A simple digital megohmmeter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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An article by S. Biryukov with the same title ("Radio", 1996, No. 7, pp. 32, 33) describes a resistance meter with an upper limit of 2G0m, a lower limit of 200 Ohm (resolution - 0,1 Ohm). Many radio amateurs in their letters ask to talk about the possibility of expanding the measurement range towards low resistances, for example, by introducing limits of 20 and 2 ohms. The author tells about such a wide-range ohmmeter.

It would seem that everything is very simple - just add two measurement limits in the SA1 switch, introduce additional reference and current-setting resistors 10 and 100 times less in resistance than for the 200 Ohm limit - and you can measure resistance up to fractions of an ohm. However, the resistance of the connecting wires, as well as the instability of the resistance of the contacts of switches and clamps for connecting the measured resistors, will not allow realizing the required accuracy.

The four-wire resistance measurement method will help here (Fig. 1). A relatively stable current is passed through the resistor under test and one pair of clamps, set by the power source and one of the resistors R31, R32. The voltage drop across the measured resistance is taken by the second pair of clamps and fed to the measuring input of the ADC. With this measurement scheme, the voltage drop across the switch contacts, clamps and wires does not affect the result. In addition, the accuracy of setting the current in the circuit does not affect, since the ADC measures the ratio of voltages across the controlled resistance and the reference one (one of the resistors R29, R30).

The ohmmeter circuit switching circuit is shown in fig. 2, the numbering of the newly introduced elements continues the previous one. The measuring circuits (see Fig. 1) are powered by the voltage difference between the battery and the internal stabilizer of the ADC KR572PV5 microcircuit (-3 V). The load capacity of this stabilizer for the outflowing current is increased by connecting an emitter follower on the transistor VT1 to its output.

Additional section SA1.4 eliminates the summation of the resistance of the switch contacts and reference resistors R29, R30.

Resistors R2 and R33 bridge pins 1 and 4,5 and 3, respectively. This does not affect the accuracy in any way, since their resistance is much greater than that of contacts and wires, but it greatly simplifies switching.

Connecting pin 2 of the XS2 socket to the +U06p input of the ADC and placing it between pins 1,4 and 5,3 helps to reduce the effects of connector leakage currents on measurement accuracy at high-resistance limits.

As indicated in the main article, it is useful to reduce reference resistors operating at limits of less than 200 kOhm by 0,1 ... 0,2% relative to the values ​​\u29b\u30bspecified in the diagram. To do this, in parallel with resistors R0,1 and R0,2 (their tolerance must be no worse than 750 ... 7,5%), resistors with a resistance of XNUMX Ohm and XNUMX kOhm should be connected, respectively.

In the design of the SA1 switch, the PG2-8-12P4N type is used. Transistor VT1 - any p-pn structure, with a dissipation power of at least 350 mW and a base current transfer coefficient h21E of at least 100 at a collector current of 100 mA.

Due to the fact that the current consumption is high at low-resistance limits (up to 100 mA), it is advisable for the ohmmeter to make a stabilized power supply with a voltage of 9 ... 10 V. You can use an adapter for a voltage of 12 V and a current of up to 300 mA, supplementing it with a stabilizer chip KR142EN8A (or KR142EN8G). For the stability of its operation, a 1 μF ceramic capacitor should be connected in parallel with the output, placing it next to the microcircuit.

Recommendations for the selection of elements, the design of the printed circuit board, the design, and the adjustment are the same as for the previously described version of the device. As XS1 and XS2, you can use standard low-frequency ONTS-VG connectors with the appropriate number of sockets. To the four pins of the mating plug, solder multi-colored wires with crocodile clips at the ends.

When measured within 2; The 20 and 200 Ohm plug of the measuring cable connector is connected to the XS1 socket and the controlled resistor is connected to the meter with four clamps (1 and 4 - to one output, 5 and 3 - to the other). Within 2; 20 and 200 kΩ, you can use two clamps connected to pins 4 and 5. Within 2 MΩ - 2 GΩ, the plug is switched to the XS2 socket and the clamps connected to pins 1 and 3 are used. It is better to turn on the power source after connecting a controlled resistor - this will reduce time of establishment of indications.

You can increase the convenience of using the device by making clamps with insulated jaws. To do this, cut off the teeth of one of the "crocodile" sponges and solder a plate of double-sided foil fiberglass in their place. The role of one of the clamps will be performed by the sponge remaining with the teeth, the role of the second will be the surface of the plate. The remaining teeth should be trimmed so that they do not touch the insert during measurement. These clamps can be used on all measuring ranges.

When using mains power in devices with CMOS microcircuits, which include KR572PV5, it is necessary to protect from static electricity those microcircuit inputs to which external elements can be connected during operation. In this ohmmeter, these are pins 30, 31, 35 and 36 of the microcircuit. The easiest way to do this is as inputs 30 and 31 are protected in the multimeter previously described by the author ("Radio", 1996, No. 5, p. 34, Fig. 3) - using 510 kΩ resistors for inputs 30 and 31 and 51 kΩ for inputs 35 and 36 and capacitors 0.01 uF connected to each protected input. Elements R25.C5 are not installed.

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