Electronic ohmmeter in haste. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic ohmmeter in haste. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the manufacture of electrical measuring instruments, some difficulties may arise associated with the manufacture of instrument shunts. These shunts are usually low impedance. and you need to choose them carefully, since the accuracy of the meter depends on this. To do this, it is proposed to make a simple electronic ohmmeter, which can measure small resistances with a linear scale on four limits: 10, 25.100 and 250 ohms.

The scheme of the device is shown in the figure. It consists of a stabilized current source on the transistor VT1. the mode of operation of which is set by the zener diode VD1 and resistors R3. R4, R5, and a voltmeter (microammeter RA1 and resistors R1, R2).

Quick electronic ohmmeter

The collector current of the transistor VT1 creates a voltage across the resistor Rx proportional to its resistance. Therefore, if you calibrate (i.e. set the pointer of the microammeter to the last division of the scale) the measuring part according to a certain exemplary resistor Roop. then the measured resistance can be read on the linear scale of the measuring device.

Working with the device is as follows. To the "Rx" terminals, connect the resistor to be tested (for example, a shunt being manufactured), and to the "Ro6p" terminals, an exemplary resistor corresponding to the selected measurement limit. Switch SA2 is transferred to the appropriate measurement limit, and switch SA1 is in the "K" position (calibration). After the supply voltage is applied, by pressing the SB1 button, the tuning resistor R4 sets the pointer to the last division of the scale. Then the switch SA1 is moved to the "AND" (measurement) position and the resistance Rx is measured. The measurement accuracy will mainly depend on the accuracy of the reference resistors.

If an auxiliary device uses a power source with a voltage of 8 ... 9 V or a less sensitive head, then the D814A zener diode must be replaced with a KS139A or KS147A, the resistance of the resistor R5 should be reduced to 100 ohms. a R4 - up to 470 - 680 ohms. In addition, if the resistance of the reference resistor does not correspond exactly to the required measurement limit, then it is permissible to calibrate the meter with the reading corresponding to the nominal value of this resistor if it is at least 80% of the limit.

The device can be used exemplary resistors types MT, BLP, S2-29V. C2-36. C2-14: MLT resistors (R1. R3. R4. R5): resistor R2 of types SPO-0.5, SP3-4b or similar; transistors of the KT814 series. KT816 with a base current transfer coefficient of more than 50. As a RA1 microammeter, a measuring head is applicable, which will be installed in the manufactured device (for example, 50 or 250 μA). Switches SA1 and SA2 are TV2-1 toggle switches. Generally speaking, the SA1 switch can be omitted, leaving one pair of terminals, to which the Rocp resistor is first connected. and after calibration - resistor Rx.

If more common p-pn transistors are used in the device, the polarity of switching on the power supply of the stabilizer of the throne and the microammeter should be changed.

Author: V.Sychev, Moscow

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