JPS5853769Y2 - Noise prevention circuit - Google Patents

Description

[Detailed description of the invention] The present invention relates to a noise prevention circuit for preventing unpleasant noise from being generated when power is turned on and off, and in particular, a noise prevention circuit that can achieve reliable operation with a simple circuit configuration. This is what we intend to provide.

In the past, various circuits have been proposed to prevent the noise generated when the power is turned on and off, but generally, the internal circuit of the amplifier is modified to prevent the noise itself from being generated from the amplifier. be.

However, when the internal circuit of an amplifier is modified, the circuit becomes complicated and sufficient balance must be maintained, which makes it suitable for integrated circuits but unsuitable for discrete circuits.

The present invention has been developed in view of the above points, and will be described below based on embodiments and with reference to the drawings.

In the figure, 1 is a final stage amplifier, 2 is a speaker serving as a load, 1 is a relay consisting of a relay contact 4 and a relay coil 5 inserted between the output end of the final stage amplifier 1 and the speaker 2, and 1 is a relay. In the drive circuit, the relay drive circuit includes:
A drive transistor circuit consisting of a pair of Darlington-connected first and second transistors T and 8, and a first capacitor 1 to which a power supply voltage is supplied via a power switch 9.
0, first and second voltage dividing resistors 11 and 12 that divide the voltage of the first capacitor 10, and the first and second resistors 1
A third resistor 14 and a second resistor connected to the voltage dividing points of 1 and 12
a time constant circuit 16 consisting of a capacitor 15; and a shorting transistor 1 for bypassing the signal applied to the base of the first transistor 7 of said relay drive transistor circuit to ground.
7, and a differentiating circuit 20 having a third capacitor 18 and a fourth resistor 19, and having one end connected to the first capacitor 10 and the other end connected to the shorting transistor 17.

Next, the operation will be explained.

When power switch 9 is turned on at time to, first capacitor 10 is immediately charged and point A reaches a predetermined voltage.

Therefore, the voltage dividing point B of the first and second voltage dividing resistors 11 and 12
The voltage also reaches the predetermined voltage immediately.

On the other hand, the base voltage (voltage at point C) of the first transistor T of the drive transistor circuit is determined by the time constant T of the time constant circuit 160.
Rise according to □.

Therefore, when approximately T0 has elapsed after the power switch 9 is turned on, the first and second transistors 7 of the drive transistor circuit
and 8 are brought into conduction, the relay coil 5 is excited, the relay contact 4 is closed, and the final stage amplifier 1 and the speaker 2 are connected.

During this time, the short-circuit transistor 17 remains non-conductive because it is transiently reverse-biased and has no effect on the drive transistor circuit.

The time constant T1 is set to be longer than the time it takes for the final stage amplifier 1 to reach a steady state after power is turned on.

By setting the time constant T1 in this way, noise caused by imbalance in the final stage amplifier 1 when the power is turned on can be completely prevented.

Next, the operation when the power is cut off will be explained.

If the power switch 9 is cut off at time t1, the first
The voltage of capacitor 10 gradually decreases according to the discharge time constant.

At that time, the falling edge at the start of discharge of the first capacitor 10 is differentiated by the differentiating circuit 20 and applied as a negative pulse to the base of the shorting transistor 17, so the shorting transistor 17 immediately becomes conductive and the driving transistor The base of the first transistor I of the circuit is grounded and becomes non-conductive, and the second transistor 8 accordingly becomes non-conductive, and the relay is temporarily cut off, so that no signal is transmitted from the final stage amplifier 1 to the speaker 2. .

The signal supplied from the differentiating circuit 20 to the shorting transistor 17 suddenly becomes negative when the power switch 9 is turned off, and then gradually returns to zero.

However, when the voltage suddenly becomes negative, the short circuit transistor 1
7 conducts, the second capacitor 1 of the time constant circuit 16
5 is rapidly discharged, and it takes a predetermined time T1 to charge the second capacitor 15.
After the capacitor 15 is discharged, it is charged again and the first
The transistor 7 does not become conductive again, and there is no problem even if the short-circuit transistor 17 becomes non-conductive after a predetermined period of time has elapsed.

The voltage changes at one end of the first capacitor 10 (point A), the voltage changes at one end of the second capacitor 15 (point C), and the base voltage changes of the shorting transistor 17 are shown in FIGS. 2, 3, and 4, respectively. As shown in the figure.

As described above, by using the noise prevention circuit according to the present invention, it is possible to completely eliminate the shock sound when the power is turned on and the unpleasant residual sound when the power is turned off.

Since control is performed using a relay and a time constant circuit, it has the advantage of achieving reliable operation, and is especially effective when used in discrete circuits that can be inexpensive and have a simple circuit configuration. It is a practical product with

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a characteristic diagram showing the time change of point A, Fig. 3 is a characteristic diagram showing the time change of point C, and Fig. 4 is a characteristic diagram showing the time change of point A. FIG. 3 is a characteristic diagram showing a temporal change in the base voltage of the short-circuit transistor. Explanation of main drawing numbers, 3... Relay, 6...
・Relay drive circuit, 10, 15.18... Capacitor, 16... Time constant circuit, 1T...
・Short-circuit transistor, 20... Differential circuit.

Claims (1)

[Scope of utility model registration request] It consists of a relay having contacts connected between an amplifier and a load, a drive transistor that drives the relay, a time constant circuit that controls the drive transistor, and a first capacitor that serves as a power source for the time constant circuit. , a differential circuit inserted between a short-circuit transistor that controls the drive transistor, a base of the short-circuit transistor, and the first capacitor, in a circuit that delays connection of a relay contact when the power is turned on to prevent noise when the power is turned on; Noise during power-on and power-off, characterized in that a circuit is added that immediately brings the short-circuit transistor into conduction state when the power is cut off, disables the drive transistor and the relay, and prevents noise when the power is cut off. prevention circuit.