JP3634664B2 - Current detection circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a current detection circuit for detecting an output current or the like of a power supply circuit, and more particularly to a technique for detecting a current and setting a voltage polarity of an obtained detection signal to a voltage polarity suitable for a control circuit system.
[0002]
[Prior art]
Many electronic devices and devices having various functions are mounted on recent electronic devices. In order to operate these electronic elements / devices, naturally, it is necessary to supply power with specifications (for example, voltage value, polarity, frequency, etc.) suitable for the electronic elements / devices. There may be circuit systems with different power supply specifications. As an example, consider a fluorescent display tube. In order to drive the fluorescent display tube, a first power source for obtaining a current necessary for heating the filament and a voltage required for displaying the fluorescent display tube are obtained. 2 power supplies must be configured. Specifically, conventionally, the fluorescent display tube and its peripheral circuit are configured as shown in FIG.
[0003]
In the circuit shown in FIG. 4, a transformer T is provided, and both ends of the primary winding N <b> 1 are connected to the output terminal of the conversion circuit 2. The input terminal of the conversion circuit 2 is connected to the input terminals 1a and 1b, and the control circuit 3 for controlling the operation is connected to the conversion circuit 2. The transformer T, the conversion circuit 2, and the control circuit 3 constitute an inverter type power supply circuit 4, and the filament of the fluorescent display tube FIT is connected between both ends of the secondary winding N2 of the transformer T. Then, an intermediate tap P is provided in the secondary winding N2 of the transformer T, and the intermediate tap P is connected to one end of the voltage source 5 through the primary winding N3 of the current transformer CT constituting the current detection circuit 9c. One end of the secondary winding N4 of the current transformer CT is connected to the ground, and the other end is connected to the control circuit 3 of the power supply circuit 4. The other end of the voltage source 5 is connected to the anode and grid of the fluorescent display tube FIT via the ground and the lighting control circuit 6.
[0004]
In the circuit of FIG. 4 having such a configuration, when a voltage is supplied between the input terminals 1a and 1b from the outside, the conversion circuit 2 operates, and the conversion circuit 2 applies a pulse shape to the primary winding N1 of the transformer T. Supply voltage or AC voltage. Then, an AC voltage having a waveform similar to the waveform of the voltage applied to the primary winding N1 is generated in the secondary winding N2, and an AC current is supplied from the secondary winding N2 to the filament of the fluorescent display tube FIT. . At this time, the voltage V _B1 generated in the voltage source 5 is applied between the filament and the grid of the fluorescent display tube FIT and between the filament and the anode. The fluorescent display tube FIT is turned on by supplying a current to the filament and applying a voltage between the filament and the grid and between the filament and the anode.
[0005]
Here, when the display state of the fluorescent display tube FIT is changed by the operation of the lighting control circuit 6, the current passing between the filament and the grid and between the filament and the anode, that is, the flow rate of the cathode current is changed. In the circuit shown in FIG. 4, this cathode current is detected by the current transformer CT, and a detection signal is sent to the control circuit 3 constituting the power supply circuit 4. Then, the control circuit 3 changes the control amount of the control signal supplied to the conversion circuit 2 according to the received detection signal, for example, the voltage value and the pulse width, and changes the amount of energy passing through the conversion circuit 2. As a result, the current supplied from the secondary winding N2 to the filament increases or decreases in accordance with the display state of the fluorescent display tube FIT, and acts to stabilize the display brightness.
[0006]
By the way, the fluorescent display tube is lit by emitting electrons from the filaments, accelerating the electrons to strike the anode, and causing the phosphor applied to the anode to emit light. Therefore, it is basically necessary to keep the filament at a potential lower than that of the anode and the grid. In the circuit shown in FIG. A voltage having a lower potential than that, that is, a voltage having a negative voltage polarity is supplied.
In such a configuration, if a current detection circuit 8c, which is commonly used in the resistance detection method, is applied to detect the cathode current, the polarity of the voltage generated in the voltage source 5 is negative. The voltage of the detection signal also becomes negative.
[0007]
However, the conversion circuit 2, the control circuit 3, and the like that constitute the power supply circuit 4 in FIG. 4 are generally so-called positive logic circuits that process a signal having a voltage higher than the ground potential, that is, a voltage having a positive voltage polarity. It consists of Usually, a detection signal having a negative voltage polarity cannot be directly supplied to the positive logic control circuit 3. Therefore, in the circuit shown in FIG. 4, a current transformer CT is used for the detection circuit 9c for detecting the cathode current, not a circuit based on the resistance detection method. The detection signal is supplied to the control circuit 3 after the connection configuration is such that the voltage polarity of the detection signal obtained at the secondary winding N4 is positive.
[0008]
[Problems to be solved by the invention]
As described above, a current transformer is useful when a current is detected and a detection signal obtained thereby is exchanged between circuit systems having different voltage polarities.
However, the current transformer is a winding component, and stray capacitance and leakage inductance exist in each winding. When the display state of the fluorescent display tube FIT changes and the cathode current changes stepwise, noise generated due to stray capacitance and leakage inductance is superimposed on the signal waveform sent from the current transformer CT to the control circuit 3. There was a problem. If noise is superimposed on the signal waveform, the control system cannot accurately detect the amount of current, and output control of the power circuit to be controlled may not be performed correctly.
[0009]
As a result, if the load is a fluorescent display tube, the display brightness may become unstable, and there may be a disadvantageous phenomenon that the life of the tube is shortened.
If the connection configuration of the secondary winding N2 of the current transformer CT is fixed as shown in FIG. 4, the relationship between the voltage polarity of the detection signal and the positive / negative logic type of the control circuit 3 is limited, and the load and control There is also a disadvantage that a product having a different configuration of the current detection circuit 9c has to be prepared for each circuit specification.
Accordingly, the present invention provides a current detection circuit in which the voltage polarity of the detection signal of the current flowing through the power transmission line can be set to a polarity suitable for the circuit system in the subsequent stage, and noise is not easily superimposed on the detection signal. With the goal.
[0010]
[Means for Solving the Problems]
In the current detection circuit of the present invention, a detection resistor is connected in series to a power transmission line, and a current flowing through the transmission line is detected from a difference between two terminal voltages respectively appearing at both ends of the detection resistor. And a reference voltage source having one terminal connected to the reference potential point and a switching circuit connected to the reference voltage source, and the voltage of the transmission line corresponds to two terminal voltages when the transmission line voltage has the first polarity. The two voltage signals are biased by the reference voltage output from the reference voltage source, respectively, and a signal corresponding to the flow rate of the current flowing through the transmission line is obtained from the biased two voltage signals. On the other hand, when the transmission line voltage has the second polarity, the switching circuit stops the supply of the reference voltage, and the signal corresponding to the flow rate of the current flowing through the transmission line from the two voltage signals corresponding to the two terminal voltages is generated. It is characterized by obtaining.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
A reference voltage source having a detection resistor connected in series to the power transmission line and one terminal connected to ground is provided. A first voltage dividing circuit having a series circuit of a first resistor and a second resistor is connected between one end of the detection resistor and the other terminal of the reference voltage source, and the third resistor and the fourth resistor A second voltage dividing circuit having a series circuit of resistors is connected between the other end of the detection resistor and the other terminal of the reference voltage source. An error amplifier is provided, and a connection point between the first and second resistors and a connection point between the third and fourth resistors are connected to two input terminals of the error amplifier, respectively. And the switching circuit which stops supply of a reference voltage according to the voltage polarity of an electric power transmission line is connected to a reference voltage source.
[0012]
Specifically, the reference voltage source is connected to the three-terminal regulator having the anode connected to the ground and the cathode connected to the two voltage dividing circuits, and to the anode, the gate, and the cathode and the gate of the three-terminal regulator. The configuration includes two resistors. As an example, the switching circuit includes a transistor as a switch in which the main current path is connected between terminals of a reference voltage source, and a resistor and a diode connected between the base of the transistor and the other end of the detection resistor. And a circuit. As another example, the switching circuit includes a transistor as a switch in which the main current path is connected between the reference voltage source and the power supply point of the driving voltage, and the main current path between the base of the transistor and the ground. The control transistor is connected, and the resistor is connected between the base of the control transistor and the other end of the detection resistor.
[0013]
【Example】
FIG. 1 shows a fluorescent display tube and its peripheral circuit including an embodiment of a current detection circuit according to the present invention in which the voltage polarity of a detection signal of a current flowing in a power transmission line can be set to a polarity suitable for a circuit system in the subsequent stage. It is shown in FIG.
FIG. 1 shows the configuration of the fluorescent display tube and its peripheral circuit when the voltage polarity of the power transmission line is made negative in turning on the fluorescent display tube FIT, and FIG. 2 shows that the voltage polarity of the power transmission line is made positive. The structure of the fluorescent display tube and its peripheral circuit is shown. In the circuits of FIGS. 1 and 2, the current detection circuit 9a according to the first embodiment of the present invention is configured as follows.
[0014]
A detection resistor R1 is connected in series to a power transmission line connecting the intermediate tap P of the transformer T and one end of the voltage source 5 (or voltage source 11), and a three-terminal regulator IC is provided in which the anode terminal is connected to the ground. A resistor R7 is connected between the cathode and gate of the three-terminal regulator IC, and a resistor R8 is connected between the anode and gate, and the reference voltage source 7 is constituted by the three-terminal regulator IC and resistors R7 and R8. A voltage dividing circuit comprising a resistor R2 and a resistor R3 is connected between one end (A) of the detection resistor R1 and the cathode of the three-terminal regulator IC, and the other end (B) of the detection resistor R1 and the three-terminal regulator IC. A voltage dividing circuit including a resistor R4 and a resistor R5 is connected between the cathode and the cathode. An error amplifier EA is provided, one input terminal is connected to a connection point (voltage dividing point) between the resistors R2 and R3, and the other input terminal is connected to a connection point (voltage dividing point) between the resistors R4 and R5, The output terminal of the error amplifier EA is connected to the control circuit 3.
[0015]
A resistor R6 is connected between the supply point of the driving voltage (V _C ) and the cathode of the three-terminal regulator IC, and as a switch, between the terminals of the reference voltage source 7, that is, between the anode and cathode of the three-terminal regulator IC. The main current path of transistor Q1 is connected. A diode D1, a resistor R9, and a resistor R10 are connected in series between the base of the transistor Q1 and the other end (B) of the detection resistor R1, and a diode D2 is connected between the connection point of the resistors R9 and R10 and the ground. Connecting. The transistor Q1, resistors R9 and R10, and diodes D1 and D2 constitute a switching circuit 8a.
[0016]
Note that the circuit portion of FIG. 1 has the same configuration as the circuit of FIG. 4 except for the current detection circuit 9a.
On the other hand, in the circuit of FIG. 2, a voltage source 11 having a polarity opposite to that of the voltage source 5 of the circuit of FIG. 1 is connected between the other end (B) of the detection resistor R1 and the ground, and the fluorescent display tube FIT. Connect the grid to ground. Furthermore, a voltage source 10 having one end on the low potential side connected to the ground is provided, and a positive voltage is supplied from the voltage source 10 to the anode of the fluorescent display tube FIT through the lighting control circuit 6. As a result, a current flows from the voltage source 10 through the lighting control circuit 6, the anode of the fluorescent display tube FIT, the filament, the secondary winding N2, the detection resistor R1, the voltage source 11, and the ground path. .
[0017]
1 and 2 configured as described above, the current detection circuit 9a according to the present invention operates as follows according to the voltage polarity of the power transmission line.
First, when the fluorescent display tube FIT is turned on with the voltage polarity of the power transmission line in a negative state as in the circuit of FIG. 1, the negative voltage of the voltage source 5 is connected to the other end (B) of the detection resistor R1. V _B1 appears. On the other hand, a negative voltage having a magnitude obtained by subtracting the voltage drop generated in the detection resistor R1 from the voltage V _B1 appears at one end (A) of the detection resistor R1. The voltages appearing at both ends (A) and (B) of the detection resistor R1 are divided by resistors R2 and R3, or resistors R4 and R5 connected to both ends of the detection resistor R1, respectively. Is done.
[0018]
Here, since the voltage at the other end (B) of the detection resistor R1 is negative, the transistor Q1 of the switching circuit 8a is turned off. At this time, in the reference voltage source 7, the three-terminal regulator IC is operated by the driving voltage (V _C ) supplied via the resistor R6, and a reference voltage having a predetermined voltage value is generated between the anode and the cathode. Let
Then, the reference voltage is supplied from the reference voltage source 7 to the voltage dividing circuit of the resistors R2 and R3 and the voltage dividing circuit of the resistors R4 and R5, and the divided voltage appearing at the connection point between the resistors R2 and R3 and the resistors R4 and R3 The divided voltage appearing at the connection point of R5 is biased by the reference voltage.
[0019]
Specifically, the voltage value of the reference voltage is determined from the value obtained from the equation {(R4 / R5) · V _B1 } from the resistance value of each resistor R4, R5 (or R2, R3) and the voltage value of the voltage V _B1. If it is increased, the polarity of the voltage at each voltage dividing point becomes positive. Each divided voltage having the positive polarity in this way is input to the error amplifier EA. Then, the output signal of the error amplifier EA has a positive voltage polarity, and when the cathode current flowing through the detection resistor R1 changes, the amount of change in the signal corresponds to the amount of change in the cathode current.
[0020]
Next, when the fluorescent display tube FIT is turned on with the voltage polarity of the power transmission line being positive as in the circuit of FIG. 2, the other end (B) of the detection resistor R1 is connected to the positive polarity of the voltage source 11. A voltage V _B3 appears. On the other hand, one end (A) of the detection resistor R1, a positive polarity voltage of a magnitude plus the voltage drop resulting from the voltage V _B3 to the detecting resistor R1 appears. The voltages appearing at both ends (A) and (B) of the detection resistor R1 are divided by resistors R2 and R3, or resistors R4 and R5 connected to both ends of the detection resistor R1, respectively. Is done. The voltage source 11 is for supplying an offset voltage between the filament and the grid, and the relationship between the voltages generated in the voltage sources 10 and 11 is {V _B2 > V _B3 }.
[0021]
Here, since the polarity of the voltage at the other end (B) of the detection resistor R1 is positive due to the voltage V _B3 generated in the voltage source 11, the transistor Q1 of the switching circuit 8a is connected via the diode D1 and the resistors R9 and R10. In response to a forward bias, it is turned on. At this time, the anode and cathode of the three-terminal regulator IC are short-circuited by the transistor Q1, and the supply of the reference voltage from the reference voltage source 7 to each voltage dividing circuit is stopped.
Since the divided voltage at each voltage dividing point is originally positive in polarity without being biased by the reference voltage, each divided voltage is input to the error amplifier EA as it is. Then, the output signal of the error amplifier EA has a positive voltage polarity, and the change amount corresponds to the change amount of the cathode current.
[0022]
As can be seen from the above description of the operation, the current detection circuit 9a according to the present invention can adapt the voltage polarity of the detection signal to the control circuit 3 in the subsequent stage regardless of the voltage polarity of the power transmission line. Also, the current detection circuit 9a of the present invention has no elements such as stray capacitance and leakage inductance in the circuit, unlike the case where a current transformer is used. For this reason, there is no noise superposition on the signal, and a signal that correctly reflects the change in the flow rate of the current can be sent to the control circuit 3 in the subsequent stage.
[0023]
In principle, even if the switching circuit 8a is omitted from the current detection circuit 9a of the present invention, the voltage polarity of the detection signal can be adapted to the control circuit 3 in the subsequent stage regardless of the voltage polarity of the power transmission line. It is. However, if the switching circuit 8a is excluded, even when the voltage polarity of the power transmission line is positive, a bias due to the reference voltage is applied to the voltage dividing points of the resistors R2, R3 (or resistors R4, R5). Then, the voltage value of the signal obtained at the voltage dividing point of the resistors R2 and R3 (or resistors R4 and R5) exceeds the allowable operating voltage of the error amplifier EA, and the current detection circuit cannot actually be used. There is. The presence of the switching circuit 8a suppresses an increase in the voltage value of the signal input to the error amplifier EA when the voltage polarity of the power transmission line is positive, and the use of the current detection circuit 9a can be used regardless of the voltage polarity of the power transmission line. It has an important meaning in making it possible.
[0024]
Such a switching circuit is formed with a circuit configuration different from that shown in FIG. 1 in the current detection circuit 9b according to the second embodiment of the present invention shown in FIG.
FIG. 3 shows the configuration of the fluorescent display tube and its peripheral circuit when the voltage polarity of the power transmission line is made negative, including the current detection circuit according to the present invention, as in FIG. The configuration is as follows.
[0025]
The main current path of the transistor Q2 as a switch is connected between one end of the reference voltage source 7, that is, the cathode of the three-terminal regulator IC and the supply point of the driving voltage (V _C ). A resistor R11 is connected between the base and emitter of the transistor Q2, and the base of the transistor Q2 is connected to the ground via the diode D3 and the main current path of the control transistor Q3. A resistor R9 and a resistor R10 are connected in series between the base of the control transistor Q3 and the other end (B) of the detection resistor R1, and a diode D2 is connected between the connection point of the resistors R9 and R10 and the ground. . The transistor Q2, the control transistor Q3, the resistors R9 and R10, and the diodes D2 and D3 constitute a switching circuit 8b. With respect to the circuit portions other than the switching circuit 8b, the circuit configurations of FIGS. 1 and 3 are the same.
[0026]
The operation of the switching circuit 8b configured as described above and the current detection circuit 9b having the switching circuit 8b is as follows.
When the fluorescent display tube FIT is turned on with the voltage polarity of the power transmission line in a negative state as in the circuit shown in FIG. 3, the negative voltage V _B1 of the voltage source 5 is connected to the other end (B) of the detection resistor R1. Appears. Since the voltage at the other end (B) of the detection resistor R1 is negative, the control transistor Q3 of the switching circuit 8b is turned off, and the transistor Q2 as a switch is caused by the forward bias supplied via the resistor R11. Turns on.
[0027]
Inside the reference voltage source 7, the three-terminal regulator IC is operated by the driving voltage (V _C ) supplied through the main current path of the transistor Q2, and a reference voltage having a predetermined voltage value is applied between the anode and the cathode. generate. Then, the reference voltage is supplied from the reference voltage source 7 to the voltage dividing circuit of the resistors R2 and R3 and the voltage dividing circuit of the resistors R4 and R5, and the divided voltage appearing at the connection point between the resistors R2 and R3 and the resistors R4 and R3 The divided voltage appearing at the connection point of R5 is biased by the reference voltage.
Here, if the voltage value of the reference voltage is set larger than the value obtained from the above equation, the polarity of the voltage at each voltage dividing point becomes positive. By inputting each divided voltage having the positive polarity in this manner to the error amplifier EA, a signal whose voltage polarity is positive and whose change amount corresponds to the change amount of the cathode current is obtained at its output terminal. .
[0028]
On the other hand, assuming that the current detection circuit 9b having the configuration shown in FIG. 3 is applied to a circuit for turning on the fluorescent display tube FIT with the voltage polarity of the power transmission line as shown in FIG. It becomes as follows.
In this case, since the voltage at the other end (B) of the detection resistor R1 is positive, the control transistor Q3 of the switching circuit 8b is turned on and the transistor Q2 as a switch is turned off. Then, since the transistor Q2 is in the OFF state inside the reference voltage source 7, the driving voltage (V _C ) is not supplied, and the anode and cathode of the three-terminal regulator IC are equivalently opened.
[0029]
At this time, one end (A) of the detection resistor R1 is connected to the ground via resistors R2, R3, R7 and R8, and the other end (B) is connected to the ground via resistors R4, R5, R7 and R8. The The circuit portions of the resistors R1 to R5, R7 and R8 in such a state are substantially the circuit portions of the resistors R1 to R5 of the circuit of FIG. 2 in which the terminals of the reference voltage source 7 are short-circuited by the transistor Q1. Fulfills the same function. Therefore, even in the current detection circuit 9b having the configuration shown in FIG. 3, when the voltage polarity of the power transmission line is in a positive state, the divided voltages at the voltage dividing points of the resistors R2 and R3 and the resistors R4 and R5 are directly used as error amplifiers. A signal having a positive voltage polarity and a change amount corresponding to the change amount of the cathode current is obtained at the output terminal of the error amplifier EA.
[0030]
In the above description of the embodiments of the current detection circuit according to the present invention, a positive logic circuit is assumed as the error amplifier EA of the circuits shown in FIGS. The voltage polarity system of the error amplifier EA is substantially the same as that of the control circuit 3, and the error amplifier EA can be configured integrally with the control circuit 3. Further, the driving voltage (V _C ) necessary for driving the error amplifier EA and the reference voltage source 7 can be supplied from a power supply point for driving the control circuit 3.
In each embodiment, the configuration of the apparatus that supplies the detection signal to the positive logic type circuit system is shown. Conversely, the present invention is applied to an apparatus that supplies the detection signal to the negative logic type circuit system. It doesn't matter. Naturally, the circuit system in which the current detection circuit of the present invention is incorporated is not limited to the inverter circuit for lighting the fluorescent display tube as shown in the embodiment.
[0031]
【The invention's effect】
As described above, the current detection circuit according to the present invention has a detection resistor connected in series with the power transmission line, and a reference voltage from the reference voltage source is supplied to each voltage signal corresponding to the voltage appearing at both ends of the resistance. A configuration in which a bias is applied and a detection signal corresponding to the current flowing through the power transmission line is obtained from the two voltage signals to which the bias is applied, and a switching circuit is connected to the reference voltage source, in accordance with the voltage polarity of the power transmission line. And a structure for stopping the supply of the reference voltage.
[0032]
In such a current detection circuit of the present invention, the voltage polarity of the signal can be manipulated by applying a bias with the reference voltage, and the voltage polarity of the detection signal can be adapted to the circuit system of the subsequent stage. Also, by stopping the supply of the reference voltage according to the voltage polarity of the power transmission line, it is possible to prevent a situation in which the voltage value of the signal becomes too high due to the application of a bias and the circuit cannot actually be used. Further, since there are no factors such as stray capacitance and leakage inductance in the circuit, noise superimposition is eliminated in the signal. Therefore, according to the present invention, a highly versatile and highly reliable current detection circuit can be provided.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of a first embodiment of a current detection circuit according to the present invention and a fluorescent display tube and its peripheral circuits when the voltage polarity of a power transmission line is negative.
FIG. 2 is a circuit diagram of a first embodiment of a current detection circuit according to the present invention and a fluorescent display tube and its peripheral circuits when the voltage polarity of a power transmission line is positive.
FIG. 3 is a circuit diagram of a second embodiment of a current detection circuit according to the present invention, and a fluorescent display tube and its peripheral circuits.
FIG. 4 is a circuit diagram of a conventional current detection circuit, a fluorescent display tube and its peripheral circuits.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1a, 1b: Input terminal 2: Conversion circuit 3: Control circuit 4: Power supply circuit 5: Voltage source 6: Lighting control circuit 7: Reference voltage source 8a, 8b: Switch circuit 9a, 9b: Current detection circuit 10: Voltage source ( 11: Voltage source (for offset voltage supply) CT: Current transformer EA: Error amplifier FIT: Fluorescent display tube R1: Resistance for detection R2: Resistance (first resistance) R3: Resistance (second resistance) R4: Resistance (third resistance) R5: Resistance (fourth resistance) Q1, Q2: Transistor (switch) Q3: Control transistor V _C : Driving voltage

Claims (5)

In a current detection circuit for connecting a detection resistor in series to a power transmission line and detecting a current flowing through the transmission line from a difference between two terminal voltages respectively appearing at both ends of the detection resistor,
A reference voltage source having one terminal connected to a reference potential point, and a switching circuit connected to the reference voltage source;
When the voltage of the transmission line has the first polarity, the two voltage signals corresponding to the two terminal voltages are biased by the reference voltage output from the reference voltage source, respectively, and the two voltages that have received the bias Obtain a signal corresponding to the flow rate of current flowing through the transmission line from the signal,
When the voltage of the transmission line has the second polarity, the switching circuit stops the supply of the reference voltage, and corresponds to the flow rate of the current flowing through the transmission line from the two voltage signals corresponding to the two terminal voltages. A current detection circuit characterized by obtaining a signal. In a current detection circuit for connecting a detection resistor in series to a power transmission line and detecting a current flowing through the transmission line from a difference between two terminal voltages respectively appearing at both ends of the detection resistor,
First and second voltage dividing circuits connected to both ends of the detection resistor, a reference voltage source having one terminal connected to a reference potential point, and a switching circuit connected to the reference voltage source Equipped,
When the voltage of the transmission line has the first polarity, the first divided voltage appearing at the voltage dividing point of the first voltage dividing circuit and the second voltage appearing at the voltage dividing point of the second voltage dividing circuit. Each of the divided voltages is biased with a reference voltage output from the reference voltage source, and a signal corresponding to the flow rate of the current flowing through the transmission line is obtained from the biased first and second divided voltages. And
When the voltage of the transmission line has the second polarity, the switching circuit stops the supply of the reference voltage, and the first divided voltage appearing at the voltage dividing point of the first voltage dividing circuit and the second voltage A current detection circuit, wherein a signal corresponding to a flow rate of a current flowing through the transmission line is obtained from a second divided voltage appearing at a voltage dividing point of the voltage dividing circuit. The current detection circuit according to claim 2, wherein the first polarity is negative and the second polarity is positive. The voltage value of the reference voltage output from the reference voltage source is such that both the first and second divided voltages biased with the reference voltage are converted from a negative voltage value to a positive voltage value. The current detection circuit according to claim 3, wherein: A detection resistor connected in series with the power transmission line;
A reference voltage source having one terminal connected to a reference potential point;
A switching circuit connected to the reference voltage source and stopping the supply of the reference voltage according to the voltage polarity of the power transmission line;
A first voltage dividing circuit having first and second resistors connected in series and connected between one end of the detection resistor and the other terminal of the reference voltage;
A second voltage dividing circuit having third and fourth resistors connected in series and connected between the other end of the detection resistor and the other electrode of the reference voltage;
A current detection comprising: an error amplifier having a first input terminal connected to a connection point of the first and second resistors, and a second input terminal connected to the connection point of the third and fourth resistors. circuit.

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