JPH0830995B2 - Positive / negative voltage output switching circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive / negative voltage output switching circuit, and in particular, a predetermined DC voltage (intermediate voltage) is applied to a developing device in, for example, an electrophotographic copying machine or a laser beam printer. The present invention relates to a positive / negative voltage output switching circuit used when supplying power.

[Prior art]

An example of this type of conventional technology is June 13, 1988.
Japanese Patent Application No. 63-145322 filed on the date or May 1991
There is Japanese Patent Application No. 1-118167 filed on the 10th.

The positive / negative voltage output switching circuit of Japanese Patent Application No. 63-145322 is constructed as shown in FIG. When the phototransistor PT included in the photocoupler PC is turned off, the transistor
Q1 to Q3 turn on and the voltage Va drops to almost (-) potential,
The voltage Vb (divided by resistor R1 + resistor R2 / variable resistor VR1) also becomes (-). Therefore, the diodes D2 and D3 turn on, and the anode potential of the diode D1 becomes (-) and turns off. Therefore, in this state, the output voltage Vc becomes (-), and its variable range is determined by the variable resistor VR1.

On the contrary, when the phototransistor PT of the photocoupler PC is turned on, all the transistors Q1 to Q3 are turned off,
Voltages Va and Vb become (+) and diodes D2 and D3
Turns off. On the other hand, (+) potential is variable resistance VR2 and resistance R
The voltage is divided by 3 and R4 and further applied to the anode of the diode D1 through the series circuit of the resistors R5 to R7, so that the diode D1 is turned on. Therefore, in this state, the output voltage Vc is (+) and the variable range is the variable resistance VR2.
Is determined.

The multi-output power supply circuit of Japanese Patent Application No. 1-118167 is constructed as shown in FIG. The switching control circuit 1 is the switching transistor Q4, that is, the primary winding of the transformer.
The induced voltage generated in the secondary winding N2 of the transformer by disconnecting N1 is generated by the diodes D4 and D5 and the capacitor C1 and
It is rectified by C2, smoothed, and taken out as a predetermined (+) voltage to the high voltage terminal OUT1. As a result, the load current IL1 flows through the charger 2 and the Zener diode ZD1 and the point A becomes a (−) constant voltage (ZD1 Zener voltage). Further, the Zener current IZ2 flows through the Zener diode ZD2 through the resistor R8, and the point B becomes a (+) constant voltage (Zener voltage of ZD2). Therefore, at the medium voltage terminal OUT2, point A-B
A voltage corresponding to the divided voltage value by the variable resistor VR3 of the potential difference between the points is generated, and the load current IL2 flows in the developing device 3. Since the voltage at point A is (-) and the voltage at point B is (+), the variable resistance
By adjusting VR3, the developing bias of the developing device 3, that is, the intermediate voltage output voltage can be changed in the range of (-) to (+).

[Problems to be Solved by the Invention]

In the conventional circuit shown in FIG. 2, the diodes D1 to D3 are
It was necessary to use a high breakdown voltage diode, and it was necessary to use high breakdown voltage regulators as the variable resistors VR1 and VR2. Moreover, since the number of parts is large, improvement in reliability cannot be expected, and it is expensive as a whole.

Moreover, in the conventional circuit shown in FIG.
ZD1 and ZD2 set the medium voltage voltage variable region output to the medium voltage terminal OUT2, but large-capacity Zener diodes ZD1 and ZD2 are required to set the variable region in a wide range. Since the conventional circuit shown in FIG. 3 has a large number of components as in the conventional circuit shown in FIG.
The reliability was not expected to be improved, and it was expensive as a whole.

Therefore, a main object of the present invention is to provide a positive / negative voltage output switching circuit which has a small number of parts, is highly reliable and is inexpensive.

[Means for Solving the Problems] The present invention relates to a direct current power supply having first and second output terminals, and a first zener voltage connected between a first output terminal of the direct current power supply and ground. A series circuit of a Zener diode, a second Zener diode connected between the second output end of the DC power supply and the first Zener diode and having a second Zener voltage larger than the resistor, the regulator circuit and the first Zener voltage; An output terminal drawn from a connection point between the second Zener diode and the regulator circuit, a bias resistor connected between the first output end of the DC power supply and the ground, a switching means for turning on / off the regulator circuit, and a regulator circuit. Is a positive / negative voltage output switching circuit including voltage drop changing means for changing the voltage drop of the regulator circuit when the switch is off.

[Action]

When the negative DC voltage is normally output from the output terminal, the second Zener voltage is set higher than the first Zener voltage, so that the cathode of the second Zener diode is irrelevant regardless of the switching state of the regulator circuit. The potential is always higher than the cathode potential of the first Zener diode. Therefore, the current from the DC power supply always flows into the first Zener diode through the bias resistor,
The anode of the Zener diode is kept at a negative constant potential determined by the magnitude of its Zener voltage.

When the regulator circuit is turned on by the switching means, a negative constant voltage that is substantially equal to the anode potential of the first Zener diode is output from the output terminal.

When the regulator circuit is turned off by the switching means, the voltage output to the output terminal becomes a value obtained by adding the voltage drop in the regulator circuit to the anode voltage of the first Zener diode. Therefore, if the voltage drop changing means sets the voltage drop in the regulator circuit larger than the first Zener voltage, a positive voltage is output to the output terminal. If the voltage drop changing means sets the voltage drop in the regulator circuit to be smaller than the first Zener voltage, a negative voltage is output to the output terminal.

〔The invention's effect〕

According to the present invention, the positive / negative switching of the voltage of the output terminal and the voltage variable region thereof can be set only by turning on / off the regulator circuit and appropriately adjusting the magnitude of the voltage drop of the regulator circuit when it is off. . Therefore, even if a wide voltage variable region is set, it is not necessary to use an expensive element as in the conventional case, and the cost can be reduced.

The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

〔Example〕

Referring to FIG. 1, the positive / negative voltage output switching circuit 10 of this embodiment is configured as a multi-output power source used in, for example, an electrophotographic copying machine, and includes a transformer 12. The collector of a switching transistor Q11, which interrupts the current flowing through the primary winding N1, is connected to one end of the primary winding N1 of the transformer 12, and the emitter thereof is grounded. Then, the control signal from the switching control circuit 14 is given to the base of the switching transistor Q11, and the switching transistor Q11 is turned on / off. Secondary winding N of transformer 12
2 includes diodes D11 and D12 and capacitor C11 and
A rectifying / smoothing circuit 16 including C12 is connected.

From the (+) side end 16a of the rectifying / smoothing circuit 16, the high voltage terminal OUT1
Is drawn out, and a current limiting resistor R11 for preventing arc discharge is inserted between the (+) side end 16a and the high voltage terminal OUT1. And, between the high voltage terminal OUT1 and the ground, a charger
18 is connected. The charger 18 charges the photoreceptor (not shown) with (+) charges.

Between the (−) side end 16b of the rectifying / smoothing circuit 16 and the ground,
Zener diode ZD11 is connected so that the cathode is grounded. In addition, high voltage terminal OUT1 and Zener diode ZD11
A series circuit including a discharge resistor R12 for the capacitors C11 and C12, a Zener diode ZD12, and a regulator circuit 20 is connected between the anode and the. At this time, the cathode of the Zener diode ZD12 is connected to the discharge resistor R12, and the anode is connected to the regulator circuit 20. The Zener voltage V12 of the Zener diode ZD12 is
It is set higher than the Zener voltage V11 of D11.

Regulator circuit 20, in this embodiment, includes transistors Q12 and Q13 and a zener diode ZD13 connected in series between the respective anodes of zener diodes ZD11 and ZD12. A bias resistor R13 is connected between the base and collector of the transistor Q12, and the transistor Q12
A bias resistor R14 is connected between the bases of Q13 and Q13. Further, between the base of the transistor Q13 and the anode of the Zener diode ZD13, a resistor R15, a variable resistor VR11 and a switching element, that is, a photocoupler is provided.
A series circuit consisting of PC1 is connected. Further, a series circuit including a resistor R16, a variable resistor VR12 and a photocoupler PC2 is connected in parallel with this series circuit.

The intermediate voltage terminal OUT2 is drawn out from the collector of the transistor Q12 of the regulator circuit 20, and the developing device 22 is connected to the intermediate voltage terminal OUT2.

A bias resistor R17 is connected in parallel between the cathodes of the Zener diodes ZD11 and ZD12.

In operation, the switching control circuit 14 turns on / off the switching transistor Q11,
The current flowing in the primary winding N1 of the transformer 12 is interrupted to generate an induced voltage in the secondary winding N2 of the transformer 12. diode
D11 and D12 and capacitors C11 and C12 rectify and smooth the induced voltage, and a predetermined (+) high voltage is taken out to the high voltage terminal OUT1. As a result, the charger 18 is discharged and the load current IL flows into the Zener diode ZD11.

Also, the current from the rectifying / smoothing circuit 16 is the current limiting resistor R11.
Through the discharge resistor R12. Since the Zener voltage V12 of the Zener diode ZD12 is set higher than the Zener voltage V11 of the Zener diode ZD11, the cathode potential of the Zener diode ZD12 is set regardless of the switching (ON / OFF) state of the regulator circuit 20. It is always higher than the cathode potential of ZD11. Therefore, the current IA passes through the bias resistor R17 and the zener diode Z
It constantly flows into D11, and the anode of the Zener diode ZD11 is maintained at a constant (-) voltage (Zener voltage V11 of the Zener diode ZD11) and becomes the reference potential.

When both photocouplers PC1 and PC2 are turned off, transistors Q12 and Q13 are turned on and medium voltage terminal OUT2
The voltage of Zener diode ZD13 and transistor Q1
Although there is a voltage drop in 2 and Q13, it becomes almost equal to the anode potential of the Zener diode ZD11. As described above, since the anode of the zener diode ZD11 has a constant (-) voltage, the (-) intermediate voltage is output from the intermediate voltage terminal OUT2.

When either photocoupler PC1 or PC2 turns on, both transistors Q12 and Q13 turn off. At this time, the voltage of the medium voltage terminal OUT2 becomes (Zener diode ZD
11 anode voltage V11) + (voltage drop Vc in the regulator circuit 20). That is, if the voltage drop Vc <the Zener voltage V11, the (-) intermediate voltage is output to the intermediate voltage terminal OUT2. If Vc> V11, a (+) medium voltage is output. One regulator circuit like this
By 20, the positive / negative switching control of the voltage output to the intermediate voltage terminal OUT2 can be performed.

The voltage drop Vc in the regulator circuit 20 is obtained by the following formula.

Therefore, the voltage drop Vc can be changed by adjusting the combined resistance Rf, that is, the variable resistance VR11 or VR12 to change the voltage dividing condition, and the voltage range output to the intermediate voltage terminal OUT2 can be arbitrarily set.

Further, even when the corona discharge of the charger 18 is stopped and the load current IL does not flow, the current IA always flows from the bias resistor R17 to the zener diode ZD11, so the anode potential of the zener diode ZD11 is It is kept constant. That is, regardless of the ON / OFF state of the charger 18, a stable output can be obtained at the intermediate voltage terminal OUT2 using the anode potential of the Zener diode ZD11 as a reference potential.

In the regulator circuit 20 in this embodiment, the voltage comparison circuit of the transistors Q12 and Q13 is used simply, but it may be configured with a Darlington connection circuit or a differential amplifier to improve the accuracy. Of course good things.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. 2 and 3 are circuit diagrams showing different conventional techniques. In the figure, 10 is a positive / negative voltage output switching circuit, 12 is a transformer, 16 is a rectifying / smoothing circuit, 18 is a charger, 20 is a regulator circuit, 22 is a developing device, R11 is a current limiting resistor, R12 is a discharging resistor, and R13 to R17. Is a bias resistor, ZD11, ZD12, ZD13 are zener diodes, VR11, VR12 are variable resistors, OUT1 is a high voltage terminal, O
UT2 indicates a medium voltage terminal.

Claims (1)

[Claims] 1. A DC power supply having first and second output terminals, a first zener diode having a first zener voltage connected between the first output terminal of the DC power supply and ground, and the DC power supply. A series circuit of a resistor, a regulator circuit, and a second Zener diode having a second Zener voltage higher than the first Zener voltage, the serial circuit being connected between the second output end of the second Zener diode and the first Zener diode. 2, an output terminal drawn from a connection point between the Zener diode and the regulator circuit, a bias resistor connected between the first output end of the DC power supply and the ground, switching means for turning on / off the regulator circuit, And a voltage drop changing means for changing the voltage drop of the regulator circuit when the regulator circuit is off,
Positive / negative voltage output switching circuit.