FIGURE 8-123. D'Arsonval meter movement.
scale, units of measurement, and meter uses. The terminals are made of materials having very low electrical resistance. Their funetion is to conduct the required current into and away from the meter.
Pivot FIGURE 8-122. Method of mounting moving elements.
designed in which the pivot points are reversed and the bearings are inside the moving-coil assembly. A method of mounting moving elements is shown in figure 8-122.
The bearings are highly polished jewels such as sapphires, synthetic jewels, or very hard glass.
These are usually round and have a conical depression in which the pivots rotate. They are set in threaded nuts which allow adjustment. The radius of the depression in the jewel is greater than the radius of the pivot point. This limits the area of contact surfaces and provides a bearing which, when operated dry, probably has the lowest constant friction value of any known type of bearing.
The case houses the instrument movement and protects it from mechanical injury and exposure.
It also has a window for viewing the movement of the pointer across a calibrated scale. The dial has printed on it pertinent information such as the Operation of the Meter Movement The major units are mounted in their relationship to one another (figure 8-123). Note that the coil portion of the moving element is in the magnetic field of the permanent magnet.
In order to understand how the meter works, assume that the coil of the moving element is placed in a magnetic field as shown in figure 8-124.
The coil is pivoted so that it is able to rotate back and forth within the magnetic field set up by the magnet. When the coil is connected in a circuit, current flows through the coil in the direction indicated by the arrows and sets up a magnetic field within the coil. This field has the same polarity as the adjacent poles of the magnet.
The interaction of the two fields causes the coil to rotate to a position so that the two magnetie fields are aligned. This force of rotation (torque) is proportional to the interaction between the like poles of the coil and the magnet and, therefore, to FIGURE 8-124. Effect of a coil in a magnetic field.
the amount of current flow in the coil. As a result, a pointer attached to the coil will indicate the amount of current flowing in the circuit as it moves across a graduated scale.
In the arrangement just discussed, note that any torque sufficient to overcome the inertia and friction of moving parts causes the coil to rotate until the fields align. This uncontrolled movement would cause inaccruate current readings. Therefore, the turning motion of the coil is opposed by two springs. The value of the current flowing through the coil determines the turning force of the coil. When the turning force is equal to the opposition of the springs, the coil stops moving and the pointer indicates the current reading on a calibrated scale. In some meters the springs are made of conducting material and conduct current to and from the coil. The pole pieces of the magnet form a circular air gap within which the coil is pivoted.
To obtain a clockwise rotation, the north pole of the permanent magnet and that of the coil must be adjacent. The current flowing through the coil must, therefore, always be in the same direction. The D'Arsonval movement can be used only for d.c. measurements and the correct polarity must be observed. If the current is allowed to flow in the wrong direction through the coil, the coil will rotate counterclockwise and the pointer will be damaged. Since the movement of the coil is directly proportional to the current through the coil, the scale is normally a linear scale.
Damping in order that meter readings can be made quickly and accurately, it is desirable that the moving pointer overshoot its proper position only a small amount and come to rest after not more than one or two small oscillations. The term "damping" is applied to methods used to bring the pointer of an electrical meter to rest after it has been set in motion. Damping may be accomplished by electrical means, by mechanical means, or by a combination of both.
Electrical Damping A common method of damping by electrical means is to wind the moving coil on an aluminum frame. As the coil moves in the field of the permanent magnet, eddy currents are set up in the aluminum frame. The magnetic field Closed air chamber Light vane swinging in chamber
•with small clearance FIGURE 8-125. Air damping.
produced by the eddy currents opposes the motion of the coil. The pointer will therefore swing more slowly to its proper position and come to rest quickly with very little oscillation.
Mechanical Damping Air damping is a common method of damping by mechanical means. As shown in figure 8-125, a vane is attached to the shaft of the moving element and enclosed in an air chamber.
The movement of the shaft is retarded because of the resistance which the air offers to the vane.
Effective damping is achieved if the vane nearly touches the walls of the chamber.
Meter Sensitivity The sensitivity of a meter movement is usually expressed as the amount of current required to give full-scale deflection. In addition, the sensitivity may be expressed as the number of millivolts across the meter when full-scale current flows through it. This voltage drop is obtained by multiplying the full-scale current by the resistance of the meter movement. A meter movement, whose resistance is 50 ohms and which requires 1 milliampere (ma.) for full-scale reading, may be described as a 50-millivolt 0-1 milliammeter.
Extending the Range of an Ammeter A 0-1 milliammeter movement may be used to measure currents greater than 1 ma. by connecting a resistor in parallel with the movement. The parallel resistor is called a shunt because it bypasses a portion of the current around the movement, extending the range of the ammeter.
10 ma.
0-1 ma 10 ma. 1 ma. 9 ma.
FIGURE 8-126. Meter movement with shunt.
A schematic drawing of a meter movement with a shunt connected across it to extend its range is shown in figure 8-126.
Determining the Value of a Shunt The value of a shunt resistor can be computed by applying the basic rules for parallel circuits.
If a 50 millivolt 0-1 milliammeter is to be used to measure values of current up to 10 ma., the following procedure can be used: The first step involves drawing a schematic of the meter shunted by a resistor labeled R, (shunt resistor), as shown in figure 8-127.
Since the sensitivity of the meter is known, the meter resistance can be computed. The circuit is then redrawn as shown in figure 8-128, and the branch currents can be computed, since a maximum of I ma. can flow through the meter.
The voltage drop across R, is the same as that across the meter, Rm:
E = IR = 0.001 × 50 = 0.050 volt.
R, can be found by applying Ohm's law:
Rs = Irs Kirs
0.050 = 0.009 = 5.55 ohms.
The value of the shunt resistor (5.55 Q) is very small, but this value is critical. Resistors used as shunts musts must have close tolerances, usually 1 percent.
FIGURE 8-127. Circuit schematic for shunt resistor.
Universal Ammeter Shunt The schematic drawing in figure 8-129, the universal shunt, shows an arrangement whereby two or more ranges are provided by tapping the shunt resistor at the proper points. In this arrangement, a 0-5 ma. movement with a resistance of 20 ohms is shunted to provide a 0-25 ma. range and a 0-50 ma. range.
Ammeters having a number of internal shunts are called multirange ammeters. A scale for each range is provided on the meter face (figure 8-130).
Some multimeters avoid internal switching through the use of external shunts. Changing ammeter ranges involves the selection and installation on the meter case of the proper size shunt.
MULTIMETERS Ammeters are commonly incorporated in multiple-purpose instruments such as multimeter or volt-ohm-milliammeters. These instruments vary somewhat according to the design used by different manufacturers, but most incorporate the functions of an ammeter, a voltmeter, and an ohmmeter in one unit. A typical multimeter is shown in figure 8-131. This multimeter has two 10 ma.
> 50 g 1 1 ma. 9 ma.
10 ma.
FIGURE 8-128. Equivalent meter circuit.
0.5 ma.
2012
• ONE AMPERE 50 ma. 25 ma.
FIGURE 8-129. Universal ammeter shunt.
selector switches: a function switch and a range switch. Since a multimeter is actually three meters in one case, the function switch must be placed in proper position for the type of measurement to be made. In figure 8-131, the function switch is shown in the ammeter position to measure d.c. milliamperes and the range switch is set at 1000. Set in this manner, the ammeter can measure up to 1,000 milliamperes or 1 ampere.
Multimeters have several scales, and the one used should correspond properly to the position of the range switch. If current of unknown value is to be measured, always select the highest possible range to avoid damage to the meter. The test leads should always be connected to the meter in the manner prescribed by the manufacturer. Usually the red lead is positive and the black lead is negative, or common. Many multimeters employ color coded jacks as an aid 3 - - 300 FIGURE 8-130. A multirange ammeter.
DCMA Function 1000 switch Range switch FIGURE 8-131. A multimeter set to measure one ampere.
in connecting the meter into the circuit to be tested. In figure 8-132, a multimeter properly set to measure current flow is connected into a circuit.
The precautions to be observed when using an ammeter are summarized as follows:
1. Always connect an ammeter in series with the element through which the current flow is to be measured.
2. Never connect an ammeter across a source of voltage, such as a battery or generator.
Remember that the resistance of an ammeter, particularly on the higher ranges, is extremely low and that any voltage, even a volt or so, can cause very high current to flow through the meter, causing damage to it.
3. Use a range large enough to keep the deflection less than full scale. Before measuring a current, form some idea of its magnitude. Then switch to a large enough scale or start with the highest range and work down until the appropriate scale is reached. The most accurate readings are obtained at approximately half-scale deflection. Many milliammeters have been ruined by attempts to measure amperes.
Therefore, be sure to read the lettering either on the dial or on the switch positions and choose proper scale before connecting the meter in the circuit.
4. Observe proper polarity in connecting the meter in the circuit. Current must flow 326
• DCMA BLACK + RED FIGURE 8-132. A multimeter set to measure current flow.
through the coil in a definite direction in order to move the indicator needle up-scale.
Current reversal because of incorrect connection in the circuit results in a reversed meter deflection and frequently causes bending of the meter needle. Avoid improper meter connections by observing the polarity markings on the meter.
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
VOLTMETER 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 FIGURE 8-133. Simplified diagram of a resistance may be connected externally. When voltmeter.
meter coil and the series resistance is 100 мa 100 22 R=] - = 0.0001 - = 10,000 ohms, M 1 VOLT 100 VOLT and the series resistance alone is
• -.01 V -- 1000 VOLT R, = 10,000 - 100 = 9,900 ohms.
9,900 g 25 006 666 wn 9,999,900 R ment with the required resistances connected in Multirange voltmeters utilize one meter moveseries 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:
1 R = 7 = 100 = 0.01 megohm FIGURE 8-134. Multirange voltmeter schematic. 100 100 = 1 megohm 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 fullscale deflection. The total resistance, R, of the VOLTS -I5V
0-1.5V- — O-150V FIGURE 8-135. Typical multirange voltmeter.
1,000 100 = 10 megohms.
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 centralzero 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 d.c. 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 d.c. volts position and the range switch is placed in the 50-volt position.
Red DCV Black 0-30V
VARIABLE SOURCE FiGURE 8-136. A multimeter connected to measure a circuit voltage drop.
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 is less noticeable in low-resistance circuits because the shunting effect 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.
Voltmeter Accuracy 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 Voltmeter Sensitivity OHMMETERS The sensitivity of a voltmeter is given in ohms Two instruments are commonly used to check per volt (Q/E) and is determined by dividing the the continuity or to measure the resistance of a resistance (Rm) of the meter plus the series circuit or circuit element. These instruments are resistance (Rs) by the full-scale reading in volts. the ohmmeter and the megger, or megohmmeter.
The ohmmeter is widely used to measure resistance sensitivity = Rm + R. and to check the continuity of electrical circuits and devices. Its range usually extends to a few E inegohms. The megger is widely used for measuring This is the same as saying that the sensitivity is insulation resistance, such as the resistance equal to the reciprocal of the current (in amperes); between the windings and the frame of electric that is, machinery, and for measuring the insulation resistance of cables, insulators, and bushings. Its sensitivity = ohms 1 range may extend to more than 1,000 megohms.
volts volts amperes When measuring very high resistances of this ohms nature, it is not necessary to find the exact value of resistance, but rather to know that the insula- Thus, the sensitivity of a 100-microampere tion is either above or below a certain standard.
movement is the reciprocal of 0.0001 ampere, or 10,000 ohms per volt.
The sensitivity of a voltmeter can be increased When precision measurements are required, some type of bridge circuit is used. Ohmmeters may be of the series or shunt type.
OHMS E. _ 4.5V 37.5K 13.5K 9K 4.5K ISK B 0-1 ma 2.5
MILLIAMPERES FIGURE 8-137. Ohmmeter circuit.
Series-type Ohmmeters A simplified schematic of an ohmmeter is shown in figure 8-137. E is a source of EMF; R, is a variable resistor used to zero the meter; R2 is a fixed resistor used to limit the current in the meter movement; and A and B are test terminals across which the resistance to be measured is placed.
If A and B are connected together (shortcircuited), the meter, the battery, and resistors R1 and R2 form a simple series circuit. With R1 adjusted so that the total resistance in the circuit is 4,500 ohms, the current through the meter is 1 ma. and the needle deflects full scale. Since there is no resistance between A and B, this position of the needle is labeled zero (figure 8-138). If a resistance equal to 4,500 ohms is placed between terminals A and B, the total resistance is 9,000 ohms and the current is .5 ma.
This causes the needle to deflect half scale. This half-scale reading, labeled 4.5 K ohms, is equal to the internal resistance of the meter, in this instance 4,500 ohms. If a resistance of 9,000 ohms is placed between terminals A and B, the needle deflects one-third scale. Resistances of 13.5 K and
1.5 K placed between terminals A and B will cause a deflection of one-fourth and three-fourths scale, respectively.
If terminals A and B are not connected (opencircuited), no current flows and the needle does not move. The left side of the scale is, therefore, labeled infinity to indicate an infinite resistance.
A typical ohmmeter scale is shown in figure
8-138. Note that the scale is not linear and is crowded at the high resistance end. For this reason, it is good practice to use an ohmmeter range in which the readings are not too far from mid-scale. A good rule is to use a range in which the reading obtained does not exceed ten times, or is not less than one-tenth, the mid-scale reading.
FIGURE 8-138. A typical ohmmeter scale.
The useful range of the scale shown is, by this rule, from 450 ohms to 45,000 ohms.
Most ohmmeters have more than one scale.
Additional scales are made possible by using various values of limiting resistors and battery voltages. Some ohmmeters have a special scale called a low-ohm scale for reading low resistances.
A shunt-type ohmmeter circuit is used for this scale.
Shunt-Type Ohmmeter Shunt-type ohmmeters are used to measure small values of resistance. In the circuit shown in figure 8-139, E (voltage) is applied across a limiting resistor R and a meter movement in series.
Resistance and battery values are chosen so that the meter movement deflects full scale when terminals A and B are open. When the terminals are short-circuited, the meter reads zero; the short circuit conducts all the current around the meter.
The unknown resistance Rz is placed between terminals A and B in parallel with the meter movement. The smaller the resistance value being measured, the less current flows through the meter The value of the limiting resistor R is usually made large compared to the resistance of the meter movement. This keeps the current drawn from the battery practically constant. Thus, the value of R.
determines how much of this constant current flows through the meter and how much through Rza Note that in a shunt-type ohmmeter, current is always flowing from the battery through the meter 330
(2) Short the leads together and set the meter to read zero ohms by setting the zero adjustment. If the scale is changed, readjust to zero ohms.
(3) Connect the unknown resistance between the test leads and read its resistance from the scale. Never attempt to measure resistance in a circuit while it is connected to a source of voltage. Disconnect at least one end of the element being measured to avoid reading the resistance of parallel paths.
FIGURE 8-139. Shunt-type ohmmeter circuit.
movement and the limiting resistor. Therefore, when using an ohmmeter with a low-ohm scale, do not leave the switch in low-ohm position.
Use of the Ohmmeter The ohmmeter is not as accurate a measuring device as the ammeter or the voltmeter because of the associated circuitry. Thus, resistance values cannot be read with greater than 5 to 10 percent accuracy. While there are instruments which read the resistance of an element with very great accuracy, they usually are more complicated to use.
In addition to measuring the resistance, the ohmmeter is a very useful instrument for checking continuity in a circuit. Often, when troubleshooting electronic circuits or wiring a circuit, visual inspections of all parts of the current path cannot be readily accomplished. Therefore, it is not always apparent whether a circuit is complete or whether current might be flowing in the wrong part of the circuit because of contact with adjacent circuits.
The best method of checking a circuit under these conditions is to send a current through the circuit.
The ohmmeter is the ideal instrument for checking circuits in this manner. It provides the power and the meter to indicate whether the current is flowing.
Observe the following precautions when using an ohmmeter:
(1) Choose a scale which will contain the resistance of the element to be measured. In general, use a scale in which the reading will fall in the upper half of the scale (near full-scale deflection).
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 (figure 8-140) consists of two primary elements, both of which are provided with individual magnetic fields from a common permanent magnet: (1) A hand-driven d.c. generator, G, which supplies the necessary current for making the measurement and (2) 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 Coil A is connected in series with R3 and the R23 xis O I Hand generator Lero Ohmmeter FIGURE 8-140. Simplified megger circuit.
unknown resistance, Ra, to be measured. The series combination of coil A, R3, and Rz is connected between the + and - brushes of the d.c. 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 coil 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 utilizes 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.
BASIC CIRCUIT ANALYSIS AND TROUBLE-
SHOOTING Troubleshooting is the process of locating causes for malfunctions or trouble in a circuit. The following definitions serve as a guide in the troubleshooting discussion:
(1) Short circuit-a low resistance path. It can be across the power source or between the sides of a circuit. It usually creates high current flow which will burn out or cause damage to the circuit conductor or components.
(2) Open circuit-a circuit that is not complete or continuous.
(3) Continuity-the state of being continuous, or connected together; said of a circuit that is not broken or does not have an open.
(4) Discontinuity-the opposite of continuity, indicating that a circuit is broken or not continuous.
Figure 8-141 includes some of the most common sources of open circuits (commonly called "opens" or "an open"). A loose connection or no connection is a frequent cause of an open circuit. In A of figure 8-141, the end of a conductor has separated from the battery terminal. This type of malfunction opens a circuit and stops the flow of current.
Another type of malfunction that will cause an open circuit is a burned-out resistor, shown in B of figure 8-141. When a resistor overheats, its resistance value changes; and, if the current flow through it is great enough, it can burn and open the circuit. In C, D, and E of figure 8-141, three more likely causes of open circuits are shown.
The opens shown can often be located by visual inspection; however, many circuit opens cannot be seen. In such cases, a meter must be used.
The circuit shown in figure 8-142 is designed to cause current to flow through a lamp, but because of the open resistor, the lamp will not light. To locate this open, a voltmeter or an ohmmeter can be used.
If a voltmeter is connected across the lamp, as shown in figure 8-143, the voltmeter will read zero.
Since no current can flow in the circuit because of the open resistor, there is no voltage drop across the lamp. This illustrates a troubleshooting rule that should be remembered: When a voltmeter is connected across a good (not defective) component in an open circuit, the voltmeter will read zero.
Next, the voltmeter is connected across the open resistor, as shown in figure 8-144. The voltmeter has closed the circuit by shunting (paralleling) the burned-out resistor, allowing current to flow.
Current will flow from the negative terminal of the battery, through the switch, through the voltmeter and the lamp, back to the positive terminal of the battery. However, the resistance of the voltmeter is so high that only a very small current flows in the circuit. The current is too small to light the lamp, but the voltmeter will read the battery voltage. Another troubleshooting point worth 332
Burned out Burned out Broken lamp bulb fuse wire D FIGURE 8-141. Common causes of open circuits.
remembering is: When a voltmeter is placed across an open component in a series circuit, it will read the battery, or applied voltage.
This type of open circuit malfunction can also be traced by using an ohmmeter. When an ohmmeter is used, the circuit component to be tested must be isolated and the power source removed from the circuit. In this case, as shown in figure 8-145, these requirements can be met by opening the circuit switch. The ohmmeter is zeroed and placed across (in parallel with) the lamp. In this circuit, some value of resistance is read. This illustrates another important troubleshooting point: when an ohmmeter is properly connected across a circuit component and a resistance reading is obtained, the component has continuity and is not open.
When the ohmmeter is connected across the open resistor, as shown in figure 8-146, it indicates infinite resistance, or a discontinuity. Thus, the circuit open has been located with both a voltmeter and an ohmmeter.
An open in a series circuit will cause the current flow to stop. A short circuit, or "short," will cause the opposite effect. A short across a series circuit produces a greater than normal current flow. Some examples of shorts, as shown in figure 8-141, are two bare wires in a circuit that are touching each other, two terminals of a resistor connected together, etc. Thus, a short can be described as a Break Break FIGURE 8-143. Voltmeter across a lamp in an FIGURE 8-142. An open circuit. open circuit.
Break Break- FIGURE 8-144. Voltmeter across a resistor in an open circuit.
connection of two conductors of a circuit through a very low resistance.
In figure 8-148, a circuit is designed to light a lamp. A resistor is connected in the circuit to limit current flow. If the resistor is shorted, as shown in the illustration, the current flow will increase and the lamp will become brighter. If the applied voltage were high enough, the lamp would burn out, but in this case the fuse would protect the lamp by opening first.
Usually a short circuit will produce an open circuit by either blowing (opening) the fuse or burning out a circuit component. But in some circuits, such as that illustrated in figure 8-149, there may be additional resistors which will not allow one shorted resistor to increase the current flow enough to blow the fuse or burn out a component. Thus, with one resistor shorted out, the circuit will still function since the power dissipated by the other resistors does not exceed the rating of the fuse.
To locate the shorted resistor while the circuit is functioning, a voltmeter could be used. When it is connected across any of the unshorted resistors, a portion of the applied voltage will be indicated on Break FIGURE 8-146. Using an ohmmeter to locate an open in a circuit component.
the voltmeter scale. When it is connected across the shorted resistor, the voltmeter will read zero.
The shorted resistor shown in figure 8-150 can be located with an ohmmeter. First the switch is opened to isolate the circuit components. In figure 8-150, this circuit is shown with an ohmmeter connected across each of the resistors. Only the ohmmeter connected across the shorted resistor shows a zero reading, indicating that this resistor is shorted.
The procedures used in troubleshooting a parallel circuit are sometimes different from those used in a series circuit. Unlike a series circuit, a parallel circuit has more than one path in which current flows. A voltmeter cannot be used, since, when it is placed across an open resistor, it will read the voltage drop in a parallel branch. But an ammeter or the modified use of an ohmmeter can be employed to detect an open branch in a parallel circuit.
If the open resistor shown in figure 8-151 was not visually apparent, the circuit would appear to be functioning properly, since current would continue to flow in the other two branches of the circuit. To determine that the circuit is not operating properly, the total resistance, total current, and the branch currents of the circuit should be calculated as if there were no open in the circuit:
RI = R N
- 3o = 10 Q total resistance.
FIGURE 8-145. Using an ohmmeter to check a Since the voltage applied to the branches is the circuit component. same and the value of each branch resistance is 334