JP2004234064A - Constant current circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant current circuit which can be manufactured at low costs without separately providing any special temperature compensating circuit, and which can stably supply constant currents in a wide surrounding temperature range while suppressing the increase of the size of a device or the increase of power consumption. <P>SOLUTION: This constant current circuit 1 is provided with a first transistor Tr1 whose emitter is connected to an input terminal 2, and whose collector is connected to an output terminal 8, a second transistor Tr2 whose collector is connected to the base of the first transistor Tr1, and whose emitter is connected through a constant voltage element 4 to a circuit ground G and a voltage control means 7 for detecting collector currents I<SB>C1</SB>of the first transistor Tr1, and for controlling the base potential of the second transistor Tr2, and for controlling the collector currents I<SB>C1</SB>to constant currents. In the constant current circuit 1, a capacitor Cx connected between the base of the second transistor Tr2 and the circuit ground G has ≥1.5μF capacitance. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a constant current circuit for supplying a constant current, and more particularly to a constant current circuit suitable for stably supplying a constant current even when the ambient temperature changes.
[0002]
[Prior art]
Conventionally, a constant current circuit has been used as a power supply for charging a capacitor or a secondary battery, a power supply for driving a light emitting diode, or a power supply for charging a solar battery in which a capacitor or a secondary battery is combined with a solar battery. Have been. Such a constant current circuit is generally configured to exhibit constant current characteristics by combining semiconductor elements such as transistors and constant current diodes.
[0003]
FIG. 5 shows an example of such a conventional constant current circuit. The conventional constant current circuit 11 includes a first transistor Tr1 having an emitter connected to a first input terminal 2 to which a DC voltage is applied and a collector connected to an output terminal 8, and a collector connected to the base of the first transistor Tr1. base There emitter with being connected to the second transistor Tr2, which is connected to circuit ground G through a constant voltage element 4, by detecting the collector current I _C1 of the first transistor Tr1 of the second transistor Tr2 by controlling the potential and a voltage control unit 7 for controlling the collector current I _C1 of the first transistor Tr1 to the constant current.
[0004]
Wherein between the emitter and base of the first transistors Tr1, it is connected to the field effect transistor 3 so as to reduce the ripple of the collector current I _C1 of the first transistor Tr1.
[0005]
The second transistor Tr2 is for controlling the base current _{I B1} of the first transistor Tr1. Therefore, the between the base and the collector of the second transistor Tr2 of the first transistors Tr1, a first resistor R1 is inserted, first by defining the maximum value of the base current I _B1 The collector current I _C1 of the transistor Tr1, that is, the maximum value of the output current is specified. The resistance value of the first resistor R1 is selected so that the first transistor Tr1 is not damaged by an overcurrent.
[0006]
The constant voltage element 4 inserted between the emitter of the second transistor Tr2 and the circuit ground G is constituted by a Zener diode or the like, and gives a reference potential to the emitter. Thereby, the voltage V _BE between the base and the emitter can be kept constant with respect to the change in the ambient temperature.
[0007]
Further, the base of the second transistor Tr2 is connected to a connection point between the second resistor R2 and the third resistor R3 connected in series. The second resistor R2 and the third resistor R3 are inserted between the second input terminal 5 and the circuit ground G to divide the DC voltage applied to the second input terminal 5. Accordingly, the Zener voltage Vz of the zener diode 4 are determined voltage plus the base-emitter voltage V _BE, the base voltage V _B2 is set to a predetermined voltage value.
[0008]
On the other hand, the voltage control unit 7, the current detection unit 6 for detecting a collector current I _C1 of the first transistor Tr1 is connected. The current detection unit 6 is arranged between the collector of the first transistor Tr1 and the output terminal 8, and outputs a detection result to the voltage control unit 7. Then, the voltage controller 7 receives the detection result of the collector current _IC1 from the current detector 6, and if the current value is lower or higher than a predetermined set value, the second transistor Tr2 It is adapted to move up and down the base potential V _B2 of. This feedback control, since the base current I _B1 of the first transistor Tr1 collector current I _C2 of the second transistor Tr2 is controlled is controlled, so that the collector current I _C1 is the final output current is finely adjusted It has become.
[0009]
Next, the operation of the conventional constant current circuit 11 will be described.
[0010]
In the constant current circuit 11, when a DC voltage is applied to the emitter of the first transistor Tr1, a collector current _IC1 and a base current _IB1 are output from the collector and the base, respectively. The collector current _IC1 is output from the output terminal 8 as an output current. The collector current I _C1 is determined in proportion to the base current I _B1 according to the static characteristics of the transistor, and the base current I _B1 is determined in proportion to the collector current I _C2 of the second transistor Tr2. The collector current _{I C2} of the second transistor Tr2 is determined in proportion to the base current _{I B2} of the second transistor Tr2 accordance static characteristics of the transistor. Therefore, the constant current circuit 11, by controlling the base voltage _{V B2} of the second transistor Tr2, the base current _{I B2} of the second transistor Tr2 is Sadamari, thereby, the first transistor Tr1 collector current _{I C2} is defined result base current I _B1 of being controlled, the collector current I _C1 is the final output current is defined.
[0011]
Then, for example, the collector current I _C1 of the first transistor Tr1 is reduced, which current detector 6 outputs to the voltage control unit 7 detects. The voltage control unit 7 increases the base potential _VB2 of the second transistor Tr2 according to the current value _IC1 detected by the current detection unit 6 so as to compensate for the decrease. As a result, the base current I _B2 and the collector current I _{C2 of} the second transistor Tr2 and the base current I _B1 of the first transistor Tr1 respectively increase, and finally compensate for the decrease in the collector current I _C1 of the first transistor Tr1. A constant current is output from the output terminal. Similarly, if the case where the collector current I _C1 of the first transistor Tr1 is increased, which current detector 6 outputs to the voltage control unit 7 detects. Voltage control unit 7 reduces the base potential _{V B2} of the second transistor Tr2 so as to compensate the increase of the collector current _{I C1.} As a result, the base current I _B2 and the collector current I _{C2 of} the second transistor Tr2 and the base current I _B1 of the first transistor Tr1 decrease, respectively, and finally the increase in the collector current I _C1 of the first transistor Tr1 is canceled. A constant current is output from the output terminal.
[0012]
However, a semiconductor element such as a transistor used in the constant current circuit 11 has temperature dependency and is easily affected by an ambient temperature. For example, in the case of a transistor, the voltage V _BE between the base and the emitter has a negative correlation with temperature, and fluctuates by about −2 mV per 1 ° C. of temperature change. Therefore, the constant current circuit 11 composed of such a semiconductor element is easily affected by the ambient temperature, and the output current becomes unstable with the temperature change. For this reason, in the conventional constant current circuit 11, the usable temperature range is limited, and it cannot be used in cold regions or in a closed environment exposed to direct sunlight.
[0013]
In order to solve such a problem, conventionally, some constant current circuits having a temperature compensation circuit have been proposed. For example, Japanese Patent Application Laid-Open No. 6-45674 proposes a laser diode drive circuit with a temperature compensation circuit for keeping the light output constant with respect to a temperature change (see Patent Document 1). This laser diode driving circuit with a temperature compensation circuit connects a first variable resistor, an FET (Field Effect Transistor) and a second variable resistor in series, and connects this to the laser diode and the transistor. Connected in parallel. Then, a constant current circuit is configured by the FET and the second variable resistor to set a circuit current corresponding to a temperature change, and is converted to a base voltage of the transistor by the first variable resistor, and the light output of the laser diode is kept constant. Is supplied from the transistor. It is stated that this makes it possible to take out a constant optical output from the laser diode by controlling each variable resistor even when the temperature changes.
[0014]
Japanese Patent Application Laid-Open No. 2000-201073 proposes a constant current source circuit for supplying a stable current by compensating for a change in performance caused by a change in temperature (see Patent Document 2). In this constant current source circuit, a first current source having a negative temperature coefficient and a second current source having a positive temperature coefficient are connected to a mixer. The mixer mixes the current from each current source at a desired ratio. Thus, even if the current amount of each current source increases and decreases with a change in temperature, a stable current can be supplied by mixing the current amounts to have a constant value.
[0015]
[Patent Document 1]
JP-A-6-45674 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-201073
[Problems to be solved by the invention]
However, the temperature compensation techniques described in each of the above-mentioned publications have disadvantages in that the manufacturing cost increases, the entire device increases, and the power consumption increases. Further, there is a problem that the allowable range of the circuit constant is narrow and adjustment is not easy.
[0017]
The present invention has been made in order to solve such problems, and can be manufactured at low cost without separately providing a special temperature compensation circuit, and suppresses an increase in the size of the device and an increase in power consumption. It is another object of the present invention to provide a constant current circuit capable of stably supplying a constant current over a wide ambient temperature range.
[0018]
[Means for Solving the Problems]
A feature of the constant current circuit according to the present invention is that a first transistor having an emitter connected to the input terminal and a collector connected to the output terminal, a collector connected to the base of the first transistor, and an emitter connected to the constant voltage. A second transistor connected to a circuit ground via an element and a collector current of the first transistor are detected to control a base potential of the second transistor to control a collector current of the first transistor to a constant current. A constant current circuit including a voltage control means, wherein a capacitor connected between the base of the second transistor and circuit ground has a capacitance of 1.5 μF or more, or A resistor having a resistance of 5Ω or more connected in parallel, wherein the capacitor has a capacitance of 1.5 μF or more and 470 μF or less; A.
[0019]
By setting the capacitor connected to the bias circuit in this way to a predetermined capacitance, or setting the capacitive impedance in which a resistor and a capacitor are connected in parallel to a predetermined resistance and capacitance, The temperature dependence of the base potential of the two transistors is reduced, and a constant current can be stably output from the first transistor in a wide temperature range from -50 ° C to 80 ° C. If the capacitance of the capacitor exceeds 1.5 μF and exceeds 470 μF, the voltage control means oscillates and does not function as a constant current control circuit. If the resistance value of the resistor connected in parallel to the capacitor is less than 5 kΩ, the effect of the capacitor is reduced and the temperature dependence of the output current increases.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of a constant current circuit according to the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing a constant current circuit 1 of the present embodiment. Note that among the components of the present embodiment, the same or corresponding components to those of the above-described conventional constant current circuit 11 are denoted by the same reference numerals, and detailed description thereof will not be repeated.
[0021]
In the constant current circuit 1 of the present embodiment, a first transistor Tr1 having an emitter connected to the first input terminal 2 and a collector connected to the output terminal 8, and a collector connected to the base of the first transistor Tr1. second transistors Tr2, the first transistor and the second transistor Tr2 to detect a collector current I _C1 of Tr1 whose emitter is connected to circuit ground G via a configured constant-voltage element 4 with Zener diodes with and a voltage control unit 7 for controlling the collector current I _C1 of the first transistor Tr1 to the constant current by controlling the base potential. Further, a field effect transistor 3 for reducing ripple is connected between the emitter and the base of the first transistor Tr1. Further, a first resistor R1 is provided between the base of the first transistor Tr1 and the collector of the second transistor Tr2 to determine a maximum base current of the first transistor Tr1 and prevent an overcurrent from flowing. Is inserted. Note that a resistor may be connected instead of the field effect transistor 3.
[0022]
In the constant current circuit 1 of the present embodiment, a capacitor Cx having a predetermined capacitance and a resistor Rx having a predetermined resistance are connected in parallel between the base of the second transistor Tr2 and the circuit ground G. The connected capacitive impedance 9 is connected. The capacitive impedance 9 is connected in series to the second resistor R2 between the second input terminal 5 and the circuit ground G, and the connection point is connected to the base of the second transistor Tr2. . The capacitive impedance 9 defines the base potential of the second transistor Tr2 by the resistor Rx, and reduces the temperature dependency of the base potential by setting the capacitor Cx to a predetermined capacitance. It has become. As the capacitor Cx, for example, various capacitors such as a ceramic capacitor, an electrolytic capacitor, a mica capacitor, and a polyester film capacitor can be used. As the resistor Rx, various resistors such as a carbon resistor, a carbon film resistor, a metal oxide film resistor, and a metal thin film resistor can be used.
[0023]
The base of the second transistor Tr2 is connected to a voltage controller 7 composed of an OP amplifier or the like for controlling the base potential. The voltage controller 7 further includes a first transistor Tr1. Is connected to a current transformer 6 for detecting the current value of the collector current _IC1 .
[0024]
Then, the constant current circuit 1 of the present embodiment, by defining the base potential of the second transistor Tr2, which defines a collector current I _C1 of the first transistors Tr1, if this collector current I _C1 is set value If the current increases or decreases, the current is detected by the current detector 6, and the voltage controller 7 controls the output current to be constant by reducing or increasing the base potential of the second transistor Tr2.
[0025]
Next, with respect to the capacitive impedance in the present embodiment described above, an experiment was performed to find a combination of the resistance value of the resistor Rx and the capacitance of the capacitor Cx that can avoid the temperature dependence of the second transistor. In the experiment, various types of resistors Rx having different resistance values and capacitors Cx having different capacitances were prepared, and the combinations thereof were changed to detect an output current with respect to a change in ambient temperature.
[0026]
Here, the type of the resistor Rx has a resistance value of 1 kΩ, 5 kΩ, 10 kΩ, 100 kΩ, and 1000 kΩ, and the type of the capacitor has a capacitance of 0.1 μF, 1.5 μF, 10 μF, 470 μF, and 1000 μF. Was used. Then, the constant current circuit 1 was placed in a housing insulated with styrene foam, and the ambient temperature was changed from -50C to 80C. Then, as shown in FIG. 2, the results were obtained by variously combining the resistance value of the resistor Rx and the capacitance of the capacitor Cx. Note that Test No. 1 is a result of a conventional constant current circuit 11 as a comparative test.
[0027]
According to the experimental results shown in FIG. 2, Test No. 1 is obtained by connecting only the resistor Rx of 10 kΩ alone. However, the temperature dependency appears strongly, and the output current value becomes −10 ° C. at the stage of −10 ° C. The output current value could not be detected at −50 ° C. and 80 ° C.
[0028]
Next, in Test Nos. 2 to 6, in order to determine a suitable capacitance, the resistance value of the resistor Rx was fixed at 5 kΩ, and the capacitance was varied from 0.1 μF to 1000 μF. As a result, when the capacitance was 0.1 μF (test number 2) and 1000 μF (test number 6), the voltage control unit 7 oscillated and the constant current control was not performed. On the other hand, when the capacitance is 1.5 μF (Test No. 3), 10 μF (Test No. 4) and 470 μF (Test No. 5), the output current value becomes almost constant in a wide temperature range from −50 ° C. to 80 ° C. Indicated. FIG. 3 shows the relationship between the output current value and the ambient temperature in Test No. 1 and Test No. 3. Thus, a remarkable improvement was recognized as compared with the conventional constant current circuit 11. In addition, due to the performance of the digital thermometer used, the temperature could not be measured below −50 ° C. However, when the temperature of the inside of the housing was directly reduced and the output current value was measured, no sharp drop was observed, and probably − It is expected that practical use is possible up to about 70 ° C.
[0029]
Next, based on the results of Test Nos. 2 to 6, the capacitance of the capacitor Cx was fixed at 1.5 μF, and the resistance value was varied from 1 kΩ to 1000 kΩ. As a result, when the resistance value is 1 kΩ (test No. 7), the temperature dependency appears strongly, and the output current value decreases to almost half at the stage of −10 ° C., and the output current value decreases at −50 ° C. and 80 ° C. Could not be detected. On the other hand, when the resistance value is 5 kΩ (test number 8), 100 kΩ (test number 9) and 1000 kΩ (test number 10), the output current value is almost constant in a wide temperature range of −50 ° C. to 80 ° C. in any of the tests. The value was shown. Further, when the resistor was opened and the capacitor Cx of 1.5 μF was connected alone as shown in FIG. 4 (test number 11), the output current value was similarly reduced in the temperature range of −50 ° C. to 80 ° C. Was constant. Note that although Test No. 3 and Test No. 8 have the same resistance value and capacitance, the output current values differ because the resistor Rx and the capacitor Cx used are different. As described above, there is an error in the output current value depending on each product.
[0030]
From the above test results, at least, the capacitance of the capacitor Cx connected between the base of the second transistor Tr2 and the circuit ground G is set to 1.5 μF or more, or the capacitor Cx and the resistor Rx Are connected in parallel, and the resistance value of the resistor Rx is set to 5 kΩ or more and the capacitance of the capacitor Cx is set to 1.5 μF or more and 470 μF or less, the temperature dependence of the base potential of the second transistor Tr 2 It has been found that the characteristics can be remarkably reduced and a stable current can be supplied in a wide temperature range of -50 ° C to 80 ° C.
[0031]
Therefore, according to the constant current circuit 1 of the present embodiment, it is possible to connect the predetermined capacitor Cx between the base of the second transistor Tr2 and the circuit ground G without separately providing a special temperature compensation circuit. Alternatively, a stable current can be output in a wide temperature range only by providing the capacitive impedance 9 including the capacitor Cx and the resistor Rx connected in parallel. Further, since the capacitive impedance 9 is constituted only by the capacitor Cx and the resistor Rx, it can be manufactured at an extremely low cost, and it is possible to suppress an increase in the size of the entire device and an increase in power consumption.
[0032]
Note that the constant current circuit 1 according to the present invention is not limited to the above-described embodiment, and can be appropriately changed.
[0033]
For example, one Zener diode 4 in the present embodiment may be connected, or a plurality of Zener diodes 4 may be connected in series. Further, a capacitor may be connected in parallel to the Zener diode 4 to remove noise.
[0034]
【The invention's effect】
As described above, according to the present invention, it is possible to manufacture at low cost without separately providing a special temperature compensation circuit, and stably in a wide ambient temperature range while suppressing an increase in the size of the device and an increase in power consumption. This produces effects such as the ability to supply a constant current.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing one embodiment of a constant current circuit according to the present invention.
FIG. 2 is a diagram showing a result of measuring an output current value with respect to an ambient temperature by changing a capacitance of a capacitor and a resistance value of a resistor in the embodiment.
FIG. 3 is a graph in which test numbers 1 and 3 in FIG. 2 are plotted.
FIG. 4 is a circuit diagram showing another embodiment of the constant current circuit according to the present invention.
FIG. 5 is a circuit diagram showing a conventional constant current circuit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Constant current circuit 2 1st input terminal 3 Field effect transistor 4 Constant voltage element (Zener diode)
5 second input terminal 6 current detection unit 7 voltage control unit 8 output terminal 9 capacitive impedance Cx capacitor G circuit grounds _IC1 , _IC2 collector currents _IB1 , _IB2 base current R1 first resistor R2 second resistor R3 Third resistor Rx Resistor Tr1 First transistor Tr2 Second transistor

Claims (2)

A first transistor having an emitter connected to the input terminal and a collector connected to the output terminal; and a first transistor having a collector connected to the base of the first transistor and having an emitter connected to circuit ground via a constant voltage element. A constant current circuit including two transistors and voltage control means for detecting a collector current of the first transistor and controlling a base potential of the second transistor to control the collector current of the first transistor to a constant current. So,
A constant current circuit, wherein a capacitor connected between the base of the second transistor and circuit ground has a capacitance of 1.5 μF or more. 2. The constant current circuit according to claim 1, further comprising a resistor of 5Ω or more connected in parallel to the capacitor, wherein the capacitor has a capacitance of 1.5 μF or more and 470 μF or less. 3.

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