hereinbefore described with reference to the accompanying drawings.