JPH0344498B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching device that switches the power applied to a load circuit from a power source with a large internal impedance at a constant cycle, and even when the switching cycle is relatively long, fluctuations in the power supply voltage caused by the switching can be suppressed. The object of the present invention is to provide a device in which the power supply voltage supplied from the power supply circuit to other circuits does not fluctuate.

Same as color television broadcasting method.
The NTSC system also has color subcarrier frequencies of 3.58 MHz and 4.43 MHz, and even within the same area, broadcasts using different frequency systems may coexist depending on the channel. In such a case,
When a color broadcast signal of any frequency is not being received, for example, when receiving a black and white broadcast, the reception state of 3.58MHz and 4.43M in a color television receiver.
Hz reception status can be switched alternately at a fixed period, and when a broadcast signal of one of the frequencies is received, it can be detected and the status can be switched to continuously receive that frequency. It is being done. However, when switching in this way, the current value from the power supply circuit fluctuates each time it is switched, but the internal impedance of the power supply changes as when rectifying and smoothing horizontal flyback pulses is used as a power supply circuit. If it is relatively large, the power supply voltage will also fluctuate due to the current fluctuation, which is disadvantageous in that it will adversely affect other circuits that are supplied with operating power from the same power supply.

As a means to prevent such fluctuations in the power supply voltage, it is possible to connect a capacitor with a large capacitance value or to connect a constant voltage diode, but the former method cannot be stabilized if the switching cycle is long, and The latter method had disadvantages such as limitations on the voltage that could be used, and was not an effective means.

SUMMARY OF THE INVENTION An object of the present invention is to provide a switching device capable of eliminating such conventional disadvantages, and an embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, 1 is a power supply circuit with a large internal impedance as described above, 2 is a load circuit in which the power from the power supply circuit 1 is switched at a constant cycle,
Reference numeral 3 denotes another circuit to which power is supplied from the power supply circuit 1. Power is supplied from the power supply circuit 1 to the load circuit 2 via a resistor 4, and a first switching transistor 5 is connected between the resistor 4 and the ground, and this transistor 5 is cut off. Power is supplied to the load circuit 2 only during the period,
When transistor 5 becomes conductive, power is not supplied to it. Note that the other end of the load circuit 2 is coupled to a low-voltage stabilized power supply circuit 6 here.

On the other hand, 7 is an unstable multivibrator for switching the power supply to the load circuit 2 at regular intervals, and is composed of transistors 8 and 9, resistors 10 to 13, and capacitors 14 and 15. Then, by switching between conduction and cutoff at the constant period, the power is supplied or not supplied to the load circuit 2 at the constant period.

Furthermore, since the voltage from the power supply circuit 1 fluctuates due to switching of the power supply only in the above portion, a compensation circuit 17 is provided. In the compensation circuit 17, a series circuit of a resistor 18 and a second switching transistor 19 is provided between the power supply circuit 1 and the ground, and the collector output of the transistor 5 is applied to the base of the resistor 18 through a resistor 20. Therefore, this transistor 19 is the transistor 5.
It is designed so that it can be switched in the opposite phase.
The resistance value of the resistor 18 is set to such a value that a current equal to the difference between the current flowing through the resistor 4 when the transistor 5 is turned on and when the transistor 5 is turned off flows.

According to this configuration, the total current value flowing from the power supply circuit 1 to the resistors 4 and 18 is always the same regardless of whether the transistor 5 is turned on or off, and regardless of whether the power supply to the load circuit 2 is switched. The voltage of the power supply from the power supply circuit 1 can be made constant at all times. Therefore, the influence on other circuits 3 due to this switching can be minimized.

FIG. 2 shows a specific example of a circuit in which a switching circuit based on this principle is applied to a receiving frequency switching circuit in a color television receiver as described above. Note that the same parts as in FIG. 1 will be described with the same reference numerals.

Here, a diode switch circuit for switching the color subtone frequency of an NTSC color television signal is used as the load circuit 2, and when the first switching transistor 5 is cut off, the power supply circuit 1 24V through resistor 4 from
voltage is applied, current is passed through the path of resistor 21 → coil 22 → diode 23 → resistor 24, and the path of diode 25 → resistor 26 → resistor 27 → coil 28, and diodes 23 and 25 are made conductive, resulting in 4.43 Switched to MHz reception status.
On the other hand, when transistor 5 is conducting,
Resistor 29 → Diode 30 → Coil 22 → Resistor 2
1 path, and coil 28 → resistor 27 → resistor 3
Current flows through the path from 1 to diode 32, making diodes 30 and 32 conductive, resulting in a frequency of 3.58MHz.
is switched to the receiving state.

As described above, according to the present invention, when supplying power to a load circuit by periodically switching the first switching element using the output of a multivibrator from a power source with a large internal impedance, switching is performed in the opposite phase to the switching. Since a second switching element is provided to prevent changes in the total current from the power supply circuit, fluctuations in the power supply voltage can be prevented regardless of the length of the switching cycle or the level of the power supply voltage used. This makes it possible to realize a useful device that does not have any adverse effects when power is supplied to other circuits from the same power supply circuit.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a switching device according to an embodiment of the present invention, and FIG. 2 is a specific circuit diagram of a color demodulation circuit to which the same device is applied. DESCRIPTION OF SYMBOLS 1... Power supply circuit, 2... Load circuit, 3... Other circuit, 4... Resistor, 5... First switching transistor, 7... Unstable multivibrator,
19...Second switching transistor.

Claims (1)

[Claims] 1. A multivibrator that reverses at a constant cycle,
A first switching element that is switched at a constant cycle by the output of the multivibrator to switch the power supply to the load circuit from a power source with a large internal impedance; a second switching element that is switched periodically and in an opposite phase to the first switching element to switch the power supply from the power supply to the pseudo load; A switching device characterized in that the value of the total power supply current from the power supply is kept approximately constant.