Title:
TOUCH CONTROL FOR ELECTRICAL APPARATUS
United States Patent 3641410


Abstract:
A touch control to energize the load of a tool or appliance. A timing circuit gates a triac ON to energize the load in response to a touch or touch device having no moving parts. The touch device in one embodiment is an impedance without moving parts and the value of which varies with the location of the operator's touch to vary the timing of the circuit and, correspondingly, to provide for variable energization of the load. The impedance can be a capacitor or a resistor. Another embodiment includes several impedances and several touch devices to provide for selective energization of the load of a predetermined extent.



Inventors:
VOGELSBERG WALTER H
Application Number:
05/033202
Publication Date:
02/08/1972
Filing Date:
04/30/1970
Assignee:
BLACK AND DECKER MFG. CO.:THE
Primary Class:
Other Classes:
200/522, 388/838, 388/919, 388/937
International Classes:
H02P25/14; H03K17/96; (IPC1-7): H02P7/00
Field of Search:
200/DIG.1,157 318
View Patent Images:



Primary Examiner:
Hix L. T.
Assistant Examiner:
Hickey, Robert J.
Claims:
I claim

1. In combination with a load adapted to be energized from a source of alternating current, a touch or proximity control for energizing the load, said control comprising

2. A touch control according to claim 1 wherein said solid-state bidirectional controllable conduction means is a triac.

3. A touch control according to claim 1 wherein said circuit means includes

4. A touch control according to claim 1 wherein one of said solid-state control means is a PNP-transistor; and

5. A touch control according to claim 1 wherein

6. A touch control according to claim 5 which further includes

7. A touch control according to claim 6 wherein

8. A touch control according to claim 6 wherein

9. A touch control according to claim 1 wherein

10. In combination with a load adapted to be energized from a source of alternating current, a touch or proximity control for energizing the load, said control comprising

11. A touch control according to claim 10 wherein

12. In combination with a load adapted to be energized from a source of alternating current, a touch or proximity control for energizing the load, said control comprising

13. In combination with a portable power tool or appliance including an electrical load adapted to be energized from a source of alternating current, a variable touch control without moving parts for energizing the load, said control comprising

14. The combination of claim 13 wherein

15. The combination of claim 14 wherein

16. The combination of claim 14 wherein

17. The combination of claim 14 wherein

18. The combination of claim 13 wherein

19. A control circuit without moving parts for controlling the energization of a load comprising, in combination

20. A control circuit without moving parts for controlling the energization of a load comprising, in combination

21. A control circuit according to claim 20 wherein

22. A control circuit without moving parts for controlling the energization of a load comprising, in combination

Description:
This invention relates generally to electronic switch arrangements with a touch-type circuit element having no moving parts to provide for manual actuation of the switch to energize a load.

More particularly, the invention relates to solid state proximity switch arrangements in which a touch button or plate requiring only a light touch or engagement by the finger of the operator controls the operation of a switch capable of carrying substantial current, and in which accidental touching of the touch button to activate the switch and energize a load is positively prevented.

Further, one embodiment of the invention relates to a touch control circuit for controlling the energization of a load from an alternating current supply, wherein the touch element provides for varying the ON time of the switch during each cycle of the AC current to permit selective manual control of the extent of energization of the load as a function of the position of an operator's finger or hand relative to the touch element.

While there have been considerable recent developments in electronic and solid state controls for various devices, there has been little if any recent development directed to the replacement of mechanical switches with electronic switches to perform the same function. While touch-type electronic switch arrangements have been used for elevator service, the elevator service circuitry is quite complex and has found little if any application in other fields.

It is well known that the extent of energization of a load from an AC source can be controlled by controlling the firing angle of a controllable conduction device so the load is energized during only a predetermined portion of each cycle of the alternating current supply. Among the controllable conduction devices which have been used to regulate the portion of each AC cycle which energizes the load are the SCR and the triac. The SCR is a unidirectional conducting device which can conduct current in only one direction and has the unique characteristic of remaining conducting, after it is gated ON, until the current through the SCR falls below its minimum holding current. The triac, sometimes considered the equivalent of two SCR's in inverse parallel relation, can conduct current in both directions, but must be gated ON during each half cycle of the alternating current because it too is commutated during each half cycle when the line current falls below its minimum holding current.

Applicant has found that a touch control including a triac or its equivalent in SCR's can readily be used in place of the usual mechanical switch for operating various tools and appliances such as a portable electric drill. In the usual electric drill, a mechanical trigger switch is provided and the drill is turned ON when the operator pulls the trigger switch to an ON position. When the switch is released the drill is deenergized. In applicant's touch control arrangement, touching a touch element activates a trigger circuit which gates a controlled conduction device, in series with the load, ON during each half cycle of the alternating current to energize the drill motor or other load. In one preferred embodiment, the controlled conduction device is a triac, the trigger circuit has a very small time constant, and correspondingly, the triac is gated ON for substantial full wave conduction so long as the operator maintains a touch on the touch element.

In a second embodiment of the invention, the touch element is so constructed that the time constant of the trigger circuit, and correspondingly, the firing angle of the controlled conduction device, is varied at the option of the operator by changing the position of his hand or finger on a touch element having no moving parts. Such change in the position of the operator's hand or finger changes the time constant of the circuitry so the operator can select and control the firing angle of the controlled conduction device merely by moving his finger along the touch element. As applied to a motor control, for example, a speed control for an electric drill, the invention finds particular advantage. The "variable" touch plate is located in a position on the handle of the drill where it can conveniently be engaged by one or, if desired, several fingers of the hand of the operator. The basic concept involved is to vary the time constant of the trigger circuit by varying the effective impedance of the touch element. Where the touch element is of the capacitive or resistive type, the impedance can be varied by changing the area of engagement of, or the location of engagement of the operator's finger or hand with the touch element. As the effective impedance of the element is increased, the conduction angle of the controlled conduction device is decreased, and less current is supplied to the load. As the effective impedance of the element is decreased, the conduction angle of the controlled conduction device is increased and more current is supplied to the load. Hence, the operator of the tool can regulate the extent of energization of the load, for example, the speed of the motor, by the location of his finger on the touch plate or by the pressure exerted on the touch plate.

To prevent accidental energization of the load where the switch of this invention is used in a portable tool, such as an electric drill, the touch plate is recessed so the operator must make a conscious effort to engage the plate in order to energize the load. An additional safety feature resides in a cover or shield which prevents contact of the touch plate without a conscious effort on the part of the operator to first move the shield.

The trigger circuit for the controllable conduction device includes a pair of transistors, one a PNP and the other an NPN. The arrangement is such that the PNP-transistor supplies a gating signal to the conduction device during each half cycle of the AC current source of one polarity, and the NPN-transistor supplies a gating signal during each half cycle of the other polarity so long as the touch or proximity element is activated. The transistors conduct at a very low voltage and correspondingly, provide for full-wave gating of the controllable conduction device.

Correspondingly, an object of this invention is a unique touch or proximity-type switch for controlling a load, the switch being particularly characterized by the absence of moving parts.

Another object is a touch-controlled switch including a solid-state controllable conduction device for supplying full wave power from an AC source to energize a load.

Another object is a touch controlled switch having particular utility for use with a hand held appliance and in which means are provided to prevent accidental operation of the switch without a conscious effort on the part of the operator.

A further object is a touch-controlled switch without moving parts where the effective impedance of the touch element is varied by the position of the operator's hand or finger on the element to control the time constant of a gating circuit for a controlled conduction device so the firing angle of the conduction device can be regulated to control the energization of the load from an AC source.

A still further object is a control arrangement of the type described which has improved sensitivity, is inexpensive to manufacture, is reliable, is of small size so it can be packaged as an integral part of the tool, appliance, or other load controlled by the conduction device, is convenient to use, and is instrumental in virtually eliminating operator fatigue.

Numerous other objects and features of the invention will become apparent with reference to the following drawings, which form a part of this specification and in which:

FIG. 1 is a pictorial view showing an electric drill including the touch or proximity control of this invention;

FIG. 2 is a circuit diagram of a first embodiment of the control circuit including a capacitance-type proximity or touch element which can be variable;

FIG. 3 is a partial view in front elevation of the handle of the drill of FIG. 1 showing one embodiment of the touch element;

FIG. 4 is a partial view in section taken along line 4--4 of FIG. 3;

FIG. 5 is a partial view in section taken along line 5--5 of FIG. 3;

FIG. 6 is a view corresponding to FIG. 5 and showing a hinged shield overlying the touch element;

FIG. 7 is a view corresponding to FIG. 6 and showing a second embodiment of shield for the touch element;

FIGS. 8A- E show the relative timing of voltage and current in the control circuits;

FIG. 9 is a partial front elevational view of the handle of the drill of FIG. 1 showing a variable capacitance-type touch plate;

FIG. 10 is a partial view in section taken along line 10--10 of FIG. 9, and showing a first embodiment of a shield for the touch element of FIG. 9;

FIG. 11 is a view corresponding to FIG. 10 and showing a second embodiment of the shield for the touch element;

FIG. 12 is a circuit diagram showing a second embodiment of the control circuit of this invention, wherein a plurality of capacitors are used as the touch elements to provide for step-type variable speed control;

FIG. 13 is a front elevational view of the handle of a drill showing the arrangement of the capacitors for the control circuit of FIG. 11;

FIG. 14 is a partial view in section taken along line 14--14 of FIG. 13;

FIGS. 15A- D show waveforms of current conduction in the control circuit;

FIG. 16 is a circuit diagram showing another embodiment of the variable speed control of this invention, wherein a resistor is the touch element;

FIG. 17 is a partial front elevational view of an electric drill handle including the variable resistance element of the control circuit of FIG. 16;

FIG. 18 is a view in section taken along line 18--18 of FIG. 17;

FIG. 19 is a circuit diagram showing another embodiment of a variable speed control according to this invention;

FIG. 20 is a partial view in front elevation of a drill handle incorporating the touch elements used with the control circuit of FIG. 19; and

FIG. 21 is a partial view in section taken along line 21--21 of FIG. 20.

Referring now to the drawings in detail, and particularly to FIG. 1, there is shown a portable electric drill 1 having a hollow housing 2 containing a motor 3 (FIG. 2) which drives the drill chuck 4 located at the forward end of the housing. Housing 2 also includes the usual pistol grip-type handle 5 into the lower end of which the power cord 6 extends. The drill is connected to a suitable alternating current source by plugging the plug 7 into an appropriate alternating current electrical outlet.

Mounted within handle 5 of housing 2 is a control circuit 8. Control circuit 8 includes input terminals 9 and 10 and output terminals 11 and 12. Input terminal 9 is connected directly to output terminal 11 via line 13. Input terminal 10 is connected to align 14 which is connected to one of the anodes 15 of a triac 16, and output terminal 12 is connected to the other anode 17 of the triac. Output terminals 11 and 12 are connected respectively to the input terminals of the motor 3, which as seen at FIG. 3 includes field windings 18 and an armature 19. Motor 3 is a 120 volt AC universal series field-type motor.

Line cord 6 is of the three wire type and includes a "hot" wire 22 connected to input terminal 10 via an ON-OFF switch 23. Wire 24 of the line cord is connected to input terminal 9. There is also a third or ground wire 25 which is grounded to the drill housing in the usual manner as at 26. Plug 7 is of the polarized type and has a "hot" prong 27, a ground terminal 28 and a prong 29 which is adapted to be connected to the ground side of the power supply into which the plug is inserted.

The control circuit 8, which is particularly characterized by the absence of any moving parts includes a capacitor 32 connected to line 14, and a resistor 33 connected to the capacitor at a junction 34. Connected to the opposite end of resistor 33 is a touch plate assembly 34 which functions as a capacitor and includes a first plate 35 connected to resistor 33 and a second plate 36 separated from the first plate by a layer of dielectric material which insulates the plates from each other.

Connected in shunt across triac 16 are serially connected resistors 39 and 40 which function as a voltage divider and are connected together at a reference junction 41. The control circuit 8 also includes an NPN-transistor 42 and a PNP-transistor 43. Transistors 42 and 43 have their bases connected together at junction 44. Similarly, the emitters of transistors 42 and 43 are connected together and are connected to gate terminal 45 of triac 16. The collector of transistor 42 is connected to reference junction 41 via a diode 46 and the collector of transistor 43 is connected to reference junction 41 via a diode 47. Diode 46 has its anode connected to the junction 41 and its cathode connected to the collector of transistor 42 to prevent the occurrence of a negative signal at the collector of this transistor. Diode 47, on the other hand, has its anode connected to the collector of transistor 43 and its cathode connected to the junction 41 to prevent the occurrence of a positive signal at the collector of this transistor. A breakover device in the form of a neon bulb 48 is connected between junction 34 and junction 44 to which the bases of transistors 42 and 43 are connected.

As shown at FIGS. 1 and 3--5, touch assembly 35 is mounted on the handle 5 of the drill at the position of the usual trigger switch. As previously mentioned, handle 5 is hollow, and at least the front portion 52 of the handle is formed from an electrically insulating material, such as plastic, so touch assembly 35 is insulated from the housing 2 of the drill. As will subsequently be explained in detail, touching the outer conductive plate 37 of touch assembly 35 (switch 23 closed) gates triac 16 ON to energize motor 3 of the drill. To prevent accidental engagement of the operator's finger with the outer touch plate 37 of the touch assembly, handle 5 is provided with a recess 53, and the touch assembly is mounted adjacent the bottom wall 54 of the recess. As shown at FIGS. 3-5, the recess is formed in part by an upper edge 56, a lower sloping wall 57, and side projections or ribs 58 and 59, respectively. The outer touch plate 37 is spaced from the front of these ribs and edges to prevent accidental engagement of outer plate 37 by the operator of the tool. Advantageously, touch assembly 35 has its inner plate 36 seated on rear wall 54 of the recess and an opening 60 is provided through the wall to accommodate wire 61, which is connected between resistor 33 and inner plate 36.

As shown at FIG. 6, a hinged shield 65, pivotally connected to the front of handle 5, can also be provided. A pivot pin 66 mounts the shield for pivotal movement to and from the solid and phantom line positions of FIG. 6. The pivot pin 66 passes through a journal 67 which is formed integral with the front portion of handle 5. Advantageously, shield 65 extends downwardly beneath recess 53 to provide a projecting end 68. Handle 5 curves inwardly, as at 69, at a location opposite the projecting end 68, so the operator of the tool can slide his finger upwardly between the handle and the projecting end 68 to lift shield 65 to the phantom line position, where touch plate 35 is exposed and can be touched by the finger of the operator to energize the drill. A leaf spring 70 is secured to handle 5 adjacent the upper end of shield 65 and has an end which engages the outside surface of the shield to normally maintain the shield in the solid line position to close recess 53.

FIG. 7 shows an alternative arrangement, wherein a shield 72 extends across recess 53 to prevent the operator of the tool from accidentally engaging touch assembly 35. The flexible shield 72 is advantageously formed from electrically insulating sheet material which is resilient, such as polyethylene. Shield 72 is secured to the handle above the recess by a rivet 73. As shown at FIG. 7, shield 72 has a projecting tip 74 which allows the operator to lift the shield to the phantom line position by sliding his finger up the drill handle, in the manner explained for shield 65, whereupon he can easily touch the touch assembly 35 to energize the drill.

OPERATION OF THE CIRCUIT OF FIG. 2

With plug 7 inserted in a 120 volt AC receptacle and switch 23 closed, motor 3 is energized for substantially full-wave operation, when the operator of the tool touches his finger to outer plate 37 of touch assembly 35. For substantial full-wave operation of the motor it is necessary for triac 16 to conduct for a substantial portion of each positive and negative half cycle of the alternating current supply. However, the triac will conduct only for that portion of each positive half cycle and for that portion of each negative half cycle after the triac is triggered into conduction by an appropriate trigger signal applied to its gate 45.

First consider the operation of control circuit 8 during half cycles of the power supply when line 13 is positive relative to line 14 (See voltage wave 75, FIG. 8A) and the operator's finger is on touch assembly 35. As a result of body capacitance between outer touch plate 37 and line 13, capacitor 32 charges through the operator's body, the touch assembly, and resistor 33 until the potential at junction 34 is sufficient to cause neon bulb 48 to fire. When neon bulb 48 fires and conducts, positive potential is applied to the bases of both transistors 42 and 43. At the same time, the voltage at anode 17 of the triac is positive relative to the voltage at anode 15 (triac not yet conducting) and correspondingly, the potential at reference junction 41 is positive (the resistance of resistor 39 is less than 40). Because of the direction in which diode 46 is connected, the collector of transistor 42 is positively biased and the transistor conducts because of the positive bias at both its base and collector and correspondingly supplies the trigger signal 76 (FIG. 8B) necessary at gate 45 to cause the triac to conduct during the remainder of the positive half cycle, as shown for current 77, FIG. 8D. During the half cycle when line 13 is positive relative to line 14, transistor 43 does not conduct and is protected from a large positive signal at its collector by the diode 47.

As the AC voltage wave 75 at line 13 changes from positive to negative at zero voltage 78, triac 16 commutates and line 13 then becomes more negative, as shown by waveform 79. With the continued touch of the operator's finger on touch plate 37, the upper plate of capacitor 32 now charges negatively through the touch assembly and resistor 33 until the breakover value of neon bulb 48 is again reached. At the instant neon bulb 48 breaks over, capacitor 32 discharges to provide a negative bias at the bases of both transistors 42 and 43. By the time neon bulb 48 breaks over, reference junction 41 has become sufficiently negative to apply a negative bias to the collector of transistor 43 through diode 47. With negative bias at its collector, and positive bias at its base, transistor 43 conducts to apply a negative trigger signal 80' to gate 45 of triac 16. This action again occurs during the first few degrees of the negative half cycle, and hence, triac 16 is triggered into conduction by transistor 43 during half cycles when line 13 is negative and remains in conduction for the remainder of the half cycle, as shown by current wave form 81'.

THE VARIABLE CAPACITOR TOUCH ARRANGEMENT

FIGS. 9-11 show a touch assembly 80 without moving parts which, when substituted for the touch assembly 35, provides for variable speed operation of motor 3. Touch assembly 80 is vertically elongated and of triangular outline. The touch assembly 80 is mounted in a recess 81 formed in the forward portion of handle 5. Touch assembly 80 takes the form of a capacitive impedance and includes an outer triangularly shaped conductive touch plate 82 and an inner triangular plate 83 separated by a layer of dielectric material 84. Inner plate 83 is connected to resistor 33 by the wire 61, which extends through bottom wall 85 of recess 81. Outer touch plate 82 has a height of approximately 21/2 inches to permit the operator of the tool to engage the plate with one or several fingers, as shown in phantom lines at FIG. 8.

With the control circuit of FIG. 2 connected to the touch assembly 80, substantially full-wave conduction of current to the motor is attained when the operator only lightly touches the outside plate 82 of touch assembly 80. A light touch adjacent the apex 86 of plate 82 results in very low capacitance between the body of the operator and the resistor 33. Correspondingly, since the capacitance of the timing circuit including the operator's finger, touch assembly 80, resistor 33 and capacitor 32 remains essentially unchanged, and the upper plate of capacitor 32 will charge to the breakover voltage of neon bulb 48 very early during each half cycle of the alternating current. Correspondingly, triac 16 will be triggered early during each half cycle and will provide the substantially full-wave current of FIG. 8D to the motor.

To control the conduction angle of the triac 16 in order to controllably vary the speed of motor 3, the operator need merely increase the pressure of his finger on touch plate 82, vary the location of his finger on the touch plate, or, if desired grip the touch plate with several fingers. As the operator increases his grip on touch plate 83, the value of the capacitance between the operator's finger or fingers and the resistor 61 increases. Correspondingly, the time constant of the circuit including touch assembly 80, resistor 33, and capacitor 32 increases with the result that it takes longer for capacitor 32 to charge to the breakover value of neon bulb 48. Since transistors 42 and 43 can conduct sufficient current to gate the triac ON only when the bases are properly biased, the transistors will conduct later during each half cycle because of the later breakover of the neon bulb 48 during each half cycle. Correspondingly, triac 16 is triggered ON at a later time during each half cycle of the alternating current, as shown at FIG. 8E with the result that less power is supplied to motor 3. Hence, the speed of motor 3 can be decreased by the operator by increasing his grip on touch plate 82, and the current can be varied from full-wave (FIG. 8D) to a very low value (FIG. 8E) depending on the operator's touch.

SECOND EMBODIMENT OF VARIABLE CONDUCTION CONTROL

FIGS. 12-14 show a second embodiment of a step-type variable conduction control without moving parts. As will be observed by comparison of FIGS. 2 and 12, these circuits are identical, save that capacitor 95 may have a value different from capacitor 32, resistor 96 may have a value different from resistor 33, and resistors 97-99 and touch assemblies 100-103 are added to the circuit of FIG. 11. Touch assemblies 100-103 are similar in construction to touch assembly 35, but the capacitive impedances are different. Conduction control of the triac 16 of FIG. 12 is obtained in steps by selectively touching one of the touch assemblies 100-103. Resistors 97-99 are connected in series with resistor 96 and capacitor 95. The capacitive impedance-type touch assembly 100 is connected directly to resistor 96, the capacitive impedance-type touch assembly 101 is connected to the junction between resistors 97 and 98, the capacitive impedance-type touch assembly 102 is connected to the junction between resistors 98 and 99, and the capacitive impedance-type touch assembly 103 is connected in series with resistor 99. The values of the capacitive impedance of touch assemblies 100-103 are different. Touch assembly 100 advantageously has a very low capacitance, the capacitance of touch assembly 101 is greater than that of touch assembly 100, the capacitance of touch assembly 102 is greater than that of touch assembly 101 and the capacitance of touch assembly 103 is greater than that of touch assembly 102.

The relative values of the capacitance of touch assembly 100, resistor 96 and capacitor 95 are such that the time constant of the circuit, when the operator touches the outer plate of touch assembly 100 is very small. Correspondingly, neon bulb 48 breaks over very early during each half cycle, the appropriate one of transistors 42 and 43 is biased ON very early during each half cycle, and triac 16 is gated ON for substantially full-wave conduction, as shown at FIG. 15A. When the operator places his finger on the outer plate of touch assembly 101 both capacitance and resistance are added to the timing circuit with the result that the time constant of the circuit is increased and it takes longer for capacitor 95 to charge to the breakover value of neon bulb 48. Correspondingly, a touch on the touch assembly 101 results in a somewhat smaller conduction angle for the triac, as shown at FIG. 15B. When the outer plate of touch assembly 102 is touched, resistors 97 and 98 are added to the circuit, as is the capacitance of touch assembly 102. This increased capacitance and resistance further increases the time constant of the circuit, with the result that the conduction angle of triac 16 is further reduced, as shown at FIG. 15C. Similarly, when the outer plate of touch assembly 103 is touched, the value of the capacitance of the touch assembly plus the value of the resistors 97-99 are added to the timing circuit, with the result that the time constant is further increased and the conduction angle of triac 16 is further decreased, as shown at FIG. 15D.

As shown at FIGS. 13 and 14, touch plates 100-103 are mounted in appropriate recesses 105 in the front of handle 5 of the drill. These recesses are defined in part by transverse ribs 106 which separate the respective compartments for the touch assemblies from each other. The respective recesses and ribs prevent accidental touching of the several touch assemblies by the operator and hence, accidental energization of the tool is avoided. Shields, such as the shields 86 (FIG. 10) or 89 (FIG. 11), can of course be provided with the embodiment of FIGS. 13 and 14. The wires 107 connect the plates to the several resistors.

VARIABLE RESISTANCE CONDUCTION CONTROL

FIGS. 16-18 show another embodiment of the control circuit of this invention, wherein continuously variable conduction of the triac 16, with corresponding continuously variable speed control, is obtained. In this embodiment, capacitor 110 and resistor 111 are provided in the timing circuit and a resistor 112, which functions as the touch element, is connected to the resistor 111. The remainder of the circuit is identical to those previously described. Resistor 112 is of the type having an exposed surface so the resistance of the resistance element, and correspondingly, the time constant of the circuit, including resistance element 112, resistor 111, and capacitor 110 varies with the position of the operator's finger on the resistance element. As shown at FIGS. 17 and 18, resistance element 112 advantageously takes the form of a thin film 113 of cermet or carbon resistance material deposited on the surface of an electrically insulating base material 114. As shown at FIG. 17, the resistance material 113 is deposited in a zigzag layer of generally W configuration. A conductor 115 is connected to the tip 116 at one side of the W-shaped layer, and this wire connects the resistance element 112 to resistor 111. The front face of resistance element 112 can be covered with a thin sheet of tough insulating material 117, such as MYLAR, to prevent wearing away of the resistance film 113 during operation of the drill. Resistance film 113 may, however, be left exposed if its value does not change during continued use.

The value of resistor 111 is sufficiently high (for example, several hundred K ohms) that there is no danger of an electrical shock to the operator when he touches resistance element 112. The value of resistance element 112, from the corner 116 to the corner 118, is on the order of several Megohms to provide for continuously variable conduction of the triac from substantially full ON to almost completely OFF. When the operator touches his finger to the corner 116, the time constant of the circuit, including resistance element 112, resistor 111, and capacitor 110 is very small and the triac is triggered on for substantially full conduction, as shown, for example, at FIG. 15A. As the operator moves his finger along the film 113 in a direction away from the corner 116, the resistance of, and correspondingly, the time constant of the timing circuit increases with the result that the transistors are biased ON later during each half cycle and the conduction angle of triac 16 is decreased. The value of resistance element 112 is so selected that when the operator touches the corner 118, the conduction angle of the triac is as little as 20°-30°.

STEP TYPE VARIABLE RESISTANCE CONDUCTION CONTROL

FIGS. 19-21 show another embodiment of variable conduction control for triac 16. In this embodiment, resistors 120-122 are connected in series with resistor 111 and electrically conductive touch plates 123-126 are provided. Touch plate 123 is connected directly to resistor 111 by wire 126, and because of the low time constant of the circuit, including capacitor 110 and resistor 111, triac 16 is triggered into conduction early during each half cycle and provides the substantial full-wave conduction shown, for example, at FIG. 15A. When touch plate 124 is touched, resistor 120 is added to the timing circuit and its time constant is increased. Correspondingly, touching touch plate 124 provides a decrease in the conduction angle so the conduction angle is similar to that shown at FIG. 15B. Touching touch plate 125 adds resistors 120 and 121 to the timing circuit, thereby increasing its time constant and triggering the triac 16 later during each half cycle so the current conducted by the triac is as shown at FIG. 15C. Similarly, touching touch plate 126 further increases the time constant of the circuit by adding the value of resistors 120-122 to the circuit, with the result that the conduction angle of the triac becomes very small, as shown at FIG. 15D.

Touch plates 123-126 are advantageously thin plates of stainless steel, which are mounted in recesses 127 formed in the front of handle 5, so the desired one of the touch plates can be conveniently touched by the finger of the operator. The touch plates 123-126 are separated by transversely extending ribs 128, which define the several recesses 127 and also prevent accidental touching of any of the touch plates by the operator. If desired, a shield, such as the shield 86 or 89 of FIGS. 10 and 11 respectively, may be provided for the embodiment of FIGS. 20 and 21 to completely prevent accidental engagement of any of the touch plates by the operator.

While several preferred embodiments of this invention are shown and described herein, it is contemplated that the drill shown and described can be any electrical appliance or tool and that the circuitry described herein may have uses other than the ones recited in detail.