Electromagnetic Meter Construction

Permanent Magnets

The moving-coil meter movement uses a horseshoe-shaped permanent magnet. The moving coil is placed within the magnetic field between the magnet’s two poles. However, if a simple horseshoe magnet were used, many of the magnetic lines of force would not cut through the moving coil. Magnetic lines of force travel the path of least resistance. Soft iron offers less resistance to lines of force than air. Therefore, soft iron pole pieces are attached to the poles of the magnet to concentrate the lines of force between the magnetic poles.

To further concentrate the lines of force between the magnet poles, a circular, soft-iron core is placed between the pole pieces. The core not only causes a very strong, uniform magnetic field between the poles, but it also acts as a keeper to help the permanent magnet retain its magnetism. The moving coil rotates around the soft iron core, which is fixed in place.


Moving-Iron Meters

The moving-coil and the moving-iron meter movements contain essentially the same parts except for the permanent magnet of the moving-coil movement and the iron bars of the moving-iron movement. A moving-iron meter contains two vanes mounted within a coil. One vane is fixed and the other, with a pointer attached, is free to rotate. Current through the coil induces a magnetic field of the same polarity in both vanes. The free vane, therefore, is repelled by the fixed vane. It rotates a distance that depends on the strength of the magnetic field, and, therefore, on the strength of the current. The pointer, which is attached to the free vane, also rotates and swings across a calibrated scale to indicate the amount of current flowing.


Iron Vanes

The differences between the concentric-vane and radial-vane iron meters are the shape of the vanes and the physical placement of the vanes with respect to each other. The concentric-vane meter contains two semicircular soft iron vanes. One vane is essentially inside the other, which is why the meter is called a concentric-vane meter. The outside vane, which is tapered at one edge, is fixed; the inner vane, which has square edges, is pivoted. When current flows through the coil, the lines of force cut through both vanes, but the distribution of the lines of force is not the same in one vane as in the other. The lines of force are distributed uniformly through the movable (inner) vane because its dimensions are uniform, but they are not distributed evenly in the stationary (outer) vane because of its tapered edge. Fewer lines of force pass through the tapered end than through the rest of the vane because the tapered end is smaller than the rest of the vane, and, therefore, has a higher magnetic reluctance.

When both vanes become magnetized in the same polarity, they will repel each other, causing the movable vane to rotate on its pivot. The strongest repulsion will occur in the area where the stationary vane is not tapered since there will be more magnetic lines than in the tapered area. This means that the movable vane will swing toward the tapered end of the stationary vane since there are fewer lines of force there.






Voltage Tests

Where there is an option for the types of tests or measurements that can be done, opt for the resistance check, which allows for all power to be “off. ” To play it safe in these instances, wherever possible, throw the mainline switch or associated circuit breaker off. Lock the switch off if possible or tag it. Wear clothing that covers as much of your body as possible to reduce the chances of inadvertent contact with “hot” points. Try to use one hand at a time to make connections. Wear rubber or plastic soled shoes that do not use nails and are not worn or frayed. Wear gloves for as many tests as possible and use tools that have insulated handles and test leads that are not frayed. When wiring must be disconnected for a voltage or current test, always disconnect the power before the wiring is disconnected or reconnected and discharge all capacitors.

Be especially cautious in hazardous environments- wet locations of all kinds are dangerous: atmospheres with metal, fibers, carbon, and grain dust are explosive, and can be set off with an electrical spark or hot equipment. Be careful around dangerous fluids and fuels, and gaseous vapors.


Neon Bulb Outlet Test

Meters are usually used to make tests where rather specific, or sometimes, approximate readings are required. There are many tests that only have to indicate whether a line is “hot” or not. Low-cost devices which are easy to use are available for making quick checks like these. The neon bulb tester is one example.

The neon lamps used in line voltage testers typically will not glow if the voltage is under 80 volts and can usually be used safely with voltages up to 250 volts, and sometimes to about 500 volts. Be aware of the rating of the tester you buy. Voltages above the lamp’s upper limit will damage the tester.

The neon tester will glow when it is connected between the hot line and the ground terminals and will not glow between the grounded terminals. This is a good test to determine “hot” and grounded lines. The neon lamp has two elements in it. Only one element glows with a DC voltage and both glow with an ac voltage.

A similar plug-in tester uses three glow lamps, which with six combinations of glow/no glow lights, can indicate whether the outlet voltage is correct, or whether there is an open ground, open neutral, open hot line, reversed hot and ground lines, or reversed hot and neutral lines.


Thermostat Voltage Test

The heating and/ or cooling system thermostats can be either a high- or low-voltage type. The high-voltage type uses the power line voltage to directly activate the equipment being controlled. This hot line test must be handled as carefully as any power line. The low-voltage variety, used in residential and small commercial applications, is like the low-voltage bell circuits. A transformer in the power line reduces the power line voltage to 24-volts dc, which requires inexpensive, safe, low-voltage wiring. The 24 volts at the thermostat is also safe to the user in the event of a short circuit with a metal thermostat case.

While high-voltage power line thermostats can control equipment directly, the 24-volt thermostat usually operates a 24-volt relay, which closes its contacts to energize a high voltage power line circuit. A meter should be used at the thermostat wire to determine the precise reading.





Electromagnetic Current Meters

How Current Affects a Magnetic Field

Current flowing through a coil produces the magnetic field that surrounds the coil. The strength of the magnetic field is proportional to the amount of current flowing through the coil. As the current increases, the strength of the magnetic field increases, and as the current decreases, the strength of the magnetic field decreases. For example, if the current through a certain coil is increased from 1 to 1.6 amperes, the magnetic field around the coil will be stronger for 1.6 amperes than it was for 1 ampere.

Now suppose a spring is attached to the iron bar so that it holds back the bar. The magnetic field, therefore, will have to overcome the spring tension. The stronger the field, the more the spring tension will be overcome. Therefore, the greater the current flowing through the coil, the greater will be the magnetic field and the further the iron bar will be drawn into the coil. The greater the current flowing through a coil enclosing two iron bars, the further each iron bar will be repelled from the other. Similarly, the moving coil will rotate further as the current through the coil increases. All electromagnetic current meters operate on the principle that the strength of the magnetic field about a coil is proportional to the amount of current flowing through it.


Types of Electromagnetic Current Meters

Magnetic fields can be used to cause motion between magnetized objects and the amount of motion is proportional to the strength of the magnetic field, which, in turn, is proportional to the current that produces the field.

There are two basic types of electromagnetic current meter movements in use today: the moving-coil type and the moving-iron type. Both types operate on electromagnetism, but each type uses magnetic fields in a slightly different way to indicate the amount of current flowing in a circuit. Also, each type has certain advantages and disadvantages.

It is not easy to tell the difference between the different types of meters just by looking at them or by using them. From the outside, they appear the same, and they are generally used in the same manner to take current measurements. But, when you know how each type works, you can easily identify them when you examine their movements.


The Moving-Coil Meter Movement

In 1882, Arsene d’Arson val, a Frenchman, invented the galvanometer, named in honor of Italian scientist Galvani. The meter was basically a device that used a stationary permanent magnet and a moving coil. Although the early galvanom­eter was very accurate, it could only measure very small currents and was very delicate. Over the years, many improvements were made that extended the range of the meter and made it very rugged.

Because it is extremely accurate and rugged, the moving-coil movement is by far the most common meter movement used today. It is the basic meter movement used to measure current, voltage, resistance, and a wide variety of other electrical quantities. Therefore, a thorough understanding of the moving­ coil meter is a “must” for anyone studying electricity.

In its simplest form, the moving-coil meter uses a coil of fine wire wound on a light aluminum frame. A permanent magnet surrounds the coil. The aluminum frame is mounted on pivots to allow it and the coil to rotate freely between the poles of the permanent magnet. When current flows through the coil, it becomes magnetized, and the polarity of the coil is repelled by the field of the permanent magnet. This causes the coil frame to rotate on its pivots, and the distance it rotates depends on how much current flows through the coil. Therefore, by attaching a pointer to the coil frame and adding a scale calibrated in units of current, the amount of current flowing through the meter can be measured.







Damping a Moving-Coil Meter


Keeping friction to a minimum permits measuring small currents, but creates a major problem when reading the meter.

When a meter measures current, the pointer should move across the scale and stop immediately at the correct reading. However, because of the very little friction of the rotating parts, the pointer does not come to rest immediately at the correct reading; it overshoots because of inertia, and then the spring pulls it back; it overshoots slightly again, and so on. As a result, the pointer tends to swing back and forth or vibrate, about the correct reading point many times before coming to rest.

To overcome this problem, the meter movement must be damped. Damping can be thought of as a braking action on the rotating parts. It almost completely eliminates the vibrating action of the pointer, resulting in quick, correct pointer indication.

Damping also eliminates another problem. When a meter is removed from an external circuit or when the circuit is de-energized, the pointer returns to zero. Because of the very low friction of the rotating parts, the springs tend to pull the parts back to zero very quickly; so quickly in fact, that the pointer could bend as it overshoots and strikes the left retaining pin. This is particularly true when the meter returns to zero from near full -scale deflection.


Damping a Moving-Coil Meter

Moving-coil meter movements make use of the aluminum frame on which the coil is wound to provide damping. Since aluminum is a conductor, the frame acts as a one-turn coil. When the coil assembly and pointer rotate to register current, the aluminum frame cuts through the field flux lines of the permanent magnet. Small currents, called eddy currents, are induced in the frame, which set up a magnetic field about the frame.

The polarity of this magnetic field is opposite to that of the magnetic field about the coil. Therefore, the magnetic field about the frame opposes the magnetic field about the coil. This action reduces the overall field of the moving coil so that it swings more slowly. In effect, the faster the coil swings, the more the aluminum frame’s field slows it down. This causes the coil and pointer to rotate relatively slowly and smoothly to the correct reading without vibrating. As soon as the coil assembly and pointer come to rest, no further eddy currents are induced in the frame and its magnetic field disappears.

When the meter is disconnected from the circuit, or when the circuit is de-energized, essentially the same action occurs. The coil assembly and pointer start to rotate very rapidly toward zero, but now, since it is rotating in the opposite direction, the eddy currents produced in the frame flow in the opposite direction. This results in a magnetic field with a polarity opposite to the one previously. As the springs pull the coil assembly and pointer closer and closer to zero, the like poles of the permanent magnet and the frame repel each other more and more so that the coil assembly is again braked or slowed down, and slowly comes to rest at zero. Thus, the pointer is prevented from striking the left retaining pin and, perhaps, bending around it.







RMS and Average Values of a Sine Wave

Alternating current flows periodically first in one direction and then in the opposite direction. One direction is called a positive alternation and the other direction is called a negative alternation. A complete positive and negative alternation is called one cycle. The number of complete cycles that occur each second is the frequency and is designated in hertz, abbreviated Hz. Therefore, if one complete cycle occurs per second, the frequency is 1 Hz; if 5 cycles are completed per second, the frequency is 5 Hz, and so on.

An ac wave can have many wave shapes; for example, it can be a sine wave, square wave, sawtooth wave, etc. AC meters are calibrated based on sine waves. When an ac meter is used to measure non-sinusoidal waveforms, only an approximate indication of values is obtained. Sometimes the indicator could be so far off that the reading is meaningless. Therefore, other measuring instruments such as oscilloscopes should be used instead of ac meters to measure non-sinusoidal waveforms.


 RMS and Average Values of a Sine Wave

The root-mean-square (RMS) value of a sine wave is very important in the study of meters. The basic electrical units, that is, the ampere and the volt, are based on de. Therefore, a method had to be derived to relate AC to DC. The maximum, or peak, value of a sine wave could not be used because a sine wave remains at its peak value for only a very short time during an alternation. Thus, a sine wave with a peak current of 1 ampere is not equal to a dc current of 1 ampere from an energy standpoint since the dc current always remains at 1 ampere.

A relationship based on the heating effects of ac and dc was derived. It was found that a current equal to 0.707 of the peak ac wave produced the same heat, or lost the same power, as an equal DC current for a given resistance. For example, a sine wave with a peak value of 3 amperes has a heating effect of 0.707 x 3 or 2.121 amperes of de.

The value of 0.707 can be derived in the following manner: The heating effect of current is based on the basic power formula; that is, P = 12 R, where P is the power dissipated as heat. From the formula, you can see that the heat varies as the square of the current.

When a sine wave reaches its peak value, the heat dissipated becomes maximum. Lesser heat values are dissipated for all values of current below the peak value. To find the heat dissipated during an entire sine wave cycle, each instantaneous value of current is first squared and then added. Then the mean (or average) of this sum is found. After this, the square root of the mean is found, and the answer is called the root-mean-square (RMS) value of the sine wave. Often the RMS value of a sine wave is called the effective value because 0.707 of the peak value of a sine wave has the same effect as an equal amount of de.

Another sine wave characteristic that is important in the study of meters is the average value of the sine wave. The average value is obtained during one alternation and is equal to 0.637 of the peak value of the sine wave.







Rectifier Meters

The moving-coil meter movement can only be used to measure dc, however; there is no way that it can be used to measure ac directly. If ac was applied directly to the meter, one half of the cycle would try to make the meter pointer move in one direction and the other half would try to make the pointer move in the opposite direction. Even at very low frequencies, the pointer would not be able to move fast enough to follow the positive and negative alternations of the ac wave. Therefore, instead of moving across the scale, the pointer would simply vibrate about zero. But, if the ac is first changed to DC before applying it to the meter movement then the moving-coil meter can be used in AC applications as well as DC.

Alternating current can be converted to DC by special devices called rectifiers. These devices offer a high opposition to current flow through them in one direction and a low opposition to current flow in the other direction. Therefore, when a sine wave is applied to a rectifier, it will pass either the positive alternation or the negative alternation, depending upon how the rectifier is connected into the meter circuit. In no case, however, will it pass both alternations. Therefore, a rectifier changes a sine wave to a pulsating dc wave. In a rectifier symbol, the arrow points in the direction of high resistance, so current flow through it is in the opposite direction. Rectifier meters use semiconductor rectifiers.


The Half-Wave Rectifier Meter

There are two basic types of rectifier circuits: the half-wave type and the full-wave type. In the half-wave type, one alternation of current passes through the meter movement and the opposite alternation is bypassed by the rectifier. Even though current through the meter is pulsating, the meter pointer, because of its inertia, will not have sufficient time to follow these fluctuations. Therefore, the meter pointer will rest at the average value of the current flowing through it.

The average current for one alternation is 0.637 of peak value, but, for the next alternation, it is zero and that alternation is bypassed by the meter. Therefore, the average current for a complete cycle is the sum of both alternations divided by 2, or 0.637/2 = 0.313 of the peak value. The meter pointer then deflects to the position on the scale that represents 0.318 of the peak value of the current flowing through the meter. But, for the reading to be meaningful the scale is usually calibrated to show the equivalent effective or RMS value. Therefore, the points on the scale are calibrated at 0.707 of the equivalent peak values.






Introduction to Electromagnetism

The Basic Meter

Meters, except for the few that operate on electromagnetic principles can only measure the amount of current flowing through them. However, they can be calibrated to indicate almost any electrical quantity. For example, you know that according to Ohm’s law, the current that flows through a meter is determined by the voltage applied to the meter and the resistance of the meter: I=        V/ R

Therefore, for a given meter resistance, different values of the applied voltage will cause specific values of current to flow. As a result, although a meter measures current, the meter scale can be calibrated in units of voltage.

Similarly, for a given applied voltage, different values of resistance will cause specific values of current to flow. Therefore, the meter scale can also be calibrated in units of resistance, rather than in units of current. The same holds true for power since power is proportional to current:    P = VI or P = I 2 R


The Current Meter

When current flows through a wire, it produces three effects:

1.         It creates a magnetic field that surrounds the wire.

2.         It generates heat in the wire.

3.         It produces a voltage drop across a resistance.

The amount of current flowing through the wire determines both the strength of the magnetic field and the amount of heat produced. These effects are used in the two basic types of current meters: the electromagnetic current meter and the thermal current meter. From their names, you can see that the electromagnetic meter makes use of the magnetic field to measure the amount of current flow and the thermal meter makes use of the heat produced by the current flow to measure the amount of current flow.


Review of Electromagnetism

The electromagnetic current meter is, by far, the one used most often to measure current, voltage, resistance, and power. This type of meter is easy to understand if you know the basic magnetic principles upon which the meter operates.

Magnetic fields interact in certain ways. For example, the like poles of two iron magnets will repel each other and the unlike poles will attract each other. The same is true for the like and unlike poles of electromagnets. Furthermore, an iron magnet and an electromagnet will repel each other if they are positioned so that their like poles are facing each other, and they will attract each other if their unlike poles are facing each other.



If you place a soft iron bar close to a magnetized solenoid, the iron bar will become magnetized. The magnetic lines of force set up in the iron will line up in the same direction as those of the solenoid. As a result, the poles set up in the iron bar will also be in the same direction. Therefore, the poles of the solenoid and iron bar that face each other are opposite. Since opposite poles attract each other, the iron bar will be drawn into the coil. The plunger-type moving iron meter operates on this principle.






Calculating the Resistance of Multirange Multipliers

There are two methods of calculating the values of multiplier resistors for a multirange voltmeter. In the first method, each multiplier is calculated the same as for a single-range voltmeter. Assume that you wish to extend the range of a 1-mA movement to measure 0- 10, 0- 100, and 0- 1000 volts, and you also want a 0- 1-V range. Since full-scale deflection equals 1 Von the 0- 1-V range (V = IM RM = 0.001 A x 1000 Q = I volt), no multiplier is needed. The total resistance (RT OT) needed to limit meter current (IM) to 1 mA on the 0- 10-V range is RrnT = V w vJ IM   =   10 V/ 0.001A=    10,000 Q

Since the resistance of the meter (RM) is 1000 ohms, then the multiplier resistance RMuLT is 9000 ohms. A second method of calculating the values of voltmeter multiplier resistors is the series-multiplier arrangement in which the multiplier resistors are connected in series. R1 is the multiplier resistor for the 0-10 volt range. For the 0- 100-V range, R1 is in series with R2. Therefore, the value of the multiplier resistance for the 0- 100-V range is equal to R1 plus R2. Similarly, the multiplier resistance for the 0-1000-V range is equal to R1 plus R2 plus R3.

Now, let’s calculate the values for a series multiplier voltmeter. We will use the same 1-mA, 1000-Q meter movement that we used previously. Since this movement indicates 1 volt for a full-scale deflection, no multiplier resistor is needed for the 0- 1-V range. Therefore, your first step is to calculate the multiplier resistance needed for the 0- 10-V range. Again, using Ohm’s law, find the total resistance (RToT) needed to limit meter current (IM) to 1 mA at this range:

RrnT = V w.xl lM     = 10 V/ 0.001 A = 10,000 Q

Therefore, multiplier resistor R1 for the 0-10-V range equals 10,000n minus the 1000-Qmeter resistance, or 9000 n. Thus far, the procedure is the same as in the other method, and the value of the multiplier resistor is the same for the 0- 10-V range. Having found the series multipliers for the 0- 1- and 0- 10-V ranges, let’s calculate the total resistance needed for the 0- 100-Vrange:

RrnT = vM AXl’IM = 100 v; o.001 A = 100,000 n

Subtracting the meter resistance from the total resistance, you find that the multiplier resistance for the 0- 100-V range is 99,000 ohms. Thus far, this method is the same as the previous, but now the multiplier resistance is made up of R I plus R2 in series. Therefore, since you need 99,000 n for the multiplier resistance and R I equal s 9000 n, R2 must equal 90,000 n.

Similarly, for the 0-1000-V range:

RT oT = VMNJ’IM = 1000 v; o.001 A = 1,000,000 n

Thus, RMuLT = RToT –   RM = 1,000,000 –   1000 = 999,000 n. But RMuLT = R1    + R2   + R3.

Thus, RMuLT =   999,000 n =   9000 + 90,000 + R3

And R3 =   999,000 –   99,000 =   900,000 n

No matter which method you use, the value of the multiplier resistance for each range remains the same. However, in the first method, the multiplier is a single resistor, while in the second method, on all but the first extended range, it is made up of resistors in series.






Current Tests: Part 1

Current tests are somewhat more difficult than voltage tests because although voltage tests are done while the circuit is energized, the test probes need only touch test points to get a voltage reading. Using the clamp-on ammeter is similarly easy since it needs simply to be clamped to the energized wire. With the in-line ammeter, though, a current test requires the power to be shut down, the circuit opened, the meter connected in place, the circuit closed again, and the power turned on to get a reading. There are similar steps when the test is completed and the wiring must be reconnected. Care must be taken in handling disconnected, loose wiring, and the ammeter must be firmly wired into the circuit with good resistance free connections. Be sure the meter is firmly supported.

Make sure all power is shut down and capacitors are discharged before you touch bare wires.


Power Line Current Tests

In a power line, whether it is the main service entrance or a separate branch circuit, the size wire and circuit breaker that are used should have been originally determined by the maximum current that the circuit was expected to carry under full load. The more current the circuit was to have carried, the larger the wire size and the larger the circuit breaker rating should have been. It was probably evaluated this way originally, but as time went on, more than expected loads could have been added to the system.

The first step in testing a power line current is to turn the main line breakers OFF. When using a clamp-on ammeter, clamp it to the appropriate wire. With the in-line ammeter, disconnect a branch wire from its circuit breaker. Next, connect a short extra test lead into the breaker for the circuit under test and connect the ammeter between these leads. Set the ammeter to its highest range, higher than that of the circuit breaker or wire rating, in case there is a short circuit causing excessive current flow. Turn the mainline breakers ON and energize appliances or equipment on the branch circuit one at a time. The ammeter reading should be below the rating of the breaker and the wire. If the reading climbs too close to the breaker rating before all appliances are on, then the line is overloaded. You can restrict the use of appliances or change the branch into two separate branches. Do not merely put in a bigger circuit breaker. The rating of the circuit breaker should match the wire size. For example, the 14-gauge wire should carry no more than 15 amps; and 12-gauge wire, no more than 20 amps. Otherwise, the wires will overheat.


Wire Sizes

Wire has resistance. When current flows through the wire, this resistance causes heat to be produced. The smaller the wire diameter, the more the resistance, and the greater the heat for a given current flow. Since larger wire sizes produce less heat with the same current, larger wire sizes have higher current ratings. For the American Standard Wire Gauges and their ampere capacity, the gauge number goes up as the wire size goes down. The ampere capacity also goes down as the gauge number goes up. The actual ampere capacity also depends on the insulation used, since the insulation must be able to withstand the heat. Note that 14-gauge wire, which is normally rated at 15 amps, can carry from 15 to 43 amps, depending on the specific insulation used; but it is usually limited to 15 amps.






Component Testers

The Wheatstone Bridge

When extremely accurate resistance measurements are required, a Wheatstone bridge is used. A Wheatstone bridge consists of four resistors connected in a diamond-shaped array. One of the resistors is the unknown one to be measured. A current source is connected to two opposite junctions and a sensitive meter is connected between the other two junctions. The meter has a zero-center reading.

To understand how a Wheatstone bridge measures resistance, assume that resistors R I and R2 each equal 400 ohms, and resistor R3 is variable from 0 to 1000 ohms. Now connect resistor Rx into the bridge circuit and close the switch. You can see that R1 and R3 form one divider network, and R2 and Rx form another divider network.  Therefore, since R1 equals R2, if R3 is made equal to Rx, the current and voltage drops in both dividers will be identical. Thus, the potentials at points C and D will be the same so that no current will flow through the meter. Therefore, when R3 is adjusted for a zero reading, you know its value equals that of Rx. The dial of variable resistor R3 is calibrated to show its exact resistance when adjusted. Therefore, its setting is also the value of unknown resistor Rx. Usually, the Wheatstone bridge contains many components so that different values of R 1, R2, and R3 can be switched in to test a wide range of resistances accurately.


Capacitors and Inductors

Prior to the development of the inexpensive digital meter, meters that measured capacitors and inductors were limited to expensive lab-type equipment. Today, though, many multimeters, as well as specialized meters, provide for routine testing of these components. Many digital volt-ohm-ammeters can check capacitors, with typical values from the low picofarads range to about 20 microfarads. Specialized test meters can test values up to 1 farad, as well as for capacitor leakage, equivalent series resistance, and dielectric absorption; some can also test inductors for values and for shorted turns.

The Wheatstone bridge can also be used to measure unknown values of capacitors and inductors in the same way as it does for resistors. However, since capacitors and inductors are reactive devices, an ac source must be used.

The same diamond-shaped array is used for the bridge, and a sensitive meter is connected between the same opposite junctions. The ac currents that are produced by the capacitive or inductive reactance’s in each leg will be the same when L3 or C3 equals Lx or Cx, as the case may be. This will cause points C and D to be at the same potential, and zero current will flow through the meter. The calibrated setting of L3 or C3 will show the value of Lx or Cx.


Diodes and Transistors

As explained for capacitors and inductors, the progress in digital meter design has allowed the meter to be used for a wide variety of sophisticated functions. In the past, analog multimeters were limited to testing resistive components; and then were able to handle capacitors and inductors. But these are all considered to be passive components with relatively fixed values. Active components, such as diodes and transistors, always required highly specialized test equipment.  They still do, for a complete and reliable analysis.

A few simple static tests can be made with diodes and transistors to give an initial idea of their reliability. With simple battery circuits and the diode connected with forward or reverse bias, the forward (high) current or the reverse (low) current can be measured. These quantities and the ratio of these quantities give an indication of the reliability of the diode.

Bipolar transistors are more difficult to check because of the complex characteristics and interaction among its three elements- the base, emitter, and collector.  Highly sophisticated equipment is needed to make complete dynamic tests. A typical digital multimeter, though, provides for testing the static forward current transfer ratio, which is the gain of the transistor in a circuit.