JPH09145750A - Constant-current circuit for digital multimeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To supply a prescribed constant current to a resistor to be measured even when the power supply voltage supplied to an operational amplifier is low by supplying a constant voltage which is opposite in polarity to the voltage supplied to the high potential-side measurement terminal of the resistor to the low potential-side measurement terminal of the resistor. SOLUTION: A constant-current circuit is provided with an opposite-polarity constant-voltage supplying section 50 which supplies a constant voltage which is opposite in polarity to a compliance voltage VR2 supplied to a constant-current generating section 80 from a compliance voltage generating section 70 to the low potential-side measurement terminal Lo6 of a resistor Rx15 to be measured. Since the section 50 supplies a constant voltage of, for example, +3V to the terminal Lo6 so as to give a 6-V potential difference to the terminal Lo6 as the voltage drop of a constant-current circuit system, the section 70 can generate a -3-V voltage as the compliance voltage VR2. Since the power supply voltage supplied to an operational amplifier 21 can be as low as -4V to -5V, a stable constant current can be supplied to the resistor Rx15 even when the power supply voltage supplied to an operational amplifier 21 is low (for example, +5V). Therefore, a small-sized inexpensive resistance measuring circuit can be constituted, because no exclusive high-voltage power source exclusively is required for the resistance value measuring range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resistance value measuring function of a digital multimeter, which uses a low power supply voltage (for example, ± 5 V) to apply a predetermined constant current to a resistance to be measured. A corresponding resistance measuring circuit.

[0002]

2. Description of the Related Art Generally, a plurality of voltage sources of ± 15 V and ± 5 V are used as a power source for a digital multimeter. The high power supply voltage ± 15V is mainly required for analog circuits such as operational amplifiers. As shown in FIG. 2, the circuit configuration of the main part of the resistance measurement of the conventional digital multimeter includes a compliance voltage generation part 70, a constant current conversion part 80, a protection circuit 4, a voltage measurement part 5, and a resistance to be measured. R
x and.

The compliance voltage generator 70 includes a reference voltage source REF1 and divider resistors R11, R12, R1.
3 and an operational amplifier U21, which receives the reference voltage source REF1 and supplies a compliance voltage VR2, for example, −6V, which is inverted and amplified by the operational amplifier U21, to one end of the constant current conversion unit 80. The constant current conversion unit 80 includes an operational amplifier U22, a standard resistor Rs14, and a FET Q32.
The compliance voltage VR2 is used as one end, and a predetermined constant current IS3 that makes the potential difference across the standard resistor Rs14 constant is supplied to the high potential side measurement terminal Hi7 via the protection circuit 4. Here, although the resistance measurement range switching circuit is omitted in FIG. 2, a circuit example in which a plurality of standard resistors Rs14 are provided and the standard resistors are switched to generate a desired constant current (on the order of μA to mA) is provided. It is common. Here, at least 1 V or more is required to maintain the FET Q32 in a normal active state. The voltage across the reference resistor Rs14, which determines the constant current value, is given a constant potential difference (for example, 1 V) due to the relation of measurement accuracy.

The protection circuit 4 is provided with a protection circuit of a transistor circuit in order to protect the internal constant current circuit system even when a voltage is applied from the outside due to an incorrect use condition. A voltage drop of 1 V or more is required at both ends. However, depending on the measuring instrument, there is a circuit configuration in which the protection circuit 4 is not provided. The voltage measuring unit 5
The voltage generated between both terminals of the measured resistance Rx by the constant current is received, differentially amplified, AD-converted, and converted into a resistance value. After that, although not shown, it is displayed and output by a display device or the like. The high potential side measurement terminal Hi7 is connected to one end of the protection circuit 4, and the low potential side measurement terminal Lo6 is connected to the circuit ground. By the way, it is assumed that a maximum of 3 V is applied to both ends of the measured resistance Rx.

As described above, the high potential side measuring terminal H
As a required voltage between i7 and the compliance voltage VR2 end, a voltage value obtained by adding at least each voltage drop,
That is, 3V + 1V + 1V + 1V = 6V is required, and as a result, the compliance voltage VR2 is required to generate -6V. As a power supply voltage for the operational amplifier to output this voltage, it is necessary to supply a power supply voltage higher than this, and a power supply of 7 V or higher is required. Therefore, in general, a power supply voltage of 10V to 15V is used to supply the constant current IS3 accurately and stably.

[0006]

As described above, in the resistance value measuring range, a high power supply voltage is required in order to provide a desired stable constant current IS3. In other voltage measurement ranges and current measurement ranges, low power supply voltage is sufficient because the constant current application function is unnecessary. As described above, providing a dedicated high-voltage power supply for the resistance value measuring range causes a cost increase, which is a difficulty in realizing a small size and low cost. Therefore, the problem to be solved by the present invention is to realize a resistance value measuring circuit system capable of applying a predetermined constant current to a resistance to be measured even at a low power supply voltage (for example, ± 5 V) without using a high power supply voltage. And

[0007]

First, in order to solve the above-mentioned problems, in the structure of the present invention, the low-potential-side measuring terminal Lo6 applied to the other end of the measured resistance Rx is connected to the high-potential-side measuring terminal Hi7. A reverse polarity constant voltage supply means 50 for supplying a constant voltage having a polarity opposite to that of the voltage applied to the circuit is provided. As a result, a constant current from the constant current conversion unit 80 is supplied to the high-potential-side measurement terminal Hi7 that is applied to one end of the resistance to be measured Rx, and the resistance measurement circuit stabilizes the resistance to be measured even at a low power supply voltage (± 5 V). A predetermined constant current can be applied.

More specifically, the low-potential-side measurement terminal Lo6 applied to the other end of the resistance Rx to be measured has a polarity opposite to that of the compliance voltage VR2 supplied to one end of the constant current conversion unit 80 by the compliance voltage generation unit 70. There is a constituent means for providing a reverse polarity constant voltage supply means 50 for supplying a constant voltage. Accordingly, the compliance voltage generating section 70, the constant current converting section 80, the protection circuit 4, and the voltage measuring section 5 are provided, and a constant current is applied to one end of the measured resistance Rx from the high potential side measurement terminal Hi7. In the circuit for measuring the resistance value, a stable predetermined constant current can be applied to the resistance to be measured even with a low power supply voltage (± 5 V).

Further, a switching relay 10 for switching and supplying a circuit ground potential or a constant voltage having a polarity opposite to that of the compliance voltage VR2 is provided to the low potential side measurement terminal Lo6 which is applied to the other end of the measured resistance Rx. There is a means of configuration. This case can be used for resistance measurement in the low power measurement mode.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below in detail with examples.

[0011]

EXAMPLE In the present invention, paying attention to the fact that a low power supply voltage has a polarity of, for example, ± 5 V, a reverse constant voltage is supplied to the low potential side measurement terminal Lo6 of the measured resistance Rx. The characteristic feature is that the voltage drop of the constant current circuit system can be effectively used up to about 10V.

(1) As shown in FIG. 1A, the circuit configuration of the constant current circuit for applying a constant current to the measured resistance Rx of the present invention is as shown in FIG.
In place of the connection to the circuit ground, a reverse polarity constant voltage supply means 50 for providing a constant voltage having a reverse polarity to the compliance voltage VR2 is provided. In addition, this is an example in which the power source voltage can be applied only to the power source of ± 5V. Reverse polarity constant voltage supply means 50
Supplies a constant voltage of, for example, + 3V in order to provide a 6V potential difference as a voltage drop of the constant current circuit system in the conventional example. As a result, the compliance voltage VR2 may be -3V. This means that the power supply voltage applied to the operational amplifier 21 may be -4V to -5V, and it can be realized with a low power supply voltage of ± 5V.

In the description of the above embodiment, the case where the constant voltage of +3 V is supplied from the reverse polarity constant voltage supply means 50 to the low potential side terminal Lo6 has been explained.
Some have a resistance measurement function in a low power measurement mode by applying a small constant current. Here, the low power measurement mode is a measurement mode that is used mainly when it is desired to reduce resistance value fluctuations due to heat generation in the measured resistance Rx.
In this case, the potential difference across the measured resistance Rx is reduced from 3 V to 0.3 V. Therefore, as shown in FIG. 1B, a switching relay 10 for connecting the low potential side terminal Lo6 to the circuit ground is added. The circuit configuration may be such that the control means of the switching relay 10 is provided in correspondence with the low power measurement mode, and the same operation can be performed with a low power supply voltage of ± 5V.

[0014]

Since the present invention is configured as described above, it has the following effects. By supplying the constant voltage having the opposite polarity to the compliance voltage VR2 to the low potential side terminal Lo6 by the reverse polarity constant voltage supply means 50, it becomes possible to operate the circuit normally even at a low power supply voltage. It is only necessary to use a low power supply voltage of ± 5V, and it is possible to eliminate the dedicated high voltage power supply for the conventional resistance value measurement range.
It became feasible at low cost.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a main part of a resistance measuring circuit part according to the present invention; and (b) a main part circuit structure of a resistance measuring circuit part to which a switching relay 10 having a low power measurement mode function is added. It is a figure.

FIG. 2 is a circuit diagram of a main part of a resistance value measuring circuit of a digital multimeter according to a conventional technique.

[Explanation of symbols]

 REF1 Reference voltage source VR2 Compliance voltage IS3 Constant current 4 Protection circuit 5 Voltage measurement unit Lo6 Low potential side terminal Hi7 High potential side terminal 8 Positive potential supply circuit 10 Switching relay Rs14 Standard resistance Rx15 Measured resistance U21, U22 Operational amplifier Q32 FET 50 Reverse Polarity constant voltage supply means 70 Voltage generator 80 Constant current converter

Claims (3)

[Claims] 1. A resistance measuring circuit for supplying a constant current from a constant current converting section (80) to a high potential side measuring terminal (Hi7) applied to one end of the resistance to be measured (Rx) A low polarity constant voltage supply means (50) for supplying a constant voltage having a polarity opposite to the voltage applied to the high potential side measurement terminal (Hi7) is provided to the low potential side measurement terminal (Lo6) given to the other end of the A constant current circuit for a digital multimeter characterized by being provided. 2. A compliance voltage generation section (70), a constant current conversion section (80) and a voltage measurement section (5) are provided, and a constant current is supplied from the high potential side measurement terminal (Hi7) to the resistance (R) to be measured.
x), the compliance voltage generator (70) supplies one end of the constant current unit 80 to the low potential side measurement terminal (Lo6) supplied to the other end of the measured resistance (Rx). A constant current circuit for a digital multimeter, which is provided with a reverse polarity constant voltage supply means (50) for supplying a constant voltage having a reverse polarity to (VR2), and is provided with the above. 3. A compliance voltage generation section (70), a constant current conversion section (80) and a voltage measurement section (5) are provided, and a constant current is supplied from a high potential side measurement terminal (Hi7) to a resistance (R) to be measured.
In the resistance measuring circuit given to x), the low potential side measuring terminal (Lo6) given to the other end of the resistance to be measured (Rx) is the circuit ground potential or the constant voltage having the opposite polarity to the compliance voltage (VR2). A constant current circuit for a digital multimeter, which is provided with a switching relay (10) for switching and supplying.