Battery voltage discriminator circuit

Abstract: A battery voltage discriminator circuit for an apparatus having a battery and a plurality of loads which are driven by the battery and operate under various kinds of load conditions is disclosed. When the output voltage of said battery means drops under a predetermined reference voltage, the circuit generates a signal indicative of the voltage drop. The circuit has a reference voltage setting circuit for setting the reference voltage to one of predetermined values, which corresponds to the kind of the current load condition, and a reference voltage change circuit for activating the reference voltage setting circuit for a period of time in response to the change in the kind of current load condition.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to a battery voltage discriminator circuit for use in an electronic apparatus such as a portable word processor, and more particularly to a battery voltage discriminator circuit for determining whether or not the battery has been consumed to the extent that it is no longer able to normally drive the apparatus. 2. Description of the prior art: A conventional battery voltage discriminator circuit provided in such an electronic apparatus using a battery as a drive power source is usually arranged as shown in Figure 7. The terminal voltage V B of a battery B a which is used as a drive power source is converted by a DC-DC converter PS into a constant voltage adjusted to a predetermined load voltage V C which is in turn supplied to a load L O . The battery voltage V B is divided by two resistors R₁ and R₂ to generate a detection battery voltage V₁ the level of which corresponds to that of the battery voltage V B as shown in (a) of Figure 8. The detection battery voltage V₁ is inputted to an inverting input terminal of a comparator CP. The load voltage V C is divided by two resistors R₃ and R₄ to generate a predetermined reference voltage V₂ which is input to a non-inverting input terminal of the comparator CP. When the detection battery voltage V₁ drops to the reference voltage V₂, the output terminal of the comparator CP becomes high to output an abnormal voltage detection signal LB as shown in (b) of Figure 8, so that an alarm is given to indicate that the battery B a has been consumed to the extent that it is no longer available for use. In Figure 7, capacitors C₁ and C₂ are provided for eliminating ripples from the battery voltage V B and the load voltage V C . The case will be considered in which the circuit of Figure 7 is applied to a portable computer or word processor having a variety of loads such as a printer, a modem, a buzzer, an LED indicator or the like. When any abrupt change in load occurs as in the starting of an X-directional drive motor for the printer or on the connection of the modem to a communication line, there may occur an overcurrent flowing to cause a drop in the battery voltage V B and a momentary sudden drop in the detection battery voltage V₁ to a lower voltage level than the reference voltage V₂, with the result that an abnormal voltage detection signal LB is produced despite the fact that the battery B a has not been consumed to the extent of being no longer available for use. In order to obviate such inconvenience, measures have been taken such that when an abnormal voltage detection signal is output upon the occurrence of any abrupt load fluctuations, no alarm of such occurrence is given, or such that the detection conditions for battery consumption detection are set to comply the maximum conceivable load condition in which, for example, the printer prints black on the entire surface of a recording paper sheet. However, these measures have been found unsatisfactory. With the first mentioned measure, it is impossible to accurately detect, as originally intended thereby, that the battery voltage has dropped to the extent that it is no longer available for driving the apparatus. With the latter measure in which, even when the load condition is low, the degree of battery consumption is determined according to detection conditions corresponding to the maximum possible load condition, it is likely that an abnormal voltage detection signal will be erroneously produced even if the battery is still available for effective use. Therefor, the latter measure permits no efficient use of the battery, which is disadvantageous from the standpoint of economy. DE-A-34 03 798 describes a power-off detection circuit in which a voltage comparison- circuit compares a power source proportional voltage with a fixed reference voltage and outputs a power-off signal when the level of the power source proportional voltage is smaller than that of the reference voltage. The power source proportional voltage is set at one of two possible values according to whether any of a number of loads is supplied with power or none of the loads is supplied with power. The setting of the power source proportional voltage is thus independent of which particular load is supplied with power. SUMMARY OF THE INVENTION The invention provides a circuit for checking the supply voltage of a battery usable for supplying power to a variety of electrical loads which impose respective predetermined different load conditions on the battery, the circuit comprising means for generating a reference voltage, means for generating a battery detection voltage corresponding to the battery voltage, and means for generating an output signal when said battery detection voltage falls below said reference voltage, characterized by means for temporarily changing said reference voltage selectively by a respective predetermined amount according to which one of the electrical loads is supplied power by the battery. In a preferred embodiment the changing means is arranged to change said reference voltage by each said predetermined amount for a respective predetermined period of time. Thus, the invention described herein makes possible the objectives of: (1) providing a battery voltage discriminator circuit which can accurately detect the degree of battery consumption even when the load condition is changed over a wide range; (2) providing a battery voltage discriminator circuit which can accurately detect the degree of battery consumption even when the load condition is changed abruptly; (3) providing a battery voltage discriminator circuit which can output a signal indicative of the consumed state of a battery even when the load condition is changed over a wide range; and (4) providing a battery voltage discriminator circuit which can output a signal indicative of the consumed state of a battery even when the load condition is changed abruptly. According to the invention, when there is no occurrence of abrupt load fluctuation, the reference voltage changing means is not driven, and hence the output voltage of the battery is compared with a predetermined reference voltage. Upon the occurrence of any foreseeable abrupt load change such as, for example, the start of an X-directional printer drive motor of the printer or the connection to a line of a modem, the reference voltage changing means changes the reference voltage by a predetermined amount corresponding to the current load condition. This change of the reference voltage is maintained preferably only for a predetermined period of time which corresponds to the kind of the current load condition. Since the degree of battery consumption is determined in this way according to the detection conditions in which the reference voltage corresponds to the respective current load condition, it is possible to accurately determine the condition of battery consumption irrespective of load fluctuations and thus to ensure economical use of the battery. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a circuit diagram showing a embodiment of the invention. Figure 2 is a flow chart of the embodiment shown in Figure 1. Figure 3 is a graph showing the variation of the battery voltage when loads are changed. Figure 4 is a graph showing the variation of the load current when loads are changed as shown in Figure 3. Figure 5 is a timing chart of the embodiment shown in Figure 1. Figure 6 is a graph showing the variation of the battery voltage in the embodiment of Figure 1. Figure 7 is a circuit diagram showing a conventional circuit. Figure 8 is a graph showing the relation between a period of using a battery and the battery voltage. DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the invention will now be described with reference to the accompanying drawings. Figure 1 is a circuit diagram showing one embodiment of the invention which is applied to a portable word processor having loads such as a printer, a modem, a buzzer, and an LED indicator (which are not shown). In Figure 1, elements identical with or similar to those in Figure 7 are designated by like reference numerals, and their description is omitted. The circuit shown in Figure 1 comprises four circuits which have respectively resistors R5 - R8 and switching transistors Q1 - Q4 connected in series. As described later, these resistors R5 - R8 have different resistances, and are used for setting the level of the reference voltage input to the non-inverting input terminal of the comparator CP (hereinafter, each of these resistors is referred to as "the reference voltage setting resistor"). The four circuits are connected in parallel and inserted between the ground and the non-inverting input terminal of the comparator CP to which the reference voltage V₂ is input. The bases of the transistors Q1 - Q4 are respectively connected with separate control signal terminals SA - SD through which a control signal is supplied from a CPU (not shown) of the word processor. In the word processor, there are foreseeable abrupt load changes which will occur in such instances as when the X-directional drive motor of the printer is driven, the modem is connected to a communication line, the buzzer sounds, and the LED indicator lights. Figure 4 illustrates the changes in the load current caused in the case where the above mentioned load changes have occurred in sequence, and Figure 3 illustrates changes in the battery voltage V B caused when the load current changes of Figure 4 occur. More specifically, in these figures are shown changes in the load current and battery voltage in the case where the power supply is turned on at time T₀, the motor is started at time T₁, the modem is connected at time T₂, the buzzer sounds at time T₃, the motor is restarted at time T₄, the LED indicator is lighted at time T₅, the buzzer sounding is stopped at time T₆, the LED indicator is turned off at time T₇, and the power supply is interrupted at time T₈. The duration of battery voltage changes due to the starting of the motor at time T₁ and time T₄ is t₁ (100 msec), and the battery voltage change at the time is v₁ (-0.5 V). The duration of the battery voltage change due to the connection of the modem at time T₂ is t₂ (50 msec), and the battery voltage change at that time is v₂ (-0.7 V). The duration of the battery voltage change due to the buzzer sounding at time T₃ is t₃ (75 msec), and the battery voltage change at that time is v₃ (-0.2 V). The duration of the battery voltage change due to the LED lighting at time T₅ is t₄ (25 msec), and the battery voltage change at that time is v₄ (-0.1 V). On the basis of these actual measurement data, the resistance of the reference voltage setting resistor R5 is selected so that the reference voltage applied to the non-inverting input terminal of the comparator CP is caused to drop by 0.5 V when the transistor Q1 is turned on. Similarly, the resistance of the reference voltage setting resistor R6 is selected so that the reference voltage is caused to drop by 0.7 V when the transistor Q2 is turned on; that of the reference voltage setting resistor R7 is selected so that the reference voltage is caused to drop by 0.2 V when the transistor Q3 is turned on; and that of the reference voltage setting resistor R8 is selected so that the reference voltage is caused to drop by 0.1 V when the transistor Q4 is turned on. The operation of the discriminator circuit during the load changes shown in Figures 3 and 4 will be described in detail with reference to Figures 2, 5 and 6. When the power supply is turned on, the sequence shown in Figure 2 starts to sequentially judge whether the motor is started (step S1), whether the modem is connected (step S3), whether the buzzer sounds (step S5), and whether the LED indicator is lit (step S7). If judgment is NO at any of these judgment steps, the process step following the questioned judgment step is jumped, and the process loops. At time T₁, judgment is YES at step S1, and as shown in (a) of Figure 5 a control signal of HIGH is supplied to the first control signal terminal SA for a period of 100 msec (step S2), whereupon the first switching transistor Q1 turns on so that the resistor R5 is connected to the non-inverting input terminal of the comparator CP. Therefore, the reference voltage which has been divided by the resistors R3 and R4 becomes determined by the combination of the resistors R3, R4 and R5 so as to be lowered by v₁ (0.5 V) as shown in Figure 6. If the drop v₃ of the battery voltage V B (i.e., the detection battery voltage V₁) due to motor starting is not greater than 0.5 V, no abnormal voltage detection signal LB will be output from the comparator CP. Similarly, at time T₂, judgment is YES at step S3, and as shown in (b) of Figure 5 a control signal of HIGH is supplied to the second control signal terminal SB for a period of 50 msec (step S4), whereupon the second switching transistor Q2 turns on so that the resistor R6 is connected to the non-inverting input terminal of the comparator CP. The reference voltage is determined by the combination of the resistors R3, R4 and R6 and is lowered by v₂(0.7 V) as shown in Figure 6. If the drop of the detection battery voltage V₁ due to the connection of the modem is not greater than 0.7 V, no abnormal voltage detection signal LB will be output from the comparator CP. At time T₃, judgment is YES at step S5, and as shown in (c) of Figure 5, a control signal of HIGH is supplied to the third control signal terminal SC for a period of 75 msec (step S6), whereupon the third switching transistor Q3 turns on so that resistor R7 is connected to the non-inverting input terminal of the comparator CP. The reference voltage is determined by the combination of the resistors R3, R4 and R7 and is lowered by v₃ (0.2 V) as shown in Figure 6. If the drop of the detection battery voltage V₁ due to the sounding of the buzzer is not greater than 0.2 V, no abnormal voltage detection signal LB will be output from the comparator CP. At time T₄, the procedures in steps S1 and S2 which have been described above are repeated. At time T₅, judgment is YES at step S7, and as shown in (d) of Figure 5, a control signal of HIGH is supplied to the fourth control signal terminal SD for a period of 25 msec (step S8), whereupon the fourth switching transistor Q4 turns on so that resistor R8 is connected to the non-inverting input terminal of the comparator CP. The reference voltage is determined by the combination of the resistors R3, R4 and R8 and is lowered by v₄ (0.1 V) as shown in Figure 6. If the drop of the detection battery voltage V₁ due to the lighting of the LED indicator is not greater than 0.1 V, no abnormal voltage detection signal LB will be output from the comparator CP. As described above, according to the invention, the reference voltage is changed to one of the predetermined values according to various foreseeable abrupt load changes each time when such load change occurs. Therefore, it is possible to perform accurate capacity management during the service life of the battery without erroneous output of any abnormal voltage detection signal during the occurrence of any abrupt load fluctuation, and thus to achieve efficient use of the battery without waste. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of the claims.

Claims: A circuit for checking the supply voltage of a battery usable for supplying power to a variety of electrical loads which impose respective predetermined different load conditions on the battery, the circuit comprising means (R3,R4) for generating a reference voltage (V₂), means (R1,R2) for generating a battery detection voltage (V₁) corresponding to the battery voltage (V B ), and means (CP) for generating an output signal (LB) when said battery detection voltage falls below said reference voltage, characterized by means (Q1,R5;Q2,R6;Q3,R7;Q4,R8) for temporarily changing said reference voltage (V₂) selectively by a respective predetermined amount (v₁;v₂;v₃;v₄) according to which one of the electrical loads is supplied power by the battery. The circuit of claim 1, characterized in that the changing means is arranged to change said reference voltage (V₂) by each said predetermined amount (v₁;v₂;v₃;v₄) for a respective predetermined period of time (t₁;t₂;t₃;t₄).

Inventors: SENOO SEIITI, WATANABE YOSHIO, SENOO, SEIITI, WATANABE, YOSHIO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G01R19
IPC codes: H02J7, G01R19, G01R31