What is a Voltmeter Explained

Voltmeters

What is a Voltmeter?

A Voltmeter provides a method to accurately measure voltage, which is the difference in electric potential, between two points in a circuit while not changing the voltage in that circuit. It is an instrument used for measuring voltage drop.

Electrical current consists of a flow of charge carriers. Voltage, also called electromotive force (EMF) or potential difference, manifests as "electrical pressure" that makes current possible. Given an electric circuit under test, having a constant resistance, the current through the circuit varies in direct proportion to the voltage across the circuit.

A voltmeter can be tailored to have various full-scale ranges by switching different values of resistance in series with the microammeter as shown in Fig. 3-6. The voltmeter exhibits high internal resistance because the resistors have large ohmic values. The greater the supply voltage, the larger the internal resistailce of the meter because the necessary series resistance increases as the voltage increases.

Fig 3-6. A simple circuit using a microammeter (tA) to measure DC voltage.

Voltmeters, whether digital meters or digital voltmeters, should have high resistance and the higher the better! You don't want the meter to draw much current from the power source. (Ideally, it wouldn't draw any current at all.) The power-supply current should go, as much as possible, towards operating whatever circuit or system you want to use, not into getting a meter to tell you the voltage. Also, you might not want to keep the voltmeter constantly connected in parallel in the circuit. You might need the voltmeter for testing many different circuits. You don't want the behavior of a circuit to be affected the moment you connect or disconnect a voltmeter. The less current a voltmeter draws, the less it affects the behavior of anything that operates from the power supply.

Alternative types voltmeters use electrostatic deflection, rather than electromagnetic deflection, to produce its readings. Remember that electric fields produce forces, just as magnetic fields do. Therefore, a pair of electrically charged plates attract or repel each other. An electrostatic voltmeter takes advantage of the attractive force between two plates having an opposite electric charge, or having a large potential difference. It is used to measure the potential difference. Figure 3-7 portrays the functional mechanics of an electrostatic voltmeter. It constitutes, in effect, a sensitive, calibrated electroscope. The device draws essentially no current from the power supply. Nothing but air exists between the plates, and air constitutes a nearly perfect electrical insulator. A properly designed electrostatic meter can indicate AC voltage as well as DC voltage. However, the construction tends to be fragile, and mechanical vibration can influence the reading.

Fig 3-7. Functional drawing of an electrostatic voltmeter movement.

It's always good when a voltmeter has a high internal resistance. The reason for this is that you don't want the meter to draw much current from the power source. This cur­rent should go, as much as possible, towards working whatever circuit is hooked up to the supply, and not into just getting a reading of the voltage. Also, you might not want, or need, to have the voltmeter constantly connected in the circuit; you might need the voltmeter for testing many different circuits. You don't want the behavior of the circuit to be affected the instant you connect the voltmeter to the supply. The less current a voltmeter draws, the less it will affect the behavior of anything that is working from the power supply.

If you connect an ammeter directly across a source of voltage—a battery, say—the meter needle will deflect. In fact, a milliammeter needle will probably be "pinned" if you do this with it, and a microammeter might well be wrecked by the force of the needle striking the pin at the top of the scale. For this reason, you should never connect mil-liammeters or microammeters directly across voltage sources. An ammeter, perhaps with a range of 0-10 A, might not deflect to full scale if it is placed across a battery, but it's still a bad idea to do this, because it will rapidly drain the battery. Some batteries,-such as automotive lead-acid cells, can explode under these conditions. This is because all ammeters have low internal resistance. They are designed that way deliberately. They are meant to be connected in series with other parts of a circuit, not right across the power supply.

But if you place a large resistor in series with an ammeter, and then connect the ammeter across a battery or other type of power supply, you no longer have a short cir­cuit. The ammeter will give an indication that is directly proportional to the voltage of the supply. The smaller the full-scale reading of the ammeter, the larger the resistance to get a meaningful indication on the meter. Using a microammeter and a very large value of resistor in series, a voltmeter can be devised that will draw only a little current from the source.

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