JP2584350B2 - DC voltage detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC voltage detecting circuit for generating a detection output when a positive DC voltage is higher than a predetermined level and when a negative DC voltage is lower than a predetermined level. In particular, the present invention relates to a DC voltage detection circuit suitable for use in a protection circuit that detects a DC voltage generated at an output point of an OCL amplifier and thereby protects a load connected to the output point from being destroyed.

[0002]

2. Description of the Related Art At present, the mainstream of hi-fi acoustic amplifiers, in an OCL amplifier to which an output capacitor is not connected, two positive and negative power supplies are used, and a DC voltage at an output point is set to a ground voltage. The output point and the speaker are directly connected in a DC manner. Therefore, if the internal circuit of the OCL amplifier fails, for example, if the output transistor is destroyed and a DC voltage is generated at the output point, the speaker will be damaged. Therefore, the OCL amplifier is provided with a protection circuit to protect the speaker. ing.

Such a protection circuit detects a DC voltage generated at an output point of the OCL amplifier, operates a relay or the like in accordance with the detected output, and disconnects the speaker from the output point. Therefore, the protection circuit is provided with a DC voltage detection circuit as shown in, for example, Japanese Utility Model Application Laid-Open No. 63-129315.

By the way, in the DC voltage detecting circuit as described above, the positive and negative DC voltages are detected so that the positive and negative DC voltages are generated at the output point of the OCL amplifier correctly. It is an important requirement to match the detection sensitivity of. Therefore, the above-mentioned actual opening 63-1
In the DC voltage detection circuit shown in No. 29315,
The detection sensitivities of the positive and negative DC voltages are set independently so that the respective detection sensitivities are matched.

[0005]

However, the DC voltage detection circuit described above requires the use of a Schottky diode and an increase in the number of elements to be used, thereby reducing the cost, the conduction voltage of the Schottky diode, and the resistance of each resistor. There was a problem in design due to the need to consider values.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing points, and has a grounded-type first transistor in which a detected DC voltage is applied to a base, and a detected DC voltage which is applied to an emitter. A grounded-base second transistor to be applied, and a grounded-collector third transistor having a base connected to the collector of the second transistor and an emitter connected to the collector of the first transistor, A detection output is obtained from a connection point between the collector of the first transistor and the emitter of the third transistor.

[0007]

In the DC voltage detecting circuit of one power supply, a transistor used for detecting a DC voltage has a different grounding method for positive and negative polarity. Focusing on the fact that the difference in the current amplification factor due to the difference in the grounding scheme is caused by the difference in detection sensitivity between the positive and negative DC voltages, the present invention uses a DC emitter of one polarity using a transistor with a common emitter. The DC voltage of the other polarity is detected by using a common-base transistor and a common-collector transistor whose base is connected to the collector of the common-base transistor. Thus, the combined current amplification factor of the two transistors for detecting the DC voltage of the other polarity is matched with the current amplification factor of the common emitter type transistor.

[0008]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the figure, reference numeral 1 denotes an input terminal to which an output voltage generated at an output point of an OCL amplifier (not shown) is applied, 2 and 3 are connected in series with each other, and are connected between the input terminal 1 and ground. 1 and 2 resistors, 4 is a bypass capacitor, 5 is a first transistor of a common-emitter type to which a DC voltage generated at a voltage dividing point A of the first and second resistors 2 and 3 is applied to a base, 6 is The grounded-type second transistor 7 to which the DC voltage generated at the voltage dividing point A is applied to the emitter is a grounded-collector type third transistor whose base is connected to the collector of the second transistor 6.

Reference numeral 8 denotes a relay contact for separating the output point of the OCL amplifier from a speaker (not shown), 9 denotes a relay coil for driving the relay contact 8, and 10 denotes a relay coil 9.
A driving transistor 11 for supplying a driving current to the driving transistor 11 has an input terminal 12 connected to the collector of the first transistor 5 and the emitter of the third transistor 7, and outputs a control signal for controlling the driving transistor 10. A microcomputer having an output terminal 13 for detecting the state of the output point of the OCL amplifier in accordance with the signal level applied to the input terminal 12 and providing the input terminal 12 with a power supply voltage level (+ V _DD ) of "H". Signal is applied, an "H" signal is output from the output terminal 13 while the ground voltage level (0
V) when the "L" signal is applied, the output terminal 13
Outputs an “L” signal.

In FIG. 1, an output voltage generated at an output point of the OCL amplifier is applied to an input terminal 1 and divided by first and second resistors 2 and 3 to set a voltage level to be detected. At the same time, the AC component is removed by the bypass capacitor 4. Therefore, a DC voltage in which the DC component of the output voltage at the output point is set by the first and second resistors 2 and 3 is generated at the voltage dividing point A. The DC voltage generated at the voltage dividing point A is applied to the base of the first transistor 5 and to the emitter of the second transistor 6.

The DC voltage generated at the voltage dividing point A of the first transistor 5 is higher than the ground voltage by the rising voltage V _BE1 between the base and the emitter of the first transistor 5.
It turns on when it rises above, while the second transistor 6
Is turned on when the DC voltage generated at the voltage dividing point A falls below the ground voltage by the rising voltage V _BE2 between the base and the emitter of the second transistor 6. And the second
When the transistor 6 turns on, the third transistor 7 turns on. Therefore, when the DC voltage of the voltage dividing point A becomes V _{BE 1} or more, and the DC voltage of the voltage dividing point A is -V _BE2
When the voltage falls below, an “L” signal is applied to the input terminal 12 of the microcomputer 11, and when the DC voltage at the voltage dividing point A is higher than −V _{BE2 and} lower than V _BE1 , the input terminal 12 is connected to the input terminal 12. Is applied with an "H" signal. The first
As the second transistors 5 and 6, transistors whose rising voltage between the base and the emitter is about 0.6 V are usually used.

When an "L" signal is applied to the input terminal 12, an "L" signal is output from the output terminal 13 of the microcomputer 11, and in this case, the drive transistor 10 is turned off. Therefore, no drive current flows through the relay coil 9 and the relay contact 8 is opened.

Therefore, the output point of the OCL amplifier and the speaker are separated. On the other hand, when the "H" signal is applied to the input terminal 12, the "H" signal is output from the output terminal 13, and in this case, the drive transistor 10 is turned on. Therefore, a drive current flows through the relay coil 9,
The relay contact 8 is closed.

Therefore, the output point of the OCL amplifier and the speaker are connected. Now, the OCL amplifier is in a normal state, and the DC voltage at the output point is equal to the ground voltage (0 V).
And At this time, the DC voltage V _A generated in the voltage dividing point A is also ground voltage, since the _{-V BE2 <V A <V BE1} , the first and second transistors 5 and 6 are both turned off. Therefore, an “H” signal is applied to the input terminal 12 of the microcomputer 11, and an “H” signal is output from the output terminal 13.

Therefore, since the relay contact 8 is in the closed state, a signal generated at the output point of the OCL amplifier is supplied to the speaker.

[0016] On the other hand, OCL amplifier fails, positive or negative DC voltage becomes as generated at the output point, the DC voltage V _A generated in the voltage dividing point A is higher than the ground voltage of the first transistor 5 base-emitter The first or second transistor 5 or 6 is turned on when the voltage rises by more than the rising voltage V _BE1 between them or falls by more than the rising voltage V _BE2 between the base and the emitter of the second transistor 6 from the ground voltage. Therefore, the input terminal 1 of the microcomputer 11
2 is applied with an “L” signal, and the output terminal 13 outputs an “L” signal.
A signal is output.

Accordingly, since the relay contact 8 is opened, no DC current flows through the speaker, and the speaker is protected from damage.

In the circuit of FIG. 1, since the positive and negative DC voltages are divided by the common first and second resistors 2 and 3, the difference between the positive and negative DC voltage detection sensitivities is as follows. The difference between the operating sensitivity of the first transistor 5 and the combined operating sensitivity of the second and third transistors 6 and 7 has an effect. Here, the operational sensitivity of the transistor is mainly determined by the current amplification factor.

By the way, the current gain is determined by the transistor grounding method, and based on the DC current gain h _FE at the common emitter, the DC current gain h _FB at the common base is

[0020]

[Equation 1] h _FB = -h _FE / (1 + h _FE )

The current amplification factor h _FC of the collector-grounded direct current is

[0022]

[Equation 2] h _FC =-(1 + h _FE )

## EQU2 ## Here, the first transistor 5 for detecting a positive DC voltage is grounded to the emitter, and the DC current amplification factor is h _{FE as} described above. On the other hand, the second and third transistors 6 and 7 for detecting a negative DC voltage are connected to a common base and a common collector, respectively. The current amplification rate obtained by combining the second and third transistors 6 and 7 can be regarded as the product of the current amplification rates h _FB and h _FC of the second and third transistors 6 and 7 from the connection relationship. Therefore, the second and third transistors 6 and 7 synthesized current amplification factor is, h _FB × h _FC, i.e., the use of those h _FE is equal characteristics as the second and third transistors 6 and 7 , The combined current amplification factor is h _FE .

Therefore, if the characteristics of the respective transistors are made uniform, the current amplification factor of the first transistor 5 and the combined current amplification factor of the second and third transistors 6 and 7 can be made equal, and the current amplification factor can be increased. , The difference between the positive and negative DC voltage detection sensitivities can be eliminated.

Even if the characteristics of the respective transistors are not uniform, a positive DC voltage can be detected by a grounded base transistor having a current amplification factor h _FB ≒ 1, which is used for detecting a negative DC voltage. since the transistor and the current amplification factor of the emitter-grounded used (h _F parameter) connects the transistor near the collector grounded, the difference in detection sensitivity of the DC voltage at the positive and negative can be greatly reduced.

Therefore, in the circuit of FIG. 1, the detection sensitivities of the positive and negative DC voltages are substantially equalized.

Incidentally, the potential difference between the collector and the emitter of the first transistor 5 when the first transistor 5 operates is + V _DD , and the second transistor when the second and third transistors 6 and 7 operate. 6 and the potential difference between the emitter of the third transistor 7 is + V _DD-
(−V _BE2 ). Therefore, the first transistor 5
Is equal to the combined current gain of the second and third transistors 6 and 7, the negative DC voltage detection sensitivity is slightly better than the positive DC voltage detection sensitivity. However, in order to compensate for this sensitivity difference, a resistor 14 is connected between the base and the emitter of the third transistor 7 as shown by the broken line in FIG. 1 to reduce the base current flowing through the third transistor 7 slightly. Can be solved.

[0028]

As described above, according to the present invention, a common base transistor used for detecting a negative DC voltage is combined with a common collector transistor, and the combined current of those transistors is combined. Since the amplification factor is adjusted to the current amplification factor of the transistor with the grounded emitter used to detect the positive DC voltage, the DC voltage detection with the same sensitivity for positive and negative DC voltage with a simple circuit configuration Circuit can be provided.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

[Explanation of symbols]

 2,3 resistor 5 first transistor 6 second transistor 7 third transistor

Claims (1)

(57) [Claims] 1. A DC voltage detecting circuit for detecting that a positive DC voltage is higher than a predetermined level and a negative DC voltage is lower than a predetermined level, wherein the detected DC voltage is applied to a base. Grounded-type first transistor having the same conductivity type as the first transistor
Together with a second transistor grounded base type DC voltage detection is applied to the emitter, the second transistor
And the conductive type are opposite in polarity, and the base is
A third transistor of a common-collector type connected to the collector of the transistor, wherein the collector of the first transistor is connected to the emitter of the third transistor ;
The collector of the first transistor and the third transistor
DC voltage detection circuit that obtains the detection output from the connection point with the emitter .