Megger-prefix to the multimeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This easy-to-use add-on, together with the 83x Series multimeter, which has a maximum resistance measurement limit of 2 MΩ, allows you to directly measure the resistance of resistors and high-resistance circuits up to 20 MΩ. An additional power supply for the set-top box is not required.

It is known that inexpensive and popular among radio amateurs multimeters of the 83x series without additional nodes or calculations do not allow measuring resistance more than 2 MΩ. The proposed prefix expands the measurement limits up to 20 MΩ. The value of the measured resistance is displayed on the display of the multimeter. As in other set-top boxes developed by the author, power (+3 V) is supplied to it from the internal stabilizer of the multimeter's ADC microcircuit.

The attachment diagram is shown in fig. 1. A current source (IT) is assembled on the op-amp DA1.1 and resistors R3-R6 according to the scheme known in the radio engineering literature as Howland IT. The author has already used such a node in his earlier development [1]. Its output current is calculated based on the following conditions: R3 = R5, R4 = R6 for the convenience of their subsequent selection; the current through the resistor R6 is equal to the algebraic sum of the currents through the resistor R3 and the measured resistor R_x the input currents of the op-amp DA1 are negligible. The op-amp is covered by a deep DC OOS through the R4R5 divider, therefore, equal voltages are set at its both inputs (inverting and non-inverting) if the output voltage is less than the maximum at a given supply voltage. In this case, the IT output current (I_O) will be equal to: I_O = U_R2/R3, where U_R2 - voltage at the output of the resistive divider R1R2 (i.e., across the resistor R2). This voltage serves as a model for IT, since the resistance of the resistor R2 is significantly less than the resistance of the resistor R3.

Rice. 1. Attachment scheme

The IT output current is chosen to be 0,1 μA, and it is quite enough to measure the resistance of resistors up to 20 MΩ, since the voltage drop across it will not exceed 2 V, which is less than the supply voltage of the set-top box (3 V). With the resistances of the resistors R3-R6 indicated in the diagram, the DA1.1 op-amp is guaranteed to operate in a linear mode, providing high stability and constancy of the IT output current flowing through the measured resistor R_x, and hence the high linearity of the dependence of the voltage drop across the measured resistor or circuit. This voltage is fed to the input of the buffer amplifier, made on the op amp DA1.2 (input resistance - not less than 1 GΩ) with unity voltage gain. To interface with a multimeter, a resistive voltage divider R7R8 is used, which reduces the voltage at the output of the op-amp DA1.2 by ten times. From the output of the divider, it goes to the input "VΩmA" of the multimeter for subsequent measurement.

The current consumption of the set-top box is almost equal to the current consumption of the DA1 chip. Measurement error in the range from 2 to 19,99 MΩ - no more than 3%.

The prefix is ​​assembled on a board made of fiberglass laminated on one side, its drawing is shown in fig. 2, the location of the elements on it - in fig. 3. MCP602 op amp can be replaced by domestic op amp KR1446UD4A (in DIP8 package) [2]. When replacing with another Rail-to-Rail op amp, it should be borne in mind that its inputs must be made on field-effect transistors (input resistance - at least 1 GΩ), the minimum supply voltage - no more than 3 V and the current consumption (per case) - no more than 3 mA. To reduce the error when measuring resistances less than 2 MΩ, the zero bias voltage should not exceed 1...2 mV. Blocking capacitor C1 - tantalum K53-1, resistors - MLT, C2-33, high-resistance - KIM.

Pairs of resistors R3 and R5, R4 and R6 should be selected using a multimeter with a resistance deviation of not more than 1% in each pair. At the same time, the deviation of the resistance from the nominal does not affect the measurement accuracy - their equality is important. The resistances in each pair can be reduced to 1,5 MΩ and 300 kΩ, respectively. In this case, the voltage across the resistor R2 must be reduced based on the equality UR2 (B) = 0,1xR3 (MΩ). For example, if R3 = R5 = 1,6 MΩ, R4 = R6 = 330 kΩ, then R1 = 27 kΩ, R2 = 1,6 kΩ. Pin XP1 - suitable from the connector or a piece of tinned wire of suitable diameter. A hole for it in the board is drilled "in place" after installing the pins XP2, XP3. Pins XP2 and XP3 - from probes for a multimeter. Input sockets XS1, XS2 - screw terminal block ED350V-02P from DINKLE or similar.

Rice. 2. Drawing of a board made of fiberglass laminated on one side

Rice. 3. The location of the elements of the set-top box on the board

The photo (Fig. 4) shows the attachment connected to the multimeter when measuring the KIM-0,125 resistor with a nominal resistance of 15 MΩ and a tolerance of ± 10% from the nominal value.

Rice. 4. Prefix connected to the multimeter

When working with a prefix, the switch of the type of work of the multimeter is set to the position of measuring direct voltage at the limit of "200mV". Before calibration, in order to avoid failure of the internal stabilizer +3 V of the ADC, the set-top box is first connected to an autonomous power supply with a voltage of 3 V (two 1,5 V galvanic cells connected in series can be used) and the current consumption is measured, which should not exceed 3 mA, and then connected to a multimeter. Next, the calibration is carried out by connecting to the sockets XS1, XS2 "Rx" a resistor with a resistance of several megaohms with a known measured resistance or an accuracy class of at least 1%. A selection of resistor R7 achieves the desired readings on the indicator. Indications, taking into account the comma, are divided by ten. Please note that to facilitate calibration, the R7 resistor on the board is made up of two resistors connected in series. On fig. 3 they are designated as R7' and R7".

Literature

1. Glibin S. LC-meter - attachment to the multimeter. - Radio, 2014, No. 8, p. 21-24.
2. KR(KF)1446UDxx operational amplifiers. - URL: qrz.ru/reference/micro/datasheet/1446ud. pdf.

Author: S. Glibin

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