JPH0317262Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] This invention relates to an auto-ranging circuit for a resistance meter.

[Conventional technology]

For resistance meters with multiple measurement ranges,
Auto-range circuits that automatically switch ranges depending on the magnitude of the resistance to be measured have come into use, one example of which is shown in FIG.

Referring to the figure, for example, a predetermined constant DC current I generated in a constant current generating section 1 is applied from terminals 2 and 3 via probes 4 and 5 to a resistor to be measured ₆ having a resistance value R . As a result, a voltage _VX is generated across the resistor 6 to be measured.

This voltage _VX is taken out via the probes 4 and 5 and applied to the measuring section 9 from the terminals 7 and 8.
The measuring section 9 measures this voltage V _X and calculates R _X =V _X ÷I to determine the resistance value R _X of the resistor 6 to be measured. The measuring section 9 is provided with, for example, an amplifier A ₁ , an A/D converter 10 , and a measuring circuit 11 , and the obtained resistance value _R It's getting old.

The auto range circuit of this conventional device includes, for example, four reference resistors provided in the constant current generating section 1.
There are four ranges with R _S1 and R _S4 , and a switch that switches between these four ranges using a range up signal or a range down signal generated from the A/D converter 10 in the measurement section.
It consists of SW. In this conventional example, on the input side of the A/D converter 10, for example, permissible upper and lower limit values of the signal level applied via the amplifier _A1 are set, and a resistor to be measured 6 is set.
When the voltage _VX generated in the range exceeds this upper limit, a range up signal is issued, and on the other hand, when it falls below the lower limit, a range down signal is issued.

The constant DC current I flowing through the resistor 6 to be measured is
For example, it is applied from the DC voltage +V inside the device via the protective resistor R and the transistor Q, and in the example shown, it is applied to the reference resistance of the second range via the switch SW.
It is designed to flow from R _S2 to the ground side. In this case, on the base side of the transistor Q, there is, for example, a control amplifier that maintains the DC current I at a constant value.
A ₂ is provided, and a reference voltage V _S is applied from a reference voltage source 12 to the input end of the amplifier A ₂ . In addition, for example, a switch can be connected to the input terminal.
The reference resistors of each range are connected via SW, and in the illustrated example, the reference resistor R _S2 of the second range is connected.

Here, for example, if a current I ₂ flows from the DC power supply +V to the resistor 6 to be measured, this current I ₂ causes a voltage across the reference resistance R _S2 of the second range to be V ₂ = R _S2 × I _2. A voltage V ₂ is generated and applied to the amplifier A ₂ mentioned above.

Since the amplifier _A2 controls the transistor Q so that the reference voltage V _S applied to the input terminal is equal to the voltage _V2 applied to the input terminal, V _S = V ₂ = R _S2 ×I ₂ Therefore, the value of current I ₂ is I ₂ =V _S ÷R _S2 . Therefore, the current I ₂ flowing through the resistor 6 to be measured
is a known constant current.

[Problem that the invention attempts to solve]

However, in this conventional device, for example, the probe 4 or 5 that supplies a constant DC current for measurement is
If there is a poor contact etc. with the resistor to be measured 6,
Depending on the degree, the auto range circuit may not operate properly, and as a result, the predetermined constant current may not be able to flow completely, resulting in an error in the measured value.

That is, as shown in FIG. 3A, if the current supply side probe 5a of the probe 5 happens to be in a state of poor contact with the resistor to be measured 6 due to the presence of foreign matter such as dust, the contact due to this The resistor r is interposed between the current supply probe 5a and the voltage detection probe 5b, as shown in the equivalent circuit of FIG.

Here, for example, if the measurement range is full scale R
[ _Ω ] range, if R _X <R r > _R , therefore _R In this case, when the amplifier A _Z controls the transistor Q to cause a predetermined constant current I to flow, the voltage between terminals 2 and 3 becomes (R _X +r)×I, and the resistance _R
It will be larger than if only. Since the power supply voltage +V is constant, the operating voltage range of the transistor Q is narrowed by a large amount of the voltage image r×I, and depending on the degree, it becomes impossible to respond to the control from the amplifier A _Z. In this state, the predetermined constant current I cannot flow through the transistor Q, and a smaller current I' is allowed to flow.

In this case, a voltage V _X ' _{= R}
However _{, for} the voltage _{V X} = _R Since V _X ′< _VR for I), it is below the allowable upper limit for the A/D converter 10, and no range up signal is sent.

Therefore, the measuring circuit 11 assumes that this voltage V _X ' is generated by a predetermined current I, and obtains the resistance value R _X ' by the calculation R _X '=V _X '÷I. Therefore, an error occurs with respect to the correct resistance value R _X.

This idea was devised in view of the above points, and its purpose is to select an appropriate range without being affected by contact resistance, etc., between the resistor under test and the probe. An object of the present invention is to provide an auto-ranging circuit for a resistance meter that allows accurate measurement values to be obtained with high resolution.

[Means for solving problems]

Referring to FIG. 1 showing an embodiment of this invention, in order to solve the above-mentioned problems, in this auto-ranging circuit, for example, a current supply terminal 2 which applies a constant DC current for measurement to a resistor 6 to be measured is used. It consists of a comparator 25 that monitors voltage, and a gate circuit 26 that selectively forms a range up signal or a range down signal from the range under signal or range over signal issued from the A/D converter 23 and the monitoring output of the comparator 25. It has a range switching signal generating section 24, and range switching is performed by driving a switch SW using an output signal from the range switching signal generating section 24.

[Effect]

In FIG. 1, it is assumed that a contact resistance r (not shown) is applied to the current supply side probe 5a of the probe 5, for example. When a measurement current is passed through this contact resistance _r and the resistance _R Comparator 25
An H level signal is output from. As a result, even if the A/D converter 23 does not issue a range under signal indicating that the current range is low, a range up signal is sent from the OR circuit 27 to the switch SW, and the range is switched to the appropriate range.

Furthermore, if the set range is too high for the resistance value R _X of the resistor to be measured 6, an overrange signal is generated from the A/D converter. in this case,
Although the output of the comparator 25 is at the L level, a signal made to the H level via the inverting circuit 28 is applied to the AND circuit 29. As a result, a range down signal is sent from the AND circuit 29 to the switch SW, and the range is switched to the appropriate range.

When the range is switched to an appropriate range, neither a range under signal nor a range over signal is generated from the A/D converter 23, and the output of the comparator 25 also becomes L level. Therefore, the outputs of the OR circuit 27 and the AND circuit 29 are both at L level, and the switch
The SW will not operate and the range will be maintained.

[Example] Referring again to FIG. 1, a resistance meter to which this invention is applied includes a constant current generator 21 that applies a constant DC current for measurement to a resistor 6 to be measured, and a constant current generator 21 that applies a constant DC current to the resistor 6 to be measured, and a It consists of a measuring section 22 that measures the voltage generated in the body 6 and determines its resistance value _R.sub.X , and the range switching signal generating section 24 mentioned above.

In this embodiment, the constant current generating section 21 is constructed almost in the same way as the constant current generating section 1 of the conventional device illustrated in _FIG . and four measuring ranges with reference resistances R _S1 to R _S4 .

The measuring section 22 is also configured in substantially the same manner as the measuring section 9 of the conventional device, and includes, for example, an amplifier A ₁ , an A/D converter 23 , and a measuring circuit 11 . This A/D converter 23 includes the A/D converter 23 of the conventional device described above.
Like the D converter 10, for example, a double integral type A/D converter has an allowable upper limit value and a lower limit value set for the signal range input from the amplifier _A1 , taking into account its dynamic range and required resolution. is used.

If the voltage _V In the embodiment, for example, a range under signal is sent from the A/D converter 23 to the range switching signal generating section 24 to notify that the current range is low. Also,
If the input voltage _V A signal is being sent out.

In the range switching signal generating section 24, the voltage at the current supply terminal 2 is compared with a reference voltage V _S ' of a reference voltage source 30 by a comparator 25. In this case, the magnitude of the reference voltage V _S ' is the upper limit value V _FS of the A/D converter 23, and
The voltage generated in the reference resistor R _S is set to a voltage (V _S + V _FS ) that is approximately equal to the sum of the voltage V _S of the reference voltage source 12, so that the voltage at the current supply terminal 2 is When the upper limit value V _FS of the converter 23 is reached, a range up signal is appropriately output.

The voltage _V
Even if the upper limit of the D converter 23 is not exceeded and the range under signal is not sent, if the voltage at the terminal 2 rises due to the contact resistance of the probe 4a or 5a and exceeds the reference voltage V _S ', the comparator 25 An H level signal is emitted from the terminal. As a result, as described in the explanation of the operation above, a range up signal is sent from the OR circuit 27 of the gate circuit 26, and the switch SW is activated to switch to a higher range.

In normal range switching when there is no contact resistance, a range up signal or a range down signal is generated from the OR circuit 27 or AND circuit 29 of the range switching signal generator 24 in response to a range under signal or range over signal from the A/D converter 23. is sent. Note that the operation in the case where neither the range under signal nor the range over signal is output from the A/D converter 23 and the output of the comparator 25 is at L level is omitted since it has been described in the section of the explanation of the operation above.

[Effect of idea]

As explained above in detail, the autoranging circuit of the resistance meter according to the invention has a comparator that compares the voltage of the current supply terminal that applies a predetermined constant DC current to the resistor to be measured with the reference voltage. It receives a range under signal or a range over signal from an A/D converter that digitally converts the voltage generated in the resistor to be measured, and selectively forms a range up signal or a range down signal from this signal and the comparison output of the comparator. It is equipped with a gate circuit that drives the range selector switch.

Therefore, according to this auto-range circuit, even if there is contact resistance between the measurement probe connected to the current supply terminal and the resistor to be measured, the appropriate range can be selected without being affected by it. It is possible to perform accurate measurements with the resolution provided by the range.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the auto-ranging circuit of the resistance meter according to this invention, Fig. 2 is a block diagram of the conventional circuit, and Fig. 3 A and B show the range that occurs between the probe and the resistor to be measured. It is a circuit diagram for explaining contact resistance. In the figure, 2 is a current supply terminal, 21 is a constant current generator, 23 is an A/D converter, 24 is a range switching signal generator, 25 is a comparator, 26 is a gate circuit, 27 is an OR circuit, and 28 is an inversion circuit. , 29 is an AND circuit, 30 is a reference voltage source, R _S is a reference resistor,
V _S ′ is the reference voltage.

Claims (1)

[Claim for Utility Model Registration] A constant DC current for resistance measurement is passed through a resistor to be measured through a switchable reference resistor, and the voltage generated in the resistor to be measured is set to predetermined allowable upper and lower limits. In the auto-ranging circuit of the ohmmeter, which switches the reference resistance based on the range-up signal or range-down signal obtained by comparison, a comparison output is made between the voltage generated across the resistor to be measured and the allowable upper and lower limit values. two gate elements each serving as one input, and a comparator that compares the voltage generated between the current input terminal of the resistor under test and ground with the other allowable upper limit value, and a comparison output from the comparator. A resistor comprising a range switching signal generating section that selectively forms the range up signal or the range down signal in relation to the generated voltage by applying its inverted output to the gate as the other input. Auto range circuit of meter.