JPH0432811Y2 - - Google Patents

Description

[Detailed explanation of the invention] (1) Purpose of the invention [Field of industrial application] This invention is designed to solve the problem of transistor amplifiers, especially amplifiers in which four transistors are connected in a bridge configuration, due to short circuits on the sides of the transistors caused by overcurrent. This invention relates to a protection circuit that prevents destruction.

[Prior Art] Figure 2 is a conceptual diagram of a conventional bridge type amplifier using four transistors, in which the emitters and collectors of transistors Q ₁ , Q ₂ , Q ₃ , and Q ₄ are sequentially connected in a ring. Then, connect the (+) power supply to the connection point of the collectors of Q ₁ and Q ₃ , and the (-) power supply to the connection point of _the collectors of _{Q 2} and Q ₄ , respectively.
The A terminal of the motor serving as a load is connected to the connection point between the emitter and collector of Q3, and the connection point of the emitter and collector of _Q3 and _Q4 is connected to the B terminal of the motor.

Furthermore, the output of the input control circuit S is connected to the bases 1, 2, 3, and 4 of each transistor, and the energization state of each transistor is controlled by the control command input to the distribution circuit S, thereby controlling the rotation of the electric motor M. It is something.

Now, in order to rotate the electric motor M clockwise, the outputs are output from the control circuit S 1 and 4, and the transistors Q ₁ ,
When Q ₄ is turned on, the motor M is energized from A to B to rotate clockwise, and when rotating counterclockwise, the output is output from 2 and 3 of the distribution circuit S and the transistor
Turn on Q ₃ and Q ₂ and energize motor M from B to A.
Rotate counterclockwise. If no output is output from the control circuit S, all transistors are turned off and the motor M is stopped.

[Problem to be solved by the invention] However, in the circuit shown in Fig. 2, when the motor M is suddenly reversed from a certain direction, for example from clockwise to counterclockwise, transistors Q ₁ and Q ₄ are used.
is turned OFF and then Q ₃ and Q ₂ are turned ON.
Q ₂ may turn ON while Q ₁ is not completely OFF, causing the power supply to be shorted through Q ₁ and Q ₂ , causing a large current to flow through Q ₁ and Q ₂ and destroying the transistor. There was a problem.

In the conventional technology for solving the above-mentioned problems, for one or more of the transistors constituting the sides of the _bridge , for example, in FIG. A resistor is inserted in series, and the collector of the fifth transistor driven by the voltage of the resistor is Q ₂
and the base of the fifth transistor Q ₂
There is a device in which a first protection circuit is provided at the emitter of the fifth transistor by connecting the emitter of the fifth transistor to the (-) terminal of the power supply. However, in this protection circuit, if the voltage generated in the current detection resistor is higher than the voltage that activates the fifth transistor, the transistor operates, lowering the base current of Q ₂ and making it non-conducting to protect against a short circuit. However, the emitter current of Q ₂ decreases and the voltage drop of the current detection resistor, that is, Q ₅
When the base voltage of Q 2 decreases, the emitter current of Q ₂ increases again, which repeats the process, increasing the power loss of Q ₂ , and if the overcurrent continues for a long time, the transistors that make up the bridge will overheat. . Furthermore, in order to avoid this, if the current detection value is set to a low value, the loss of the current detection section becomes large, which is not practical.

(2) Structure of the invention [Means for solving the problem] In order to solve the above problems, the present invention, together with the first protection circuit implemented in the prior art,
The voltage of the current detection resistor is passed through the first photocoupler, the time delay circuit, and the second photocoupler,
A feature of the present invention is that it includes a second overcurrent protection circuit that controls the input control circuit of the amplifier.

[Operation] The overcurrent protection circuit of the present invention has a first protection circuit configured to directly control the current of the transistor forming the bridge side using the detected voltage, and a first protection circuit configured to directly control the current of the transistor forming the bridge side, and a first protection circuit configured to directly control the current of the transistor forming the bridge side, and a first protection circuit configured to directly control the current of the transistor forming the bridge side, and a first protection circuit configured to directly control the current of the transistor forming the side of the bridge. The first protection circuit protects against large overcurrents in a short period of time, and the second protection circuit protects against overcurrents that continue for a long time. Since the protection circuits can operate, the drawbacks of using only a single protection circuit can be compensated for, and a more reliable overcurrent protection circuit can be constructed.

[Embodiment of the invention] FIG. 1 shows an embodiment of an overcurrent protection circuit for a transistor amplifier to which the invention is applied.

In Figure 1, transistors Q ₁ and Q ₂ form the sides of the bridge, with the emitter of Q ₂ and the (-) side of the power supply.
A current detection resistor R ₁ is inserted in series between Q ₂ and
A fifth transistor is connected to the base of the power source and the (-) of the power source.
The collector and emitter of _Q5 are connected respectively, and the base of _Q5 is connected to the emitter of _Q2 to form a first protection circuit.

Next, the voltage of the resistor _R1 is inputted to the first photocoupler _P1 via the resistor _R2 , and the output of the photocoupler is inputted to the time delay circuit. Time delay circuit AND circuit U ₁
the above first input terminal through the resistor R ₃ to the first input terminal of
The output of the above photocoupler _P1 is inputted to the second input terminal of _U1 via a time delay circuit consisting of resistors _R5 , _R6 , diode D, and capacitor C. The output of the AND circuit U ₁ is inputted to one side of the AND circuit U ₃ via the latch circuit U ₂ , and the start command signal of the motor control circuit is inputted to the other input of _{U 3 .} A second protection circuit is formed by a circuit that inputs the output of the photocoupler P2 to the second photocoupler _P2 , and inputs the output of the photocoupler to the base of the bridge transistor _Q2 via the control circuit S of the bridge type amplifier.

In the first protection circuit, overcurrent flows through transistor Q ₂ and voltage across current detection resistor R ₁ flows through transistor Q _5.
When the base voltage exceeds VBE, transistor _Q5 turns on, lowering the base of _Q2 to (-) potential, turning _Q2 off, and protecting the transistor from overcurrents such as short circuits in the power supply.

Next, if the voltage of current detection resistor _R1 does not reach VBE and an overcurrent continues to flow exceeding the operating voltage of the photocoupler, the voltage of _R1 is input to the first photocoupler _P1 , and the photocoupler The output of is directly connected to the first terminal of the AND circuit _U1 of the time delay circuit via the resistor _R3 , and to the second terminal via the delay circuit formed by the resistor _R5 , the diode D, and the capacitor C. Since it is configured to input, the output of the AND circuit _U1 is output after the second voltage becomes the same voltage as the first terminal after a certain time delay.

When the AND circuit U ₁ output is output, it is held by the latch circuit U ₂ and input to the second terminal of the AND circuit U ₃ , and the start command of the motor control circuit input to the first terminal of U ₃ is cut off. The output of U ₃ becomes H level and the second
No current flows through the photocoupler P ₂ , and no output is output from the control circuit S of the bridge circuit, so the base current of Q ₂
It is turned OFF and operates as a protection circuit that prevents power supply short circuit caused by transistors Q ₁ and Q ₂ and also prevents destruction of the transistors.

As mentioned above, when the voltage of the resistor that detects the current of the transistor to be protected becomes higher than VBE, the first protection circuit directly activates the fifth transistor of the protection circuit.
Since the configuration drives Q ₅ , the response is fast, and transistors Q ₁ and Q ₂ that are likely to occur when the motor is reversed, etc.
This is effective in preventing short-circuits in the power supply due to simultaneous conduction. On the other hand, the second protection circuit is a current detection resistor.
When the voltage of R ₁ is below VBE and above the operating voltage of the photocoupler and continues for longer than the time constant of the time delay circuit, it is activated and turns off transistor Q ₂ to protect the transistor from overcurrent.

In addition, in the case of an overcurrent that returns to a steady current after a certain period of time, such as when starting a motor, the output from the AND circuit U ₁ can be delayed by setting the time constant of the above-mentioned time delay circuit to be delayed from the time when the current returns to a steady state. The feature is that the current returns to a steady state before the output occurs, and no output is output from photocoupler P ₁ , making it possible to prevent it from reacting to such overcurrents.A protection circuit is activated depending on the magnitude and duration of the overcurrent. It is possible to automatically select and operate the protection appropriate to the nature of the overcurrent.

[Effect of the invention] Since the protection circuit of the transistor amplifier according to the invention has the above-mentioned configuration, in the case of a large overcurrent where the voltage of the current detection resistor exceeds VBE, the first protection circuit directly connects the transistor _Q5 . make it work
While the base of Q ₂ is dropped to (-) potential and Q ₂ is turned OFF to protect the transistor with an extremely fast reaction, most of the overcurrent that occurs during actual operation is transient like the starting current and becomes steady in a short period of time. Since there are many types that return to current, by appropriately setting the time constant of the second protection circuit, it is possible to select and protect the second protection circuit. Further, since the second protection circuit has a photocoupler inserted between the main amplifier circuit and the overcurrent detection circuit, it has the effect of insulating both circuits.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a protection circuit for a transistor amplifier embodying the present invention, and FIG. 2 is a conceptual diagram of a conventional bridge type amplifier. Explanation of symbols, Q ₁ , Q ₂ , Q ₃ , Q ₄ , Q ₅ ... Transistor, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ ... Resistor, C ... Capacitor , D...diode, P ₁ ,
P ₂ ...Photocoupler, M...Electric motor, U ₁ ...AND
Circuit, U ₂ ... Latch circuit, U ₃ ... AND circuit, S
...Input control circuit.

Claims (1)

[Scope of utility model registration request] A bridge-type amplifier in which the emitters and collectors of each of four transistors are sequentially connected to form a bridge, a power supply is connected to one pair of diagonals of the bridge, and a load device is connected to the other pair of diagonals, and the above-mentioned An appropriate resistor R ₂ is inserted in series between the emitter of at least one transistor Q ₂ constituting one side of the bridge and the power supply, and a collector of a fifth transistor Q ₅ is inserted between the base of the transistor Q ₂ and the power supply. In the protection circuit for a transistor amplifier, the protection circuit includes a first protection circuit in which the emitter of the transistor Q ₂ is connected to the emitter of the transistor Q ₂ and the base of the fifth transistor Q ₅ is connected to the connection point between the resistor R 2 and the emitter of the transistor Q 2 . Connect the base of the transistor Q5 of No. ₅ to the input terminal of the first photocoupler _P1 , and
The output of P ₁ is passed through the time delay circuit to the latch circuit U ₂
The output of the latch circuit U ₂ and the output of the main control circuit are input to the second photocoupler P ₂ via the AND circuit U ₃ , and the output of the photocoupler P ₂ is used to control the input of the bridge amplifier. 1. A protection circuit for a transistor amplifier, characterized in that it is constituted by a second protection circuit forming a circuit that is input to the base of a transistor _Q2 constituting a side of the bridge via a circuit S.