The D'Arsonval meter movement can be used either as an ammeter or a voltmeter (figure 8-133). Thus, an ammeter can be converted to a voltmeter by placing a resistance in series with the meter coil and measuring the current flowing through it. In other words, a voltmeter is a current measuring instrument, designed to indicate voltage by measuring the current flow through a resistance of known value. Various voltage ranges can be obtained by adding resistors in series with the meter coil. For low range instruments, this resistance is mounted inside the case with the D'Arsonval movement and usually consists of resistance wire having a low temperature coefficient which is wound either on spools or card frames. For higher voltage ranges, the series resistance may be connected externally. When this is done, the unit containing the resistance is commonly called a multiplier.
Extending the Voltmeter Range
The value of the necessary series resistance is determined by the current required for full scale deflection of the meter and by the range of voltage to be measured. Because the current through the meter circuit is directly proportional to the applied voltage, the meter scale can be calibrated directly in volts for a fixed series resistance.
For example, assume that the basic meter (microammeter) is to be made into a voltmeter with a full scale reading of 1 volt. The coil resistance of the basic meter is 100 ohms, and 0.0001 ampere (100 microamperes) causes a full scale deflection. The total resistance, R, of the meter coil and the series resistance is
and the series resistance alone is
|Multirange voltmeters utilize one meter movement with the
required resistances connected in series with the meter by a convenient
switching arrangement. A multirange voltmeter circuit with three ranges
is shown in figure 8-134. The total circuit resistance for each of the
three ranges beginning with the 1 volt range is:
|Multirange voltmeters, like multirange ammeters, are used frequently.
They are physically very similar to ammeters, and their multipliers are
usually located inside the meter with suitable switches or sets of terminals
on the outside of the meter for selecting ranges (see figure 8-135).
Voltage measuring instruments are connected across (in parallel with) a circuit. If the approximate value of the voltage to be measured is not known, it is best, as in using the ammeter, to start with the highest range of the voltmeter and progressively lower the range until a suitable reading is obtained.
In many cases, the voltmeter is not a central zero indicating instrument. Thus, it is necessary to observe the proper polarity when connecting the instrument to the circuit, as is the case when connecting the dc ammeter. The positive terminal of the voltmeter is always connected to the positive terminal of the source, and the negative terminal to the negative terminal of the source, when the source voltage is being measured. In any case, the voltmeter is connected so that electrons will flow into the negative terminal and out of the positive terminal of the meter. In figure 8-136 a multimeter is properly connected to a circuit to measure the voltage drop across a resistor. The function switch is set at the dc volts position and the range switch is placed in the 50 volt position.
The function of a voltmeter is to indicate the potential difference between two points in a circuit. When the voltmeter is connected across a circuit, it shunts the circuit. If the voltmeter has low resistance, it will draw an appreciable amount of current. The effective resistance of the circuit will be lowered, and the voltage reading will consequently be lowered.
When voltage measurements are made in high resistance circuits, it is necessary to use a high resistance voltmeter to prevent the shunting action of the meter. The effect if less noticeable in low resistance circuits because the shunting effect is less.
The sensitivity of a voltmeter is given in ohms per volt (ohms/E) and is determined by dividing the resistance (Rm) of the meter plus the series resistance (Rs) by the full scale reading in volts. Thus,
This is the same as saying that the sensitivity is equal to the reciprocal of the current (in amperes); that is,
Thus, the sensitivity of a 100 microampere movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.
The sensitivity of a voltmeter can be increased by increasing the strength of the permanent magnet, by using lighter weight materials for the moving element (consistent with increased number of turns on the coil), and by using sapphire jewel bearings to support the moving coil.
The accuracy of a meter is generally expressed in percent. For example, a meter with an accuracy of 1 percent will indicate a value within 1 percent of the correct value. The statement means that, if the correct value is 100 units, the meter indication may be anywhere within the range of 99 to 101 units.