JPS5815307A - Overload detecting circuit for amplifier - Google Patents

Abstract

PURPOSE:To surely detect overload, by avoiding the fluctuation of a collector potential of a detecting transistor (TR) by an output signal level. CONSTITUTION:A resistor R4 is connected betweeb an emitter of a TRQ5 and an emitter of a power TRQ1 of the output stage of an amplifier and a resistor R5 is connected between the emitter of the TRQ5 and a power supply Vcc. The base of the TRQ5 is connected to an input terminal of a load RL via a diode D3 to compensate an emitter-to-base voltage of the TRQ5 and grounded via a resistor R6. The condition of detection of an overload is RE/(R4.RL)>1/R5.

Description

[Detailed description of the invention] The present invention relates to an amplifier overload detection circuit.

When an amplifier that drives a load such as a speaker becomes overloaded due to an unexpected accident or the like, an excessive current flows through the power transistor at the output stage, resulting in destruction of the power transistor, the speaker, etc. In order to avoid this, an overload detection circuit is provided to detect an overload condition to protect the power transistor and the like.

FIG. 1 shows a conventional example of an overload detection circuit connected to the output stage of an amplifier. Supplied to load RL. Overload detection circuit 11 that detects the overload state of this load R
consists of a voltage divider circuit including resistors R1+R2 connected in series between the emitter of transistor Q1 and ground to form a bridge with resistor RB and load RL, and a base connected to the voltage dividing point of this voltage divider circuit. The transistor Q2 whose emitter is connected to the connection point (B) between the i-resistance RE and the load RL becomes disconnected, and when the potential difference between the points (A) and (B) reaches a predetermined level, the transistor Q2 is activated. The structure is as follows.

Note that the diode D1 inserted in the voltage dividing circuit is used to compensate for the voltage drop between the base and emitter of the transistor Q2. Moreover, the resistors R1 and R2 are the load RL
is set so that the voltage at point A1 is the potential vA and the potential at point (B) is at the rated load.

The collector output of transistor Q2 is supplied to drive circuit 13 that drives relay 12 via diode D2. The drive circuit 13 is composed of a transistor Q31Q4%, a Zener diode ZD, a resistor R3, and a capacitor c1, and turns on the relay 12 at rated load, and turns off the relay 12 in response to the collector output of transistor Q2 at overload. Make it.

A make contact 12α of the relay 12 is connected in series to a load RL, for example.

In such a configuration, in the event of overload, the resistance R1+"2
Since the bridge circuit consisting of sRB and load RL is out of balance, transistor Q2 is turned on and an overload condition is detected, and drive circuit 13 turns off relay 12 in response to the collector output of transistor Q2. This opens the contact 12G of the relay 12 to cut off the excessive current, thereby protecting the power output transistor Q1, the load RL, etc.

However, in the overload detection circuit 11, since the collector potential of the transistor Q2 changes at the same time depending on the output signal level (if a signal such as a pulse wave and higher than the Zener voltage vz of the Zener diode ZD is applied to the 81 point, The drawback is that overload cannot be detected.

An object of the present invention is to provide an amplifier overload detection circuit that can reliably detect an overload condition without being affected by the output signal level.

In the overload detection circuit according to the present invention, a voltage corresponding to the input terminal voltage of a load connected to the emitter of the output transistor via a resistor is applied to the base of the detection transistor, and the emitters of the detection transistor and the output transistor are Resistors are connected between the power supply and the emitter of the detection transistor, respectively, and an overload state is detected based on the collector output of the detection transistor.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a basic circuit diagram of an overload detection circuit according to the present invention connected to the output stage of an amplifier, in which parts equivalent to those in FIG. 1 are designated by the same reference numerals.

The overload detection circuit 21 according to the present invention includes a detection transistor Q5, a first resistor TL4 connected between the emitter of the transistor Q5 and the emitter of the power structure quadrant Ql of the output stage of the amplifier. , transistor Q5
and a second resistor R5 connected between the emitter of the transistor Q5 and the power supply +VCC, and the base of the transistor Q5 is connected to the input terminal of the load RL via the diode D3 and grounded via the resistor R6. has been done. Diode D3
is only for compensating the voltage drop between the emitter and base of transistor Q5, and a resistor may be inserted in place of the diode, or the base of transistor Q5 may be directly connected to the input terminal of load RL. It's okay.

In such a configuration, the currents flowing through resistors RB+R4 and R5 are 4B, respectively, and the output voltages of the jl and Z2e amplifiers are τ. When the collector current i6 of the transistor Q5 is expressed as . Then, it is possible to detect whether the RL is overloaded.

FIG. 3 is a circuit diagram showing a first embodiment using the overload initial detection circuit according to the present invention. has been added to the configuration shown in Figure 2. The drive circuit 13α consists of a resistor R7 connected between the collector of the transistor Q5 in the overload detection circuit 21 and ground, a capacitor C2 and a resistor R8 that differentiate the voltage across the resistor R7 and apply it to the base of the transistor Ql. The configuration is the same as the drive circuit 13 shown in FIG. 1 except that a differentiating circuit is newly provided.

FIG. 4 is a circuit diagram showing a second embodiment using the overload detection circuit according to the present invention, in which parts equivalent to those in FIG. 3 are designated by the same reference numerals. In this embodiment, the overload detection circuit 21 is connected to the negative output stage of the push-pull amplifier, and the transistor Q≠conductivity type transistor Q-: hx detection as shown in FIG. It is used as a tracoresta. Further, the drive circuit 136 basically has the same circuit configuration as the drive circuit 13α shown in FIG.

In the first and second embodiments mentioned above, the overload detection circuit 2
If the resistance values of R4, R, and R5 are set so that R, = -, the detection transistor Q5 will be activated at the rated load.

Therefore, the drive circuits 13α and 13b detect 6c<Vaa and turn off the relay 12 which is on at the rated load, thereby opening the relay contact 12a and cutting off the excessive current. .

As described above, according to the present invention, since the collector potential of the detection transistor does not vary depending on the output signal level, an overload state can be detected reliably, and the interface with the relay drive circuit is easy.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional example of an overload detection circuit, Fig. 2 is a basic circuit diagram of an overload detection circuit according to the present invention, and Fig. 3 is a first implementation using the overload detection circuit according to the present invention. FIG. 4 is a circuit diagram showing a second embodiment using an overload detection circuit according to the present invention. Explanation of symbols of main parts 11, 21... Overload detection circuit 12... Relay 13, 13α, 13h... Drive circuit RL-.
・Applicant: Pioneer Co., Ltd. Agent: Patent Attorney: Motohiko Fujimura, Figure 2, Figure L3

Claims (2)

[Claims] (1) An overload detection circuit for an amplifier that drives a predetermined load connected to the emitter of an output transistor via a resistor, in which a voltage corresponding to the input terminal voltage of the load is applied to the base. A well-known detection transistor,
a first resistor connected between the emitter of the detection transistor and the emitter of the output transistor; and a second resistor connected between the emitter of the detection transistor and a power supply; An overload detection circuit characterized in that an overload condition is detected based on a collector output. (2) The overload detection circuit according to claim 1, wherein the base of the detection transistor is connected to the input terminal of the load via a diode.