JPS6111780Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a series type constant voltage circuit equipped with a short circuit protection circuit that prevents the circuit from being burnt out due to a short circuit on the load side. It was developed especially for use in electrical equipment that uses batteries as a power source.

If a battery is used as a power source, the output voltage will drop over time as the battery is used, so when designing a circuit, the circuit should be designed in advance so that it can be used within an appropriate voltage range. In that case, in the conventional load short-circuit protection circuit, a resistor for detecting a load short-circuit is inserted in series with the constant voltage circuit, so the usable voltage range of the above battery is reduced by the voltage drop across this detection resistor. It had the disadvantage of being too narrow. Since the circuit according to the invention does not use a sensing resistor, the above-mentioned drawbacks are eliminated.

The circuit shown in FIG. 1 is an example of a conventionally known series type constant voltage circuit, and the part indicated by a broken line in the figure is a short circuit protection circuit 1 consisting of a resistor R and a transistor _Q1 . When the current I becomes excessive, the base-emitter voltage of the transistor _Q1 increases, and the transistor _Q1 conducts to cut off the constant voltage control transistor _Q2 , thereby preventing the circuit from burning out. Therefore, in this circuit, a voltage drop of RxI always occurs across the resistor R during normal operation. By the way, especially when a battery is used as a power source, it is preferable to design the device so that it can be used even if the battery voltage becomes as low as possible, and it is desirable that such a voltage drop does not occur. However, in the conventional overcurrent protection circuit shown in FIG. 1, the current value is detected by the voltage drop caused by the detection resistor R, so the above-mentioned problems cannot be avoided.

The present invention has been made in order to provide overcurrent protection without using such a voltage drop due to the detection resistor R. One embodiment is shown in FIG.

In this invention, without connecting a current detection resistor in series with a constant voltage transistor, both input terminals are charged appropriately by the normal voltage generated between both output terminals during normal operation, and are connected to a battery or the like. A short-circuit detection capacitor that is appropriately charged by the input voltage between the two output terminals is provided, and the charge discharged from this capacitor is used to cut off the constant voltage control transistor when both output terminals are short-circuited. Take.

FIG. 2 shows a circuit diagram of an embodiment of the present invention. Constant voltage control transistor _Q2 is the resistance of the output voltage
A constant voltage is supplied between output terminals A and B by controlling the base voltage by a value appropriately divided by R ₆ and R ₇ . Therefore, when switch _S1 is closed, if output terminals A and B are open or a normal load is connected between them, a stable normal voltage Vo is applied between output terminals A and B.
A novel protection circuit 2 is incorporated into this well-known series type constant voltage circuit.

During normal operation, the short circuit detection capacitor
C ₁ is charged with a constant charge from the input terminal via switch S ₁ , resistor R ₁ , and diode D ₄ . The voltage across the capacitor _C1 , that is, the potential Vo at the terminal D, is appropriately determined by a Zener diode _D3 inserted in parallel with the capacitor C1. At this time, the potential Vc of terminal C is the output voltage Vo, and the diode
If the forward drop voltages of D ₁ and D ₂ are V _D1 and V _D2 , then
Vc=Vo-(V _D1 +V _D2 ). Since V _D is set lower than V _C in this circuit, no current flows through diode D ₅ and cut-off transistor Q ₁ is in a cut-off state.

Next, consider a case where both output terminals A and B of this circuit are short-circuited. FIG. 3 shows a circuit diagram in a short-circuited state. At this time, the potential of terminal C is terminal D
Therefore, the electric charge stored in the capacitor _C1 flows through the diode _D5 , and after a delay of the time constant of the resistor _R3 and the capacitor _C2 , flows into the base terminal of the cut-off transistor _Q1 , and this transistor By making _Q1 conductive, the constant voltage control transistor _Q2 is cut off to protect the constant voltage circuit.

The function of resistor R ₃ and capacitor C ₂ is as follows. That is, the circuit of the device used as the load inserted between the output terminals A and B is often equipped with a large-capacity smoothing capacitor for smoothing the power supply. If the switch _S1 is closed at this time, a large current will instantaneously flow through the load, which may cause the protection circuit to operate and cut off the constant voltage control transistor _Q2 . To prevent this, resistor R ₃ and capacitor C ₂
An RC circuit is added, and the operation of the protection circuit is delayed by the time constant of this RC circuit, as mentioned above, so that the protection circuit will not operate due to the instantaneous large current when the switch is turned on. . Since a delay circuit is provided on the path through which the charge stored in the capacitor _C1 flows into the base terminal of the transistor _Q1 , it is possible to prevent the delay circuit from erroneously operating in response to an instantaneous short-circuit condition caused by an interruption in the circuit. It is now possible to definitely prevent this. That is, malfunctions can be prevented whether the switch is opened or closed on the input side or the output side of the circuit.

According to the present invention, a protection operation can be performed in a series type constant voltage circuit without using a detection resistor connected in series with a constant voltage control transistor. This saves constant power consumption by the protection circuit, and when using a battery as an input power source, it can be used until the battery voltage drops to a specified voltage, contributing to the effective use of the power source. Ta. Furthermore, in this invention, the forward voltage drop of diodes D ₁ and D ₂ is used to set the voltage V _BC between BC during normal operation, so V _BC remains constant even if the output voltage falls below the rated value. Furthermore, the charging voltage of capacitor _C1 is determined by the falling voltage of the Zener diode, so it remains constant even if the input voltage changes. Therefore, the protection circuit of the present invention provides the same operation regardless of changes in input and output voltages. This is a feature of the present invention, compared to general circuits that perform accurate protection only at rated output.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an example of a conventional constant voltage circuit with a protection circuit, Fig. 2 is a circuit diagram of an embodiment of the present invention,
FIG. 3 is a circuit diagram showing a load short-circuit condition in this embodiment. In the figure, _Q1 is a cutoff transistor, _Q2 is a constant voltage control transistor, and _C1 is a short circuit detection capacitor.

Claims (1)

[Claims for Utility Model Registration] A constant voltage control transistor whose conductivity is controlled by the output voltage, and a constant voltage control transistor that is inserted between the control terminal and the output terminal of the constant voltage control transistor and is cut off during normal operation. A capacitor that is appropriately charged during normal operation and discharged when the load is short-circuited to make the aforementioned cut-off transistor conductive, and a capacitor that is connected between the aforementioned capacitor and the control terminal of the cut-off transistor. A rectifying element is inserted, and is cut off by the output voltage during normal operation, but becomes conductive when the load is short-circuited and introduces the discharged charge from the capacitor mentioned above into the cut-off transistor mentioned above, and a diode. A means for supplying a voltage that is a constant voltage drop from the output voltage to a terminal on the control terminal side of a cut-off transistor of the rectifying element by using a forward voltage drop, and a Zener diode to bring the capacitor to a constant potential. It is equipped with a charging circuit that charges the capacitor, and a delay circuit that is provided on the discharging path of the capacitor described above and delays the conduction of the cut-off transistor described above. A constant voltage circuit with a short-circuit protection circuit characterized by making a transistor conductive and thereby cutting off a constant voltage control transistor.