Measuring Resistance in Proteus VSM

Proteus VSM offers a powerful range of measuring instruments, as may be displayed by clicking on the Virtual Instruments Mode icon.

A notable omission from the above list is a resistance meter, which is a feature found with the least expensive multimeter. All is not lost however, since Proteus VSM offers an ohmmeter component which may be inserted in the same way as any other electronic device, appearing as shown below.

Rather strangely, the ohmmeter has four terminals instead of the two expected. More on this shortly, but for the moment, just connect the left and right terminal pairs together, and then link these across the resistor network to be be measured.

As expected, the ohmmeter confirms that the total resistance of two resistors in series is the sum of the individual resistances.

The ohmmeter may be fine tuned by right-clicking and selecting the Edit Properties option, as shown below.

Basic configuration options include the instrument range (ohms, kilo-ohms or mega-ohms) and the display precision, which defaults to five digits.

The Excitation Current option gives a first clue to the function of the four terminals, defaulting to a value of 1 mA. A little experimentation confirms the outer two terminals act as a constant current DC source, while the inner two measure the resulting potential difference. A basic calculation using Ohm's law allows the resistance of the connected network to be determined. This behaviour may be illustrated by connecting DC ammeters to each terminal, as shown below.

It is also possible to make the voltmeter polarity sensitive by enabling the Sign Polarity setting, in which case swapping the inner two terminals will cause a negative resistance to be displayed!

The ability to find the equivalent resistance of a network of resistors can be useful in circuit analysis, and is one of the basic steps in finding a Thévenin or Norton equivalent circuit.

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