JPH055503Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a power supply circuit suitable for a resistance measuring device such as an AC mΩ meter.

BACKGROUND ART Conventionally, a constant current power supply circuit as shown in FIG. 2 has been used in a resistance measuring instrument. The core of the circuit configuration is a differential input type operational amplifier 10.
Generally, operational amplifiers have the advantage that their gain and other characteristics are not affected by the electrical characteristics of the operational amplifier itself, and can be determined freely by external circuitry. Therefore, it is easy to obtain highly accurate linearity and gain, making it convenient to use in resistance measuring instruments. A reference voltage source 12 is connected between the non-inverting input terminal (+) of the operational amplifier 10 and ground. This reference voltage source 12 can be used with either alternating current or direct current, and its voltage value V is constant. operational amplifier 1
The output of 0 is applied to a series circuit of a load 14 corresponding to the resistance to be measured and a reference resistor 16, and is voltage-divided. The voltage divided by the reference resistor 16 is negatively fed back to the operational amplifier 10 and input to the inverting input terminal (-). Note that the resistance value of the load 14 is R _X and the resistance value of the reference resistor 16 is R _S
shall be. Therefore, if the output current of the operational amplifier 10 is I, the voltage across the reference resistor 16 will increase due to the imaginary short between the input terminals of the operational amplifier 10.
V _R is equal to the voltage V of the reference voltage source 12, and V _RS =
The formula I·R _S =V holds true. Note that all operational amplifiers are designed so that the potential difference between input terminals is 0, and both terminals are at the same potential. FIG. 3 is a characteristic curve diagram of the load resistance value R _X and the voltage V _RX across it. Its V ₀ is the saturation voltage of the operational amplifier 10.
As is clear from this characteristic curve diagram and the above equation, even if the load resistance value R _X is varied, the equation I=V/R _S holds true as long as the output of the operational amplifier 10 does not reach the saturation voltage V ₀ . Since the reference voltage V and the reference resistance R _S in this equation are both known constant values, the output current I of the operational amplifier 10 is constant and calculation is possible. As a result, the constant current I flows through the load 14. Therefore, the voltage across the load 14 V _RX
When measuring, V _RX = I・R _X , so R _X =
The resistance value _RX can be determined from V _RX /I. Note that the constant current value I can be made variable by changing the reference voltage V and the reference resistance value R _S .

Further, a constant voltage power supply circuit as shown in FIG. 4 is also used in the resistance measuring device. As is clear from this figure, in the constant voltage power supply circuit, the load 14 and the reference resistor 16 in the constant current power supply circuit are simply switched and connected, and the other circuit configurations are the same.
Therefore, if the output current of operational amplifier 10 is I, then
Due to the imaginary short between the input terminals of the operational amplifier 10, the voltage V _RX across the load 14 is constant and equal to the voltage V of the reference voltage source 12, and V _RX =
The formula I·R _x =V holds true. FIG. 4 is a characteristic curve diagram of the load resistance value _RX and the current I flowing therein. The I ₀ is the saturation current value of the operational amplifier 10.
As is clear from this characteristic curve diagram and the above equation, even if the load resistance _{value R RX} , and the voltage V _RS at both ends thereof is V _RS = _I.R.sub.S = _V.R.sub.S / _R.sub.X . Therefore, when we measure the voltage V _RS across it, we find that R _X = R _S・
Since both the reference voltage V and the reference resistance R _S are known constant values, the resistance value R _X can be determined from V/V _RS . Note that if the reference voltage V is changed, the voltage V _RX across the load 14 will naturally also change.

Problems to be solved by the invention However, when measuring low resistance such as relay contacts using such a constant current power supply circuit,
If the terminal voltage (output voltage of operational amplifier 10) is high,
There is a risk of destroying the oxide film. This is because an oxide film is formed on the surface of the relay contacts, but such an oxide film must not be destroyed, and the resistance value must be measured in that state. However, if the measurement terminal is open, it is the same as if a load with an infinite resistance value is connected to it, and the voltage between both terminals reaches the saturation voltage V ₀ and moves from the constant current region. It's off. Therefore, when a load 14 having a low resistance _Rx is connected thereto, a high saturation voltage _V0 is instantaneously applied as a terminal voltage, and the oxide film is destroyed before the constant current I is reached.

Furthermore, when measuring the resistance of a semiconductor element such as a Zener diode using a constant current circuit, if an excessive current flows therein, there is a risk of destroying the element. This is because semiconductor elements have a particularly small allowable current compared to other elements, and must not exceed this.

The present invention was developed by focusing on these conventional problems, and by improving the voltage applied to the load or the current flowing through it, it can be applied to resistors with low allowable voltage such as relay contacts, and semiconductor elements with low allowable current. The purpose of the present invention is to provide a power supply circuit equipped with an output limiting function suitable for measuring small resistances, etc.

Means for Solving the Problems Means for achieving the above object will be described below with reference to FIG. 1, which corresponds to an embodiment.

This constant current power supply circuit equipped with an output limiting function includes a first operational amplifier 20 that inputs a reference voltage _V1 ,
A voltage divider 24 that divides the output of the operational amplifier 20
and a second operational amplifier 30 that inputs the divided voltage _V2 . Then, the output of the second operational amplifier 30 is applied to a series circuit of a reference resistor 32 and a load 34 to divide the voltage, and the voltage drop of the reference resistor is negatively fed back to the one operational amplifier 20, and the voltage drop of the load resistor is is negatively fed back to the other operational amplifier 30.
Note that in the constant voltage power supply circuit with an output limiting function, the feedback destination operational amplifier is the opposite of the constant current power supply circuit, and the other configurations are the same.

Effect The above means works as follows.

The output of the first operational amplifier 20, which inputs the reference voltage _V1, is divided, and the second operational amplifier inputs the divided voltage _V2 .
The output of the operational amplifier 30 is connected to the reference resistor 32 and the load 34.
The voltage drop across the reference resistance is negatively fed back to one of the operational amplifiers 20, and the voltage drop across the load resistance is applied to the other operational amplifier 3.
When negative feedback is applied to 0, the voltage-divided output V ₂ of the first operational amplifier 20 is applied to the series circuit of the reference resistor 32 and the load 34 through the second operational amplifier 30. The open terminal voltage between the terminals is _V2 , which is smaller than the saturation voltage _V10 of the first operational amplifier 20. As a result, if the voltage divider 24 connected to the output terminal of the first operational amplifier 20 is changed and the open terminal voltage V ₂ is appropriately controlled to a small value by changing the voltage division ratio, the relay is activated while the constant current I ₁ is flowing. It is possible to measure even low-resistance objects such as contacts whose allowable voltage is small and whose oxide films are easily destroyed. In addition, in a constant voltage power supply circuit, the voltage drop across the reference resistor is negatively fed back to an operational amplifier different from the constant current circuit mentioned above, and the voltage drop across the load resistor is negatively fed back to another operational amplifier, so a constant voltage is applied to the load resistor. In addition, by operating the voltage divider to change its voltage division ratio and selecting a reference resistance value, it is possible to appropriately limit the current flowing thereto to be smaller than the saturation current of the first operational amplifier.
Therefore, it is possible to measure resistances such as semiconductor devices that have a small allowable current.

Embodiments Hereinafter, embodiments of the present invention will be described based on the accompanying drawings.

FIG. 1 is a diagram showing a constant current power supply circuit for a resistance measuring instrument equipped with an output limiting function according to a first embodiment of the present invention. In the figure, 20 is a differential input type first operational amplifier. 22 is a reference voltage source connected between the non-inverting input terminal (+) of the first operational amplifier 20 and the ground. Its voltage value V ₁ is constant. 24 is a voltage divider that divides the voltage of the output of the first operational amplifier 20. For the voltage divider 24 connected between the output terminal of the first operational amplifier 20 and the ground, a series circuit of resistors 26 and 28, for example, is employed. Note that their resistance values are R ₁ for the resistor 26 and R ₂ for the resistor 28. 3
0 is a differential input type second operational amplifier. A connection point between both resistors 26 and 28 is connected to the non-inverting input terminal (+), and a divided voltage V ₂ from the first operational amplifier 20 is input thereto. 32 is a reference resistor, 34 is a load, both form a series circuit, and the second operational amplifier 3
Connected between the 0 output terminal and ground. The resistance value of the reference resistor 32 is R _S1 and the resistance value of the load 34 is R _X1 . Note that both terminals A and B to which the load 34 is connected
are the measurement terminals. 36 is the reference resistance 32
This is a third operational amplifier whose input terminals are connected to both ends of the operational amplifier. The output terminal of the first operational amplifier 20
By connecting the inverting input terminal (-) of the reference resistor 32 to the inverting input terminal (-) of the first operational amplifier 20, the voltage V _RS1 across the reference resistor 32
to leave negative feedback. Further, the measurement terminal A, which corresponds to the connection point between the reference resistor 32 and the load 34, is connected to the inverting input terminal (-) of the second operational amplifier 30. Therefore,
The voltage across the load 34 V _RX1 is the second operational amplifier 30
to leave negative feedback.

Next, the operation of such a constant current power supply circuit will be explained. First, a reference voltage V ₁ is applied to the non-inverting input terminal (+) of the first operational amplifier 20. Then, the first
The output of the operational amplifier 20 is divided by a voltage divider 24 according to their resistance values R ₁ and R ₂ . The voltage V ₂ across the resistor 28 thus divided is input to the non-inverting input terminal (+) of the second operational amplifier 30 . Next, the voltage V ₂ is connected to the inverting input terminal (-) by an imaginary short between the input terminals of the second operational amplifier 30.
The voltage at the measurement terminal A, that is, the voltage across the load 34, V _RX1 (=V ₂ ), remains unchanged. or,
The voltage V _RS1 across the reference resistor 32 is the voltage across the third operational amplifier 3.
6, the imaginary signal of the first operational amplifier 20
By short, it is equal to the reference V ₁ . Note that in the design of an operational amplifier, the voltage between the input terminals is the output voltage. Therefore, if the current flowing through the reference resistor 32 is I ₁ , then V _RS1 =I ₁ ·R _S1 =V ₁ holds true. In addition,
Figure ₆ is _a characteristic curve diagram of the load resistance value _R
The saturation voltage is 0. _{Therefore ,} even if _{the load resistance value R}
The current I ₁ flowing through is a constant current of I ₁ =V ₁ /R _S1 .
When measuring resistance using this constant current power supply circuit, the voltage between open terminals A and B is V ₁₀ .
R ₂ /(R ₁ +R ₂ ), which is smaller than the saturation voltage V ₁₀ of the first operational amplifier 20. Therefore, by operating the voltage divider 24 connected to the output terminal of the first operational amplifier 20 and changing the voltage division ratio determined by the ratio of the resistance values R ₁ and R ₂ , the open terminal voltage can be set to 0.02 to 0.2V, for example.
If the voltage is controlled to be as small as possible, it becomes possible to measure even low-resistance devices such as relay contacts whose permissible voltage is small and whose oxide films are easily destroyed.

However, there is a limit to the output current _I1 of the second operational amplifier 30, and only about 10 to 30 mA can be obtained. Therefore, if it is desired to increase the output current _I1 , a current booster circuit, for example, a complementary circuit 40 is added to the output side of the second operational amplifier as shown in FIG. Then, the output current is
It can be increased to about 200mA.

FIG. 8 is a diagram showing a constant voltage power supply circuit for a resistance measuring instrument having an output limiting function according to a second embodiment of the present invention. In this constant voltage power supply circuit, the feedback destination operational amplifier is the opposite of the constant current power supply circuit described above, and the other circuit configurations are the same. In the figure, 5
0, 60, and 66 are the first, second, and third operational amplifiers. 52 is a reference voltage source, and its voltage value
_V3 is constant. 54 is a voltage divider such as a resistor 56
and 58 in series, and their resistance values are R ₃ for the resistor 56 and R ₄ for the resistor 58. The second operational amplifier 60 receives the divided voltage V ₄ . 62 is a reference resistor with a resistance value R _S2 , and 64 is a load with a resistance value R _X2 . Note that both terminals C and D to which the load 64 is connected serve as measurement terminals. Of the voltages across each of the reference resistor 62 and the load 64, the voltage drop V _RS2 of the reference resistor is negatively fed back to the second operational amplifier 60 via the third operational amplifier 66, and the voltage drop V _RX2 of the load resistor is applied to the measurement terminal. 1st operational amplifier 5 from C
Negative feedback is given to 0. Therefore, the voltage V _RS2 across the reference resistor 62 is equal to the divided output V ₄ of the first operational amplifier 50, and the voltage V _RX2 across the load 64 is equal to the reference voltage V ₃ . Therefore, if the current flowing through the reference resistor 62 is I ₂ , then V _RS2 =I ₂ ·R _S2 =V ₄ holds true. Note that the same current I ₂ also flows through the load 64. FIG. 9 is a characteristic curve diagram of the load resistance value R _X2 and the current I ₂ flowing therein. The I ₁₀ is the saturation current of the first operational amplifier 50. Therefore, the load resistance value
Even if R _X2 is made variable, when the condition I ₁₀ · R ₄ ≧ I ₂ · R _S2 is satisfied, the current I ₂ flowing through the reference resistor 62 and the load 64 becomes I ₂ =V ₄ /R _S2 . However, V ₄ ,
Since both R _S2 are known constant values, I ₂ is also a constant value. Therefore, the load 64 has a constant current.
By selecting V ₄ and R _S2 while applying V ₃ , the current I ₂ flowing there can be connected to the first operational amplifier 50.
The saturation current _I10 can be controlled appropriately. Therefore, it is possible to measure resistances such as semiconductor devices that have a small allowable current.

Effects of the invention According to the invention explained above, while passing a constant current through a resistor with a small allowable voltage such as a relay contact,
By controlling the voltage applied there to an appropriate small value, and applying a constant voltage to resistors with small allowable currents such as semiconductor elements, the current flowing through them can be controlled to an appropriate small value, thereby preventing the destruction of those components. You can measure resistance, etc. without any trouble.

[Brief explanation of the drawing]

FIG. 1 shows a constant current power supply circuit for a resistance measuring instrument equipped with an output limiting function according to a first embodiment of the present invention. FIG. 2 shows a conventional constant current power supply circuit for a resistance measuring instrument. FIG. 3 is a characteristic curve diagram showing the relationship between the load resistance value _RX and the voltage V _RX across it in the constant current power supply circuit shown in FIG. FIG. 4 shows a conventional constant voltage power supply circuit for a resistance measuring instrument. FIG. 5 is a characteristic curve diagram showing the relationship between the load resistance value _RX and the current I flowing therein in the constant voltage power supply circuit shown in FIG. FIG. 6 is a characteristic curve diagram showing the relationship between the load resistance value R _X1 and the voltage V _RX1 across it in the constant current power supply circuit shown in FIG. FIG. 7 is a diagram showing a modification of the constant current power supply circuit shown in FIG. 1. FIG. 8 shows a constant voltage power supply circuit for a resistance measuring instrument having an output limiting function according to a second embodiment of the present invention. FIG. 9 is a characteristic curve diagram showing the relationship between the load resistance value R _X and the current I ₂ flowing therein in the constant voltage power supply circuit shown in FIG. 20,50...first operational amplifier, 22,52...
... Reference voltage source, 24, 54 ... Voltage divider, 30, 6
0...Second operational amplifier, 32,62...Reference resistor, 34,64...Load, 36,66...Third operational amplifier.

Claims (1)

[Scope of utility model registration request] The second operational amplifier includes a first operational amplifier inputting a reference voltage, a voltage divider dividing the output of the operational amplifier, and a second operational amplifier inputting the divided voltage.
The output of the operational amplifier is applied to a series circuit of a reference resistor and a load to divide the voltage, the voltage drop across the reference resistor is negatively fed back to one of the operational amplifiers, and the voltage drop across the load resistor is negatively fed back to the other operational amplifier. A power supply circuit equipped with an output limiting function characterized by: