A power supply circuit which maintains the voltage across a first capacitor (C.sub.1) substantially constant over a wide range of mains voltages, the capacitor (C.sub.1) energizes a motor (M) and is charged during the rising edges of the applied mains voltage in that above a specific input voltage (U.sub.min) a first transistor (T.sub.1) is turned on via a second capacitor (C.sub.2) and a first resistor (R.sub.2). When the rectified mains voltage at the output of a diode (D.sub.1) exceeds a specific value (U.sub.2) defined by a first zener diode (Z.sub.2), a second transistor (T.sub.2) is turned on. Consequently, the first transistor is turned off. When the first transistor (T.sub.1) is cut off, the first capacitor is discharged through the motor. The second capacitor is then discharged via a second zener diode (Z.sub.1) and prevents the first transistor from being turned on.

BACKGROUND OF THE INVENTION 
This invention relates to a power-supply circuit for energizing a load and 
comprises two input terminals for the application of an input voltage. 
Coupled to the input terminals are a rectifier, a first capacitor having 
terminals for the connection of the load, a first transistor switch having 
a control electrode, and second transistor switch for turning off the 
first transistor switch when the rectified input voltage exceeds a 
specific value. 
Such a circuit is suitable for energizing a load with different input 
voltages. Such a circuit is particularly suitable for use in a dryshaver 
in which the circuit is utilized for energizing the motor. This enables 
the shaver to be used with different mains voltages available in various 
countries without the need for an adaptor or switching over. 
Such a circuit is known from U.S. Pat. No. 4,001,668. In this circuit the 
first capacitor is maintained at an average constant voltage in that both 
during the rising edge and the falling edge of the rectified input voltage 
the first capacitor is recharged to the value of the rectified input 
voltage defined by the second transistor switch. A drawback of this 
circuit is that it generates a comparatively large amount of spurious 
radiation. This is because during the falling edge of the rectified input 
voltage the first transistor switch is turned on at the value of the input 
voltage defined by the second transistor switch, while at this instant the 
voltage across the first capacitor is lower than this value. As a result 
of this, the first capacitor is charged with a comparatively large 
charging current, so that the voltage across this capacitor increases 
stepwise to the value of the rectified input voltage defined by the second 
transistor switch. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the invention to provide a power-supply 
circuit which produces practically no spurious radiation. According to the 
invention a power-supply circuit of the type defined in the opening 
paragraph is characterized in that the control electrode of the first 
transistor switch is coupled to one input terminal by a second capacitor 
and the other input terminal by at least one diode. During the rising 
edges of the input voltage the first transistor switch is turned on by the 
charging current of the second capacitor, and during the falling edges the 
second capacitor discharges through the diode so that the first transistor 
switch is kept in the non-conductive state. As a result, the first 
capacitor is charged only during the rising edges of the rectified input 
voltage. The voltage across the first capacitor then follows the gradual 
increase of the input voltage, which precludes the occurrence of spurious 
radiation. Moreover, as compared with the known circuit, the second 
capacitor reduces the dissipation in the control circuit of the first 
transistor switch. 
A first embodiment of the invention is characterized in that the circuit 
comprises first protection means for turning off the first transistor 
switch when a specific current through the load is exceeded. For example, 
in the event of a short-circuit of the load the first protection means 
turn off the first transistor switch in order to prevent said transistor 
switch from being damaged by an excessive current. The first protection 
means may be characterized further in that it comprises a resistor which 
is arranged in series with the load and which is coupled to the control 
electrode of the second transistor switch by at least a first 
reference-voltage element. 
A second embodiment of the invention is characterized in that the circuit 
comprises second protection means for turning off the first transistor 
switch when a specific current through the first capacitor is exceeded. 
The second protection means prevent the first transistor switch from being 
damaged by an excessive charging current when the power-supply circuit is 
switched on in the case of a large instantaneous value of the rectified 
input voltage and in the case of voltage transients. In a further 
embodiment these second protection means may be characterized further in 
that they comprise a resistor which is arranged in series with the first 
transistor switch, and which is coupled to the control electrode of the 
second transistor switch by at least one diode. 
A third embodiment of the invention is characterized in that the circuit 
comprises a correction device for increasing the value of the rectified 
input voltage above which the second transistor switch turns off the first 
transistor switch when the load current increases. By means of the 
correction device the first capacitor is charged to a voltage which 
increases as the load current increases in order to ensure that the 
average voltage across this capacitor and hence across the load, 
increases. 
In a further embodiment, if the control electrode of the second transistor 
switch is coupled to an output of the rectifier by a series arrangement of 
at least one zener diode and a resistor, the correction device may 
comprise a transistor whose base and emitter are connected to the 
terminals of a resistor arranged in series with the load and whose 
collector is connected to the junction point between the zener diode and 
the resistor in the series arrangement between the control electrode of 
the second transistor switch and the output of the rectifier. A further 
embodiment may be characterized in that the correction device comprises 
limiting means for limiting the correction provided by the correction 
circuit above a specific load current.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the basic diagram of a power-supply circuit in accordance with 
the invention. Between the two terminals 2 and 3 for the application of 
the mains voltage the circuit comprises a series arrangement of a resistor 
R.sub.1, a rectifier, which in the present example comprises a diode 
D.sub.1, a first capacitor C.sub.1 having terminals 4 and 5 for connecting 
a load which in the present example comprises a motor of, for example, a 
dry-shaver, and a first transistor switch which in the present example 
comprises a single transistor T.sub.1. By means of the series arrangement 
of a resistor R.sub.2 and the capacitor C.sub.2 the control electrode of 
this transistor T.sub.1 is connected to the anode of the rectifier 
D.sub.1. The junction point between the resistor R.sub.2 and the capacitor 
C.sub.2 is connected to the input terminal 3 via a zener diode Z.sub.1. 
The circuit further comprises a second transistor switch, which in the 
present example comprises a transistor T.sub.2 whose collector-emitter 
path is arranged in parallel with the base-emitter junction of the 
transistor T.sub.1. The control electrode of transistor T.sub.1 is 
connected to the cathode of the rectifier D.sub.1 by means of a series 
arrangement of a zener diode Z.sub.2 and a resistor R.sub.3. 
The operation of the circuit will now be explained with reference to FIG. 
2, which illustrates how the voltages at various points in the circuit 
vary. The sinusoidal mains voltage V.sub.i applied between the input 
terminals 2 and 3 is shown in broken lines in FIG. 2. During the rising 
edge in the time interval O-t.sub.3 of this mains voltage the capacitor 
C.sub.2 is charged, the charging current flowing from the input terminal 2 
to the input terminal 3 via the resistor R.sub.1, the capacitor C.sub.2, 
the resistor R.sub.2 and the base-emitter junction of the transistor 
T.sub.1. The base current of the transistor T.sub.1 is then limited by the 
resistor R.sub.2 and by the zener diode Z.sub.1, which is turned on above 
a specific value of the voltage across this resistor and directly drains 
the charging current to the input terminal 3. Initially there is no 
current in the collector line of the transistor T.sub.1 because the 
rectifier diode D.sub.1 is cut off as a result of the voltage still 
present across the capacitor C.sub.1. In this time interval the capacitor 
C.sub.1 is discharged through the load M. At the instant t.sub.1 the 
instantaneous value of the mains voltage becomes higher than the voltage 
U.sub.min across the capacitor C.sub.1, so that the rectifier diode 
D.sub.1 is turned on. As a result of this, the capacitor C.sub.1 is 
charged, the charging current flowing from the input terminal 2 to the 
input terminal 3 via the resistor R.sub.1, the diode D.sub.1, the 
capacitor C.sub.1 and the collector-emitter path of the transistor 
T.sub.1. The charging current is then limited by the resistor R.sub.1. The 
voltage across the capacitor C.sub.1 now increases in conformity with the 
increase of the mains voltage V.sub.i. At the instant t.sub.2 when the 
rectified input voltage has an instantaneous value U.sub.2 the zener diode 
Z.sub.2 and the transistor T.sub.2 are driven into conduction. This causes 
the transistor T.sub.1 to be cut off so that the capacitor C.sub.1 is not 
charged any further and the maximum voltage across this capacitor is 
consequently equal to the voltage U.sub.2. The transistor T.sub.2 remains 
conductive until at the instant t.sub.4 the instantaneous value of the 
rectified input voltage becomes smaller than the voltage U.sub.2. In the 
time interval t.sub.2 -t.sub.3 the capacitor C.sub.2 is charged to the 
peak value of the input voltage, the charging current flowing through the 
collector-emitter path of the transistor T.sub.2. During the falling edge 
of the mains voltage in the time interval t.sub.3 -t.sub.6 the capacitor 
C.sub.2 is discharged completely via the zener diode Z.sub.1 which now 
operates as a diode, thus preventing the transistor T.sub.1 from being 
turned on. The transistor T.sub.1 is turned on again if during the next 
rising edge, the instantaneous value of the mains voltage exceeds the 
capacitor voltage U.sub.1. When the load is constant the average value 
Uc.sub.1 of the voltage across the capacitor C.sub.1 is thus maintained 
constant in the above manner. The operation of the circuit is then 
independent of the mains-voltage amplitude over a wide range. 
FIG. 3 shows a first embodiment of the invention, identical parts bearing 
the same reference numerals as in FIG. 1. In this embodiment a resistor 
R.sub.4 is arranged in series with the load M, the junction point between 
this resistor R.sub.4 and the load M being connected to the control 
electrode of the transistor T.sub.2 via the series arrangement of a zener 
diode Z.sub.3 and a resistor R.sub.5. These additional elements constitute 
the first protection means which turn off the power-supply circuit in the 
event of, for example, a short-circuit of the motor or blocking of the 
motor. Above a specific value of the current through the load and hence 
above a specific value of the voltage across the resistor R.sub.4, the 
zener diode Z.sub.3 is turned on so that the transistor T.sub.2 is turned 
on and consequently the transistor T.sub.1 is turned off. The resistor 
R.sub.5 then limits the base current of the transistor T.sub.2. During 
normal operation of the circuit the protection means have another 
advantage. If the transistor T.sub.2 turns off the transistor T.sub.1, the 
collector voltage of the latter increases. This increase is transferred to 
the base of the transistor T.sub.2 via the resistor R.sub.4, the zener 
diode Z.sub.3 and the resistor R.sub.5, resulting in a positive-feedback 
effect which causes the transistor T.sub.1 to be turned off very rapidly. 
In this embodiment a negative voltage-dependent resistor R.sub.6 is 
arranged between the input terminals 2 and 3 to limit the input voltage 
to, for example, 600 V. 
FIG. 4 shows a second embodiment of the invention, in which identical parts 
bear the same reference numerals as in FIG. 3. In this embodiment a 
resistor R.sub.8 is arranged in the emitter line of the transistor T.sub.1 
and the emitter of the transistor T.sub.1 is connected to the base of the 
transistor T.sub.2 via the series arrangement of a resistor R.sub.9 and a 
diode D.sub.2. These elements constitute the second protection means which 
turn off the power-supply circuit in the event of an excessive charging 
current through the transistor T.sub.1. If the power-supply circuit is 
switched on, for example, at the instant at which the mains voltage has 
its maximum value, the transistor T.sub.1 is turned on rapidly via the 
capacitor C.sub.2. At this instant the capacitor C.sub.1 is still without 
charge so that a very large charging current flows through the transistor 
T.sub.1, which current is limited only by the resistor R.sub.1. The 
transistor T.sub.1 is now protected in that above a specific current the 
voltage across the resistor R.sub.8 becomes so high that the transistor 
T.sub.2 is turned on via the resistor R.sub.9 and the diode D.sub.2 and, 
consequently, the transistor T.sub.1 is cut off. The resistor R.sub.9 then 
limits the base current of the transistor T.sub.2. 
FIG. 5 shows a third embodiment of the invention, in which identical parts 
bear the same reference numerals as in FIG. 4. As the load of the motor M 
increases the capacitor C.sub.1 is discharged to an increasing extent, so 
that the average voltage across this capacitor decreases. This causes the 
speed of the motor to decrease. In order to maintain the speed of the 
motor as constant as possible for an increasing load, the average voltage 
across the capacitor C.sub.1 should also increase with the increasing 
load. For this purpose the power-supply circuit comprises a correction 
circuit comprising a transistor T.sub.3 whose base-emitter junction, in 
series with a resistor R.sub.10, is arranged in parallel with the resistor 
R.sub.4 and whose collector is connected to the cathode of the rectifier 
diode D.sub.1 via a resistor R.sub.11. The series arrangement of the zener 
diode Z.sub.2 and the resistor R.sub.3 is connected to the collector of 
the transistor T.sub.3. The correction circuit operates as follows. As the 
motor current increases and consequently the voltage across the resistor 
R.sub.4 increases, the transistor T.sub.3 is driven further into 
conduction, causing the voltage across the resistor R.sub.11 to increase 
further. As a result of this, the voltage across the series arrangement of 
the resistor R.sub.3, the zener diode Z.sub.2 and the base-emitter 
junction of the transistor T.sub.2 decreases, so that the zener diode 
Z.sub.2 and the transistor T.sub.2 are turned on at an increasingly higher 
instantaneous value of the input voltage. Consequently, the transistor 
T.sub.1 is turned off at an increasing instantaneous value of the input 
voltage, so that the capacitor C.sub.1 is charged to a voltage which 
increases as the load current increases. A zener diode Z.sub.4 arranged in 
parallel with the resistor R.sub.11 limits the voltage across the resistor 
R.sub.11 above a specific load current. This ensures that the voltage 
across the capacitor C.sub.1 cannot become higher than the maximum 
permissible motor voltage at increasing load current. 
The invention is not limited to the embodiments described herein but to 
those skilled in the art, many modifications are conceivable within the 
scope of the present invention. For example, the first and the second 
transistor switches may comprise Darlington transistors or other compound 
transistors. Instead of bipolar transistors, field-effect transistors may 
be used, in which case base, collector and emitter should read gate, 
source and drain respectively. Further, the zener diode Z.sub.1 may be 
replaced by one or more series-connected diodes. The first and the second 
protection means may also be constructed in another manner and, in 
particular, the current-sensing resistors for measuring the load current 
and the charging current may be arranged at other locations in the 
circuit. Further, the transistor in the correction circuit may be replaced 
by any other amplifier circuit. Finally, it is to be noted that the 
rectifier may alternatively comprise a full-wave rectifier or a bridge 
rectifier.