Voltmeters
(808)DC and AC voltmeters measure the voltage or its drop in a circuit. In both cases, the measurement result is presented using a needle rather than a digital display. Voltmeters can be stand-alone devices or constitute a multimeter component.
How meters operate – analogue voltmeter
For measurements, analogue voltmeters most commonly use d'Arsonval galvanometers with a movable coil made of thin wire and suspended in a magnetic field. The coil rotates and moves the indicator proportionally to the current applied.
Galvanometers with movable coils are valued due to their high sensitivity. However, they are unsuitable for AC measurements, as they only respond to the average current flow (this problem is often solved by using a rectifier and transformer).
Analogue vs. digital voltmeters
At present, voltage measurements are most typically performed using digital voltmeters, due to their above-average accuracy and precision. However, analogue meters offer several advantages when compared with their digital equivalents.
Analogue voltmeter advantages
- A needle illustrates voltage fluctuation trends more effectively than a reading displayed on a digital screen.
- The measurement is continuous, which makes it possible to observe even momentary spikes.
- Most analogue voltmeters do not require any power supply other than a test current source.
- Analogue voltmeters are also mechanically robust and less susceptible to damage.
However, analogue voltmeters also come with certain disadvantages.
For example, they are not equipped with the auto-bias technology. Incorrectly connected test leads can bend the needle and damage the device.
A “parallax error” can also occur when a scale is viewed at an incorrect angle, which results in a reading error amounting even up to a few degrees. Some meters also come with a mirror attached to the display, so that the user can easily determine the correct viewing angle by checking the needle reflection.
An analogue voltmeter read-out can also be imprecise if the instrument is placed near a magnetic field source. Furthermore, the meter must be positioned properly.
Where do we use analogue voltmeters?
Digital voltmeters have replaced analogue models in most applications, but there are still certain fields of operation where the latter perform more effectively. For example, they are used to measure pulses or oscillations, as in those cases the indicator movement is more important than the accurate voltage value; battery consumption measurement is another example worth mentioning here.
Analogue voltmeter measurement scale and range
A voltmeter designed to measure just one voltage range is a simple and straightforward instrument, but numerous meters can be configured to measure multiple ranges, so the scales used become more complicated. Certain analogue multimeters with the voltmeter feature are capable of measuring both AC and DC voltages, and their users have dozens of ranges to choose from. It is therefore essential to know how to use the correct scale to display a selected range and determine the full reading scale percentage to find the measured value. For example, a needle read-out equal to 2 within the 0–10 V scale is interpreted as 200 V when a meter measurement range is set up as 0–1000 V.
When measuring unknown voltage, calibrating a meter to the highest range prevents its damage. When measuring known voltage, the meter should be set to the smallest range that can accommodate the voltage. For example, a 9 V battery should be tested at 0–10 V, and a 120 V socket should be tested at 0–250 V.
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