Ohmmeter with linear scale. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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This device is a simplified version of the design described by the author of these lines in the article "Ohmmeter with a linear scale" in "Radio", 1998, No. 3, p. 38. It differs in the range of measured resistances - from fractions of an ohm to 500 ohms.
The ohmmeter circuit is shown in the figure. The range of measured resistances is divided into three sub-ranges with limiting resistance values of 10, 100 and 500 ohms. They are selected by the SA1 switch, and the SA1.1 section connects the limiting resistors (R1 and R2, R4 and R5, R7 and R8 connected in series), and the SA1.2 section - exemplary (R3, R6, R9). The latter must be selected before installation in the device with an accuracy of 1%.
The position of the SA2 switch sections shown in the diagram corresponds to the measurement mode. When connected to the sockets X1 and X2 of the resistor, the voltage falling on it is supplied through the resistor R10 to the pointer indicator RA1. Resistor R10 limits the discharge current of capacitor C3 through the measured circuit. At the same time, the R10C3 circuit eliminates sharp throws of the indicator arrow when the measurement subrange is incorrectly selected or the resistor is disconnected from the input jacks. A similar role is played by the zener diode VD1.
Transferring the switch handle SA2 to another position (contacts SA2.1 open, SA2.2 - closed) corresponds to the calibration mode. With the help of trimmers R1, R4, R7, the indicator needle is set to the final division on the corresponding subrange.
As for the accuracy of measurements, on the sub-range "10" it corresponds to 3%, on the sub-range "100" - about 5%, on "500" - about 10%. The increase in the measurement error in the last two subranges is explained by the shunting of the resistance of the measured circuit by the resistance of the dial indicator circuit.
The ohmmeter is powered by AC power. The step-down transformer must provide a voltage of 14 ... 15 V on the winding II at a load current of at least 35 mA. The voltage rectified by the VD2-VD5 diodes is supplied to the DA1 stabilizer, and from its output to the measuring circuits.
When the measured circuit or resistor is disconnected from the input, the indicator needle goes off scale. You should not be afraid of this, since a current of no more than 1 mA flows through the indicator (with a resistance of the indicator frame of about 3500 Ohms). This can be avoided by setting the switch to the calibration position and moving it to the measurement position only when connected to the sockets X1, X2 of the monitored circuit.
Author: N. Serebrov, Nizhny Novgorod
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