Switching circuit for varying rotational speed of motor

A switching circuit for varying rotational speed of a motor includes a power supply line for connecting the motor to a power source so as to supply driving power to the motor. A parallel connecting circuit of an on-off switch and a short circuiting switch is provided in the power supply line, in such a manner that the driving power is continuously supplied to the motor when the short circuiting switch is on, while in the case that the short circuiting switch is off, the driving power is supplied to the motor during on of the on-off switch but is not supplied during off of the on-off switch. A control member is provided for operation by an operator for varying rotational speed of the motor. The control member is operative for turning on the short circuiting switch at the maximum amount of its operation. The switching circuit further includes an output element for producing an output signal corresponding to the amount of operation of the control member, a continuously conductive switch to be continuously kept on when the control member is operated between the maximum amount of operation and the amount slightly smaller than the maximum amount, and a control element connected to the output element and the continuously conductive switch for controlling the on-off switch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a switching circuit for varying rotational 
speed of a motor, especially of a power driven tool such as a power driven 
screwdriver and a power driven drill. 
2. Description of the Related Art 
A switching circuit for controlling rotational speed of a motor is shown in 
FIG. 5. The switching circuit includes a power supply line 30 connecting a 
DC power source E with a DC motor M. In the power supply line 30, there is 
provided a change-over switch 32 for changing the direction of rotation of 
the DC motor M from the forward direction to the reverse direction or vice 
verse, a parallel connection of an on-off switch 34 with a short 
circuiting switch 36, a triangle wave generator 42 connected with the 
terminals of the DC motor M via diodes 38, 40 which are forwardly 
connected, a comparator 46 for comparing a control signal a derived from 
the triangle wave generator 42 with a rotational speed setting signal b 
derived from a variable resistor 44, and a final control circuit 48 for 
controlling the on-off switch 34 based on an output signal c of the 
comparator 46. 
The variable resistor 44 changes its output or the voltage of the setting 
signal b according to the amount of operation of a control member (not 
shown) which may cooperate with a trigger of a power driven tool. The 
voltage of the setting signal b becomes lower as the amount of operation 
of the control member increases. The compartor 46 provides the output 
signal c when the voltage of the control signal a is higher than that of 
the setting signal b, and the final control circuit 48 turns on the on-off 
switch 34 based on the output signal c As the amount of operation of the 
control member increases, the time during which the on-off switch 34 is on 
becomes longer or the duty factor increases, so that the rotational speed 
of the DC motor M continuously increases. 
Further, when the amount of operation of the control member becomes 
maximum, the short circuiting switch 36 turns on and the voltage of the 
power source E is directly applied to the DC motor M, so that the DC motor 
continuously rotates at the maximum speed. 
The on-off switch 34 usually includes a power transistor which is to be 
used at the duty factor of less than 50%. When the short circuiting switch 
36 is turned on, the output signal c corresponding to the duty factor of 
about 50% is continuously supplied to the final control circuit 48, and 
the on-off switch 34 turns on at the duty factor of about 50%. As 
explained above, the on-off switch 34 and the short circuiting switch 36 
are connected in parallel in the power supply line 30. Therefore, when the 
on-off switch 34 is on, the current flows through both the on-off switch 
34 and the short circuiting switch 36. On the other hand, when the on-off 
switch 34 is off, the current flows through only the short circuiting 
switch 36. For example, if the voltage of the DC power source E is 10 V, 
this voltage is not directly applied to the short circuiting switch 36 
when the on-off switch 34 is on. However, this voltage of 10 V is directly 
applied to the short circuiting switch 36 when the on-off switch 34 is 
off, so that a spark is produced at the contact point of the short 
circuiting switch 36 because of the substantial potential difference by 
such direct application. Thus, when the short circuiting switch 36 turns 
on, the on-off switch 34 is intermittently turns on at the duty factor of 
about 50%. Therefore, as shown in FIG. 6, the short circuiting switch 36 
may turn on either at a timing T1 where the control signal c exists and 
the on-off switch 36 is on or at a timing T2 where the control signal c 
does not exist and the on-off switch 34 is off. When the short circuiting 
switch 36 turns on at the timing T2, the voltage of the DC Power source E 
is directly applied to the short circuiting switch 36, so that the spark 
is produced at the short circuiting switch 36. Such production of sparks 
also occurs when the short circuiting switch 36 turns off from on, and as 
sparks are repeatedly produced, the contact is worn and the life of the 
short circuiting switch 36 is shortened. 
In order to overcome such drawbacks, it may be considered to use as the 
on-off switch 34 a power transistor which permits large duty factor such 
as 100%, so that the on-off switch 36 can always be on when the short 
circuiting switch 36 is turned on. However, such a power transistor to 
permit large duty factor is expensive and therefore, the costs of 
manufacturing the switching circuit increases, thereby producing another 
drawback. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention to provide a 
switching circuit for varying rotational speed of a motor which prevents 
the contact of a short circuiting switch from wearing caused by sparks and 
ensures long life of the switch. 
It is another object of the present invention to provide a switching 
circuit which can prevent the contact of a short circuiting switch from 
wearing by using as the on-off switch a power transistor which permits 
relatively small duty factor and which is of small heat capacity and of 
low cost. 
According to the present invention, there is provided a switching circuit 
for varying rotational speed of a motor, comprising: 
a power supply line for connecting the motor to a power source so as to 
supply driving power to the motor; 
a parallel connecting circuit of an on-off switch and a short circuiting 
switch provided in said power supply line, in such a manner that the 
driving power is continuously supplied to the motor when the short 
circuiting switch is on, while in the case that the short circuiting 
switch is off, the driving power is supplied to the motor during on of the 
on-off switch but is not supplied during off of the on-off switch; 
a control member for operation by an operator for varying rotational speed 
of the motor, the control member being operative for turning on the short 
circuiting switch at the maximum amount of operation; 
output means for producing an output signal corresponding to the amount of 
operation of the control member; 
a continuously conductive switch to be continuously kept on when the 
control member is operated between the maximum amount of operation and the 
amount slightly smaller than the maximum amount; and 
control means connected to the output means and the continuously conductive 
switch for controlling the on-off switch, the control means being 
operative to continuously keep the on-off switch on during the time when 
the continuously conductive switch is on, and being operative to increase 
the ratio of the time during which the on-off switch is on to a unit 
period of cycle based on the output signal from the output means as the 
amount of operation of the control member increases; 
whereby as the amount of operation of the control member increases from the 
minimum amount to the maximum amount, the ratio of the time to supply 
power to the motor to the unit period of cycle increases, and thereafter, 
the power is continuously supplied to the motor when the amount of 
operation reaches the amount exceeding slightly smaller than the maximum 
amount of operation by turning of the continuously conductive switch to on 
and by subsequent turning of the short circuiting switch to on at the 
maximum amount of operation. 
Preferably, the control means includes a triangle wave generator, a 
comparator for comparing the output of the output means with the output of 
the triangle wave generator, and final control means connected to the 
comparator for keeping the on-off switch on as long as the output of the 
triangle wave generator exceeds the output of the output means. 
The continuously conductive switch is connected in parallel with the output 
means, and the grounded signal becomes input to the comparator instead of 
the outPut of the output means when the continuously conductive switch is 
on, so that the comparator judges the output of the triangle wave 
generator as exceeding the output of the output means so as to keep the 
on-off switch on. 
The ratio of the time during which the on-off switch is on to the unit 
period of cycle controlled by the control means is about 0.5, and the 
on-off switch permits switching at the ratio of the time during turn-on to 
the time during turn-off less than about 0.5. 
The invention will become more fully apparent from the claims and the 
description as it proceeds in connection with the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there Is shown a switching circuit for controlling 
rotational speed of a DC motor M of a power driven tool (not shown). The 
DC motor M is connected in series with a DC power source E in a power 
supply line 10 via a change-over switch SW1. 
The change-over switch SW1 includes contacts e1, e2 connected to a plus 
terminal of the DC power source E, contacts f1, f2 connected to a minus 
terminal of the DC power source E and switch elements g1, g2 each 
connected to corresponding terminals of the DC motor M through lines 12, 
14. The switch element g1 may be switched to contact either of the contact 
e1 or the contact f1, while the switch element g2 may be switched to 
contact either of the contact e2 or f2. By the selection of switching of 
the switch elements g1 and g2, the direction of rotation of the DC motor M 
can be changed to the reverse direction from the forward direction or vice 
verse, or the supply of the power to the DC motor M can be stopped: 
Forward Rotation 
Switch element g1--contact e1 
Switch element g2--contact f2 
Reverse Rotation 
Switch element g1--contact f1 
Switch element g2--contact e2 
Stop 
Switch element g1--contact f1 
Switch element g2--contact f2 
In the power supply line 10, there is also provided a parallel connection 
of an on-off switch SW2 comprising a power transistor and of a short 
circuiting switch SW3. The switching of the on-off switch SW2 is 
controlled by a final control circuit 16. Input to the final control 
circuit 16 is an output signal c of a comparator 24 which compares a 
control signal a derived from a triangle wave generator 22 with a setting 
signal b derived from a variable resistor VR1 (see FIG. 2). The triangle 
wave generator 22 includes a resistance, a capacitor and an amplifier (not 
shown) and is connected to the respective terminals of the DC motor M via 
diodes 18, 20. The diodes 18, 20, the triangle wave generator 22, the 
comparator 24 and the final control circuit 16 form a low speed circuit 25 
as shown in FIG. 1. 
As shown in FIG. 2(a), the control signal a is composed of a series of 
triangle waves on frequency and amplitude, while, the setting signal b 
changes its voltage level according to the position of a control member 26 
of the variable resistor VR1 shown in FIG. 3. The control member 26 is 
constructed as a brush which contacts a resistance member 28 and an output 
terminal 29, so that the voltage level of the setting signal b changes 
according to the contact position of the control member 26 with the 
resistance member 28. The control member 26 can be operated by an 
operator. In the switching circuit used for the power driven tool as with 
this embodiment, the control member 26 is operatively connected to a 
trigger (not shown) of the power driven tool and is operated by the 
operator through the trigger. Further, the resistance member 28 is 
connected to the minus terminal of the DC power source through the power 
supply line 10 at the end of direction of arrow A, while it is connected 
between the diodes 18, 20 and the triangle wave generator 22 at the end of 
direction of arrow B or opposite direction. The output terminal 29 is 
connected to the comparator 24 so as to output the setting signal b. 
In this embodiment, the voltage level of the setting signal b continuously 
decreases as the control member 26 moves in the direction of arrow A, 
while it continuously increases as the control member 26 moves in the 
direction of arrow B. 
The comparator 24 compares the control signal a with the setting signal b 
and produces the output signal c when the voltage level of the control 
signal a exceeds the setting signal b. As shown in FIG. 2(a), the period 
of time t of the output signal c becomes longer as t1, t2, t3 ... as the 
voltage level of the setting signal b is lowered as b1, b2, b3 . . . . The 
setting signal bo corresponds to the end of stroke in the direction of 
arrow 8 or the minimum operation position Po of the control member 26. The 
setting signal bo at such position Po always exceeds the control signal a, 
so that there is no output signal c. 
The output signal c as explained is then supplied to the final control 
circuit 16 for the on-off switch SW2, and the final control circuit 16 
turns on the on-off switch SW2 during only the period of time t of the 
output signal c so as to supply electric power to the DC motor M. Such 
time t continuously changes as t1, t2, t3 . . . to become longer according 
to the movement of the control member 26 in the direction of arrow A and 
consequently, the rotational speed of the DC motor M continuously changes 
from low to high. Although the time t becomes longest at the end position 
Pe of the control member 26 in the direction of arrow A, a continuously 
conductive switch SW4 is positioned at Pl which is immediately before the 
position Pe. 
As shown in FIG. 1, the continuously conductive switch SW4 is connected 
between the minus terminal of DC power source E and the comparator 24 in 
parallel to the resistance 28 of the variable resistor VR1. With this 
arrangement, when the continuously conductive switch is on, a continuous 
output signal ce is produced by the comparator 24 and is supplied to the 
final control circuit 16 so as to continuously keep the on-off switch on, 
so that the DC motor M continuously rotates at high speed. 
In this embodiment, the on-off switch SW2 is a transistor which switches at 
the duty factor not more than about 50% preferably at 40 to 50%, and the 
ratio of the period of time t of the output signal c or the time when the 
power is supplied to the DC motor M to the unit of period S of the control 
signal a is determined at 40 to 50% even if the control member 26 is 
positioned at its maximum operative position Pe. 
Further, as shown in FIG. 3, the short circuiting switch SW3 is positioned 
at the maximum operative position Pe of the control member 26. Therefore, 
when the control member 26 reaches the position Pe, the short circuiting 
switch SW3 turns on in addition to the on-off switch SW2, so that the DC 
motor M rotates at high sPeed with both the on-off switch SW2 and the 
short circuiting switch SW3 being continuously kept on. 
Conversely, when the control member 26 moves from the maximum operative 
position Pe toward the minimum operative position Po, the short circuiting 
switch SW3 firstly turns off and thereafter the on-off switch turns off. 
The timing chart of such turn-on of the on-off switch SW2 by the action of 
continuously conductive switch SW4 and the corresponding timing chart of 
turn-on of the short circuiting switch SW3 are shown in FIGS. 4(a) and 
4(b), respectively. 
In operation, the control member 26 is normally kept in the minimum 
operative position Po, and therefore, the on-off switch SW2 and the short 
circuiting switch SW3 are kept off. Firstly, the operator turns the 
change-over switch SW1 to select the direction of rotation of the DC motor 
M, and thereafter moves the control member 26 through the trigger toward 
the maximum operative position pe. By such movement of the control member 
26 between the position Po to P1, the voltage level of the setting signal 
b changes according to the position, and the corresponding output signal c 
is supplied to the on-off switch SW2 to switch the same on during the time 
t within the unit of period S. Thus, the DC motor M rotates at a speed 
corresponding to the time t and increases its speed as the control member 
26 moves toward the position Pe. 
As the control member 26 approaches the position Pe, the on-off switch SW2 
turns to continuously keep on at the position P1, and thereafter the short 
circuiting switch Sw3 turns on. For example, if the voltage of the DC 
power source E is 10 V and the dropped voltage between the emitter and 
collector of the power transistor or the on-off switch SW2 is 2 to 3 V, 
the switching voltage applied to the short circuiting switch SW3 is 2 to 3 
V which corresponds to the dropped voltage. Therefore, the full voltage of 
10 V of the DC power source E is not applied to the short circuiting 
switch SW3 and no spark is produced at the contacts of the short 
circuiting switch SW3. This is the same when the short circuiting switch 
SW3 turns off. 
Consequently, the short circuiting switch SW3 is prevented from being used 
at the voltage which produces sparks, and the short circuiting switch SW3 
may have a long life. 
Further, in this embodiment, the power transistor for the duty factor of no 
more than about 50%, preferably about 40 to 50% as the on-off switch SW2, 
As described above, the full voltage of DC power source E is applied to 
the on-off switch SW2 only before the short circuiting switch SW3 turns on 
and during very short time, and there is no problem even if the current 
exceeding the rated current flows through the on-off switch SW2 during 
such very short time. For this reason, it is not necessary to use an 
expensive power transistor which can be continuously used for the duty 
factor of 100% as the on-off switch SW2. 
Thus, in this embodiment the relatively cheep power transistor which 
permits relatively small duty factor and has relatively small heat 
capacity can be used as the on-off switch SW2 so that the switching 
circuit can be constructed at low cost. 
While the invention has been described with reference to a preferred 
embodiment thereof, it is to be understood that modifications or 
variations may be easily made without departing from the scope of the 
present invention which is defined by the appended claims.