Attachment for measuring low resistances. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The smallest resistance measurement limit available in most common digital multimeters is 200 ohms. But quite often there is a need to perform an accurate measurement of resistances with smaller values: up to 20 ohms when checking the windings of transformers, relays and up to 2 ohms in the manufacture and selection of shunts for electrical measuring systems. The prefix, the description of which is given in this article, allows, with a scale of 200 mV (direct current) of an auxiliary digital device, to measure resistance on two limits - up to 2 and 20 ohms.

The principle of operation of the attachment is based on measuring the voltage drop across the analyzed resistor when a fixed current flows through it. Schematic diagram of the device is shown in Fig.1. The current is generated by a current generator on the transistor VT1. Its high stability is ensured by the operation of the amplifier on the DA2 chip, which controls the operation of the transistor. The value of the fixed current when measuring resistances up to 2 ohms is 100 mA, and for resistances up to 20 ohms -10 mA. The DC amplifier chip, in turn, is powered by a stabilized voltage generated by the DA1 chip to increase the stability of the entire device.

The measurement limit is set by switch SA1. The SB1 button is turned on only for the duration of the measurements. Diode VD1, connected in parallel with the measured resistor, protects the multimeter when the set-top box is turned on without a resistor.

The design of the set-top box must be compact, with a length of connecting conductors no more than 10 cm. The transistor must be equipped with a small heat sink made of aluminum. It is convenient to use alligator clips to connect the measured resistor. Particular attention should be paid to the method of connecting the multimeter probes. They must be connected directly to the terminals (see Fig. 2) in which the measured resistor is installed - in this case, the resistance of the probe conductors will not affect the readings during measurements.

Instrument calibration. Before starting the calibration, the sliders of the variable resistors R4 and R7 should be set to their middle positions. Then connect a current source with a voltage of 8 ... 24 V (current not less than 150 mA) to the prefix. The fixed value of current through the measured resistor can be set in two ways.

The first method will require the use of a milliammeter of a fairly good class (better than a digital one with a number of digits of 4,5). Connect the probes of the milliammeter to the clamps for the measured resistance. Set the switch of the prefix SA1 to the position of measuring resistance 2 ohms (upper according to the diagram), and on the milliammeter - the limit is 200 mA. Press the SB1 button and adjust the variable resistor to set the current to 100 mA.

Then switch SA1 to the position of measuring resistance up to 20 ohms (lower according to the diagram), set the limit of the milliammeter to 20 mA. Press the SB1 button and set the current to 4 mA with a variable resistor R10. Repeat the specified current calibration again and then lock the variable resistor sliders with varnish or paint.

The second calibration method is to use 1 and 10 ohm reference resistors. By adjusting the specified variable resistors in each of the subranges, set the voltage drop across the exemplary resistors of 100 mV.

In the design of the device, it is possible to use the K1401UD2A microcircuit, the K142EN5A microcircuit stabilizer, the KT815B transistor and the KD103A diode

Author: S.OWSIAK

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