Ohmmeter with linear scale. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Ohmmeter with linear scale. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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If you look at the scale of the dial indicator of an ohmmeter of almost any avometer, it is easy to see that it is non-linear - it is stretched near the zero mark, and compressed at the end. It is inconvenient to use such a scale, and even if you decide to build such an ohmmeter yourself, you are unlikely to be able to calibrate its scale.

A completely different matter is an ohmmeter with a linear scale, when its own dial indicator scale remains suitable for reading the readings. The scheme of just such a measuring device is shown in the figure.

(click to enlarge)

An ohmmeter is capable of measuring the resistance of resistors or other parts, say, windings of chokes, inductors, transformers, electric motors, in the range from tenths of an ohm to hundreds of kiloohms. The entire range is divided into five subranges, each of which is set by the SA1 switch. Its section SA1.1 connects to the part under study, the conclusions of which are connected to sockets X1 and X2, a limiting resistor (R2, R4, etc.) of the divider, and section SA1.2 - an exemplary resistor (R1, R3, etc.). ).

The voltage falling on the part under study is fed to a cascade made on a field-effect transistor VT1. The second stage is assembled on a transistor VT2. The cascades are connected to each other in a bridge circuit, one of the diagonals of the bridge includes a pointer indicator RA1, on the scale of which the measurement result is read. The bridge is balanced with a variable resistor R13, setting the indicator needle to zero, and the maximum current through the indicator is limited by resistor R15, setting its arrow to the final division of the scale.

When the voltage of the controlled part (or calibration voltage) is supplied to the gate of the transistor VT1, the bridge is unbalanced, a current flows through the pointer indicator, the value of which is the greater, the greater the voltage drop at the sockets X1, X2, and therefore, the greater the measured resistance.

The linearity of the ohmmeter scale is ensured by the flow of a practically stable direct current through the part under test, which has a resistance, since the limiting resistor on each subrange is selected with a resistance 62 times greater than the maximum measured resistance. The measurement error in this case is small - no more than 1,5%, which is quite acceptable in amateur radio practice.

The ohmmeter is powered from the AC mains through a step-down transformer T1. Alternating voltage from the secondary winding of the transformer is supplied to the rectifier bridge, assembled on diodes VD4-VD7. The rectified voltage is filtered by capacitor C2 and then fed to a parametric stabilizer made on a ballast resistor R17 and zener diodes VD2, VD3 connected in series. A stable voltage of 12 V is supplied to the input voltage divider, formed by one of the current-setting resistors and the circuit under test (or a reference resistor). A voltage of 8,5 V is used to power the transistor stages. The total current consumed by the ohmmeter does not exceed 30 mA.

The R11C1 filter is installed to prevent sharp throws of the indicator arrow when a resistor of greater resistance is connected to the input sockets of the ohmmeter compared to the maximum measured on this subrange. The same task is performed by the zener diode VD1, which limits the maximum voltage at the gate of the transistor VT1.

Reference resistors R1, R3, R5, R7, R9 should be selected with an accuracy of 1%, current-supplying R2, R4, R6, R8, R10 can be with a tolerance of 10%, the remaining fixed resistors - up to 20%. Variable resistors R13, R15 - any type. Pointer indicator RA1 - M265M or another microammeter with a full deflection current of the arrow 100 μA. Transistors, in addition to those indicated in the diagram, can be with letter indices G, E. A transformer with a power of at least 1 W with a secondary winding for a voltage of 12 ... 15 V. If the rectified voltage exceeds 15 V, an oxide capacitor should be installed at the appropriate rated voltage .

There are no special requirements for the design of the device - it can be arbitrary. Of course, the dial indicator and all controls and input jacks must be located on the front panel.

How to use an ohmmeter? Having connected the tested resistor to the input sockets, press the switch button SB1 and set the indicator needle to zero with the variable resistor R13 (initial division of the scale). Then set the SA2 switch to the "Calibration" position, when the group of contacts SA2.1 opens and SA2.2 closes. Variable resistor R15 sets the indicator needle to the final division of the scale. After that, switch SA2 is returned to the "Measurement" position (shown in the diagram).

A similar procedure is carried out on each subrange, and the measurements begin with the "1OOk" subrange, then moving the SA1 switch to other positions - until a subrange is found on which it is possible to more accurately measure the controlled resistance. The range of resistance measured by an ohmmeter can be increased to 1 MΩ by setting the SA1 switch to six positions. The additional limiting resistor should be 62 MΩ, and the calibration resistor should be 1 MΩ,

When repeating the ohmmeter, you can do without the resistor R14 by connecting the gate of the transistor VT2 to the negative power wire. To reduce the influence of the leakage current of the zener diode VD1 on the measurement accuracy, it is recommended to connect any low-power diode in series with the zener diode VD1 (anode to the gate), and install an MLT-O.125 resistor with a resistance of 4,7 kΩ between the zener diode cathode and the drain of the transistor. With the introduction of the 1 MΩ range, this refinement is mandatory.

Author: N. Serebrov, Nizhny Novgorod

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