An instrument called a megohmmeter is used for insulation testing and similar high-resistance tests. The megohmmeter contains a high-voltage generator that supplies current through the series resistors and the unknown resistance (R) to the two-coil assembly that operates the pointer; see Figure 17–13.
Note in the figure that permanent magnets supply the field for the DC generator and the field for the moving coil assembly. The potential coil is connected in series with R2 across the generator output. The current coil is connected in series with the unknown resistance. The current in the coil depends on the value of the unknown resistance. The potential coil and the current coil are fastened together and can rotate only as a single unit.
Since there is no spring in the coil and pointer assembly, the pointer can take any position on the scale when the meter is not in use. If there is no external connection across the ground and line terminals, when the generator is operated the current in the potential coil causes a magnetic force that rotates the coil assembly counterclockwise, moving the pointer to the infinity (∞) end of the scale (open-circuit point). If the ground and line terminals are shorted (if the unknown R has a value of 0 ohms), there is almost no current in the potential coil. Thus, the strong field of the current coil will rotate the assembly clockwise and move the pointer to the 0 end of the scale.
When the value of the unknown R is neither very high nor very low, the currents in the two coils produce opposing torques. As a result, the coil and pointer assembly comes to rest at the position near the middle, where these torques balance each other. A low value of external resistance permits the current coil to turn the pointer assembly closer to the 0 end of the scale. As the assembly is moved closer to the 0 side, the potential coil is pushed far enough into the north-pole field to prevent further turning. In the presence of
a high external resistance, the current coil has less effect and the potential coil moves the pointer closer to the ∞ end of the scale. The scale is marked to show external resistance in megohms.
In addition to the megohmmeter, various electronic instruments operated from a 120-volt AC line can measure high resistance.
As stated in Chapter 9, Watts 5 Volts 3 Amperes in DC circuits. To measure the wattage of a circuit, a meter must have two coils; one coil is affected by the voltage and one by the current. The voltage coil is the moving coil and is connected across the current line so that the magnetic strength of the coil is proportional to the line voltage. The combination of the moving coil and its series resistor in Figure 17–14 is similar to a voltmeter, described previously. However, instead of having a permanent magnet to provide the magnetic field for the moving coil, the wattmeter has current coils to provide the magnetic field. The magnetic strength of these coils is proportional to the current through them (the current supplied to the device being tested).
The amount of movement of the coil and pointer depends on the strength of both coils. If there is a voltage but no current, then there is no magnetic field to turn the moving coil; therefore, the pointer reads 0. If the magnetic strength of either coil is
increased, the turning force increases; that is, the turning force depends on the product of the magnetic strengths of the two coils, just as the force between any two magnets depends on the product of their magnetic strengths (Chapter 16).
With this coil arrangement, the pointer reading depends on the product of the volt- age on one coil and the current in the other coil. Thus, the meter scale is calibrated in watts. The coils used have air cores, not iron cores. A wattmeter can operate on AC as well as DC, because the magnetic polarity of both coils reverses when the current reverses, and the turning force remains in the same direction.
Wattmeters are more necessary for AC measurements than for DC measurements. In DC circuits, watts are always equal to volts 3 amperes, and wattmeters are not required. In AC circuits, there are occasions when watts are not equal to volts 3 amperes, and wattmeters are needed to indicate the power consumption in the circuit.
Many wattmeters have terminals marked “V” for the voltmeter function and “A” for the ammeter function. Such meters are connected as shown in Figure 17–15. Note that in this case one of the “V” terminals always connects to one of the “A” terminals. Therefore, many instruments provide a common terminal only, which is generally labeled 6 or COM2. Figure 17–16 illustrates how the hookup looks with such instruments.
Wattmeters that contain both stationary and moving coils are generally referred to as dynamic wattmeters. Dynamic wattmeters are rapidly being replaced with electronic wattmeters due to the high cost of constructing a dynamic wattmeter. Electronic wattmeters contain an electronic circuit that permits the use of a standard d’Arsonval movement or can be output to a digital display.
17–8 BRIDGE CIRCUITS
One of the most common methods used to accurately measure values of resistance, inductance, and capacitance is with a bridge constructed by connecting four components together to form a parallel-series circuit. All four components are of the same type, such as four resistors, four inductors, or four capacitors. The bridge used to measure resistance is called a Wheatstone bridge. The basic circuit for a Wheatstone bridge is shown in Figure 17–17. The bridge operates on the principle that the sum of the voltage drops in a series circuit must equal the applied voltage. A galvanometer is used to measure the voltage between points B and D. The galvanometer can be connected to different values of resistance or directly between points B and D. Values of resistance are used to change the sensitivity of the meter circuit. When the meter is connected directly across the two points, its sensitivity is maximum.
In Figure 17–17, assume the battery has a voltage of 12 volts, and that resistors R1 and R2 are precision resistors and have the same value of resistance. Since resistors R1 and R2 are connected in series and have the same value, each will have a voltage drop equal to one-half of the applied voltage, or 6 volts. This means that point B is 6 volts more negative than point A and 6 volts more positive than point C.
Resistors RV (variable) and RX (unknown) in Figure 17–17 are connected in series with each other. Resistor RX represents the unknown value of resistance to be measured.
Resistor RV can be adjusted for different resistive values. If the value of RV is greater than the value of RX, the voltage at point D will be more positive than the voltage at point B. This will cause the pointer of the zero-center galvanometer to move in one direction. If the value of RV is less than RX, the voltage at point D will be more negative than the voltage at point B, causing the pointer to move in the opposite direction. When the value of RV becomes equal to RX, the voltage at point D will become equal to the voltage at point B. When this occurs, the galvanometer will indicate 0. A Wheatstone bridge is shown in Figure 17–18.