GB2107493A - Improvements in or relating to electrical energy control systems - Google Patents

Abstract

An electrical energy control system for controlling the supply of electrical energy to an electric blanket heating element (R2) and having, in series with the heating element (R2), a thermal switch (E1/S1) which causes, in use of the apparatus, the supply to be cyclically connected and disconnected. Sensing means (I, R30) are provided whereby a signal is derived which is related to the temperature of the heating element (R2), and a transistor (Q1) responsive to said signal is provided for controlling the flow of current through the thermal switch in a sense to stabilise said temperature. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrical energy control systems This invention relates to electrical energy control systems and has particular application to the control of electrical energy to energy-consuming apparatus provided with electrical resistor means for heat generation such as electrically-powered heating panels and heating cables as may be used, for example, for soil heating.

The term "electrically-powered heating panel" includes electrically-powered under-carpet heaters, blankets, mattresses and pads. The invention is particicularly applicable, however, to electricallypowered blankets, or, more simply, "electric blankets".

It is already an established practiceto provide electrical energy control of electric blankets with a thermal switch in series with the heating coil of the blanket. Whilst this is a simple system, there is a requirement for a number of associated waterproof protective thermostats. This tends to make the cost of the blanket high and still leaves the blanket not fully protected against overheating.

The invention seeks to provide electrical energy control systems having improved control functions and/or improved protection against overheating of the electrical resistor means.

According to the invention, an electrical energy control system for controlling the supply of electrical energy to energy-consuming apparatus provided with electrical resistor means and having, in series with said resistor means, a thermal switch which causes, in use of the apparatus, said supply to be cyclically connected and disconnected is characterisedin that sensing means are provided whereby a signal is derived which is related to the temperature ofthe resistor means, and controller means responsive to said signal are provided for controlling the flow of current through the thermal switch in a sense to stabilise said temperature.

The control of flow of current through the thermal switch is preferably obtained by providing a controllable variable shunt path to the thermal switch.

However, it is possible also to arrange for control in a variable series path.

One form of sensing device can comprise the already known dual coil blanket heating element having insulation between a blanket heating coil and a surrounding coil which increases its conductance as the blanket heating coil increases in temperature.

Typically an increase of said conductance is adapted to signal a decrease in the current flowing from the blanket heating coil into a shunt path to a thermal switch which also takes current from the coil. The decrease in shunt current acts to increase the current flowing through the thermal switch to open said thermal switch a longer period to allow the resistor means (blanket heating coil) to reduce in temperature and thereby stabilise itself.

Embodiments ofthe invention will now be described, by way of example only, with refernce to the accompanying drawings, wherein: Figures 1 and 2 are circuit diagrams of a dual coil blanket heating element each having a control system according to the invention, and Figure 3 is a circuit diagram of a single coil blanket heating element having a control system according to the invention.

In the Figures, like reference numerals refer to like components.

In Figure 1 there is shown an alternating current supply AC to a full wave rectifier bridge BR via a double pole isolation switch S. A direct current sup ply DC is generated at the bridge BR indicated by conventional (+) and (-) signs.

The -ve terminal of the supply DC is connected to the inner (heating) coil R2 of dual coil blanket heating element, having thermoplastic insulation land an outer coil R30, and thence via a heater elementEl of a thermal switch S1, via the contacts of a second normally closed thermal switch S2 and the contacts of the switch 51. The circuit having the inner coil R2 can be regarded as the main heating circuit. The switch S2 has a heater elementE2 and the switch S2 is preferably not resettable if it trips open, for example switch S2 may make use of a wax element to hold the switch contacts in a closed position.Should overheating occur, the wax element melts and sllows the contacts to open and stay open. The cyclic on/off ratio of switch S1 can be adjusted by changing the setting of an adjustable resistor VRl or by some mechanical device in the switch itself, or both.

As will be apparent, the insulation land outer coil R30 provide sensing means associated with substantially all points of inner heating coil R2 whereby a signal is derived which is related to the temperature of coil R2. Controller means 0 1 (referred to below) responsive to said signal are provided for controlling the flow of current through the thermal switch El/Si in a sense to stabilise said temperature.

At intermediate settings various degrees of cyclic on/off control of the blanket heating element can be effected. Typically at a mid-setting, the contacts of switch S 1 would desirably be closed for two minutes and open for two minutes.

The circuit provides a variable shunt path for the heater element E1 of thermal switch 51, both the shunt path and the element El drawing current from the heating coilR2. This path consists of a transistor Olin series with the heater element E4 of switch S2.

The conductivity of this shunt path is controlled by a signal at the base of the transistor Q 1 by the charging of a capacitor C which is responsive to currents leaking through the insulation / into the coil R30. The adjustable resistor VRl is provided in the charging path of C together with a current limiting resistorR7 and a fuse Fl. A current limiting resistorR6 is provided in series with VR1.

If the main heating circuit of R2:El :S2:Sl were to be used without the other components in the circuit, then El would heat up to open the contacts of switch S1. There would then be a cooling period terminated with switch S1 closing followed by another heating of El. That is, there would be bursts of heating atR2 each followed by a period of no heating. This would give rise to a specific mean blanket temperature. By changing the resistance of El the difference between the on/off periods could be changed and the blanket mean temperature changed. However, such a simple arrangement would lack safety protection.

The presence of the shuntE2/Oi allows control of the amount of current passing through El. The base of Ol is connected toR30 by way of fuse Fl and then to the variable resistor VRl via limiting resistorR7.

The base of 0 1 is also connected with the capacitor C and hence VRl acts to control the potential C reaches and the amount which 0 1 conducts and shunts El. A typical adjustment of VRl is such that, in the cold state of the blanket, 0 1 is nearly fully conducting.

Under these conditions, and with switches S, S1 and S2 closed, the blanket represented byR2 will heat up but El will not heat rapidly due to the shunting effect ofOl andE2.

As the blanket heats up, a leakage occurs through the insulation / which results in a lowering of the potential on the "R30-side" of C thus causing the transistor 01 to conduct less (i.e. effectively the shunt path ofEl increases in resistance) and hence more current passes through El which tends to open switch S1 to prevent the temperature of the blanket (R2) rising. As the temperature of the blanket rises, so the insulation/ increases its conductance, 01 decreases its conductance, and El heats more to open switch 81.

The circuit described above provides a settable self-regulated main heating circuit(R2 andEl in series with switch S1) having added circuit elements which sense the heating element (R2) temperature and modifies the regulation of the main heating cir cuitto lower the power supplied to the heating ele mentasthetemperature of the heating element increases and vice versa. The increase in the conductance of the insulation which brings this modification of regulation into effect, can arise with localised temperature increase of the heating element or an "all-over" increase.

The switch S2 and its thermal resistorE2 provides protection in the event that transistor 0 1 short circuits. If this happened, then El would appear parallel with E2, so that the self-regulation effect of El would be lost as E2 shunts it. This would cause blanket overheating which now is safeguarded against by heavier current passing through E2 (with the shorting ofOl) and opening of switch S2, which cannot be reset.

A break in the circuit which includes F1 and R30 takes Q 1 into a non-conducting state. This causes E1 to take more current whereby switch S1 opens. F1 could typically fail if conductance between R2 and R30 is heavy (e.g. R2 comes into direct contact with R30).

ThetransistorOl could be replaced bythyristor switch means.

Details ofthe components shown in Figure 1 as follows: El - 30 ohms R2 - 500 ohms R30- 500 ohms E2- 10 ohms R7 - 10Kohms RG-1OKohms VRl- 10 Kohms Q1 -Type BFY-51 C -2.2,aF In Figure 2, itemsEl,R2, RSO, !, E2, C, SI, 82 and Q1 are as in Figure 1 except that Q 1 is now in series with El. ResistorsR10 andR8 set the bias voltage on ar.

In this circuit, when the blanket (R2) is cold the transistor Q 1 is set to be "off". As R2 heats up (in series with S1 and S2 and the combined circuit comprising E2, El and 07) so the conductance through / increases and the base ofOl becomes more positive so allowing the current in El to increase and hence heat up El more quickly and open contact 81. That is, the energy supplied to R2 is decreased.

Should El orO 1 become open circuit then current increases through E2 and this causes S2 to open and stay open to give added circuit protection.

Adjustment of ElIS1 (the energy adjuster) can be effected either by a mechanical control at the switch S1 or by adjustment at a potentiometer between RIO andR8. This is shown by the dotted connector 10 (which replaces the connector marked 11) and the dotted potentiometerR9.

A magnetic relay could be used in lieu of S21E2 with normally closed contacts which latch open when the relay is operated.

The circuit diagram of Figure 3 shows a control system for a non-dual coil electric blanket heating element, wherein the sensing and resistor means of the control system are incorporated in a Wheatstone bridge.

R31 andR32 are the heating elements. R31 has a relatively high positive coefficient of resistance, whereasR32 has a relatively low positive coefficient of resistance. (Which may be zero).

ELMS1 and E2/S2 are thermal relays, which, in the cold state, are closed.

R31, R32, R33 and R34 forms the arms of a Wheatstone bridge. AsR31 andR32 increase in temperature,R31 increases in resistance. This causes the base of transistor 01 to go negative and increase its impedance, thus allowing current flow in El to be increased. By adjusting VRl the point at which 01 allows sufficient current flow in El to open S 1 can be varied.

Should 0 1 fail and short circuit, E2 heats up whereby S2 is caused to open and arrest the flow of currenttoR32 andR3l.

R33 may have a positive temperature coefficient of resistance and, if suitably positioned, can then serve as an ambient temperature sensor. Alternatively, R34 may have a negative temperature coefficient of resistance and be given the same function. If a higher degree of ambienttemperature sensing is desirable,R33 andR34 could be used together.

Further series resistances may be needed to provide a more stable bridge circuit.

In a modification, the base of transistor Q 1 could be connected to the junction between R33 andR34.

S2 would then need to be adjustable.

In another modification, R33 and R34 could comprise blanket heating elements, with R33 having a relatively high positive coefficient of resistance and R34 a relatively low positive coefficient of resistance.

The temperature coefficients of the armsR3l, R32, R33, R34 may be altered so that the base of Q1 is driven positive when the blanket heating resistances increase in temperature. E21S2 then serves as an energy regulator and El/Si as a thermal cut-out.

Typical values for the additional components of Figure 3 are as follows: R3i - 240 ohms R32 - 240 ohms R33 - 1 M.ohm R34-1 M.ohm

Claims (13)

1. An electrical energy control system for controlling the supply of electrical energy to energyconsuming apparatus provided with electrical resistor means such as an electric blanket heating coil and having, in series with said resistor means, a thermal switch which causes, in use of the apparatus, said supply to be cyclically connected and disconnected characterised in that sensing means are provided whereby a signal is derived which is related to the temperature of said resistor means, and controller means responsive to said signal are provided for controlling the flow of surrent through the thermal switch in a sense to stabilise said temperature.

2. A control system as claimed in Claim 1, in which the sensing means are associated with substantially all points of said resistor means.

3. A control system as claimed in Claim 1 or 2, in which the controller means provide a controlled variable shunt path to the thermal switch.

4. A control system as claimed in Claim 1 or 2, in which the controller means provide a variable series path with the thermal switch.

5. A control system as claimed in any preceding claim, in which the resistor means is a heating coil and the sensing means comprise insulation and an outer coil on said heating coil as already known in a dual coil blanket heating element.

6. A control system as claimed in Claim 5, in which a settable means or a mechanical control on the thermal and the control means act to control the heat of said heating coil to accord with the setting of the settable means.

7. A control system as claimed in any preceding claim, wherein the sensing and resistor means form the arms of a Wheatstone bridge.

8. A control system as claimed in Claim 6, wherein the resistor means comprise a single coil heating element.

9. A control system as claimed in any preceding claim, including a thermal switch which normally has a closed contact but, in the event of current flow above a threshold, opens the contact so that current no longer flows to the electrical resistor means.

10. A control system as claimed in Claim 9, wherein the thermal switch is not settable back to the closed condition, afterthe contact has opened.

11. An electrical energy control system for controlling the supply of electrical energy to an electric blanket heating element substantially as hereinbefore described with reference to Figure 1.

12. An electrical energy control system for controlling the supply of electrical energy to an electrical blanket heating element substantially as hereinbefore described with reference to Figure 2.

13. An electrical energy control system for controlling the supply of electrical energy to an electric blanket heating element substantially as hereinbefore described with reference to Figure 3.