GB2211679A

GB2211679A - Solar powered current supply circuit

Info

Publication number: GB2211679A
Application number: GB8725144A
Authority: GB
Inventors: Eric Paul Paterson
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-10-27
Filing date: 1987-10-27
Publication date: 1989-07-05
Also published as: GB8725144D0

Abstract

A supply circuit, particularly for a low current load such as a digital clock 12 or a calculator, has a solar cell 14 to energise the load and to charge a capacitor 18 which provides a backup power supply. The capacitor is a 1 Farad electric double layer "super" capacitor. <IMAGE>

Description

SOLAR POWERED DRIVE CIRCUIT FOR LOW CURRENT LOADS The present invention relates to a solar powered drive circuit particularly, but not exclusively, for use with low current loads. In particular, it relates to drive circuit for absorbing ambient light energy during the presence of such energy and for releasing stored electrical energy to the low current load for a predetermined period in the absence of light energy to provide continuous power to the load.

A solar powered drive circuit for use with low current loads should satisfy a number of criteria in addition to being relatively inexpensive and compact for use with low current loads, such as LCD elements. The circuit should be capable of being used with a variety of loads, each of these loads being adapted for use with the drive circuit, each load having different power requirements. The drive circuit should also provide continuous operation for a period of at least 1 day without receiving light energy. Once disadvantage of present solar powered low current devices is that they cease to operate in the absence of light energy of sufficient magnitude to power the device.

There are existing devices which rely on the conversion of light energy to electrical energy to drive a low current load such as an LCD element. These loads operate in the presence of ambient light energy, that is there is minimal energy storage capability and consequently continuous power is not provided. There are several types of existing storage devices which could be suitable for use with low current load circuits. One such device is a Nickel/Cadmium cell or battery, however, this has two main disadvantages, which are that the cell has a limited number of charge/discharge cycles due to chemical action and cell discharge currents of several hundred ssA are common, which is several times a typical CMOS or similar technology low current load requirement.Another storage device is an Electolytic Capacitor, however, this is physically large for the value of capacitance required (1 Farad) to store a substantial charge and such capacitors have internal leakage currents of several hundred aA, which as above, make them impractical for use with low current loads. A further device which could be use is a Lithium primary cell. This is not a storage device because it cannot be charged or recharged. Such Lithium primary cells would have a finite life, for example, around four years if used with loads requiring 10-20 ssA but such cells are expensive and may constitute a safety hazard because of the chemical composition of the device.

An object of the present invention is to provide a device which obviates or mitigates the disadvantages associated with the aforementioned devices.

This is achieved by providing a circuit which provides a means of energy storage using a storage element in the form of a "super capacitor" electrically connected to the solar cell. The super capacitor is an electric double layer capacitor which stores energy from the solar cell while the solar cell simultaneously provides energy to drive a low current load connected to the drive circuit. This mode is called the "day mode" operation of the circuit. When the light energy incident on the solar cell is insufficient to power the low current device the energy stored in the super capacitor is released through the low current load, providing continuous power. This is called "night mode" operation of the circuit. Thus the circuit will operate in the absence of light for a predetermined period.

In a preferred embodiment of the invention the drive circuit can be connected to an LCD digital display timepiece which requires to be powered continuously. The super capacitor is an NEC 5.VDC electric double layer capacitor with a capacity of 1 Farad connected in parallel with a solar cell via a low leakage diode. The capacitor is physically small and is a disc about 3.5.cm diameter and lem in height. In parallel with the super capacitor is a series arrangement of a current limiting resistor and an output terminal arrangement for connection to a low current load such as the digital display time-piece. In -parallel with the output terminal arrangement are a number of low leakage diodes which provide a means of voltage clamping to control the voltage applied across the time-piece.

According to one aspect of the present invention there is provided a solar powered drive circuit for driving low current loads, said drive circuit comprising: an energy conversion means for converting light energy to electrical energy, an electrical energy storage means for storing said electrical energy, said electrical energy storage means being coupled to said energy conversion means, a drive circuit output means coupled to said electrical energy storage means and to said energy conversion means for transferring electrical energy to an electric load, whereby, in use, an electrical load is connected to said drive circuit and in a first mode of operation, light energy incident on said energy conversion means is converted to electrical energy and is stored in said electrical energy storage means and is used to drive said electrical load, and in a further mode of operation and in the absence of said light energy said drive circuit releases stored electrical energy to the low current load for a predetermined period of time.

Preferably said predetermined period of time corresponds to the length of the longest period of night fall at the location of operation of said drive circuit.

Preferably said electrical energy storage means is a super capacitor connected across the energy conversion means, said capacitor having a value of 1 Farad.

These and other aspects of the invention will become apparent from the following description when taken in combination with the accompanying drawing which is a schematic diagram of a circuit in accordance with the invention connected to a digital clock.

Reference is now made to the circuit diagram which depicts a drive circuit, generally indicated by reference numeral 10 which is connected to a low current load in the form of an LCD digital clock 12 for supplying continuous power thereto, as will be later described.

A solar cell 14 is used to absorb light energy and convert absorbed energy to electrical energy. Across the solar cell 14 are serially connected a forward-biased low leakage diode 16 and a small 1 Farad super capacitor 18, which is an electric double layer capacitor, (NEC 5.VDC M310). In parallel with the super capacitor 18 is connected a current limiting resistor 20 in series with an output arrangement 22. This output arrangement consists of 3 low leakage diodes 24 and terminals 26 for connection to the low current load, in the form of an LCD digital clock 12 which is shown connected between terminals 22.

There are two main modes of operation of the circuit. The first mode of operation, called the "day mode" for convenience, occurs when the circuit is energised by electrical energy resulting from the conversion of incident light energy by said solar cell 14. The second mode of operation called the "night mode" occurs when there is no light or insufficient light incident on the solar cell 14 to power the circuit 10.

Firstly, considering the operation of the circuit in the day mode, the digital time clock 12 is directly driven by current supplied from the solar cell 14 through the low leakage diode 16 and the current limiting resistor 20.

The diodes 24 are used to clamp the voltage across the digital time clock 12 to a working level. Because the super capacitor 18 is in parallel with the solar cell 14 some current flows to charge the super capacitor 18.

Because the 1 Farad capacitor 18 is capacitively large a considerable build up of charge takes place during the day mode with the capacitor effectively storing electrical energy.

In the night mode the electrical energy provided by the solar cell is non-existant or is insufficient to drive the digital clock. Thus because of diode 16 the charged super capacitor 18 discharges only through the current limiting resistor 20 and output arrangement 22. There is sufficient charge stored in the capacitor during the day mode to provide the digital time clock 12 with sufficient current to drive the clock continuously during the night mode. The time constant controlling discharge of capacitor 18, based on the value of the capacitance and the resistance of the output arrangement 22 and current limiting resistor 20, is large enough to ensure that this occurs.The current limiting resistor 20 is large with respect to the total equivalent resistance of the output arrangement 22 to ensure that the resistance of different loads does not substantially affect the operation, by causing rapid current drain.

Because low leakage diode 16 prevents discharge of the capacitor through the solar cell 14 practically all of the capacitor's discharge current goes through the output drive arrangement.

Several modification of the overall drive circuit are possible, for example the solar cell 14 can be replaced with an alternative form of energy conversion means which will provide electrical energy to drive the circuit. It will be understood that the term solar cell used here is also synonymous with solar panel and the cell could be a single cell supplying a voltage of normally 0.45v to 0.5v or a group of N cells (a solar panel) supplying N x (o.45v to 0.5v). As is well known in the art the cells are available in series connected in one glass substrate. The digital clock can be replaced by any low current load such as a calculator provided the low current load can be used with the drive circuit. Also, the several super capacitors may be connected in parallel to provide higher output currents for other applications where loads have a higher current drain Advantages of the arrangement described are that continuous operation of the clock is possible and battery back up is not required. The circuit can be used to power any low current load provided that the load is suitable for use with the drive circuit. Also the use of the capacitively large super capacitor which is physically small, about 3.5cm diameter and lcm high), permit the construction of a compact portable solar powered drive circuit which has not been hitherto obtainable.

Claims

1. A solar powered drive circuit for low current loads comprising energy conversion means for converting light energy to electrical energy, electrical energy storage means for storing said electrical energy, said electrical energy storage means being coupled to said energy conversion means, drive circuit output means coupled to said electrical energy storage means and to said energy conversion means for transferring electrical energy to an -electric load, whereby, in use, an electrical load is connected to said drive circuit such that in a first mode of operation, light energy incident on said energy conversion means is converted to electrical energy and is stored in said electrical energy storage means and is also used to drive said electrical load, and in a further mode of operation, in the absence of said light energy, said drive circuit releases stored electrical energy to the low current load for a predetermined period of time.

2. A solar powered drive circuit for low current loads as claimed in claim 1 wherein said predetermined period of time corresponds to the length of the longest period of nightfall at the location of operation of said drive circuit.

3. A solar powered drive circuit for low current loads as claimed in claim 1 or claim 2 wherein said electrical energy storage means is a super capacitor connected across the energy conversion means.

4. A solar powered drive circuit for low current loads as claimed in claim 3 wherein said capacitor has a value of 1 Farad.

5 A solar powered drive circuit for low current loads substantially as hereinbefore described with reference to the accompanying drawing.