JPS6042501Y2 - amplifier protection circuit - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an amplifier protection circuit for protecting the output transistor, power transformer, etc. of a transistor amplifier from excessive current and overheating.

A, which has traditionally been widely used as a transistor protection circuit.
SO detection circuit (Area of 5afety Opera
The cation detection circuit (safe operating area detection circuit) detects the impedance of the load connected to the output transistor to protect the output transistor from excessive current, and its configuration is as shown in Figure 1. It is based on a bridge circuit that includes on one side.

That is, as shown in the figure, this ASO detection circuit is a bridge circuit consisting of the emitter resistor R21 of the output transistor Q20 on the first side, the resistor R2□ on the second side, the resistor R23 + resistor R24 on the third side, and the load R1 on the fourth side. a load impedance detection transistor Q2□ whose base and emitter are respectively connected between point b and point C, and a relay drive circuit connected to the collector of this transistor Q21.
A protective relay L connected to the output end of the relay drive circuit
The contact 1 of the protective relay L is connected in series to the hot side of the load R3.

Since the adaptive load impedance of a typical transistor amplifier is usually designed to be a minimum of 4 ohms, the operating set point in such an ASO detection circuit is typically set to 4 ohms or less.

Specifically, the constants on each side of the bridge circuit section are set so as to become unbalanced when the load R7 is equal to or less than 4Ω.

If both ends of load R1 are short-circuited for some reason, or if a load with a lower impedance than the operating set point (for example, a 2Ω load) is accidentally connected, the output current will flow into point a in Figure 1. Since the potential at point b of the bridge circuit section is higher than that at point C, transistor Q21 is turned on, and therefore relay drive circuit K
The protective relay L is activated through the circuit, the relay contact 1 is opened, and the load R7 is released from the output circuit.

As a result, excessive current is prevented from flowing through the output transistor Q20.

In addition, D2. in FIG. is a reverse current blocking diode, C2
1 is a capacitor for applying a reverse bias to prevent malfunction.

The above is the configuration and operation of the conventional ASO detection circuit.

By the way, it is possible to connect an amplifier to an amplifier as long as it has an impedance within its applicable load impedance range, but the lower the impedance, the more current will flow, which will place a greater burden on the elements in the amplifier, especially on the output. Overheating of elements that generate heat, such as transistors and power transformers, becomes a problem.

Therefore, if a low impedance load is used,
When the output of such an amplifier increases, the current flowing through the heating element in the amplifier increases, and the temperature of the heating element rises, this temperature rise is detected and the operating set point of the ASO detection circuit is adjusted to that of the load. If the switching is made so that the impedance is higher than the impedance, the protection relay immediately operates to release the load from the output circuit, which is effective in preventing the heating element from overheating.

The present invention was devised in view of this point, and its purpose is to change the output transistor depending on the magnitude of the output of the transistor amplifier.
An object of the present invention is to provide an amplifier protection circuit that detects the heating state of a heating element such as a power transformer, and automatically switches the operating set point of an ASO detection circuit accordingly.
Its features include an ASO detection circuit, an automatic operation set point switching circuit added to the ASO detection circuit for automatically switching the operation set point of the detection circuit, and an operation setting point that detects the temperature of the heating element in the amplifier. It has a circuit configuration consisting of an automatic switching control circuit that controls the automatic point switching circuit.

Embodiments of the present invention will be described below with reference to FIGS. 2 and 3.

FIG. 2 is a circuit configuration diagram showing an embodiment of the amplifier protection circuit according to the present invention.

Q in the diagram.

, Ql is an output transistor, Q2 to Q5 are transistors, Ro%R13 is a resistor, D□ to D are diodes, C1
~C3 is a capacitor, ■ is a thermistor, T is a power transformer, K is a relay drive circuit, L is a protection relay, 1 is a relay contact, Sl, S2 are output selector switches, R1 is a load, 1"1! -B□, B2 indicates a power input terminal, and F indicates a rectifier connection terminal.

R0~R4! R1? C□, C2, Dll Q2.
To! The ASO detection circuit in this embodiment is constituted by the L and L parts.

Note that the capacitors C□ and C2 are for preventing malfunction due to phase rotation.

Next, diode D2. D3. and resistor R6 is an automatic operation set point switching circuit, which consists of resistor R1 on the first side, resistor R2 on the second side, resistor R1 on the third side, and resistor R1 on the first side of the ΔSO detection detection circuit. is added to the resistor R4 on the third side of the bridge circuit portion constituted by the load R1.

This operation set point automatic switching circuit is controlled by the resistor R5 by turning on and off the transistor Q in the automatic switching control circuit which will be described later.
This is so that the equivalent connection state of .

That is, as shown in FIG. 3, when the transistor Q5 is on, the circuit state as shown in FIG. 3 is obtained when the transistor a1 is off.

Therefore, the resistance value of the third side of the bridge circuit part changes to R4//R5 (parallel connection resistance value) when transistor Q5 is on, and only R1 when it is off. The equilibrium condition of the bridge circuit portion, ie, the operating set point of this ASO detection circuit, is switched in two stages.

In other words, in the case of Figure 3a, since the resistance value on the third side of the bridge circuit part is low, it is natural that a low impedance load will correspond to the load R1 on the fourth side, and the operating set point will be low. In the case of the same figure, conversely, since the resistance value on the third side is high, the load R1 on the fourth side has a high impedance, and the operating set point is high.

Next, R6 to R13, Tl'l, C3, D4 in Fig. 2?
The components Qs to q detect the magnitude of the output of this transistor amplifier based on the temperature of the heating element in the amplifier, and operate an automatic operation set point switching circuit added to the ASO detection circuit. This is an automatic switching control circuit that performs control.

The thermistor Th is attached to the heat sink of the output transistor Q□ or to the iron core of the power transformer T, etc., and constantly detects the temperature of the heating element, and the temperature is set at a certain output P of the amplifier.

A constant temperature t corresponding to t. Rlo so that transistor Q3 turns off when it exceeds Rlo.

The values of each resistance R□□ and R1° are determined.

Here, the functions of the output changeover switches S□ and S2 will be explained.

These two switches are interlocked, and by switching the voltage applied to the rectifier of the power supply section, the DC output voltage of the power supply section is changed (1), and the output of this transistor amplifier is switched in two stages.

In Fig. 2, the X contact side of switches S□ and S2 is the high output side, y
The contact side indicates the small output side.

In the case of the small output side, the output of this amplifier is the constant output P.

It is designed not to exceed.

Of the two switches, switch S2 is connected to the bias circuit section of transistor Q in the automatic switching control circuit, and provides a unique operation to the control circuit.

That is, when the switch S2 is switched to the X contact side (high output side), is the temperature of the heating element constant? ,
In the following, transistor Q3 is turned on due to the action of thermistor h, and resistors R7 and R are connected through diode D.
13, the emitter potential of the transistor Q is high by the voltage drop across the resistor R7, and the transistor Q is turned on.

Here, transistor Q is turned on only during the positive half wave due to the action of diode D, but this intermittent on-current is smoothed by the integration circuit of low frequency B8 and capacitor C3, and it flows into resistor R9. Therefore, transistor Q is continuously turned on.

Further, as described above, when the switch S2 is switched to the X contact side, the temperature of the heating element is constant.

When the voltage exceeds 0, the transistor Q3 is turned off, so the above-mentioned operation does not occur, and the transistor Q4 is turned off, and accordingly, the transistor q is also turned off.

On the other hand, when the switch S2 is switched to the y contact side (low output side), the resistor R7,
Since current flows through R6, the voltage drop across resistor R7 acts as a bias regardless of whether transistor Q3 is on or off.

is always on, and transistor Q5 also remains on through the smoothing circuit of resistor R8 and capacitor C3.

The ASO detection circuit described above, the automatic operation set point switching circuit,
As a summary of the operation of each circuit in the automatic switching control circuit, the minimum adaptive load impedance of the transistor amplifier is 4Ω, and the operating set point of the ASO detection circuit is 4Ω in the circuit state shown in Figure 3a, as shown in Figure 3S. Considering a system that is set to have a resistance of 8Ω in the circuit state, the following will explain the events that may occur in different cases.

i Output selector switch S1. When S2 is switched to the X contact side (high output side), this case can be further divided into the following two cases depending on the temperature of the heating element in the amplifier.

i-a When the temperature of the heating element is below a certain temperature, that is, the output of the amplifier is a constant output P.

This is the case when it is smaller.

In this case, the automatic switching control circuit operates as described above and the AS
Since the O detection circuit is in the circuit state shown in FIG. 3a, that is, the operating set point is 4Ω, the ASO detection circuit will not operate if the load R1 is 4Ω or more.

i - b When the temperature of the heating element is above a certain temperature, that is, the output of the amplifier is a constant output P.

This is the case if it is larger.

In this case, the automatic switching control circuit operates as described above and the AS
Since the ASO detection circuit is in the circuit state shown in FIG. 3, that is, the operating set point is 8Ω, the ASO detection circuit will not operate if the load R1 is 8Ω or more.

Therefore, if the load R+ has a low impedance of 4Ω, as shown in case 1)-a above, as long as the output of the amplifier is small, the ASO detection circuit will not work and the amplifier will operate normally. However, the output of the amplifier is a constant output P.

In this case, the operating set point shifts to 8Ω, so the ASO detection circuit operates and the rejected load is released from the output circuit, and the output transistor and power transformer are protected from overcurrent and overheating. That will happen.

ii Output selector switch S1. S2 is on the y contact side (when switched to the small output side) In this case, the temperature of the output transistor Q1 is
As mentioned above, the O detection circuit is in the circuit state shown in Figure 3a, and therefore the operating set point is always 4Ω, so if the load R1 is greater than this Ω, the ASO detection circuit is always in the state shown in Figure 3a. It will not work.

However, if a low impedance 4Ω load is used as the load R1, unlike the case 1)-b above, the output of the amplifier is a constant output P.

If it becomes larger, the operating set point will not switch to 8Ω, raising concerns that excessive current will flow through the elements in the amplifier.

However, as mentioned above, in this case, the output selector switch S1. Since S2 is switched to the Y contact, the output of the amplifier is a constant output P.

, so there is no risk of excessive current flowing through the elements in the amplifier.

In this case, the operating set point is fixed at 4Ω, which corresponds to low impedance, regardless of the temperature of the heating element for this reason.

In any of the above 1) a, i) b* 11), if both ends of load R1 are short-circuited, or if a load with an impedance lower than the operating set point is accidentally connected, the ASO detection circuit will be activated immediately. Of course, the load is disconnected from the output circuit to protect the output transistor, power transformer, etc. from excessive current.

As described below, the amplifier protection circuit according to the present invention is configured so that its operating set point is automatically switched depending on the heating state of the heating element in the amplifier.

In other words, while conventional ASO detection circuits protect the circuit by only detecting load impedance, this protection circuit protects the circuit by detecting the temperature of the heating element and switching the operating set point of the ASO detection circuit. It can be said that it has the added functionality of

Therefore, if this protection circuit is used in an amplifier, it can protect the various elements in the amplifier not only from excessive current but also from destruction due to overheating.

n The thermal safety of such an amplifier can be improved.

iii. The output of the amplifier can be automatically limited depending on the impedance of the load connected to the amplifier.

iv Since it has a simple configuration, it is easy to incorporate it into such an amplifier and the cost is low.

It has many other features.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram showing a conventional ASO detection circuit, and FIGS. 2 and 3 show an embodiment of a transistor protection circuit according to the present invention. , FIG. 3 is a circuit state diagram showing the circuit state of the ASO detection circuit in the protection circuit. Q2~Q5...Transistor, R engineering~R13・
...Resistance, D1-D, ...Diode,
C1 to C3...Capacitor, Th...
Thermistor, K...Relay drive circuit, L...
...Protection relay, 1...Relay contact, R1.
...Load, staggered ...Power input terminal.

Claims (1)

[Scope of utility model registration request] A transistor amplifier includes a safe operating area detection circuit that detects the impedance of a load to protect elements in the amplifier from excessive current, and a detection resistor arranged to maintain balance with the impedance of the load of the detection circuit. an automatic operation set point switching circuit that switches a resistance value; and an automatic operation set point switching circuit that detects a temperature rise of a heating element in the amplifier and increases the resistance value of a detection resistor of the detection circuit. An amplifier protection circuit configured to automatically switch an operation set point of the safe operating area detection circuit according to a heat generation state of the heating element by an automatic switching control circuit that performs switching control.