JPH0467634B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a battery voltage monitoring circuit that backs up loads such as emergency lights during a power outage.

[Background technology] In this type of battery voltage monitor circuit, a light emitting diode lights up when the battery voltage drops, and even if the battery voltage is recovered by charging, the light emitting diode continues to light up, and once the voltage drops, the light emitting diode lights up. To check battery performance using this circuit, power is cut off for a certain period of time, the load is turned on in an emergency manner, and then normal power is restored again. If the light emitting diode is lit at this time, it is determined that the voltage of the battery is dropping quickly and has already deteriorated.

FIG. 3 shows a conventional battery voltage monitor circuit. In the same figure, CP is a constant voltage circuit
This is a self-holding voltage comparator circuit that is supplied with power through RG. Transistors Q ₁ and Q ₂ constitute a differential amplifier, and the base of one transistor Q ₁ is connected to the battery voltage through resistors R ₁ and R _2. and
A voltage Vb ₁ divided by R ₃ is applied, and a reference voltage Vref is applied to the base of the other transistor Q ₂ . If the resistors R ₁ , R ₂ , and R ₃ are set so that Vb ₁ > Vref (1) when the battery voltage is normal, only transistor Q ₁ is in the on state and transistors Q ₂ , Since Q ₃ , Q ₄ and Q ₅ are in the off state, the light emitting diode LED is off. Next, when the battery voltage decreases and Vb ₁ <Vref (2), transistor Q ₁ turns off, transistors Q ₂ , Q ₃ , Q ₄ and Q ₅ turn on, and the LED lights up. At this time, the feedback transistor
Since a part of the voltage dividing resistor, ie, the hysteresis resistor _R3 , is short-circuited by _Q4 , _Vb1 further decreases, and even if the battery voltage Vb recovers thereafter, the differential amplifier does not return to inversion. Thus the voltage comparator circuit CP
is self-maintained, and even after returning to normal energization, the LED remains
continues to light up. If the battery voltage when the LED lights up is Von, then Von=R ₁ +R ₂ +R ₃ /R ₂ +R ₃ ×Vref (3), and if the battery voltage when the LED turns off once turned on is Voff, then Voff = R ₁ + R ₂ / R ₂ × (Vref − V _CES ) + V _CES (4) Now, in the above conventional circuit, once the light is turned on,
The battery voltage Voff when the LED turns off depends on the saturation voltage _VCES of the feedback transistor _Q4 , and the value of (R ₁ + R ₂ )/R ₂ is usually several tens of times larger, so the effect is large and However, there was a problem in that the hysteresis width varied greatly due to variations in the saturation voltage of the transistor _Q4 , making it difficult to set it.

[Object of the Invention] The present invention has been made in view of the above problems, and its purpose is to set the hysteresis voltage width so that the hysteresis width is not affected by variations in the saturation voltage of the feedback transistor. The object of the present invention is to provide a simple battery voltage monitor circuit.

[Disclosure of the Invention] The present invention provides a battery voltage monitor circuit comprising a voltage comparator circuit CP for detecting a voltage drop in a battery B for power outage backup, and a light emitting diode drive circuit DR operated by the output of the voltage comparator circuit CP. , the feedback transistor Q ₄ is controlled by the output of the voltage comparator circuit CP.
A collector current is supplied from the constant current source CS ₁ to the voltage comparator circuit CP, and the feedback transistor Q ₄ is connected to a part of the voltage dividing resistor for battery voltage detection, that is, the hysteresis resistor R ₃ connected to the input terminal of the voltage comparator circuit CP. A current is supplied from the collector of the circuit through a backflow prevention diode _D1 .

If configured as above, the battery voltage Von or Voff when the LED turns on or off
is the saturation voltage of the feedback transistor _Q4 .
This eliminates dependence on Vces and improves the accuracy of the hysteresis voltage width.

FIG. 1 shows an embodiment of the circuit of the present invention. In the same figure, constant voltage circuit RG and LED
The drive circuit DR has the same configuration as the conventional circuit, and in the comparator circuit CP, a constant current source CS ₂ is connected to the common terminal of differential amplifiers Q ₁ and Q ₂ , and is controlled by the output of this differential amplifier. The collector voltage of PNP transistor Q ₃ is divided by resistors R ₆ and R ₈ and applied to the base of feedback transistor Q ₄ via resistor R ₇ .
It is similar to the conventional circuit in that it is connected to the LED driving transistor _Q5 via the resistor _R9 . The present invention supplies a collector current I from a constant current source CS ₁ to the feedback transistor Q ₄ , and supplies a part of the voltage dividing resistor for battery voltage detection connected to the input terminal of the voltage comparator circuit CP, that is, a hysteresis resistor R. ₃ , current is supplied from the collector of transistor _Q4 via diode _D1 .

Figure 2 shows an example of a specific circuit of the constant current source _CS1.The input current Iin of the current mirror circuit MR is controlled by the constant voltage Vcc, the base-emitter voltage of the input side transistor _Q6 , and the resistor _R0. Therefore, the output current of the current mirror circuit MR, that is, the collector current of the output side transistor _Q7 is also a constant current. i.e. current mirror circuit
In MR, both transistors _Q6 and _Q7 , which are formed on a single chip with a common base, are matched to each other, and the base current of the output transistor _Q7 is equal to the base current of the diode-connected input transistor _Q6 . Therefore, a current determined by the emitter area ratio flows through both transistors, and is not affected by the value of the load resistance on the output side. The constant current I obtained in this way is
When Q ₄ is off, it flows into resistor R ₃ through the backflow prevention diode D ₁ , and when transistor Q 4 is on, this current I flows into transistor _{Q 4 and} no longer flows into resistor R ₃ , so the voltage across resistor R ₃ changes regardless of the saturation voltage of transistor _Q4 .

According to the configuration shown in Figure 1, the battery voltage Von when the LED lights up is Von=R ₁ + R ₂ /R ₂ × (Vref - R ₃ I) + R ₃ I (5), and the LED turns off again. _{The battery voltage Voff when _} Vcc-V _BE )/R ₀ (7) Since both Vcc and V _BE are constant, the value of I can also be regarded as a constant value. Therefore, as is clear from equations (5) and (6),
Since Von and Voff do not depend on the saturation voltage V _CES of the transistor, which generally has large variations, the accuracy of setting the hysteresis width can be greatly improved.

[Effects of the Invention] As described above, the present invention provides a battery voltage monitoring circuit that turns on the load for a certain period of time and then returns it to normal energization and detects battery deterioration by lighting an LED. A constant current is passed through a part of the resistor, and after detection, this constant current is drawn in by a feedback transistor, so variations in transistor characteristics do not affect the detected voltage and hysteresis width.
This has the advantage that setting accuracy can be improved.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram showing one embodiment of the present invention, FIG. 2 is a specific circuit diagram of the same main part, and FIG. 3 is a block circuit diagram of a conventional example. B is a battery, RG is a constant voltage circuit, CP is a self-holding comparison circuit, DR is a light emitting diode drive circuit,
MR is a current mirror circuit, CS ₁ and CS ₂ are constant current sources,
LED is a light emitting diode, Q ₁ , Q ₂ , ..., Q ₇ are transistors, R ₁ , R ₂ , ..., R ₁₀ are resistors, and D ₁ , D ₂ are diodes.

Claims (1)

[Claims] 1. In a battery voltage monitor circuit equipped with a voltage comparator circuit for detecting a voltage drop in a battery for power outage backup, and a light emitting diode drive circuit operated by the output of this voltage comparator circuit, the battery voltage monitor circuit is controlled by the output of the voltage comparator circuit. A collector current is supplied from a constant current source to the transistor that is connected, and a current is supplied from the collector of the transistor through a diode to a part of the voltage dividing resistor for battery voltage detection connected to the input terminal of the voltage comparison circuit. A battery voltage monitor circuit comprising: