How to measure electrical voltage ?

Here we see a VOM configured as a voltmeter, used to measure voltage across resistor 3 in a three-resistor circuit:

First, note how the meter’s test leads touch the resistor’s terminals, placing the meter in parallel with the resistor. A “parallel” network is defined as one having exactly two sets of electrically common points, with all components connected between those them. Touching the red test lead to the top of fi3 makes the “-t-” jack of the VOM electrically common to the top of fi3, while touching the black test lead to the bottom of 3 make the “COMMON —” jack of the meter electrically common to the bottom of 3. arallel connections ensure components share the same voltage, and with any voltage measurement our goal is for the meter to experience the exact same voltage as the component whose voltage we are interested in measuring.

Note also how we must respect the polarité (-t-, —) of the measured voltage when connecting our meter to any direct-current (DC) circuit. The terminal of the resistor we expect to be at a greater potential (+) is the one we touch with the red probe, while the black probe touches the terminal at lower potential (—). Correct polarity is very important when using a VOM to measure voltage because this is necessary to drive the meter’s pointer up the scale (clockwise rotation). Connecting the voltmeter backwards would drive the pointer in the other direction, forcing » it against the left- hand stop. The particular VOM shown in this illustration is modeled after a Simpson 260, which has a polarity-reversing switch intended to reverse the roles of red and black test leads in case of 3This is commonly referred to as pegging the pointer, because the mechanical stops on an analog
meter movement usually consist of small metal pegs.

accidental “backwards” connection. Normally one simply reverses the placement of the two test leads upon discovering a backwards connection, but the polarity-reversing switch is useful whenever the test leads are clamped in place (e.g. when secured by terminal blocks for semi-permanent connection to the circuit). An illustration showing the effect of connecting a VOM “backwards” appears below:

The purpose of any test instrument is to merely measure and not to in rence whatever’s being measured, and for an electrical meter this means extracting as little energy as possible from the circuit. For a voltmeter this means drawing as little current away from the circuit as possible, and this necessitates the voltmeter having a very hi9h resistance between its test leads. For a good- quality VOM this insertion resistance will be tens or hundreds of thousands of Ohms depending on the voltage-measuring range selected. DMMs configured as volt meters generally have insertion resistance values in the tens of mi/lions of Ohms for even better performance. An ideal voltmeter has in note insertion resistance⁴ (i.e. an ideal voltmeter behaves as an electrical open).

Using a DMM to measure 3’s voltage is not much rlifferent from using the VOM. We must select the proper function on the meter’s switch, ensure the test plugs are inserted into the correct jacks on the meter’s face, and once again connect the meter in parallel with the component:

Since we are intending to measure voltage in a DC circuit, we must choose the « V » function with the two parallel lines (one solid, one dashed). The wavy-curve symbol above the other « V » represents alternating-current A C) voltage measurement.

As mcntioncd prcviously, DMMs typically oxhibit far grcatcr insertion resistance as voltmctcrs than VOMs, and this results in less energy being extracted from the circuit under test. This allows the circuit to function nearer to its un-measured state when the meter is connected to it, resulting in more realistic measurements. For many electric circuits such as power systems this distinction is irrelevant because either multimeter type dissipates a trivial amount of power compared to the circuit’s normal operating load. However, for sensitive electronic circuits where the power levels are much smaller it is often test to use a DMM rather than a VOM to measure voltage“. DMMs are also easier for novices to use because their displays require no interpretation. A VOM’s pointer usually does not fall exactly on one of the scale’s division-mar ks, anrl so one must read the division markings carefully to interpret the numerical value represented by the pointer’s position. Digital displays require no skill to read, being simply a decimal number.

If we happen to connect the DMM’s test leads backwards to 3 the only difference we will notice is that the digital display will be preceded by a negative sign:

Unlike most VOMs where the indicating pointer’s “zero” position is biased toward one side of the scale, a DMM’s display knows no such limits. Correct-polarity measurements generate positively-signed results and backward-polarity measurements negatively-signed results. Most users of multimeters consider this to be an advantage, not having to pay attention to correct polarity when connecting a meter to a circuit.

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