GB2046540A - Electrical heating circuits - Google Patents

Abstract

A heating circuit, for example an electrical blanket or hair dryer, comprises a heating element 2, a burst firing assembly 7 which causes an alternating current to flow through the element in a series of short bursts, and a conductor 3 separated from the element 2 by temperature-dependent insulating material so that in the event of overheating the burst firing assembly 7 is short-circuited to cause an overcurrent protective device 4 to respond. In one arrangement (Fig. 5, not shown) providing automatic temperature regulation, the burst firing assembly comprises an astable multivibrator (55) whose mark/space ratio is controlled by a thermistor (68) and which controls the triggering of a triac (78) via a zero-crossings switching circuit (56). In another arrangement (Fig. 6) providing selectable heat settings, a shift register 107 provides three outputs of differing mark/space ratio any output being selectable to control the triac 108. <IMAGE>

Description

SPECIFICATION Apparatus for and method of controlling the heat output of resistive electrical heating means The present invention relates to apparatus for and a method of controlling the heat output of resistive electrical heating means. The invention has many applications, in particular, electric blankets, floor heaters, cooking stove hot-plates and hair dryers.

In known electric blankets those blankets with overheat protection have their heating elements so wound that in the event of a fault part of the elements are shorted out thus reducing the effective resistance and allowing the current to increase. The increased current is employed to blow a current overload device such as a fuse.

According to one aspect of the present invention, there is provided apparatus comprising an electrical heating element having two coils placed next to each other but separated by a layer of electrical insulation, one of the coils being connected in series with a control circuit comprising a burst firing assembly whose mark/space ratio may be varied to vary the heating effect of the element, and a current overload device in series with the said one coil, a short circuit path being produced in operation through the insulation between the two coils in the event of overheating of the element thus short circuiting the burst firing assembly and resulting in an increased current to operate the current overload device and discontinue the electrical supply to the element.

According to another aspect of the present invention there is provided apparatus comprising an electrical heating element, and a control circuit comprising a burst firing assembly whose mark/space ratio may be varied to vary the heat effect of the element, the control circuit also comprising a temperature dependent element placed to sense the temperature of the heating element and operative to alter the mark/space ratio of the burst firing assembly to maintain the temperature at a preselected value.

In order that the invention may be more clearly understood, one embodiment of the invention will now be described, by way of example with reference to the accompanying drawings, in which: Figure 1 (a) is a double wound heating element with a block circuit diagram of a basic overheat protection circuit for an electric blanket, Figure 1 (b) is a double wound heating element with a block circuit diagram of a further overheat protection circuit for an electric blanket, Figure 1 (c) is a double wound heating element with a block circuit diagram of a further overheat protection circuit for an electric blanket, Figure 2 is a voltage versus time waveform for the circuit of Figure 1, Figure 3 is a heating element with a block circuit diagram of the basic overheat protection circuit of Figure 1 but incorporating a zero voltage switch, Figure4 is a voltage versus time waveform for the circuit of Figure 2, Figure 5 is a detailed electronic circuit diagram of the circuit of Figure 3, Figure 6 is a heating element with a block circuit diagram of the basic overheat protection circuit of Figure3 but with control means for guarding against the maximum permissible heat dissipation being exceeded in the event that component values in the circuitry charge.

Figure 7 is a more detailed circuit diagram corresponding to the diagram of Figure 6, and Figure 8 shows waveforms at various points on the circuit of Figure 7.

Referring to Figure 1(a), heating element 1 comprises two heating sections 2 and 3. These heating sections are double wound. Each section comprises a coil of wire. The two sections are concentrically wound with each other and are separated by a thin layer of a suitable insulating medium such as nylon or p.v.c., the whole then being surrounded by another layer of suitable insulating material.

One end of heating section 2 is connected in series with a fuse 4 which is in turn connected to one terminal 5 of a dual terminal electrical supply. The other end of section 2 is connected in series with an electronic burst firing assembly 7 which is in turn connected to one end of heating section 3.

The electronic burst firing assembly 7 acts as an effective regular time period short circuit followed by an effective regular time period open circuit to the flow of alternating electric current in a line. The socalled on time/off time ratio or mark/space ratio being a function of the component values of the assembly. Consequently, a selected number of half or full cycles of electric current can be passed at regulartime intervals.

The opposite end of heating section 3 is then con nected to the second terminal 8 of the dual terminal electrical supply.

Figure 2 plots the voltage variation with time across the burst firing assembly of Figure 1 or between points 9 and 10 in Figure 1. In Figure 2 the time periods indicated by 21-22, 23, 24 and 25, 26 indicate that electrical current is not flowing through the double wound heating element 1. The burst firing assembly 7 is siad to be in its effective open circuit condition. Conversely, the time periods 22-23 and 24-25 indicate that electrical current is flowing through the double wound heating element. The burst firing assembly is then said to be in its effective short circuit condition.The magnitude of the current flowing through the double wound heating element 1 with the burst firing assembly in its short circuit condition is governed by the impedance of both heating coils 2 and 3 and the much lower impe dances of components like the fuse 4 and the burst firing assembly 7. The fuse is rated to withstand the long time average electrical current.

The on time/off time ratio indicated in Figure 2 is approximately 1/2. Obviously, by varying the com ponent values of the burst firing assembly 7, the ratio can be greatly varied. On the occurrence of a fault, such as the overheating of the heating pad, the heating elements 3 and 4 of Figure 1 will come into electrical contact due to the softening or melting of the dividing nylon or p.v.c. layer. Consequently, the electrical current paths between 5 and 8 of Figure 1 by-passes the burst firing assembly 7 and the subsequent increase in current blows the suitably selected fuse 4.

With a mark/space ratio of 1/2 the method of arranging the heating elements indicated in Figure 1 (a) will give rise to a minimum increase in current of 200% when the two elements come into electrical contact. Figure 1 (a) is used by way of an example to illustrate the operation of the invention. Figure 1 (b) is a further example of the invention and illustrates a method of arranging the heating elements which, with an initial mark/space ratio of 1/2 for the burst firing assembly, will give rise to a minimum increase in current of 400 /O when the two elements make an effective electrical contact. Figure 1 (c) is a further example of arranging the heating elements that can be advantageous if only one heating element is required to be heating.The heating element 3 is short circuited and can be of a very much lower impedance than element 2.

The circuit indicated in Figure 1 (a) possesses no form of control over the point in the current or applied voltage cycle that the open and short circuit conditions are initiated. If these events take place when the alternating current is at its maximum amplitude, i.e. at a point wt/4 or3wt/4 of the cycle, then a sharp or fast current/voltage edge with respect to time occurs. This is illustrated at points 21, 22, 23, 24, 25 and 26 in Figure 2. The symbol w represents the angular frequency of the alternating current and voltage and t is the time period of the cycle. Such sharp voltage and current edges, as described, are characterised by the associated transmission of electromagnetic energy at frequencies governed by the fourier components of the edge shape.Since this can give rise to the well known r.f.i. (radio frequency interference) it is an advantage to have a zero voltage crossing switch assembly in conjunction with the burst firing assembly.

Figure 3 is a block circuit diagram of the basic overheat protection circuit plus a zero voltage switch assembly circuit. The element of the circuit numbered 1-8 are identical to those of Figure 1(a). The element 41 is the zero voltage switch assembly.

Figure 4 plots the voltage versus time across the burst firing assembly of Figure 3, or between the points 39 and 40 of Figure 3. Figure 4 illustrates that the short circuit condition, or when current flows through the double wound heating element, is comprised of complete numbers of half cycles. This means that the initiation and completion of the current flow occurs at a zero voltage or zero current level, therefore, minimising the occurrence of any r.f.i.

Figure 5 is an electronic circuit diagram illustrating a more detailed design of the block circuit diagram of Figure 3 for a double wound heating element, 2 and 3, of 300 ohms. impedance per heating section.

The dual terminal power source is a 240 volts alternating supply designated 5 and 8. The part of the circuit included within area 55 is the basic mark/space ratio generator and that marked 56 is the zero voltage switch.

The basic markispace ratio generator 55 comprises an 18v zener diode 59 connected in series with the two heating sections 2 and 3 of the heating element. A 10,uF smoothing capacitor 60 is connected in parallel with the zener diode. Two NPN transistors 61 and 62 (BC 109) are provided. Both have 47 kQ collection resistors 63 and 64 respectively and each of their bases is connected through a respective capacitor 65 and 66 (0.1 and 1 .2MF respectively) to the collector of the other transistor, and, through resistors 67 and 57, to the positive rail created by 59 and 60.

The output from generator 55 is fed to the zero voltage switching circuit 56 through a diode 70 connected between one terminal of capacitor 66 and the collector of an NPN transistor 71 (BC 109) and emitter of a PNP transistor 72 (BC 479). The base of transistor 71 is connected directly and via a 120 resistor 73 to the negative rail while the base of transistor 72 is connected to the junction of a 47 kQresistor 74 and collector of a further NPN transistor 75 whose base is connected directly to the negative line. Transistor 71 has a 47 kncollector resistor 80.The other terminal of resistor 74 is connected to the emitter of transistor 72 and to the base of another NPN transistor 76 (BC 109), the emitter of which is connected through a 560 Resistor 77 to the switching electrode of a triac 78 (TRI 400).

The emitter of transistor 75 is connected to the junction of the base of transistor 71 and one end of resistor 73. This junction is also connected to a further resistor 81 the other end of which is then connected to the junction of the fuse 4 and the heat ing element 2. The other end of element 2 is con nected to one electrode of the triac 78 and to a 47 resistor 82. The other end of the resistor 82 is connected to the zener diode 59. A neon on/off indicator 90 may be connected in series with a resistor 91 (shown in dotted line) and connected in parallel with the zener diode 59 as shown. Physically this zener could be located underneath the heat control to illuminate it during operation of a heat settable blanket.

Figure 5 is now described in more detail. The mark/space ratio generator 55 is a well known astable multivibrator circuit. It operates by transistors 61 and 62 switching on and off consecutively thereby switching pointAalternativelyfrom a high to a low voltage level. The high voltage level is that defined by point B and the low voltage level is that defined by point C. The ratio of the time for point A spent at the high voltage level to that at the low voltage level is governed by the resistor capacitor networks of 57/66 and 65/67. The length of time that transistor 61 is off is determined by the charging of capacitor 66 through resistor 57. The length of time that transistor 62 is off is determined by the charging of capacitor 65 through resistor 67. When transistor 61 is off and transistor 62 is on then point A is at the low voltage level. When transistor 61 is on and 62 is off then point A is at the high voltage level.

If point A of Figure 5 and hence point Dare at a high voltage level then transistor 76 is switched on and hence the triac 78 is allowed to conduct. To ensure that the triac switches on and off at the zero voltage crossing point transistors 71,72 and 75 are incorporated. Considering first the positive half cycle of the alternating voltage supply between points 5 and 8 of Figure 5. For the time during which the voltage between points D and C is between 0 and + 0.7 volts then point D remains at the high voltage level. For the time periods above +0.7 voltstransis- tor 71 is switched on hence dragging point D to the low voltage level thereby switching transistor 76 off.

Considering now the negative half cycle of the alternating voltage supply between points 3 and 8 of Fig ure 5. During the time for which the voltage between points D C is between 0 and -0.7 volts then once again point B will remain high, thereby switching transistor 76 on and initiating the triac 78 to switch on. For time periods below -0.7 volts transistor 75 switches on which in turn switches transistor 72 on, thereby pulling point Din the circuit to the low voltage level. Therefore, transistor 76 is switched off.

Consequently it is only when the voltage cycle of the alternating voltage supply between points 3 and 8 of Figure 5 is between + 0.7 volts and point A is at the high voltage level that transistor 76 is on hence initiating triac 78 to switch on and off. When point B is at the low voltage level transistor 76 is off and hence triac 78 does not conduct.

By varying the value of resistance 57 from 500k to 2m the markispace ratio or on time/off time ratio varies from approximately 1/1 to 1/3 respectively and the mean effective current passing through the double wound element varies from 190MAto 95MA.

Consequently, altering the value of resistor 57 in the range indicated gives a variable heat control means.

By maintaining resistor 57 at a fixed value the heating capability of the double wound element remains constant. In the event of a fault, such as over-heating in the vicinity of the double wound element, leading to an electrical short circuit between the two elements the mean effective current increases to at least 400 MA. With a suitable fuse in the circuit a current, up to 190MA maximum will flow under normal operating conditions but will blow in the event of a fault of the type described.

A further embodiment of the system involves replacing resistor 57 with a resistor or variable resistor, in series with a positive temperature coefficient thermistor 58, (shown in dotted line in Figure 5).

With the thermistor physically placed in a close association with the dual wound heating element then at switch-on the mark/space ratio will be at a maximum value but will decrease as the heating element rises in temperature due to the increase in the value of the resistance of the suitably selected thermistor 58. This facilitates a variable rate of increase in temperature with respect to time, or heating rate, the rate being a maximum when the double wound element is at a minimum temperature or at switch-on. This embodiment also acts to maintain the double wound heating element at a maximum temperature governed by the resistance/temperature characteristics of the thermistor 58 and series resistor 57. The circuit will also cater for the utilisation of a negative coefficient thermistor in series with a fixed resistor.The adoption of such components would lead to an increasing rate of heating of the double wound element with respect to time eventually stabilising at an upper temperature dependant on the resistance/temperature characteristics of the thermistor and series resistor.

The above described thermistor embodiment lends itself to the automatic control of, for example, a hair dryer. In a conventional hair dryer, the hot air flow is the more restricted the closer the dryer is held to the head. As a result the temperature inside the dryer increases. This increase is sensed by the thermistor and the power fed to the heating coils of the dryer adjusted to maintain the temperature constant irrespective of how close to the head the dryer is held.

The two heating coils have been shown connected in series with the control circuit in the embodiments described. Other electrical wiring configuration may, however, equally well be used.

Referring to Figure 6, a modified circuit is shown which acts not only to give a variable control of the electrical power dissipated in an electric blanket with a low value of radio frequency interference (R.F.I) but also acts to protect the blanket from an overheat situation. With this circuit, as compared with the circuit described above, variations in the value of any single electronic component, from an open circuit to a short condition, will not increase the power dissipated in the blanket above the maximum specified heat setting under normal operating conditions.The block diagram of this Figure 6 shows the basic coaxial heating element comprising an inner element 102 and outer element 103 together with mark space ratio generation circuitry 107 which fires the triac 108 at zero voltage crossing points with a maximum duty cycle of 50%.

Circuitry 107 is fed from a wave shaper circuit 109 and its output is governed by a rotary heat selector switch 110 with it connected to a drive circuit 111 for thetriac. An indicator 112 is connected in parallel with the triac. A fuse 113 is disposed in the live supply line to the circuitry.

Referring to Figure 7, which shows a detailed circuit diagram of the arrangement shown in block form in Figure 6, the wave shaper circuit 109 comprises a schmitt trigger circuit consisting of resistors R2, R3 and R4, gates IC1 a and 1C1 band diode D2 converts the new mains sine wave W1 (see Figure 8) in a square wave W2 (see Figure 8). Gates IC1 a and Cl bare two gates of a Quad 2 Input AND gate CMOS integrated circuit. The other two gates IC1 c and ICld will be described later. The mark space ratio generator comprises a shift register 107. This shift register 107 has a constant logical one at the data input D and its last output Q7 fed into its master reset input MR. The square wave output from circuit 109 is usedto clock the shift register 107 each time the live terminal of the mains supply is 7 volts above neutral.

When the shift register 107 is clocked with the square wave output of the schmitt trigger circuit, then the data input D is latched into the output buffers Q. This sequence continues for seven clock pulses, and on the eighth the shift register 107 is reset i.e. all outputs Q return to logical zero. The output waveforms at Q4, Q5 and Q6 are referenced W3a, W3B and W3c respectively in Figure 8.

The rail voltage of VDD for the schmitt trigger IC1 a and IC1 b is effectively set by a zener diode Z1. If this rail voltage is 14 volts then the logic switching voltage or so called a rail switching point is 7 volts for IC1 a and lC1b. Consequently when point W1 (see Figure 7) is 7 volts above neutral the schmitt trigger generates a square wave output into the clock of the shift register 107. This is illustrated by waveform W2 in Figure 8. The shift register 107 and output buffers, lCl c and IC1 d, also operate at 7 volts.

The triac 108 therefore fires when the live terminal is approximately7 volts above neutral. This is therefor not a true zero voltage crossing switch but by selecting a suitable zener diode Zl,this voltage switching point can be reduced.

The gate of the triac 108 is powered by the output of the shift register 107. The different heat settings are selected by the rotary heat selector switch 110 which in turn connects each of the shift register outputs Q to the triac drive circuitry 111.

Once fired by any one of outputs Q4, Q5 and Q6 (at which waveforms W3a, W3b and W3c are respectively present) the triac 108 will conduct for a half mains cycle longer than the output of the shift register. This is a consequence of the output changing from the high to low state after the triac 108 has been; fired. Thus for the three outputs Q4, Q5 and Q6 respective waveforms at the triac 108 are produced.

These respective waveforms are referenced W4a, W4b and W4c in Figure 8. For the maximum heat setting the shift register output utilises a mark/space ratio of 3 cycles high and 4 low thereby firing the triac for 32 cycles and switching it off for 3 < . i.e. a 50% duty cycle (see waveform W4a in Figure8).

A series connection of a neon 112 and resistor R7 is connected in parallel with the triac 108. When working correctly, the neon indicator 112 will flash continuously. If a component fault occurs the neon will either extinguish in which case the fuse 113 will blow, or if the neon operates continuously without flashing no power will be supplied to the blanket.

TABLE2 The following list details the components values used in the circuit of figure 7.

R1 47K ohms +/- 0.5 watt High Stability Carbon film resistor.

R2 1M ohms +/- 0.5 watt High Stability Carbon film resistors R3 10M ohms +/- 0.5 watt High Stability Carbon film resistor.

R4 1 M ohms +/- 0.5 watt High Stability Carbon film resistor.

R5 1 M ohms +/- 0.5 watt High Stability Carbon film resistor.

R6 2.2K ohms +/- 0.5 watt High Stability Carbon film resistor.

R7 270K ohms +/- 0.25 watt High Stability Carbon film resistor.

R8 2.2M ohms +/-0.5 watt High Stability Carbon film resistor.

C1 100 mfd Electrolytic Capacitor +/- 20% 16 Volts Dl 1N4004 D2 1N4148 Z1 BZY88 ICI HEF4081 Quad 2 Input And Gate CMOS IC2 HEF4015 Dual 4 bit Static Shift Register CMOS Triac 4001 us Max RMS Current. IT(RMS) 1.6 Amp 400 Volt VDRM.

Switch 3 Position rotary switch (flash tested to 1 500V and tested to 15000 operations).

Fuse 250 mA Anti-Surge.

With the above described electronic circuit of Figure 7 variations in component values, from an open circuit to a short circuit condition, will not increase the power dissipated in the blanket above that specified by the 1:1 mark space ratio. Table 1 describes how such variations affect the power dissipated on the blanket. The component values are given in table 2.

It will be appreciated that the above embodiments have been described by way of example only and that many variations are possible without departing from the scope of the invention claimed.

TABLE 1 COMPONENT OPEN CIRCUIT SHORT CIRCUIT R1 No Power supplied to Z1 orC1 will fail and blanket. blowthefuse.

R2 No clock pulses : no ICI will fail power supplied No Power Supplied.

R3 No change, controller Fuse blows.

works normally.

R4 Full power supplied No power supplied Fuse blows.

R5 No change unless No power supplied double fault condition.

R6 No power supplied Power decreases due to lack of charge from reservoir capacitor to drive the triac.

R7 Neon extinguished but Neon fails blanket works normally (very bright) D1 No power supplied. No power supplied.

D2 No power supplied. Power decreases.

Z1 No change but voltage No power supplied.

supplied to circuit could rise.

C1 No power supplied. No power supplied.

Neon - Neon extinguished but circuit works normally.

Triac No power supplied. Fuse blows R8 Power decreases on Two possibilities: low heat setting. 1) No power supplied 2) Full Heat supplied ie 50% duty cycle.

Claims (18)

1. Apparatus comprising an electrical heating element having two coils placed next to each other but separated by a layer of electrical insulation, one of the coils being connected in series with a control circuit comprising a burst firing assembly whose mark/space ratio may be varied to vary the heating effect of the element, and a current overload device in series with the said one coil, a short circuit path being produced in operation through the insulation between the two coils in the event of overheating of the element thus short circuiting the burst firing assembly and resulting in an increased current to operate the current overload device and discontinue the electrical supply to the element.

2. Apparatus comprising an electrical heating element, and a control circuit comprising a burst fir ing assembly whose mark/space ratio may be varied to vary the heat effect of the element, the control circuit also comprising a temperature dependant element placed to sensethetemperature of the heat ing element and operative to alter the mark/space ratio of the burst firing assembly to maintain the temperature at a preselected value.

3. Apparatus as claimed in Claim 1, or 2, in which a zero voltage switch assembly is connected to the burst firing assembly.

4. Apparatus as claimed in Claim 1,2 or 3, in which the burst firing assembly comprises an ast able multivibrator.

5. Apparatus as claimed,in Claim 1,2 or 3, in which the burst firing assembly comprises a triac.

6. Apparatus as claimed in Claim 5, when appendank to Claim 4, in which the astable multivibrator is connected to the triac and resistive capacitive net works forming part of the multivibrator governs the ratio of the time that the triac is switched on to the time it is switched off.

7. Apparatus as claimed in Claim 5, when appen dant to Claim 3, in which the zero voltage switch assembly comprises two crossing point transistors one of these transistors being arranged to be switched to switch a transistor controlling the switching of the triac at positive voltages above a certain level and the other transistor being arranged to be switched to switch a transistor controlling the switching of the triac at negative voltage below a certain level.

8. Apparatus as claimed in Claim 7, in which the said one transistor is NPN and the said othertransis tor is PNP.

9. Apparatus as claimed in Claim 4, in which the burst firing assembly comprises a variable resistor connected to the multivibrator for varying the mark/space ratio of the assembly.

10. Apparatus as claimed in Claim 1, or any of claims 3 to 9 when appendant to Claim 1, in which the two coils are concentrically wound.

11. Apparatus as claimed in Claim 1, or any of claims 3 to 9 when appendantto Claim 1 or 11, in which the layer of electrical insulation is nylon or p.v.c.

12. Apparatus as claimed in Claim 2, or in any of claims 3 to 9 when appendant to claim 2, in which the temperature dependant element comprises a thermistor.

13. Apparatus as claimed in Claim 12, when appendant to Claim 9, in which the thermistor is placed in series with the variable resistor so that at switch on of the heating element the mark/space ratio will be at a maximum value but will decrease as the element rises so as to facilitate a variable ratio of increase in temperature with respect to time, or heating rate.

14. Apparatus as claimed in Claim 1 or 2, in which the burst firing assembly comprises a shift register having a plurality of outputs connected to respective terminals of a heat selector switch.

15. Apparatus as claimed in Claim 14, in which a wave shaper produces square wave pulses from an alternating current supply and supplies them to the clock terminal of the shift register.

16. Apparatus as claimed in Claim 15, in which the wave shaper comprises a schmitt trigger circuit.

17. Apparatus as claimed in Claim 14, 15 or 16, in which the heat selector switch is connected to a triac which is switched on and off in accordance with the output of the shift register to which the switch is connected.

18. Apparatus substantially as hereinbefore described with reference to Figures 1 to 5 or to Figures 6 to 8 of the accompanying drawings.