The Voltmeter and The Ohmmeter

The Voltmeter

The voltmeter uses the same type of meter movement as the ammeter but employs a different circuit external to the meter movement.

As shown before, the voltage drop across the meter coil is a function of current and the coil resistance. In another example, 50 μA × 1,000 Ω = 50 mV. In order for the meter to be used to measure voltages greater than 50 mV, there must be added a series resistance to drop any excess voltage greater than that which the meter movement requires for a full scale deflection. The case of the voltmeter, this resistance is called multiplier resistance and is designated as R_M. [Figure 12-152] The voltmeter only has one multiplier resistor for use in one range.

In this example, the full scale reading is 1 volt. R_M is determined in the follow way:

The meter movement drops 50 mV at a full scale deflection of 50 μA. The multiplying resistor R_M must drop the remaining voltage of 1 V − 50 mV = 950 mV. Since R_M is in series with the movement, it also carries 50 μA at full scale.

Therefore, for 1 volt full scale deflection, the total resistance of the voltmeter is 20k Ω. That is, the multiplier resistance and the coil resistance.

Voltmeter Sensitivity

Voltmeter sensitivity is defined in terms of resistance per volt (Ω/V). The meter used in the previous example has a sensitivity of 20k Ω and a full scale deflection of 1 volt.

Multiple Range Voltmeters

The simplified voltmeter in Figure 12-152 has only one range (1 volt), which means that it can measure voltages from 0 volts to 1 volt. In order for the meter to be more useful, additional multiplier resistors must be used. One resistor must be used for each desired range.

For a 50 μA movement, the total resistance required is 20k Ω for each volt of full scale reading. In other words, the sensitivity for a 50 μA movement is always 20k Ω regardless of the selected range. The full-scale meter current is 50 μA at any range selection. To find the total meter resistance, multiply the sensitivity by the full scale voltage for that particular range. For example for a 10 volt range, R_T = (20k Ω/V) (10V) = 200k Ω. The total resistance for the 1 volt range is 20k Ω, so R_M for a 10 V range is 200k Ω − 20k Ω = 180k Ω. [Figure 12-153]

Voltmeter Circuit Connections

When voltmeters are used, they are connected in parallel with a circuit. If unsure about the voltage to be measured, take the first reading at the high value on the meter and then progressively move down through the range until a suitable read is obtained. Observe that the polarity is correct before connecting the meter to the circuit or damage occurs by driving the movement backwards.

Influence of the Voltmeter in the Circuit

When a voltmeter is connected across two points in a circuit, current is shunted. If the voltmeter has low resistance, it draws off a significant amount of current. This lowers the effective resistance of the circuit and change the voltage readings. When making a voltage measurement, use a high resistance voltmeter to prevent shunting of the circuit.

The Ohmmeter

The meter movement used for the ammeter and the voltmeter can also be used for the ohmmeter. The function of the ohmmeter is to measure resistance. A simplified one-stage ohmmeter is illustrated in Figure 12-154, which shows that the basic ohmmeter contains a battery and a variable resistor in series with the meter movement.

To measure resistance, the leads of the meter are connected across an external resistance, which is to be measured. By doing this, the ohmmeter circuit is completed. This connection allows the internal battery to produce a current through the movement coil, causing a deflection of the pointer proportional to the value of the external resistance being measured.

Zero Adjustment

When the ohmmeter leads are open, the meter is at a full scale deflection, indicating an infinite (∞) resistance or an open circuit. [Figure 12-155] When the leads are shorted as shown in figure “zero adjust,” the pointer is at the full right-hand position, indicating a short circuit or zero resistance. The purpose of the variable resistor in this figure is to adjust the current so that the pointer is at exactly zero when the leads are shorted. This is used to compensate for changes in the internal battery voltage due to aging.

Ohmmeter Scale

Figure 12-156 shows a typical analog ohmmeter scale. Between zero and infinity (∞), the scale is marked to indicate various resistor values. Because the values decrease from left to right, this scale is often called a back-off scale.

In the case of the example given, assume that a certain ohmmeter uses a 50 μA, 1,000 Ω meter movement and has an internal 1.5 volt battery. A current of 50 μA produces a full-scale deflection when the test leads are shorted. To have 50 μA, the total ohmmeter resistance is 1.5 V/50 μA = 30k Ω. Therefore, since the coil resistance is 1k Ω, the variable zero adjustment resistor must be set to 30k Ω – 1k Ω = 29k Ω.

Now consider that a 120k Ω resistor is connected to the ohmmeter leads. Combined with the 30k Ω internal resistance, the total R is 150k Ω. The current is 1.5 V/150k Ω = 10 μA, which is 20 percent of the full scale current and appears on the scale shown in Figure 12-156.

Now consider further that a 120k Ω resistor is connected to the ohmmeter leads. This results in a current of 1.5 V/75k Ω = 10 μA, which is 40 percent of the full scale current and marked on the scale. Additional calculations of this type show that the scale is nonlinear. It is more compressed toward the left side than the right side. The center scale point corresponds to the internal meter resistance of 30k Ω. The reason is as follows:

With 30k Ω connected to the leads, the current is 1.5 V/60k Ω = 25 μA, which is half of the full scale current of 50 μA.

The Multirange Ohmmeter

A practical ohmmeter has several operational ranges. These typically are indicated by R × 1, R × 10, R × 100, R × 1k, R × 100k and R × 1M. These range selections are interpreted in a different manner than that of an ammeter or voltmeter. The reading on the ohmmeter scale is multiplied by the factor indicated by the range setting. For example, if the pointer is set on the scale and the range switch is set at R × 100, the actual resistance measurement is 20 × 100 or 2k Ω.

To measure small resistance values, the technician must use a higher ohmmeter current than is needed for measuring large resistance values. Shunt resistors are needed to provide multiple ranges on the ohmmeter to measure a range of resistance values from the very small to very large. For each range, a different value of shunt resistance is switched in. The shunt resistance increases for higher ohm ranges and is always equal to the center scale reading on any selected range. In some meters, a higher battery voltage is used for the highest ohm range. [Figure 12-157]

Megger (Megohmmeter)

The megger, or megohmmeter, is a high range ohmmeter containing a hand-operated generator. It is used to measure insulation resistance and other high-resistance values. It is also used for ground, continuity, and short-circuit testing of electrical power systems. The chief advantage of the megger over an ohmmeter is its capacity to measure resistance with a high potential, or “breakdown” voltage. This type of testing ensures that insulation or a dielectric material will not short or leak under potential electrical stress.

The megger consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: a hand-driven DC generator, G, which supplies the necessary current for making the measurement; and the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of the opposed coil type. Coils A and B are mounted on the movable member with a fixed angular relationship to each other and are free to turn as a unit in a magnetic field. Coil B tends to move the pointer counterclockwise and coil A, clockwise. The coils are mounted on a light, movable frame that is pivoted in jewel bearings and free to move about axis 0. [Figure 12-158]

Coil A is connected in series with R3 and the unknown resistance, R_X, to be measured. The series combination of coil A, R3, and R_X is connected between the + and − brushes of the DC generator. Coil B is connected in series with R2, and this combination is also connected across the generator. There are no restraining springs on the movable member of the instrument portion of the megger. When the generator is not in operation, the pointer floats freely and may come to rest at any position on the scale.

If the terminals are open circuited, no current flows in coil A, and the current in coil B alone controls the movement of the moving element. Coil B takes a position opposite the gap in the core (since the core cannot move and coil B can), and the pointer indicates infinity on the scale. When a resistance is connected between the terminals, current flows in coil A, tending to move the pointer clockwise. At the same time, coil B tends to move the pointer counterclockwise. Therefore, the moving element, composed of both coils and the pointer, comes to rest at a position at which the two forces are balanced. This position depends upon the value of the external resistance, which controls the relative magnitude of current of coil A. Because changes in voltage affect both coils A and B in the same proportion, the position of the moving element is independent of the voltage. If the terminals are short circuited, the pointer rests at zero because the current in A is relatively large. The instrument is not damaged under these circumstances because the current is limited by R3.

There are two types of hand-driven meggers: the variable type and the constant pressure type. The speed of the variable pressure megger is dependent on how fast the hand crank is turned. The constant pressure megger uses a centrifugal governor, or slip clutch. The governor becomes effective only when the megger is operated at a speed above its slip speed, at which speed its voltage remains constant.

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