JPH01315257A - Positive/negative voltage output switching circuit - Google Patents

Abstract

PURPOSE:To reduce the number of components, and to decrease its cost by turning ON and OFF a single transistor at the times of outputting of positive and negative voltages, and selectively switching to output it. CONSTITUTION:A positive/negative voltage output switching circuit is connected to the output unit of a switching power source for generating positive/negative voltages stabilized, for example, by a Zener diode, and has a photocoupler PC, transistors(Tr)Q1-Q3, bias resistors R1-R5, second resistor voltage dividers R6-VR1, first resistance voltage dividers VR2-R9, first - second diodes D1-D2, etc. A third diode D3 is connected from the collector of the TrQ1 to the anode of the first diode D1. Thus, when the TrsQ1-Q3 are turned OFF, the output voltage of the first resistance voltage divider is generated at the output terminal. When they are turned ON, the diode D1 is interrupted through the diode D3, and the output of the second resistance voltage divider is generated.

Description

Detailed Description of the Invention (a) Industrial Application Field This invention is used in power supply circuits that generate relatively high voltages, and is a positive/negative voltage output switch that selectively switches between positive and negative voltages. Regarding circuits.

(b) Prior Art Conventionally, for example, electrophotographic copying machines have incorporated a high voltage power supply circuit that selectively supplies a relatively high positive or negative voltage to a copying process section. Such a high-voltage power supply circuit includes a high-voltage power supply circuit that generates a positive voltage and a negative voltage by a switching power supply circuit, and a positive and negative voltage output switching circuit that switches both outputs and selectively supplies them to the output terminal. We are prepared.

FIG. 3 shows an example of such a conventional positive/negative voltage output switching circuit. In the figure, PCI and PO2 are photocouplers, and PTI and PT2 are phototransistors. R1-R5 are bias resistances for transistors Q1-Q3, and R21-R25 are bias resistances for transistors Q21-Q23. Also R6. R7 and VRI are Q
R26, a resistor that forms a voltage divider circuit when l-Q3 is on.
, R27 and VR21 are resistors forming a voltage dividing circuit when Q21 to Q23 are on. PTL is off now,
If PT2 is on, Q21 to Q23 are off, Q1 to Q
3 is turned on. Therefore, R6, R7 and VRI
The negative voltage divided by is output to the output terminal via diode D2. Conversely, if PTI is on and PT2 is off, Q21 to Q23 are on and Q1 to Q3 are off. This allows R26, R27 and VR21
The positive voltage divided by is outputted to the output terminal via the diode D22. Note that the negative voltage is adjusted by the VRI, and the positive voltage is adjusted by the VR21.

(C1 Problem to be Solved by the Invention However, in such a conventional circuit, a transistor for selecting a negative voltage and a transistor for selecting a positive voltage are provided separately, so the number of components is large, Components such as photocouplers were also required to control these transistors.

An object of the present invention is to provide a positive/negative voltage output switching circuit that reduces the number of necessary parts by devising the circuit, making it possible to downsize the circuit board and reduce costs.

(d) Means for Solving the Problems The positive/negative voltage output switching circuit of the present invention includes a first resistance voltage divider circuit provided between the positive power source and the negative power source or between one of the positive and negative power sources and the ground. Connect the first diode between the output of the resistor voltage divider circuit and the output terminal, connect one end of the second resistor voltage divider circuit to one of the positive and negative power supplies or the ground side, and connect the other end to the collector of the transistor. At the same time, the emitter of this transistor is connected to the other side of the positive and negative power supplies, a second diode is connected between the output terminal and the output of the second resistor voltage divider circuit, and the first diode and the transistor A third diode is connected between the collectors of the transistor, and positive and negative voltages are switched and outputted from the output terminal by turning on and off the transistor.

(e) Effect With the above configuration, when the transistor is off, the second diode is cut off because the cathode has a more positive potential than the anode. Therefore, when the transistor is on, the output voltage of the first resistor voltage divider circuit is generated at the output terminal through the first diode, the electrode other than the output terminal side of the first diode is connected to the transistor through the third diode. The first diode is cut off because it becomes almost equal to the collector potential. As a result, the output voltage of the second resistance voltage divider circuit is generated at the output terminal through the second diode.

As described above, predetermined positive and negative voltages can be selectively output by turning on and off the transistors.

(fl Embodiment FIG. 1 is a circuit diagram of a positive/negative voltage output switching circuit which is an embodiment of the present invention. This circuit is used, for example, at the output section of a switching power supply circuit that generates positive and negative voltages stabilized by a Zener diode. It is a connected circuit, and the voltage handled is ±
Less than IKV, current capacity less than 10 μA. In the figure, PC is a photocoupler, and PT is a phototransistor which is a light receiving element thereof. R1-R5 are bias resistors for transistors Q1-Q3, and R6, R7, and VRI are second resistance voltage dividing circuits that perform resistance voltage division when Q1-Q3 are in an on state. Also, ■R2, R8 and R
9 is a first resistor voltage divider circuit. As shown in the figure, a second diode D2 and a current limiting resistor R13 are connected between the output terminal of VRI, which is the output terminal of the first voltage dividing circuit, and the output terminal.
R14 is connected. A first diode DI and current limiting resistors RIO to R12 are connected between the output of the first resistance voltage divider circuit and the output terminal. Furthermore, a third diode D3 is connected between the collector of the transistor Q1 and the anode of the first diode Dl in the direction shown.

The operation of the circuit configured as described above is as follows.

First, when PT is turned on, the base potentials of Q1 to Q3 become low and Q1 to Q3 are turned off. For this reason, D2. D
The cathode potential of D2.3 becomes almost equal to the positive power supply voltage.
D3 becomes cut off.

On the other hand, since no reverse bias resistance is applied to DI, the output voltage of the first resistance voltage divider circuit is outputted from the output terminal through RIO to R12.

Conversely, if PT is turned off, the base potentials of Q1 to Q3 become high and all of them are turned on. Therefore, the cathode potential vb of D3 becomes almost a negative power supply voltage, and Vb<Vc, D3 becomes conductive and DI is cut off. As a result, the output voltage Va of the second resistance voltage divider circuit is outputted to the output terminal via D2, R13, and R14. Note that by adjusting VRI, the output voltage of a negative voltage can be adjusted, and by adjusting VR2, the output voltage of a positive voltage can be adjusted.

In the above embodiment, the circuit was constructed using NPN transistors, but it is also possible to construct the circuit using PNP transistors. FIG. 2 shows an example of such a case. In the figure, R21 to R25 are bias resistors of transistors Q21 to Q23, and R26, R27, and VR21 constitute a second voltage dividing circuit when Q21 to Q23 are in an on state. Also, VR22, R28 and R
29 is a first resistor voltage divider circuit.

When the phototransistor PT is turned on, Q21 to Q23 are turned off, and Va and Vb become almost negative power supply voltages.
D22. D23 is cut off.

Therefore, the output of the first resistor voltage divider circuit, ie, ■R22
The output voltage is outputted to the output terminal via R30 to R32 and D21. Conversely, if PT turns off, Q21-Q
23 is turned on. As a result, V b >V c and D23 becomes conductive and D21 is cut off. Then, the output voltage Va of the second resistance voltage divider circuit is D22. It is output to the output terminal via R33 and R34.

In addition, in the example, multiple resistors were connected in series to improve voltage resistance, and multiple transistors were connected in series, but it is possible to change the circuit configuration as appropriate depending on the positive and negative voltages to be switched. It is possible.

In addition, in the embodiment, one end of the second resistive voltage divider circuit was connected to the collector of the transistor and the other end was connected to the power supply line, but the other end of the second resistive voltage divider circuit was connected to the ground potential. may be connected to.

(g1 Effects of the Invention As described above, according to the present invention, a single transistor or a set of transistors can be turned on and off without separately providing a transistor that is turned on when a positive voltage is output and a transistor that is turned on when a negative voltage is output. By turning it off, the positive and negative voltages can be selectively switched and output, reducing the number of parts, making it possible to downsize the circuit board and reduce costs.Also, there is no need to match the switching timing of the positive and negative voltage outputs. You can switch smoothly.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a positive/negative voltage output switching circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a positive/negative voltage output switching circuit according to another embodiment. FIG. 3 is a circuit diagram of a conventional positive/negative voltage output switching circuit. VR2, R8, R9 - first resistor voltage divider circuit, R6, R7
, VRI - second resistive voltage divider circuit, DI - first diode, D2 - second diode, D3 - third diode, Q1 to Q3 - NPN type transistor. Applicant Gyoda Seisakusho Co., Ltd.

Claims (1)

[Claims] (1) A first resistor voltage divider circuit is provided between the positive power supply and the negative power supply or between one of the positive and negative power supplies and ground, and a first diode is installed between the output of this first resistor voltage divider circuit and the output terminal. Connect one end of the second resistor voltage divider circuit to one of the positive and negative power supplies or the ground side, connect the other end to the collector of the transistor, and connect the emitter of this transistor to the other side of the positive and negative power supplies. a second diode is connected between the output terminal and the output of the second resistor voltage divider circuit, and a third diode is further connected between the first diode and the collector of the transistor, A positive/negative voltage output switching circuit characterized by switching output of positive/negative voltage from an output terminal by turning on/off.