JPS62266904A - Overcurrent protection circuit - Google Patents

Abstract

PURPOSE:To protect an amplifier circuit and a speaker by setting the 1st or the 2nd switching transistor (TR) to be in the OFF state thereby turning off a relay circuit and opening a signal line even if an overcurrent takes place in either TR of a couple of output TRs. CONSTITUTION:If an overcurrent takes place in either of a couple of the output TRs Q1, Q2, the overcurrent is detected by a resistor R1 or R2, and a control TR Q3 or Q4 is turned on. Since capacitors C1, C2 discharge the electric charge, even when the TR Q3 or Q4 is turned off, until the capacitor C1 or C2 is charged up, the 1st switching TR Q5 or Q6 is not immediately restored to the ON state. That is, even when the overcurrent is changed periodically in response to the signal, a relay 4 is continuously in the non-drive state, the signal line is opened and the amplifier circuit and the speaker 1 are protected in an excellent way.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Application The present invention relates to a protection circuit for amplifier circuits and speakers, and particularly to a protection circuit against overcurrent.

In conventional power amplifier circuits, the signal line is opened and closed in order to protect the circuit elements inside the amplifier circuit and the speaker from overcurrent that occurs when a speaker connected to the output side is short-circuited. It is well known in the art to utilize relay circuits that do this. A reliable method for detecting overcurrent is to detect the potential difference generated across a resistor connected to the emitter of the output stage transistor. Such a technique is disclosed, for example, in FIG. 1 of Japanese Unexamined Patent Publication No. 59-55609. However, such conventional technology detects an overcurrent in one of a pair of output transistors that are connected in a push-pull manner.

Problems to be Solved by the Invention: The present invention immediately detects the current flowing through either of the pair of output transistors, opens the signal line, and protects the amplifier circuit and speaker. This is what I am trying to do.

21 Means for solving the problem A pair of output transistors are connected in a push-pull manner and are normally in an on state, but are set to an off state when an overcurrent in one of the pair of output transistors is detected. A first switching transistor, a second switching transistor which is also normally in an on state and is set to an off state when an overcurrent in the other of the pair of output transistors is detected, and a relay circuit that opens and closes a signal line. The first switching transistor, the relay circuit, and the second switching transistor are connected in series.

E1 Effect: Even if an overcurrent occurs in either of the pair of output transistors, the first or second switching transistor connected in series is set to the OFF state in response to the overcurrent, so the relay circuit is immediately turned off. It can be set to the OFF state, and the No. 2 line can be opened to protect the amplifier circuit and speaker.

The embodiment transistors (Ql, Q2) are a pair of output transistors connected in a push-pull manner, and their output midpoint (A> and speaker (1) are connected to R8 via a relay contact.

A pair of output transistors (Ql.

Q2) is applied with positive and negative power supplies (" B ! l - B l) from the vi current circuit (3) connected to the secondary side of the power transformer (2). The emitter resistor ( R1r R2) are overcurrent detection resistors, and the voltage across these resistors increases during overcurrent.

The control transistor (Qff, Q4) is normally biased to be in the off state, and the A
When the tFE between -B or between A and C increases, it becomes an on state. First switching transistor (Qs), relay 4) and second switching transistor (Q8)
is a series connection, 11! The control transistor (Ql
, Q4) is in the off state, the resistance (R
3) Transistors (Qs, Q
s) are each biased to the on state. Transistor (Q 6 ), Relay (4) and Transistor (Q
To the series circuit e), plus and minus i[[(+B2.-B2) are applied from the vi current circuit (5) connected to the secondary side of the power transformer (2). This t
Since the midpoint of fl is connected to the output midpoint (A) of the output transistors (Ql, Q2), the voltage between DE and E (voltage for driving the relay) swings according to the output signal voltage. The voltage will be as expected (see Figure 2).

Therefore, in the normal state, the first. The second switching transistors (Qs, Qg) are both in the on state, the relay (4) is in the driven state and the relay contacts are in the open state.

Now, when an overcurrent is detected by the resistor (R1) [or (R2)], the control transistor (Ql) [or (Q 4 )] turns on, and in response, the first
Switching transistor (Q s ) [or (Q
s)] is turned off, the relay (4) is placed in a non-driven state, and the relay contacts are opened. That is, the speaker (
11: It is possible to prevent damage to the output transistors (Ql, Q2) and the speaker (1) due to overcurrent caused by the short circuit in 1).

Here, the function of the capacitor (Cs r C2) will be explained. Since the overcurrent also changes periodically according to No. 9, the capacitor (Ci +
C2), the control transistors (Qx, Q4
) are switched alternately and at intervals, and in response, the first and second switching transistors (Qs, Qe) are also switched alternately and at intervals. 4> will cause chattering. However, the capacitors (C1, C:
) is inserted, the control transistor (Ql)
When [or C (Q, a)] turns on, the charge in the capacitor (Cs) r or (C2) is completely discharged, so the control transistor (Q 3 ) [or (Q4)] turns off. Even if the capacitor (C1
) [or (C2) 1 is full! Immediately switch to the first
Switching transistor (Q s ) [or (Q
s )] never returns to the on state, i.e. even if the over-IE current changes periodically in response to the signal, the relay (4
) remains undriven continuously.

The Zener diode (DI, D2) is used to sharpen the switching operation of the switching transistor (Qs, Qs), and the diode (D3) is used to make the switching operation of the switching transistor (Qs, Qs) steeper.
This is for absorbing back electromotive force (4).

According to the present invention described above, even when an overcurrent flows to either of a pair of output transistors,
In response to this, the relay can be immediately set to a non-driven state and the signal line can be opened, so that the amplifier circuit and the speaker can be well protected.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an overcurrent protection circuit according to the present invention, and FIG. 2 is an operation waveform diagram. (1) is a speaker, (4) is a relay, (Ql, Ql) is a pair of output transistors, (Qs, Qs) are first and second
switching transistor.

Claims (2)

[Claims] (1) A pair of push-pull connected output transistors, and a first switching transistor that is normally in an on state and is set to an off state when an overcurrent in one of the pair of output transistors is detected; a second switching transistor that is normally in an on state and is set to an off state when an overcurrent in the other transistor of the pair of output transistors is detected; and a relay circuit that opens and closes a signal line; An overcurrent protection circuit characterized in that a switching transistor, a relay circuit, and a second switching transistor are connected in series. (2) Claim 1 characterized in that the midpoint of the power supply circuit for the series circuit of the first switching transistor, the relay circuit, and the second switching transistor is connected to the output midpoint of the pair of output transistors. Overcurrent protection circuit as described.