JP2000009764A - Current detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect at good accuracy a current, using a PTC(provide temperature coefficient) thermistor. SOLUTION: A PTC thermistor 2 is provided with first and second resistance portions 21, 22, and one end of both resistance portions 21, 22 is in common. The second resistance portion 22 is constituted so as to be branched from the first resistance portion 21 and a ratio of resistant values of the respective resistance portions 21, 22 is set to a predetermined ratio. A potential control circuit 3 adjusts a potential V1 of the other end of the first resistance portion 21 and a potential V2 of the other end of the second resistance portion 22 to be at the same potential. The resistant value of a resistant element 7 is known already. An over current detection circuit 6 detects a potential V0 of a connection point of a transistor 31 and the resistant element 7, and a feed current I1 to a lamp 9 is demanded from the potential V0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current detection circuit for detecting a load current supplied to a load from a power supply using a positive temperature coefficient thermistor.

[0002]

2. Description of the Related Art Conventionally, when detecting a current flowing in a circuit, a resistor having a known resistance value is inserted into a path of the current,
A method of obtaining a current from a voltage value generated at both ends of the resistor and the known resistance value is often used.

[0003]

However, when a circuit current is detected by a resistor, the amount of power consumed by the resistor is proportional to the circuit current. Since it is necessary to use a resistor having a large power, the size of the resistor is increased and the cost is increased. Further, when the circuit current increases, the amount of power consumed by the resistor increases, so that the power efficiency of the circuit decreases.

On the other hand, conventionally, a positive temperature coefficient (Positive Temperature Coefficient) thermistor (hereinafter referred to as a "PTC thermistor") whose resistance value increases as the temperature rises is inserted into a circuit, so that the circuit can be protected from abnormalities such as overcurrent and overheating. Known to be protected, this PTC
Although it is conceivable to use a thermistor as the resistor, the PTC thermistor cannot be used as it is because the resistance value changes with temperature.

[0005] The present invention has been made in view of the above,
An object of the present invention is to provide a current detection circuit that can accurately detect a current while using a PTC thermistor.

[0006]

According to the present invention, there is provided a current detecting circuit for detecting a supply current supplied from a power supply unit to a load, wherein the first resistance unit is provided between the power supply unit and the load. And a second resistor section interposed between the power supply section and the ground, and branched from the first resistor section, wherein the resistance value of the second resistor section is equal to the first resistor value. A positive characteristic thermistor set to a value larger by a predetermined ratio with respect to the resistance value of the resistance portion, and a current flowing through the second resistance portion is detected. Current detecting means for obtaining the supply current flowing through the first resistance section.

According to this configuration, a current is supplied from the power supply unit to the load via the first resistance unit, and the power supply unit supplies the current to the second resistance unit branched from the first resistance unit. The current flows, and the current flowing through the second resistor is detected.
Since the resistance value of the second resistance portion is set to a value larger by a predetermined ratio than the resistance value of the first resistance portion, the current flowing through the first resistance portion based on the detected current and the predetermined ratio. Is required, the supply current to the load can be obtained with a simple configuration. Further, when the temperature rises, the resistance value of the positive temperature coefficient thermistor increases, so that the level of the supply current decreases, and the circuit components can be protected.

[0008] Further, there may be provided a potential control means for setting the potential on the load side of the first resistor section and the potential on the ground side of the second resistor section to the same potential. According to this configuration, since the resistance of the second resistor is set to a value larger than the resistance of the first resistor by a predetermined ratio, the potential on the load side of the first resistor is reduced. By setting the potential on the ground side of the second resistor to the same potential,
The ratio between the current flowing through the resistor section and the current flowing through the second resistor section is the above-mentioned predetermined ratio. Therefore, the current supplied to the load can be easily obtained from the current flowing through the second resistor and the predetermined ratio.

The positive temperature coefficient thermistor includes a thermistor portion and a pair of electrode plates disposed so as to sandwich the thermistor portion. One of the pair of electrode plates is provided with a first electrode plate. The first resistor plate is divided into a second electrode plate and the thermistor portion sandwiched between the first electrode plate and the other of the pair of electrode plates. The second resistance portion is constituted by the thermistor portion sandwiched by the other of the pair of electrode plates, and the ratio between the area of the first electrode plate and the area of the second electrode plate is the predetermined ratio. It may be configured so that

According to this configuration, one of the pair of electrode plates disposed so as to sandwich the thermistor portion is divided into the first electrode plate and the second electrode plate, and the first electrode plate and the pair of electrode plates are separated from each other. A first resistance portion is constituted by the thermistor portion sandwiched by the other of the electrode plates, and a second resistance portion is constituted by the thermistor portion sandwiched by the other of the second electrode plate and the pair of electrode plates. Since the ratio between the area of the first electrode plate and the area of the second electrode plate is set to the predetermined ratio, the characteristics of the first resistor and the characteristics of the second resistor substantially match. As a result, the supply current to the load can be accurately obtained.

[0011]

FIG. 1 is a circuit diagram of an embodiment of a current detection circuit according to the present invention. This circuit includes a power supply unit 1,
PTC thermistor 2, potential control circuit 3, FET 4, drive circuit 5, overcurrent detection circuit 6, resistance element 7, and control circuit 8
And controls the current supply from the power supply unit 1 to the lamp (load) 9. The PTC thermistor 2 includes a first resistor 21 and a second resistor 22, and the potential control circuit 3 includes a PNP transistor 31 and a comparator 32.

One end of the first resistor 21 is connected to the power supply 1 and the other end is connected to the drain of the FET 4.
Are grounded via a lamp 9, and the gate is connected to the drive circuit 5. The other end of the first resistor 21 is further connected to a non-inverting input terminal of the comparator 32.

One end of the second resistor section 22 is connected to the power supply section 1, the other end is connected to the emitter of the transistor 31 and the inverting input terminal of the comparator 32, and the base of the transistor 31 is connected to the output terminal of the comparator 32. Connected, and the collector is grounded via the resistance element 7.

As described above, one end of the first resistor 21 and one end of the second resistor 22 are common, and the second resistor 22 is branched from the first resistor 21. The ratio of the resistance value of each of the resistance portions 21 and 22 is set to a predetermined ratio as described later.

The potential control circuit 3 sets the potential V1 at the other end of the first resistor 21 (ie, the drain of the FET 4) to the potential V2 at the other end of the second resistor 22 (ie, the emitter of the transistor 31). The operation will be described later. The drive circuit 5 includes a charge pump circuit (not shown), and applies a drive voltage to the gate of the FET 4 to turn the FET 4 on and off. Control circuit 8
Is for turning on / off the lamp 9 by controlling the on / off of the FET 4 via the drive circuit 5. The resistance element 7 has a known resistance value R0.

The overcurrent detection circuit 6 has the following functions. (1) A potential V0 at a connection point between the transistor 31 and the resistance element 7 (that is, a collector of the transistor 31) is detected,
A function of calculating a supply current I1 to the lamp 9 from the potential V0.

(2) A function of judging an overcurrent when the obtained supply current I1 exceeds a predetermined level, and controlling the drive circuit 5 to turn off the FET4.

FIG. 2 shows the PTC thermistor 2.
(A) is a perspective view, (b) is a plan view, (c) is C in (b).
FIG. 4 is a sectional view taken along line C of FIG. The PTC thermistor 2 can be applied to both an inorganic type mainly composed of ceramics such as barium titanate and a resin type obtained by kneading, molding and cross-linking a mixture of insulating polymer such as polyethylene and conductive carbon. It is.

Here, the means for performing the above-mentioned cross-linking treatment may be a method of irradiating the polymer material with ionizing radiation to form a bond between radicals generated, or a method in which a peroxide is blended with the polymer material, A method of generating a chemical bond by generating a radical in an atmosphere is also effective. By performing such a cross-linking process, when returning from the state of overcurrent abnormality or overheating abnormality to the normal state, the conductivity of the PTC thermistor can return to the same level as before the occurrence of the abnormal state.

When barium titanate is used, a part of barium atoms is replaced by strontium atoms or lead atoms, so that the temperature at which the resistance starts to increase is at least 50%.
It can be controlled in the range of ~ 200 ° C. On the other hand, in the resin type, the temperature at which the resistance starts to increase is governed by the melting point of the base resin. The melting point of the base resin can be adjusted slightly by controlling the crystallinity of the resin, but its range is considerably smaller than that of the inorganic type. In the case of polyethylene, the melting point range is 100-135 ° C depending on its crystallinity.
It changes to the extent. When manufacturing a PTC thermistor using this, the resistance increase start temperature is slightly lower than the above-mentioned melting point due to kneading of conductive carbon, but the current value can be controlled by the amount, area and thickness of kneaded carbon. The control of thickness, shape and shape is easier than that of inorganic type. Other than polyethylene, use of polyvinylidene fluoride or the like is also conceivable. In this case, the melting point changes to 140 to 150 ° C. due to the crystallinity of polyvinylidene fluoride. Therefore, by kneading this with carbon, the temperature at which the resistance starts increasing is 130
It is possible to produce PTC thermistors at 140140 ° C.

As shown in FIG. 2A, the PTC thermistor 2 includes a thermistor 23 and a pair of electrode plates 24 and 25 disposed so as to sandwich the thermistor 23.

The pair of electrode plates 24 and 25 are formed, for example, by attaching nickel foil or the like to the surface of the thermistor portion 23. The pair of electrode plates 24 and 25 enable wiring to be connected to the thermistor portion 23. ing.

As shown in FIGS. 2 (a), 2 (b) and 2 (c), one of the pair of electrode plates 24, 25 is cut out by, for example, a photo-etching method to form a first electrode plate 241. And a second electrode plate 242. At this time, the area of the first electrode plate 241 and the area of the second electrode plate 242
Are divided so that the ratio with respect to the area is N: 1.

In FIG. 1, a pair of electrode plates 2
The other electrode plates 25, 25 are connected to the power supply unit 1, the first electrode plate 241 is connected to the drain of the FET 4, and the second electrode plate 242 is connected to the emitter of the transistor 31. That is, the electrode plate 25 and the first electrode plate 2
The first resistance portion 21 is constituted by the thermistor portion 23 sandwiched by 41, and the electrode plate 25 and the second electrode plate 24
The second resistance portion 22 is constituted by the thermistor portion 23 sandwiched between the two.

Therefore, the area of the first electrode plate 241 and the area of the second
Since the ratio of the area of the electrode plate 242 is N: 1,
Assuming that the resistance of the first resistor 21 is R1 and the resistance of the second resistor 22 is R2, R1: R2 = 1: N.

Next, the operation of the circuit shown in FIG. 1 will be described. The drive voltage is supplied from the drive circuit 5 by the control circuit 8 to FE.
When the voltage is applied to T4, the FET 4 is turned on, a current is supplied to the lamp 9, and the lamp 9 is turned on.

The comparator 32 compares the potential V1 at the other end of the first resistor 21 with the potential V2 at the other end of the second resistor 22. When V1 ≧ V2, the comparator 32 outputs a low-level signal. Is output, and the transistor 3
1 is turned on, a current flows through the second resistance section 22, and the potential V2 rises. On the other hand, when V1 <V2, a high-level signal is output from the comparator 32, the transistor 31 is turned off, and the current does not flow through the second resistor portion 22, and the potential V2 decreases. Thus, the transistor 31
Is maintained at V1 = V2 by turning on and off of.

Therefore, the current flowing through the first resistance portion 21,
That is, the supply current to the lamp 9 is I1, the second resistor 2
2 is I2, V1 = V2, and R2
1: R2 = 1: N, I1: I2 = N: 1
become.

Therefore, assuming that the potential detected by the overcurrent detection circuit 6 is V0, the resistance value of the resistance element 7 is R0, so that I2 = V0 / R0. Therefore, I1 = N ・ I2 = N ・ V0 / R0, and the supply current I1 to the lamp 9 is obtained by the overcurrent detection circuit 6.

As described above, the PTC thermistor 2 is configured to include the first resistance section 21 and the second resistance section 22 branched from the first resistance section 21, and the second resistance section 2
Since the supply current to the lamp 9 is detected based on the current flowing through the lamp 2, the current can be detected with a simple configuration.

Further, P is set so that the ratio between the resistance value of the first resistance portion 21 and the resistance value of the second resistance portion 22 becomes a predetermined ratio.
Since the TC thermistor 2 is configured to maintain the potentials at the other ends of the two resistance portions 21 and 22 at the same potential, the current flowing through the first resistance portion 21, that is, the supply current to the lamp 9 is reduced. The current flowing through the second resistor 22 can be made proportional, whereby the current supplied to the lamp 9 can be determined by detecting the current flowing through the second resistor 22.

Further, as shown in FIG. 2, since the first and second resistance portions 21 and 22 are formed by using one PTC thermistor 2, the characteristics of the first resistance portion 21 and the second And the characteristics of the resistance section 22 of FIG. 1 substantially coincide with each other, and there is no difference in characteristics due to manufacturing variations such as temperature dependency. As a result, even when the temperature rises, the resistance values of the first and second resistance portions 21 and 22 similarly increase in proportion. Therefore, the current flowing through the first resistor 21, that is, the current supplied to the lamp 9 can be accurately determined from the current flowing through the second resistor 22.

Further, since the first resistance portion 21 is constituted by the PTC thermistor 2, the PTC thermistor 2 is provided by, for example, an increase in the ambient temperature or an increase in the flowing current level.
When the temperature rises, the resistance value of the first resistance portion 21 increases, so that the supply current to the lamp 9 can be reduced, thereby protecting the FET 4 against overcurrent and overheating. . Further, since the supply current is reduced, it is possible to reduce the size and cost of circuit components such as the PTC thermistor 2 and the FET 4.

It should be noted that the present invention is not limited to the above embodiment, but can adopt the following modifications.

(1) The method of dividing one electrode plate 24 into a first electrode plate 241 and a second electrode plate 242 is not limited to photo-etching. For example, a vapor deposition method can be used.

(2) As shown in FIG. 3, not only the electrode plate 24 but also the thermistor 23 may be divided. In this case, for example, a laser trimming method can be used for the division.

(3) In the above embodiment, the present invention is applied to the circuit for controlling the current supply to the lamp 9. However, the present invention is not limited to this, and can be applied to the case where the current is detected in another circuit.

[0038]

As described above, according to the present invention,
A first resistor provided between the power supply and the load; and a second resistor provided between the power supply and the ground and branched from the first resistor. A positive-characteristic thermistor in which the resistance of the second resistor is set to a value larger than the resistance of the first resistor by a predetermined ratio, and a current flowing through the second resistor; Current detection means for obtaining a supply current flowing through the first resistor portion from the detected current and a predetermined ratio is provided, so that the supply current to the load can be obtained with a simple configuration,
Circuit components can be protected against temperature rise.

Further, the potential on the load side of the first resistor section and the second
By setting the potential on the earth side of the resistor portion to the same potential, the ratio of the current flowing through the first resistor portion to the current flowing through the second resistor portion becomes the above-described predetermined ratio. The current supplied to the load can be easily obtained from the current flowing through the load and the predetermined ratio.

One of the pair of electrode plates disposed so as to sandwich the thermistor portion is divided into a first electrode plate and a second electrode plate, and is divided by the other of the first electrode plate and the pair of electrode plates. A first resistance portion is constituted by the thermistor portion sandwiched therebetween, and a second resistance portion is constituted by the thermistor portion sandwiched by the other of the second electrode plate and the pair of electrode plates. And the ratio of the area of the second electrode plate to the predetermined ratio, the characteristics of the first resistance portion and the characteristics of the second resistance portion substantially match. The supply current to the load can be obtained with high accuracy.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an embodiment of a current detection circuit according to the present invention.

FIGS. 2A and 2B are diagrams showing a PTC thermistor, wherein FIG. 2A is a perspective view, FIG. 2B is a plan view, and FIG.

FIG. 3 is a view showing a modified PTC thermistor;
(A) is a plan view, and (b) is a cross-sectional view taken along line BB of (a).

[Explanation of symbols]

 Reference Signs List 1 power supply unit 2 PTC thermistor (positive characteristic thermistor) 21 first resistance unit 22 second resistance unit 23 thermistor unit 24, 25 electrode plate 241 first electrode plate 242 second electrode plate 3 potential control circuit 4 FET 6 Overcurrent detection circuit 7 Resistance element 9 Lamp (load)

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Koichi Takagi 1-7-10 Kikuzumi, Minami-ku, Nagoya City, Aichi Prefecture Inside Harness Research Institute, Inc. (72) Inventor Takashi Hoshino 1-Kikuzumi, Minami-ku, Nagoya City, Aichi Prefecture No. 7-10 Harness Research Institute, Inc. F-term (reference) 2G035 AA03 AA15 AB02 AC02 AC15 AD00 AD02 AD03 AD10 AD23

Claims (3)

[Claims] 1. A current detection circuit for detecting a supply current supplied from a power supply unit to a load, comprising: a first resistance unit interposed between the power supply unit and the load; And a second resistor section branched from the first resistor section, wherein the resistance value of the second resistor section is smaller than the resistance value of the first resistor section. A positive-characteristic thermistor set to a value larger by a predetermined ratio and a current flowing through the second resistor, and detecting the current flowing through the first resistor from the detected current and the predetermined ratio. A current detection circuit comprising: current detection means for obtaining a supply current. 2. The current detection circuit according to claim 1, wherein
A current detection circuit comprising a potential control means for setting the potential on the load side of the first resistance portion and the potential on the ground side of the second resistance portion to the same potential. 3. The current detection circuit according to claim 2, wherein
The positive temperature coefficient thermistor includes a thermistor portion and a pair of electrode plates disposed so as to sandwich the thermistor portion. One of the pair of electrode plates is a first electrode plate and a second electrode plate.
And the thermistor portion sandwiched between the first electrode plate and the other of the pair of electrode plates constitutes the first resistance portion. The second resistance portion is constituted by the thermistor portion sandwiched by the other of the pair of electrode plates, and the ratio between the area of the first electrode plate and the area of the second electrode plate becomes the predetermined ratio. A current detection circuit characterized by being configured as described above.