GB2143094A

GB2143094A - Circuit for charging a secondary cell

Info

Publication number: GB2143094A
Application number: GB08413597A
Authority: GB
Inventors: Masahiro Minowa
Original assignee: Epson Corp
Current assignee: Epson Corp
Priority date: 1983-06-01
Filing date: 1984-05-29
Publication date: 1985-01-30
Also published as: JPS59222045A; GB8413597D0; DE3419078A1

Abstract

A current limiting resistor 3, connected in series between a charger 1 and a secondary cell 7, is short- circuited by a transistor 5 when a component drawing a relatively large current is turned on, this component being in an electronic device 8 supplied by the cell 7. The device 8 may include a thermal printer and the component may be a motor (11), (Figure 2). A signal to turn on the motor (11) may be applied to a motor control transistor (12) and to a transistor 6 controlling the shorting transistor 5. <IMAGE>

Description

SPECIFICATION A Charging Circuit for Charging a Secondary Cell This invention relates to charging circuits for charging secondary cells for use as a power supply to electronic devices.

Although the present invention is primarily directed to any novel integer or step, or combination of integers or steps, herein disclosed and/or as shown in the accompanying drawings, nevertheless, according to one particular aspect of the present invention to which, however, the invention is in no way restricted, there is provided a charging circuit for charging a secondary cell for use as a power supply of an electronic device, comprising: a charging device having output terminals for connection to a secondary cell; a resistor having one side connected to one of the output terminals of said charging device and an opposite side adapted to be connected to the secondary cell for limiting charging current flowing into the secondary cell; and switching means connected to said resistor for shortcircuiting said resistor to increase said charging current in response to an increase in current drawn by the electronic device.

Said switching means may comprise a transistor having its emitter and its collector connected respectively across said resistor, and arranged so that in operation its base is supplied with a signal responsive to the increase in the current drawn by the electronic device.

In a preferred embodiment said charging circuit includes a second charging circuit including a second transistor for energising said firstmentioned transistor in response to the increase in the current drawn by the electronic device.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a circuit diagram of one embodiment of a charging circuit according to the present invention for charging a secondary cell; Figure 2 is a circuit diagram of another embodiment of charging a circuit according to the present invention for charging a secondary cell; and Figure 3 is a circuit diagram of a known charging circuit for charging a secondary cell.

Referring first to Figure 3 there is illustrated a known charging circuit for charging a secondary cell. This charging circuit comprises an AC adaptor 31 and a diode 32 as a charging device.

A resistor 33 limits charging current to a secondary cell 34, for example, a nickel-cadmium cell, to be charged. An electronic device 35 is connected to draw power from the secondary cell 34. It is generally preferable that a nickelcadmium cell is charged over a period of ten hours (lO-hour rate). Where the electronic device 35 contains a component which draws the relatively large current, a secondary cell of relatively large capacity is required, and weight and cost are increased. When current larger than the charging current is drawn from the secondary cell 34, electrical energy stored therein is progressively reduced even with the AC adaptor 31 still connected. To compensate for the reduction of electrical energy, a higher charging current has frequently been used.However, since the charging current has to be controlled by detecting a voltage across the secondary cell, the required circuitry is relatively expensive.

Referring now to Figure 1 there is illustrated one embodiment of a charging circuit according to the present invention for charging a secondary cell. The charging circuit comprises an AC adaptor 1 and a diode 2 as a charging device. Seriesconnected resistors 3,4 limit charging current, one side of the resistor 3 being connected to one output terminal of the AC adaptor 1. A nickelcadmium secondary cell 7 is connected across the output terminals of the AC adapter 1 so as to be charged by the charging current determined by the total resistance value of the resistors 3,4. The secondary cell 7 is connected to an electronic device 8 to power the same.In the case where the electronic device 8 contains a component which draws a relatively large current, the electrical energy of the secondary cell 7 will be progressively reduced if the charging current with which the secondary cell 7 is charged is smaller than the current drawn by the electronic device 8.

A transistor 5 serving as a switching device has its emitter and its collector connected across the resistor 3 for selectively short-circuiting the resistor 3. A transistor 6 for switching the transistor 5 has its base connected to the electronic device 8, its collector connected to the base of the transistor 5, and its emitter connected to the other output terminal of the AC adaptor 1.

When the component in the electronic device 8 which draws a relatively large current is energised, a signal causing the energisation is also applied to the base of the transistor 6 to render the latter conductive. When the transistor 6 is conductive, the transistor 5 is also rendered conductive to short-circuit the resistor 3.

Therefore, the charging current flowing into the secondary cell 7 is increased. The current drawn by the electronic device 8 can be supplied by regulating the charging current with the resistor 4 or the output voltage of the AC adaptor 1. If the current flowing into the electronic device 8 is i and the charging current flowing when the transistor 5 is conductive is i2, the problem of overcharging the secondary cell 7 will be overcome, in normal conditions of use, by making the difference (i2-i1) equal to the average current that would flow when charging the secondary cell 7 at between a 1 O-hour rate and a 4-hour rate. The resistor 4 is inserted to optimise the charging current for the secondary cell 7, and may be eliminated where the resistance of a secondary winding of the AC adaptor 1 is employed for that purpose.

Figure 2 illustrates another embodiment of a charging circuit according to the present invention for charging a secondary cell. Like parts in Figures 1 and 2 have been designated by the same reference numerals. In Figure 2, an electronic device contains a motor 11 which is energisable by a transistor 12 having its base connected to an input terminal 13 to which a motor driving signal is applied.

When the motor 11 is driven, the motor driving signal is also fed to the base of the transistor 6 to render it conductive thereby short-circuiting the resistor 3. An electronic device including a thermal printer, for example, consumes a substantial amount of electrical energy while the motor is in operation: a print head of the thermal printer generally consumes a relatively large amount of electrical energy and requires a high peak current. The charging circuit of Figure 2 can be arranged to prevent the secondary cell 7 from being drained of electrical energy by increasing the charge in current in response to the motor driving signal. Therefore, the charging circuit will serve as a highly effective power supply in electronic devices containing a component requiring a relative high peak current, such as, for example, a thermal printer.

The charging circuits according to the present invention described above are advantageous in that the secondary cell, such as a nickel-cadmium cell, for use with battery-operated electronic devices can be of smaller capacity than conventionally. Even where the electronic device to be powered by the secondary cell contains a component consuming a relatively high peak current, such as a motor for example, the secondary cell 7 can supply the necessary peak current so that the number of windings of the AC adaptor 1 can be reduced and its size and capacity also reduced in consequence. The charging circuits of Figures 1 and 2 thus can be relatively small in size and lightweight so that they can be used as power supplies for hand-held electronic devices.

Claims

1. A charging circuit for charging a secondary cell for use as a power supply of an electronic device, comprising: a charging device having output terminals for connection to a secondary cell; a resistor having one side connected to one of the output terminals of said charging device and an opposite side adapted to be connected to the secondary cell for limiting charging current flowing into the secondary cell; and switching means connected to said resistor for shortcircuiting said resistor to increase said charging current in response to an increase in current drawn by the electronic device.

2. A charging circuit as claimed in claim 1 in which said switching means comprises a transistor having its emitter and its collector connected respectively across said resistor, and arranged so that in operation its base is supplied with a signal responsive to the increase in the current drawn by the electronic device.

3. A charging circuit as claimed in claim 2 including a second transistor for energising said first-mentioned transistor in response to the increase in the current drawn by the electronic device.

4. A charging circuit as claimed in any preceding claim in combination with the secondary cell and the electronic device.

5. A charging circuit for charging a secondary cell for use as a power supply of an electronic device substantially as herein described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

6. A circuit for charging a secondary cell used as a power supply for an electronic device, comprising: a charging device having output terminals for connection to the secondary cell; a resistor having one end connected to one of the output terminals of said charging device and an opposite end adapted to be connected to the secondary cell for limiting a charging current flowing into the secondary ceil; and switching means connected to said resistor for shortcircuiting said resistor to increase said charging current in response to an increase in a current consumed by the electronic device.

7. Any novel integer or step, or combination of integers or steps, hereinbefore described, irrespective of whether the present claim is within the scope of, or relates to the same or a different invention from that of, the preceding claims.