GB2133643A - Power controller - Google Patents

Abstract

A triac 10 controls the supply of power to a load 6, such as the heating element of a stove or spare heater, and is attached to a housing (20, 21), (Figures 3, 6) containing a control circuit for the triac. The housing acts as a heat sink for the triac 10, or has a heat sink (24) served thereto. Pushing in of a control shaft (22) closes switches SW1, SW2 and allows subsequent shaft rotation to first close switch SW3 and then adjust resistor 18. Means (65, 70) are provided to prevent rotation of the control shaft (22) in its OFF position. The shaft (22) is transparent and fluorescent to transmit light from a neon indicator 1 to a lens (60), (Figure 8), moulded as part of the shaft (22) above a control knob (61). A temperature sensing resistor may be connected in series with the variable resistor 18. <IMAGE>

Description

SPECIFICATION Electronic infinite heat control This invention relates to controls for adjusting the rate of application of electric power to a load, especially a resistive load, and finds particular use in controls for electric heating elements for stoves, space heaters and the like.

Typical controls at present, employed for varying the power output of heaters and stove elements, embody arrangements in which the control contacts are cycled between an ON and an OFF condition over regular, relatively short time periods, so that the ON duty cycle of the contacts determines the average power applied to or dissipated in the load. Typicallythe arrangement is such that a thermally sensitive element in the control is heated during the ON period of the duty cycle, and which cools during the OFF period. Such controls only approximate to providing constant power at any one setting due to changing environmental temperature, are difficult to regulate exactly and are particularly inaccurate and inconsistent from one unit to the next at low settings (that is low duty cycle). Nevertheless, they are in widespread use because they provide a continuous range of control.

With the development of solid state electronics, there are now triacs (switchable bi-lateral conductors) on the market capable of handling the heavy currents in (and well above) the range encountered in electric heating appliances and stoves. One problem with such devices is that they evolve heat and therefore any control mechanism involving them must be capable of dissipating this heat and in an environment which itself is often quite hot.

It is an object ofthe present invention to provide an accurate control device for medium to heavy current AC loads, particularly for resistive heating loads for stoves and space heating, which gives a more accurate control but retains the low cost advantage of prior art conrols.

Specific embodiments of the invention will now be described having reference to the accompanying drawings in which: Figure 1 shows atypical circuit diagram of a heating load circuit and control embodying the invention; Figure 2 is a perspective view of one embodiment of a control device forming part of the circuit of Figure 1; Figure 3 is a side view in section of a modified device of Figure 2 with different terminal arrangement; Figure 4 is a side view in section similar to Figure 3 but with the control device in ON condition; Figure 5 is a plan view of a typical circuit board of an embodiment of the invention with components connected below the board illustrated schematically;; Figure 6 is a detailed side view in section of part of an embodiment employing a heat sink and Figure 6A shows a similararrangement butwiththecoverforming the heat sink as in Figure 2 and shows a typical placement of components connected to and accommodated beneath the circuit board; Figure 7 and 7A show details of embodiments of the indicator light assembly and the base of the operating shaft; Figure 8 is a sectional view of a modified embodiment showing details of the slider mount and of a lens, knob and detent arrangement for the operating shaft; Figure 9 is a plan view of the base of the control for receiving the board of Figure 5; and Figure 10 is a graph of output power in the load, against shaft position.

With reference first to Figure 1, an alternating current supply is provided to terminals L1 and L2 suitably made of brass. L1 feeds a load terminal H1 connected to one side of a resistive load 6 through switch contacts SW1. Terminal L2 feeds, through switch contacts SW2, to a mounting lug L3 which may also be brought out as a further terminal. L3 is connected to terminal MT2 of a triac 10 (a switchable bi-lateral conductor). The other electrode MT1 is connected through an rf choke 3 of about 50 H, suitably comprising 49turnsof#17 gauge wire on a ferrite rod (as used in the prototype), to a second load terminal H2 joined to other side of load 6. Lug/terminal L3 and terminals H1 and H2 may also suitably be of brass.

The sides of SW1 and SW2 remote from their respective terminals L1 and L2 are bridged by a series connected neon lamp 1 and resistance 2. An rf suppres sion capacftor4ofabout.1F also bridges lug L3 and terminal H2. The gate G of the triac is connected through a diac 16 (a symmetrical bi-lateral solid state switch) to one side 17 of a phasing capacitor 19 of about .1 ,uF whose other side connects to MT1. A switch arrangement SW3 (single pole double throw, open circuited in one position) connects this one side of capacitor 19 either to a variable resistance 18 of about 930 KQ maximum orto open circuit. The resistance 18 and the switch SW3 are ganged as will be explained later.

It will be appreciated by those skilled in the art that the closing of switch contacts SW1 and SW2 will light the lamp 1 and result in the application ofthe voltage from terminals L1 andL2acrosstheseriesconnected triac 10 and load 6. Current does not flow however unles switching current also flows into gate G. The variable resistance 18 and capacitor 19 (when connected by SW3) form a timing circuit and only when the voltage at junction 17 between the two, and with respect to terminal MT1 ofthetriac, reaches approximately 7 to 8 volts will the diac 16 forced into conduction through the gate G andthetriacswitched on. The triac remains conductive during the remainder of the half cycle of the AC mains until the current reaches next zero crossing and then switches off.It will not switch on again until the junction voltage is again high enough with respect ti MT. during the next half cycle. Thus, by firstly setting switch SW3 to connect 17 to the resistance 18 and then varying the resistance 18, the timing of the point in the mains AC cycle at which the junction voltage reaches the prescribed value can be chosen and thus the conductive duty cycle of the triac be controlled. Lower values of the resistance 18, mean that the triac is switched earlier in the AC cycle and therefore high average current is applied to the load with subsequent strong heating.

Larger values of resistance give lower heating.

Reference to Figure 2 shows a novel control device for effecting the circuit action described for Figure 1 and illustrates the terminals L1, L2, H1 and H2 to which the external AC source and load are respectively connected. The entire assembly is carried in a box 20 with a cover or lid 21 from which protrudes a shaft 22 for the variable resistor 18. The triac is attached to one side of the lid by fastener 28 so that the lid, constructed of a heat conductive material such as aluminium, forms a heat sink for the triac.

Figure 3shows a sectional view through an embodiment ofthe invention and illustrates the device in the OFF position with the switch contacts SW1 and SW2 in open condition. The connect actuator 25 is pressed upwards by the upper contact springs of SW1 and SW2. Figure 3 also illustrates the wipers 29 and 30 of variable resistance 18 in the position in which the wipers are also lifted from the printed circuit board 31 on which the elements of resistance 18 are deposited (of which further details are given later). It should be observed that the wipers do not form part of the triac and load series circuit, but are part of the timing switch circuitforthe triac. They do not therefore carry the load current.

In the sectional view shown in Figure 4 the apparatus of Figure 3 is in its ON position. The connect actuator 25 is shown bearing upon the movable members of contacts SW1 and SW2 so that they are closed. The sliders 29 and 30 which are moved against the circuit board are then in position to con tactthe elements of resistance 18 on the circuit board 31 when shaft 22 is subsequently rotated.

Discrete contacts are illustrated on the spring arms forming SW1 and SW2. These contacts can be dispensed with in general however since, as explained above, SW1 and SW2 do not break or make on load current, all of which is effected by the triac. As a practical matter and in the event of failure ofthe triac and switches SW1 and SW2 would be required to make and break load current for a small number of operations (until the operating person noted the failure). Typically such switches without discrete contacts could accommodate of the order of 50 operations without undue erosion.

It will also be noted that both the upper and lower contact arms of SW1 and SW2 are springs. This ensures a wiping action between the contacting faces, and thus good low resistance contact. When discrete contacts are not used it is desirableto dimple at least one of the contact arms at the region where contact will be made with the other arm for effective localising of the contacting areas.

Figure 5 shows a plan view of the upper surface of the circuit board 31, which is an inexpensive epoxy glass paper composite board on which the resistance 18 is formed and comprises an inner conductor 40 and an outer resistive element 41. Element 41 is profiled to obtain the appropriate required variation of resistance of element 18 with respect to rotation of the bridging sliders 29 and 30 between sections 40 and 41. Other interconnections on the circuit board complete those illustrated in Figure 1 forthetriac, the choke, the capacitors and diac. Surprisingly, it is found thatthe resistive element 41 of resistive ink can be printed on the board such as by a screen printing process, without special preparation or the need to make the board of a high cost ceramic substrate.

Typically used is a polyimide resistance ink. The printed material is baked to harden and condition it, and finished with a lubricant such as Lubriplate (Trade Mark). The sliders 29 and 30 are formed so that when the device 22 is in the ON condition the pressure exerted on the elements 40 and 41 does not exceed 40 grams each. A pressure of not less than 10 grams is required for reliability and allowing for tolerances therefore the figure of 40 grams is taken as a design figure. Test points 80, 81,82, 83 and 84 are provided, if desired, connecting to selected regions of the perimeter of element 41 so that the resistance at various points along the arc of travel of the slider 30 can be checked at assembly and for quality control purposes. Resistance can be trimmed where needed using a laser knife.The temperature to which the board is raised to bake the resistive coating is chosen to be above any temperature likely to be experienced by the device in use since it was found that the resistance may change if the baking temperature is exceeded. Typically baking temperatures of about 100"C to 115"C have been found to be sufficient, though higher temperatures are envisaged dependent upon the temperature which the board can tol erate without changing of its properties. Typically it is considered desirable to maintain a "guard band" of about 70"C between the tolerance temperature of the board and the ink baking temperature. Should a ceramic board be used in place of the cheaper composite then much higher temperatures can be used without difficulty.Ceramics in general are brittle and thus present other problems in manufacture and assembly.

Figure 6 illustrates the mounting ofthetriac 10 to a heat sink 24 (as discussed for Figure 2) and to the circuit Board 31. It is found that a discrete heat sink is not usually required, and Figure 6A illustrates the mounting ofthetriac 10 directly to the cover 21 using a mounting flange 63, suitably of aluminium, as the heat sink. When electric isolation between the triac and its mounting is needed, a mica insulation washer 62 can be introduced between the triac and the heat sink and a non-conductive nylon fastener 28 used in place of the screw illustrated in Figure 2. Instead of the mica washer a composite material now on the market which is heat conductive, requires a minimum of heat sink grease, and is a good electrical insulator and is known as CHO-THERM (Trade Mark) sold by Chomerics of 77 Dragon Court, Woburn, Mass.

018011 U.S.A., may be used. Insulated triacs are also available with good isolation for voltages of approximately 240 volts AC to ground, thus avoiding the need for insulation washers. A decision in any particular instance will depend upon costs invoived. Isolated triacs are typically more expensive than unisolated ones ofthe same voltage and current rating. It is also found that heavier current rated triacs tend to be operable at higher temperatures than lower current rated triacs for the same load current.

Figure 7 illustrates the mounting ofthe neon lamp 1 belowthe connect actuator 25. Figure7A illustrates a view at 90 to this. The actuator 25 is hollow so that lightfrom the neon can fall on the base 55 ofthe shaft 22, which is made of polycarbonate and is transparent so that the light from the neon can pass up the shaft, to a lens 60 moulded as part ofthe shaft above a knob 61 (as illustrated in Figure 8). The lower end of the shaft 22 and the actuator 25 are profiled, and the end oftheshaftsplit, so that on assemblytheysnapto one another to form a single operative unit, wherein the shaft rotates in the actuator but the actuator is captured on the end of the shaft.Particularly suited to the manufacture of the shaft 22 are the fluorescent dyed LISA plastics available from Bayer A.G., Leverkusen, Federal Republic of Germany. These will accept illumination incident on the side and contain dyes which fluoresce from the illumination. Fluorescent light is then transmitted by total internal reflec tion to the end ofthe shaft 22. This will permit considerable leeway in the placing of the neon. Various colours of fluorescence are available, which allows different indication colours for the control device.

With further reference to Figures 3 and 4 a locking tab 65 extends from the lid 21 towards insulated mount 70 for the variable resistor contacts 29 and 30 and engages in a slot 64 in the mount only when the control is in the OFF position. To move from this position the shaft 22 must be depressed against the springing action ofthe contacts SW1 and SW2 and of slider contacts 29 and 30 and the knob turned, so that the lower face of tab 65 then rides against the underside ofthe mount70 withthe slidercontacts 29 and 30 pressed into connection with the elements ofvariable resistor 18, and SW1 and SW2 are in ON condition.

Figure 8 shows an embodiment in which the mount 35 for the sliders 29 and 30 is modified to be received in the board 31 for rotation. A keyway is formed in the mount and is engaged by a spline 34 on the shaft 22.

The upperside36 ofthespline contactsthe underside of the lid 21 only in the OFF position of the control to allow the shaft 22 to move upwards and allow the actuatortomove to the position in which the switches SW1 and SW2 are open (such as shown in Figure 7A).

When shaft 22 is pushed down against the springing action ofthe moveable contacts of SW1 and SW2 and rotated, upper side 36 of the spline engages against the under-surface of land 37. In all embodiments in the OFF position of the control, sliders 29 and 30 do not bridge the elements 40 and 41 of resistance 18, even though they may be resting against the board.

Reference also to Figure 5 shows that whereas slider 29 continuously engages track 40, there is an angle of about 30 of rotation of the shaft over which slider 30 does not engage track 41. It is this which constitutes the open circuit position of SW3 depicted in Figure 1.

The requirement in the structures of Figures 3,4 and 8 for both depression of the knob 61, and its turning, to place the control in ON condition therefore provides a double safety arrangement, neither one action alone being effective to turn it ON. In Figure 8, the continuous engagement of the mount 35 in the board retained by ears 38 ensures that the pressure exerted by the sliders 29 and 30 on the board (or the resistance elements) can be precalculated and remain at that value.

Figure 8 also illustrates a variation in which shaft 22 engages an acetal bullet 39 urged by spring 43. This bullet acts as a detent to allow a positive "feel" when the control is in the maximum ON position. Other detent arrangements to achieve this effect may be used.

Figure 9 illustrates in plan view the structure of base 20 of a typical embodiment. Vertical risers 51 and 52 from L3 and H2 respectively connect to and support the circuit board 31 in a manner similar to risers 53 and 54 shown in Figure 3 and 4. Additional support is provided by upperfaces 55,56, 57 and 58 of corner buttresses moulded in the base 20. Web 59 steadies and supports the rear face of the mounting flange 63 of triac 10 (see also Figure 6A).

It is found that since complete control can be had over the shaping of element 41, control of the phase angle of the switching signal applied to the triac gate and power output as shown in Figure 10 can be obtained. Thus the initial ON condition is achieved at about 5% rotation of the control at a power output of 5% of total, at 25% rotation this is increased to 10% output and at 50% rotation to 20% output. 50% output is obtained at some 75% rotation, rising to 100% output at 95% rotation. Such power increments give a practically linear relationship of temperature to shaft angle (or percentage rotation) for the heating element load 6. Because all heat regulating switching is done by the triac, heavy duty contacts of the kind required in prior art controls described above are not required for SW1 and SW2.

If the lug L3 is brought to a terminal it can provide an additional connection to the circuit so that the control can be used as a commercial replacement unit for appliances in which present controls have a separate indicator light, one side of which is permanently connected to the mains L1. The other terminal of the light is then connected to L3, when the present device is substituted.

It will be appreciated by those skilled in the art that, with minor modificationsto the circuit board, a temperature probe resistance can be included in series with resistance 18 to operate as a temperature sensor located near the burner element or in an oven if the unit is used as an oven control. In such cases, the dial ofthe unit (forming part of knob 61 for instance) could be calibrated in degrees of temperature.

In practice some radio frequency interference is generated by the switched wave form produced in the triac if the device is not suppressed. The combination of choke 3 and capacitor 4 has been found fully effective as suppressors to remove any problem in the frequency spectrum about 450 kHz.

The device is also inherently protected against any unintended switch-on by transients occurring in the AC mains (a problem encountered sometimes in some microwave ovens), since when the control is off, SW1 and SW2 are open and the unit completely isolated from the mains.

Claims (13)

1. An electrical load control which comprises: a pair of input terminals for receiving an alternating current power supply; a pair of respective switch contacts series connected with each terminal; a first load terminal connected to one pair of said switch contacts, the other pair of switch contacts series connected through a switchable bi-lateral conductorto a second load terminal, said first and second load terminals being connectible to an electric load.

said bi-lateral conductor including a switching gate electrode; a variable resistance and a capacitor electrically series connected through a junction between said one input terminal and said second load terminal and means connecting said gate electrode to the junction of said resistance and capacitor; housing means enclosing said circuit; means mounting said bi-lateral conductor for heat sinking on said housing; a control shaft protruding from said housing and movable between an ON and an OFF condition of said control to close and open said switch contacts and to vary said resistance in said ON condition for providing a variable phase signal to said gate electrode whereby varying output in a said load when connected to said load terminals.

2. Apparatus as defined in claim 1 comprising indicator means electrically connected to at least one of said pairs of said switch contacts for providing a visable indication when said'switch contacts are closed.

3. Apparatus as defined in claim 2 wherein said indicator means include an illumination means in said housing and means permitting observation of light from said illumination means from a point exterior to said housing.

4. Apparatus as defined in claim 3, said illumination means being mounted adjacent said shaft, said shaft receiving illumination from the illumination means and being light transmitting and light thereby being visible from the end of said shaft.

5. Apparatus as defined in claim 4 comprising interlock means between said housing and said shaft preventing rotation of said shaft in said OFF condition of said control, and means permitting longitudinal movement of said shaft in said OFF condition, said movement of said shaft moving said contacts between open and closed condition, and said interlock means permitting rotation of said shaft when moved to closed condition of said contacts.

6. Apparatus as defined in claim 5 said resistor comprising slider means, said slider means carried by said shaft and being enabled to contact a resistive tract defining said resistance means when said shaft is moved to closed condition for said contacts.

7. Apparatus as defined in claim 6 said slider means contacting said resistive track only when said shaft is moved in the ON condition of said control subsequent to movement from said OFF condition.

8. Apparatus as defined in claim 6 said resistive track being profiled to provide a chosen relationship of resistance to rotation of the shaft.

9. Apparatus as defined in claim 6 said resistive tract comprising resistive ink means deposited on a substrate mounted in said housing.

10. Apparatus as defined in claim 5 wherein a cover means, forms part of said housing, said shaft extending through said cover means said interlock means comprising mutually engaging means on said shaft and said housing.

11. Apparatus as defined in claim 1, heat sink means for said bi-lateral conductor comprising part of said cover.

12. A power control device including an electronic component which evolves unwanted heat during use, comprising a housing containing the power control means and a heat sink for dissipating unwanted heat from the electronic component.

13. Apparatus substantially as herein described with reference to Figures 1,5,9 and 10 as modified by Figures 2-4 and Figures 6 or 6A, and Figures 7, 7A or Figure 8.