Solar cell power supply circuit

A solar cell power supply circuit for use in a calculator or equipment is disclosed. It includes a solar cell or cells, a back-up capacitor connected to the solar cells, and a circuit element connected to be responsive whether the electromotive force from the solar cells lies within a range of operation for a load element of the solar cells, typically an LSI semiconductor device. The back-up capacitor starts charging when the electromotive force of the solar cells falls out of the range of operation for the load element. Preferably, an alarm sound is delivered when the electromotive force of the solar cells is poor.

BACKGROUND OF THE INVENTION 
This invention relates to a solar cell power supply circuit. 
Lately, with the advent of very low power LSI elements, solar cell powered 
equipment such as solar cell calculators and wristwatches have been in 
increasing use to fulfill the demand for savings of power. However, this 
equipment, for example, solar cell calculators has the disadvantage that 
information will disappear during the process of calculation when incident 
light to a solar cell or cells is blocked. There are two measures to 
overcome this disadvantage: (1) a back-up capacitor is connected in 
parallel with the solar cells to temporarily protect operation of the LSI 
element or the solar cells, load especially when incident light to the 
solar cells is blocked; and (2) a back-up battery or batteries are 
installed in the equipment to energize the same when incident light is 
shut off. Those measures have been considered as unsatisfactory as 
follows. 
In the circuit method (1) (see FIG. 1) 
As shown in FIG. 1, the back-up capacitor C.sub.1 is provided in parallel 
relationship with the solar cells SB and serves to compensate for 
interrupted incident light to the solar cells SB. There are further 
provided voltage-stabilizing LEDs (D.sub.1 and D.sub.2) in parallel 
relationship with the capacitor C.sub.1 to prevent the LSI element from 
being suppled with an overvoltage when a very large amount of incident 
light is applied to the solar cells SB. In other words, the LEDs D.sub.1 
and D.sub.2 stabilize the output voltage of the solar cells SB. In this 
circuit, when the output voltage of the solar cells SB reduces to zero the 
capacitor C.sub.1 makes up for a deficiency of voltage necessary for the 
driving of the equipment when the incident light to the solar cells SB is 
blocked. The back-up capacitor C.sub.1 therefore protects operation of the 
LSI element when the incident light is shut off. To lengthen the 
operational life of the equipment, it is necessary to employ a back-up 
capacitor C.sub.1 having a capacitance as high as possible. Nevertheless, 
it takes a long time to make the equipment ready to operate after the 
equipment is removed from the dark and subjected to solar or other 
radiation. That is, in the event that the equipment is moved somewhere 
from the dark with no charge on the capacitor C.sub.1, it will take a 
considerable amount of time for the output voltage of the solar cells to 
reach a voltage level necessary to enable the voltage at the back-up 
capacitor C.sub.1 to drive the LSI element and the operator is unable to 
use the equipment for this period of time. The length T.sub.1 of time 
required for making the equipment ready to operate (hereinafter this is 
referred to as "recovery time") can be defined approximately as follows: 
EQU T.sub.1 =(C.sub.1 .multidot.V)/(I.multidot.A) (1) 
where 
C.sub.1 the capacitance of the back-up capacitor 
V the voltage to be supplied to the LSI element for normal operation of the 
equipment 
A the brightness of the incident light to the solar cells 
the output current of the solar cells illuminated with the brightness A 
Equation (1) indicates that shortening the recovery time T.sub.1 requires 
decreasing C.sub.1 and V and increasing I and A, in which case such a 
decrease in C.sub.1 causes deteriorating the primary performance of the 
back-up capacitor and such a decrease in V leads to a yield drop and cost 
increase in the manufacture of the LSI elements. Further, an increase of A 
limits correspondingly the range of brightness usable with the solar 
cell-powered equipment and an increase of I demands an increase in the 
area of the solar cells. The solar cells are generally more expensive than 
conventional batteries and the requirement for an increase in the area of 
the solar cells leads to greater expensive. 
In the circuit method (2) (see FIG. 2) 
This method relies upon the use of the back-up batteries for protecting 
operation of the equipment when the incident light is temporarily shut 
off. The back-up batteries are generally classified as primary batteries 
(cf. FIG. 2a) and secondary batteries (cf. FIG. 2b). With the former, a 
diode D is inserted to prevent charging of the primary batteries E.sub.1 
with the electromotive force from the solar cells SB and the LSI element 
is powered by the primary batteries E.sub.1 when the electromotive force 
from the solar cells SB falls. With the latter, when the electromotive 
force of the solar cells SB is large, solar energy charges the seconary 
batteries E.sub.2 (typically, Ni-Cd batteries) by way of a charging 
current limiting resistor R and supplies power to the LSI element. In this 
method, the batteries have a limited life and need be exchanged with new 
ones, whether the back-up batteries used are the secondary batteries (the 
Ni-Cd batteries) or the conventional primary batteries. This impairs 
severely the inherent and outstanding advantages of the solar cells that 
they exhibit substantially unlimited useful life. 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a solar 
cell power supply circuit which overcomes the above discussed problems 
with the conventional methods. 
It is another object of the present invention to provide a solar cell power 
supply circuit which may back up operation of a circuit element being 
powered directly with a solar cell or cells when incident light to the 
solar cells is shut off temporarily. 
According to the present invention, there is provided a solar cell power 
supply circuit comprising a solar cell or solar cells, a back-up capacitor 
connected to said solar cells, first circuit means for deciding whether 
the electromotive force from said solar cells lies within a range of 
operation of a load for said solar cells, and second circuit means for 
charging said back-up capacitor when the electromotive force of said solar 
cells is over said range of the operation of said load. Preferably, said 
first circuit means includes a switch element which becomes conductive 
when the electromotive force from said solar cells is over said range of 
the

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 3, there is illustrated an improved solar cell power 
circuit with back-up features according to an embodiment of the present 
invention, which circuit generally includes a solar cell or cells SB, LEDs 
D.sub.1 and D.sub.2 for stabilizing the output voltage of the solar cells, 
a pair of back-up capacitors C.sub.1 and C.sub.2 and a diode D.sub.3. In 
view of the fact that incident light on the solar cells varies in 
intensity depending upon the surrounding atmosphere of the solar cells 
from time to time, this circuit arrangment is designed such that it allows 
the back-up capacitor C.sub.1 to be charged only when incident light to 
the solar cells is strong enough to activate an LSI element to be powered 
by the solar cells SB. Further, a charging path for the back-up capacitor 
is provided independently of an enabling power voltage supply for the LSI 
element. Upon application of an excessive amount of the incident light, 
current begins flowing through the voltage-stabilizing LEDs D.sub.1 and 
D.sub.2, charging the back-up capacitor C.sub.1 by way of a transistor 
Tr.sub.1. 
Should the solar cells be moved from the dark and exposed to solar 
radiations under the condition where the charge on the back-up capacitor 
C.sub.1 is zero, the length of time required for the output voltage of the 
solar cells to reach a voltage level necessary to drive the LSI element as 
a load of the solar cells depends on the charging time of the back-up 
capacitor C.sub.2 rather than that of the back-up capacitor C.sub.1. The 
capacitance of the back-up capacitor C.sub.1, therefore, may be selected 
at a higher value freely from the length of time necessary for placing the 
LSI element into ready state. 
It is understood that the back-up capacitor C.sub.2 is effective as a 
measure to safeguard operation of the equipment powered by the solar cells 
against the dark atmosphere where the quantity of the incident light is 
faint and insufficient to charge the capacitor C.sub.1 and the capacitance 
of C.sub.2 is thus correlated as C.sub.1 &gt;&gt;C.sub.2. 
In the event that the back-up capacitor C.sub.1 has fully been charged and 
the incident light on the solar cells is screened, the capacitor C.sub.1 
backs up the LSI element through the diode D.sub.3. The diode D.sub.3 
feeds the charge on the back-up capacitor C.sub.1 to the LSI element. 
FIG. 4 illustrates another embodiment of the present invention wherein 
components similar to those in the previous embodiment are identified by 
the same reference numbers. There are provided additionally a 
piezo-electric buzzer P.sub.1 and an inverter I.sub.1 for enabling the 
piezo-electric buzzer. In this circuit, the back-up capacitor C.sub.1 is 
used as a major power supply source for charging the back-up capacitor 
C.sub.1. When an instruction is given by a particular terminal K of the 
LSI element for enabling the piezo-electric buzzer P.sub.1, the back-up 
capacitor C.sub.1 compensates for enabling current for the piezo-electric 
buzzer in the absence of a output voltage of the solar cells high enough 
to enable the piezo-electric buzzer P.sub.1 due to insufficient incident 
light. 
As a result, the LSI element or other components to be powered directly by 
the solar cells are backed up even when the incident light on the solar 
cells is temporarily screened. It is further possible to shorten greatly 
the length of time necessary for making the equipment ready to operate, 
without decreasing the capacitance of the back-up capacitor or 
deteriorating yield LSI manufacture and with a minimum of the area of 
solar cells. FIG. 5 shows another embodiment of the present invention 
wherein components similar to the above embodiments are designated by the 
same reference numbers. A diode D.sub.3 is a build-in diode of the LSI 
element and switch elements SW.sub.1 and SW.sub.2 are also built-in 
components of the LSI element and made typically of conventional MOS-FETs. 
The back-up capacitors C.sub.1 and C.sub.2 are correlated as C.sub.1 
&gt;&gt;C.sub.2. When the equipment is removed from the dark, the switch element 
SW.sub.1 in the LSI element is closed and the other is open so that the 
output of the solar cells SB charges the capacitor C.sub.2. In this 
instance, the capacitance of the capacitor C.sub.2 is selected such that 
the output of the solar cells may complete charging the same for a short 
period of time. If C.sub.2 has been charged over a high level enough to 
drive the LSI element in normal manner, then the built-in switching 
elements SW.sub.1 and SW.sub.2 are switched in such a manner that SW.sub.2 
changes to open position or to closed position when SW.sub.1 is in open 
position or closed position, respectively. A central processing unit CPU 
in the LSI element operates to keep at approximately 9:1 the ratio of the 
period of time where SW.sub.1 stands in open position to that where 
SW.sub.2 stands in closed position. In other words, the capacitor C.sub.1 
starts being charged at the moment where a voltage applied to the LSI 
element exceeds the level necessary for stand-by of the LSI element. The 
length of time necessary to complete the charging of the capacitor C.sub.1 
is ten times as long as that in the conventional circuit of FIG. 1. 
However, since operation of the LSI element is backed up by the back-up 
capacitor C.sub.2 while C.sub.1 is being charged, the equipment becomes 
operative immediately after it has been fetched from the dark, without 
waiting for the back-up capacitor C.sub.1 to be fully charged. The 
capacitance of the capacitor C.sub.1 can be selected at a high value, 
regardless of the length of the stand-by period of the LSI element. After 
the charging of the back-up capacitor C.sub.1 is completed in the above 
described manner, the capacitor C.sub.1 guarantees stable and reliable 
operation of the LSI element via the diode D.sub.3. 
FIG. 6 illustrates another embodiment which is analogous to the embodiments 
of FIG. 4 and FIG. 5, wherein components similar to those in FIGS. 4 and 5 
are designated by the same reference numbers. An enable signal K.sub.a is 
given by the CPU for driving the piezo-electric buzzer P.sub.1 via the 
inverter I.sub.1. The back-up capacitor C.sub.1 supplements enabling 
current for driving the buzzer P.sub.1 even when the output voltage of the 
solar cells is insufficient. 
As set forth previously, the switching elements SW.sub.1 and SW.sub.2 shown 
in FIGS. 5 and 6 are contained in the LSI element and switched on and off 
at a duty ratio of 9:1. Those elements may be implemented with MOS 
transistor transfer gates. A timing signal generator TM is further 
provided. 
FIGS. 8 through 10 show another embodiment which includes a supply voltage 
detector a, a clock generator CG, a memory element M and a display panel 
DISP. The voltage detector a develops a voltage to disable the clock 
signal generator CG when incident light on the solar cells is shut off. 
Especially, when the incident light is shut off and the applied voltage 
V.sub.1 decreases from a level A to a level B as seen in FIG. 10, the 
voltage detector a becomes operative and the switching element SW.sub.1 is 
opened to disable the generator CG. Under the disabled state of the 
generator CG the LSI element stands in the so-called static state with 
little or no current I.sub.1 flowing therethrough (see FIG. 10b) as long 
as it is made up of C-MOS devices. As long as the LSI element is in the 
static state, it is held in the internal state envisaged before the clock 
signal generator CG is disabled, because the CPU and the memory element M 
are supplied with back-up voltage of the capacitor C.sub.1. When solar 
radiation impinges on the solar cells and the voltage V.sub.1 reaches the 
level B in FIG. 10 under the static state of the LSI element, the built-in 
switch SW.sub.1 is closed to start the clock signal generator CG and place 
the LSI element into the static state. Under these circumstances, the LSI 
element is in the internal state as viewed prior to the interrupted 
operation of the clock generator CG. 
While only certain embodiments of the present invention have been 
described, it will be apparent to those skilled in the art that various 
changes and modifications may be made therein without departing from the 
spirit and scope of the invention as claimed.