Printer With Supply Voltage Control Circuit

This invention relates to a printer having a power supply which includes a battery, and more particularly to a circuit for such printer which monitors the voltage supplied from the battery to prevent incorrect operation when the battery voltage falls too low.

Many small printers use a battery as the main power supply and the output voltage of the battery is lowered as the power is consumed in such known printers. When the output voltage of the battery falls below a predetermined value, the printer may often operate incorrectly. Therefore, it is necessary to detect when the output voltage of the battery falls below a predetermined value so that the battery can replaced or recharged. A conventional circuit for detecting when the battery voltage falls below a predetermined minimum is shown in FIG. 1.

In FIG. 1, a power supply voltage (a) of typically 9 V is supplied to a first stabilized power source circuit 3 and a second stabilized power source circuit 4 from the battery 1 through the power switch 2. A specified reference voltage (b), for example 5 V, is supplied to the printing mechanism 5 from the first stabilized power source circuit 3, while a specified reference voltage (c), for example 5 V, is supplied to a control circuit 6 from the second stabilized power source circuit 4. Moreover, in the conventional circuit shown in FIG. 1, the detecting circuit which detects a drop of the power source voltage (a) supplied from the battery 1 is comprised of a current-limiting resistor 7, a zener diode 8, a semi-fixed resistor 9, a fixed resistor 10 and a comparing circuit 11. A constant voltage (d), for example 2 V, generated by the current limiting resistor 7 and zener diode 8 is directed to one input of the comparing circuit 11, and a voltage (e) divided by the semi-fixed resistor 9 and fixed resistor 10 from the reference voltage (b) output from said first stabilized power source circuit is applied to the other input of the comparing circuit 11. By adjusting the resistance value of the semi-fixed resistor 9, the divided voltage (e) is set to equal the constant voltage (d) when the reference voltage (b) becomes the allowable minimum value 9. Therefore, by comparing said constant voltage (d) with the divided voltage (e) in the comparing circuit 11, it can be determined that the reference voltage (b) has reached the allowable minimum value due to a drop of the power source voltage (a) of the battery 1.

However, this detecting circuit used in the conventional printer includes rather complicated circuit structures and, therefore, not only is the cost increased, but also fine adjustment of semi-fixed resistor is required for increasing detection accuracy. Accordingly, a printer which can be obtained economically by utilizing a simplified detecting circuit has been awaited.

It is an object of this invention to provide a printer which has attained such requirements. For this purpose, this invention provides a printer comprising a printing mechanism, a control circuit which controls the printing mechanism and a power supply including a battery, wherein a first stabilized power source circuit converts an output voltage of the battery to a predetermined first supply voltage for the printing mechanism, and a second stabilized power source circuit converts an output voltage of the battery to the second supply voltage for a control circuit of the printer. The second supply voltage is lower than the first supply voltage and a detecting circuit is provided to detect when the difference between the supply voltages has reached a value lower than a predetermined threshold value.

FIG. 1 is a block diagram indicating the conventional structure of circuitry for a printer which detects battery voltage.

FIG. 2 is a block diagram of an embodiment of a circuitry according to the present invention.

An embodiment of the present invention is described below with reference to FIG. 2.

For FIG. 2, numerals 1 to 6 correspond to those of FIG. 1, and numeral 12 notes a transistor while numerals 13 and 14 refer to resistors.

The printer of this invention basically has the structure similar to the prior art of FIG. 1, except for a detecting circuit which is comprised of the transistor 12 and resistors 13 and 14. However, an operating voltage of the printing mechanism 5 is set to a voltage which is higher than that of the control circuit 6 in such a manner that the operating voltage of the printing mechanism 5 is set to 6 V, for example, and the operating voltage of the control circuit 6 is set to 5 V, for example. Therefore, the supply voltage (b) supplied to the printing mechanism 5 from the first stabilized power source circuit 3 and the supply voltage (c) supplied to control circuit 6 from the second stabilized power source circuit 4 are set respectively corresponding to said operating voltages. The supply voltage (b) is applied to the emitter of the transistor 12, while the supply voltage (c) is applied to the base thereof through a resistor 13. Namely, the ON-OFF operations of the transistor 12 depend on the voltage difference between the supply voltages (b) and (c). In addition, a collector voltage f of the transistor 12 changes depending on the ON-OFF operations of the transistor 12. In other words, it becomes high during ON and low during OFF.

When the supply voltages (b) and (c) drop due to a drop in the power source voltage (a) supplied from the battery 1 because of power consumption as described above, the supply voltage (b) drops first because the supply voltage (b) is set higher than the supply voltage (c). Therefore, the transistor 12 can be chosen to turn OFF from its ON state before the supply voltage to be supplied to the control circuit 6 drops by setting the operating voltage of transistor 12 to an adequate value. Accordingly, it can be detected that the power source voltage (a) of the battery 1 has dropped up to an allowable voltage value, by detecting a collector voltage f which changes depending on the ON and OFF state of transistor 12 in the control circuit 6. Such process will then be described below in more detail.

For example, when the initial value of the power source voltage (a) is set to 9 V, a supply voltage (b) is set to 6 V, a supply voltage (c) is set to 5 V and the supply voltage (b) when the power source voltage (a) drops up to the allowable voltage value is set to 5.6 V, the operating voltage of the transistor 12 should be 0.6 V. Namely, the transistor 12 turns ON when a voltage difference between the supply voltages (b) and (c) is 0.6 V or higher, or turns OFF when it is under 0.6 V. Accordingly, when the power source voltage (a) is in the sufficiently high range, a voltage difference between the supply voltages (b) and (c) is 1 V and, therefore, the transistor 12 is in the ON state and a collector voltage f is in the high level. As a result, it is detected that the power source voltage (a) is higher than the allowable voltage in the control circuit 6.

The collector voltage f remains in such a high level until the supply voltage (b) reaches a 5.6 V due to a drop of the power source voltage (a) (under this condition, said supply voltage (c) is kept at 5 V and the voltage difference between said supply voltages (b) and (c) is 0.6 V or higher). However, when voltage difference between the supply voltages (b) and (c) becomes 0.6 V or less, the transistor 12 turns OFF and thereby the emitter voltage f is set to a low level. As a result, it is detected in the control circuit 6 that the power source voltage (a) sent from the battery 1 drops to an allowable value or less and the printing operation is suspended depending on such detection signal. During this period, since a supply voltage (c) to the control circuit 6 which requires comparatively high operating voltage accuracy is kept at 5 V, it does not adversely influence operation of the control circuit 6.

In the embodiment shown in FIG. 2 described above, drop of the power source voltage (a) is detected depending on the ON and OFF operations of a transistor 12. This invention, however, is not limited to such structure and any means detecting the voltage difference between the supply voltages (b) and (c) may be employed.

As described above, this invention is capable of detecting a drop of the power source voltage with a detecting means providing a simplified circuit structure and provides a very economical printer.