ELECTRICAL SAFETY
Improper electrical wiring or misuse of electricity causes destruction of equipment and
fire damage to property. Safe working habits
are required when troubleshooting an electrical circuit or component because the electric
parts that are normally enclosed are exposed.
Personal protective equipment must be worn
when working with electrical circuits to prevent injury. Safety glasses, insulated gloves,
and safety shoes are typical requirements.
Disconnects and Overcurrent
Protection Devices
A disconnect is a switch that disengages
the supply of electric power from a circuit.
Disconnects are used to manually remove
or apply power to a circuit. With few exceptions, disconnects are required in circuits that
control large loads. For example, disconnects
are used in motors, high-intensity-discharge
lighting, and heating applications.
In a properly operating circuit, current
flows through the conductive paths provided
by conductors and other components when a
load is turned on. Every load draws a normal
amount of current when operating. This normal amount of current is the current level that
the loads, conductors, switches, and other systems are designed to handle. An overcurrent is
a condition that exists in an electrical circuit
when the normal load current is exceeded.
An overcurrent protection device (OCPD)
is a circuit breaker or fuse that provides
protection to a circuit when the current exceeds the designed safety limits.
A circuit breaker is an overcurrent protection device with a mechanism that opens
a circuit when a predetermined amount of
current has been exceeded. A circuit breaker
is manually reset after a fault is cleared and
provides short-circuit and overcurrent protection to a circuit. A fuse is an overcurrent
protection device with a fusible link that melts
and opens a circuit when an overload condition exists. Fuses are replaced in disconnects
after a fuse has blown and the identified fault
is cleared.
CIRCUIT ANALYSIS
Electricity is the movement of electrical
charge from atom to atom. A conductor,
such as copper wire, has many free electrons
that can easily be pushed through a circuit to
produce work. An insulator, such as plastic
wire insulation, has very few free electrons.
Therefore, a conductor allows the free movement of charge and an insulator blocks the
movement of charge.
The unit of resistance, the ohm, is named after the German physicist, Georg Simon Ohm
(1787-1854), who discovered the relationship
between current, voltage, and resistance.
1
INSTRUMENTATION
A circuit junction is a place in an electrical circuit where at least three wires meet. A
junction provides alternate paths for current
to flow in the circuit. A branch is a wire connecting two junctions. In a circuit containing
multiple resistances in series and parallel, the
circuit can generally be reduced to a single
equivalent resistance. This equivalent resistance can be used to calculate the total current
flowing in a circuit. Then Ohm’s law and
Kirchoff’s laws can be applied to determine
the exact current flow through each branch.
Ohm’s Law
Ohm’s law is the relationship between voltage, current, and resistance in an electrical
circuit. Ohm’s law states that current in a
circuit is proportional to the voltage and
inversely proportional to the resistance. Any
value in this relationship can be found when
the other two are known. The relationship
between voltage, current, and resistance may
be visualized by presenting Ohm’s law as a
circular diagram. See Figure 1.
Ohm's Law
CU
I
R
R
R
EN
E
E=IxR
R
I
E
R
E
I
T
E
)
L TAGE
NC
E = VOLTAGE (in V)
I = CURRENT (in A)
R = RESISTANCE (in
VO
TA
2
RE S
Voltage
Voltage is the potential energy difference between two points in an electrical circuit. The
unit of voltage is the volt (V). Voltage is often
called electromotive force, or emf. Therefore,
equations may have either the letter V or the
letter E representing voltage. Voltage can be
referred to as the amount of electrical pressure
in a circuit, analogous to pressure of water in a
hose or pipe. The electrical pressure is used to
force the movement of charge through a resistance in a circuit. While voltage pushes charge
through a circuit, voltage itself does not flow
since it is a potential difference. Voltage drop
is the potential energy difference across a load
in a circuit as work is being done.
Polarity is the relative positive (+) or negative (–) state of an object. Polarity does not
always have a zero point even though ground
is often defined as zero voltage. A part of a
circuit that is more positive than another relative to ground has a positive polarity. The less
positive part of a circuit relative to ground has
a negative polarity.
IS
VOLTAGE = CURRENT x RESISTANCE
E
I= _
R
CURRENT =
Kirchoff’s Laws
Kirchoff’s voltage law is a law stating that the
sum of the voltage drops in a series circuit
is equal to the sum of the applied voltages.
Kirchoff’s current law is a law stating that the
sum of the currents entering a junction point
in an electrical circuit is equal to the sum of
the currents leaving the junction point.
VOLTAGE
RESISTANCE
E
R= _
I
VOLTAGE
RESISTANCE =
CURRENT
Figure 1. Ohm’s law can be represented as a circular diagram to help remember the relationships between voltage, current, and resistance.
Current
Current is a flow of charge passing between
two points in a circuit. The unit of current is
the ampere (A). Current flow is energy and
is used to produce work. Early scientists
believed that charges flow from positive to
negative. Later, when atomic structure was
studied, the concept of electron flow from
negative to positive was introduced. The two
different theories of current flow are called
electron current flow and conventional current flow. Electron current flow is current
flow from negative to positive. Conventional
current flow is current flow from positive to
negative. See Figure 2.
Principles of Electricity
Current Flow
POWER SOURCE
CURRENT FLOW IS
NEGATIVE TO POSITIVE
LOAD
Electron Theory
Instrumentation Loops. A very common
method of sending electrical signals in a
control circuit is through the use of DC
signals. An instrumentation circuit typically
has a signal current of 4 mA to 20 mA. This
current is typically converted to voltage at a
receiver through the use of a precision 250 Ω
resistor wired in parallel with the receiver.
The receiver is typically a very high input
impedance device that draws virtually no
current from the circuit. See Figure 3.
POWER SOURCE
Instrumentation Loops
CURRENT FLOW IS
POSITIVE TO NEGATIVE
4 mA TO 20 mA CURRENT TRANSMITTER
LOAD
Conventional Theory
Figure 2. The two different theories of current flow are called electron current flow and
conventional current flow.
Alternating Current. Alternating current
(AC) is the flow of electricity that reverses
direction on a regular basis. In an AC
circuit, the voltage and current cycle continuously between minimum and maximum
values. A cycle is one complete positive and
negative alternation of a waveform. The
number of complete cycles occurring in
one second is the frequency of the current.
A hertz (Hz) is the unit of frequency equal
to one cycle per second. Alternating current
in the United States is supplied at 60 Hz.
Frequencies vary in other countries, with
50 Hz being common.
Direct Current. Direct current (DC) is the
flow of electricity in one direction through
a circuit. In a DC circuit, the actual current
flow may vary over time, but it is always in
the same direction. For example, batteries
provide power for many devices. The actual
voltage level in a battery declines with use
as the electrochemical circuit discharges.
However, no matter how far the voltage
declines, the polarity remains the same.
24 VDC
POWER
SUPPLY
250 Ω
T1
VERY HIGH INPUT IMPEDANCE
Figure 3. An instrumentation loop typically uses a 4 mA to 20 mA signal.
Power electronics is the application of
solid-state electronics for the control and
conversion of electric power.
Resistance
Resistance is the property of an electrical
circuit that opposes current flow. The unit
of resistance is the ohm (Ω). An ohm is the
resistance of a conductor in which a voltage
of 1 V causes a current flow of 1 A. Just as
water pressure in a long pipe is reduced by
friction, voltage is reduced by resistance.
Resistance limits the flow of current in a
circuit. All materials have some resistance
to current flow. An insulator is a material
that has a very high resistance.
3
4
INSTRUMENTATION
Series. A circuit frequently contains several
resistances. A series circuit is a circuit that
has two or more loads connected so that there
is only one path for current flow. The total
resistance is equal to the sum of the separate
resistances. As the current passes through
each resistance, there is a drop in voltage in
accordance with Ohm’s law. The voltage loss
through each load is equal to the product of
the current and the resistance. According to
Kirchoff’s voltage law, the sum of the voltage drops across each load equals the total
voltage of the power source.
Parallel. Electrical loads are often connected
in parallel. A parallel circuit is a circuit that
has two or more components connected so
that there is more than one path for current
flow. Since each individual resistance is connected to the applied voltage terminals, the
voltage across each resistance is equal to the
applied voltage. See Figure 4. The current
through each resistor varies with the value
of the resistance. The current from the power
source is equal to the sum of the currents
through the parallel resistances.
Parallel Resistance
0.648 A
POWER
SUPPLY
0.408 A
0.240 A
100 Ω
0.648 A
0.216 A
0.096 A
0.192 A
0.120 A
150 Ω
0.408 A
200 Ω
0.216 A
0.096 A
250 Ω
0.096 A
Parallel Circuit
0.648 A
POWER
SUPPLY
37.04 Ω
Equivalent Circuit
Figure 4. A parallel circuit has two or more components connected so that
there is more than one path for current flow.
Networks. Resistors in a circuit can be
arranged in combination series-parallel
arrangements. These are called resistance
networks. When two or more components
of an electrical network are in series, all
the characteristics of series circuits must
apply to these components. In the same
way, when two or more components of an
electrical network are in parallel, all the
characteristics of parallel circuits must
apply to these components. Electrical
circuits can have several sources of voltage in addition to several series-parallel
resistances.
Variable Resistors. A variable resistor is
a resistor that can change its resistance to
current flow. A potentiometer is a variable
resistor where a dial or knob is turned
to change the length of the circuit path
through a resistor. A resistance temperature
detector (RTD) uses a high-precision resistor that varies with temperature. A thermistor is a temperature-sensitive resistor
consisting of solid-state semiconductors
made from sintered metal oxides and lead
wires, hermetically sealed in glass.
Capacitance
Capacitance is the ability of an electrical
device to store electric charge. A capacitor is a circuit component that possesses
capacitance. A simple capacitor consists
of two parallel metal plates separated by
air or other dielectric. A dielectric is an
insulating material between the conductors
of a capacitor. The symbol for capacitance
is C and the unit is the farad (F). A farad is
a very large amount of capacitance. Therefore, capacitance is usually expressed
as microfarads (µF), nanofarads (nF), or
picofarads (pF).
When a switch is closed in a circuit
containing a capacitor and a resistor, current begins to flow in the circuit. Current
continues to flow until the voltage across
the terminals of the capacitor equals the
source voltage. The current flow stops when
capacitor voltage reaches this magnitude.
The capacitor stores this charge until the
voltage is reversed or removed.
Principles of Electricity
RC Time Constant. When a capacitor
and resistor are in a DC circuit, the time it
takes to charge the capacitor depends on
the capacitance and the resistance. A large
capacitance takes longer to charge than a
small capacitance. A large resistor limits the
current more than a small resistor. Therefore, it takes longer to charge a capacitor in
a circuit with the larger resistor.
The time constant, τ, of an RC circuit
equals the resistance multiplied by the capacitance. The charge time is based on an
exponential curve. After the capacitor has
been charging for a time equal to 1 time
constant, the charge on the capacitor is
equal to 63.2% of the final voltage. After a
time equal to 5 time constants, the capacitor
is essentially fully charged. See Figure 5.
The same principles apply during discharge
of a capacitor. The capacitor discharge
curve is a mirror image of the shape of the
charge curve.
A common concern in control systems is
the lag between the time a change is made
to a process and the time the response is detected. This occurs in physical systems that
have large storage capacities and resistance
to change. The same type of response curve
as in an RC circuit is used when describing
these types of systems.
Magnetism
A magnet is a device that attracts iron and
steel because of the molecular alignment
of its material. An electromagnetic field
is a force that causes interactions between
magnets and electrical current in a conductor. A ferromagnetic material is a material,
such as soft iron, that is easily magnetized.
A magnetic flux line is an invisible line of
force that makes up a magnetic field. The
denser the flux lines, the stronger the magnetic field. Flux is most dense at the ends
of a magnet. For this reason, the magnetic
force is strongest at the ends of a magnet.
See Figure 6. The flux lines leave the north
pole and enter the south pole of a magnet
or magnetic field.
Induction is the process of causing electrons to align to create a magnetic or electrical force. Placing an iron bar in a strong
magnetic field causes induction. Iron atoms
share electrons and the lines of force in the
field combine to pass through the iron bar,
causing the bar to become a magnet. Magnetic flowmeters use specially shaped coils
to set up a magnetic field at a right angle to
the direction of flow. As conductive material
flows through the magnetic field, a voltage
is induced on electrodes set perpendicular
to the field.
RC Circuits
100
R1
C1
100V
95.0
75
V
Voltage
S1
98.2
86.5
63.2
50
25
FIRST-ORDER
RESPONSE CURVE
0
SWITCH CLOSES AT
TIME = 0
0
τ
2τ
3τ
Time
Figure 5. The time constant is a measure of how quickly a change moves throughout a system.
4τ
5τ
99.3
5
INSTRUMENTATION
Magnetism
NORTH POLE
SOUTH POLE
N
S
MAGNETIC FLUX LINES
MAGNET
W
S
DIRECTION OF
CURRENT FLOW
N
6
CURRENTCARRYING
WIRE
a circuit or device that opposes a change
in the flow of current. The symbol for capacitive reactance is XC and the symbol for
inductive reactance is XL.
Capacitive reactance is a capacitor’s resistance to alternating current. The amount
of capacitive reactance in a capacitor depends on the capacitance of the capacitor
and the frequency of the alternating current
through the capacitor. Inductive reactance
is an inductor’s resistance to alternating current. The amount of inductive reactance in
a coil depends on the inductance of the coil
and the frequency of the alternating current
through the inductor.
Impedance is the total opposition of any
combination of resistance, inductive reactance, and capacitive reactance offered to
the flow of alternating current. Impedance
and reactance are measured in ohms.
SEMICONDUCTOR DEVICES
MAGNETIC FLUX LINE
Figure 6. Magnetic flux lines are the invisible lines of force that make up a
magnetic field.
Inductance
Inductance is the property of an electrical
circuit or device that resists a change in
current due to energy stored in a magnetic
field. An inductor is an electrical device
that possesses inductance. Whenever current flows through a conductor, a magnetic
field is produced around the conductor. A
conductor wrapped into a coil produces a
strong magnetic field around the coil anytime current flows through the coil.
Reactance and Impedance
The flow of alternating current in a circuit
has different properties than the flow of
direct current. Resistors have the property called resistance while inductors and
capacitors have a similar property called
reactance. Reactance is the property of
Semiconductors are electrical devices with
resistances that fall somewhere between the
low resistance offered by conductors and
the high resistance offered by insulators.
Conductors have many free electrons. This
allows easy movement of current through
the conductor. Insulators have very few free
electrons. This blocks the movement of current through the insulator. Semiconductors
have a few free electrons that are available
for current flow.
Most industrial electrical circuit diagrams use conventional current flow. Semiconductor circuits are often described with
electron flow. Care must be exercised to
determine the convention employed when
using semiconductor devices.
Doping
The basic material used in most semiconductor devices is silicon. As a purified
crystal, silicon does not have enough free
electrons to support significant current
flow. To prepare these crystals for use as
semiconductor devices, the crystal structure
must be altered to permit significant current flow. Doping is the process by which
Principles of Electricity
the crystal structure of a semiconductor is
altered by adding trace amounts of other
elements. See Figure 7.
N-Type Material. N-type semiconductor
material is created by doping a region of a
semiconductor crystal with atoms from an
element that has more electrons in its outer
shell than the crystal does. Adding these
atoms to the crystal results in more free
electrons. Since electrons have a negative
charge, the doped region is called N-type
material. As with any conductor, free electrons support current flow. When voltage is
applied across N-type material, electrons
flow from negative to positive through the
crystal. The free electrons help move current and are called carriers.
P-Type Material. P-type semiconductor
material is created by doping a region of
a semiconductor crystal with atoms from
an element that has fewer electrons in its
outer shell than the crystal does. Adding
these atoms to the crystal results in fewer
free electrons, resulting in a net positive
charge. The doped region is called P-type
material. The missing electrons in the crystal structure are called holes. The holes are
represented as positive charges.
Doping
FREE
ELECTRON
Si
Si
Si
BOUND
ELECTRON
Si
P
Si
PHOSPHORUS
ATOM
Si
Si
Si
SILICON
ATOM
Figure 7. Doping is the process by which the crystal structure of a semiconductor is altered.
Diodes
A diode is a semiconductor device that allows current to pass through in only one
direction. A diode is typically constructed of
one layer of N-type material and one layer of
a P-type material. Each of the two layers of
the diode is connected to a terminal, resulting
in a two-terminal device.
Bias. A diode conducts electricity when its
anode voltage is more positive than that of
the cathode. Forward bias is the condition
where a voltage is applied to a diode in such
a way as to allow the diode to conduct easily.
When a diode is forward biased, the polarity
of the power supply causes the free electrons
and holes to move toward the region between
the N-type and P-type materials. In this arrangement, carriers are available from one
end of the diode to the other, allowing normal
current flow with little resistance.
Fluke Corporation
Benchtop multimeters can take continuity, frequency, and diode measurements.
7
8
INSTRUMENTATION
A diode blocks the flow of electricity
when its anode voltage is less positive than
that of the cathode. Reverse bias is the condition where a voltage is applied to a diode in
such a way as to cause the device to act as
an insulator. When a diode is reverse biased,
the polarity of the power supply causes the
free electrons and holes to move away from
the region between the N-type and P-type
materials. This action blocks current flow.
Photoelectric Diodes. The major purpose of
a photoelectric device is to work as a transducer to produce a change of either resistance
or voltage when exposed to light. Typically,
photoelectric diodes, or photodiodes, are
used as ON/OFF devices, measuring devices,
or limited power sources. See Figure 8. As
measuring devices, photoelectric transducers
respond to changes in the intensity or color
of light. This results in a variable output voltage. Examples of photoelectric transducers
as instrument measuring devices are detectors, gas analyzers, and spectrometers.
Light-Emitting Diodes. A light-emitting
diode (LED) is a diode that produces light
through the use of semiconductor materials.
A diode junction can emit light when an
electrical current is present. The presence of
an electrical current produces light energy
because electrons and holes are forced to
recombine.
Photodiodes
PHOTODIODE
CHIP
WINDOW
METAL
CAN
ANODE
LEAD
CATHODE
LEAD
Figure 8. Photoelectric diodes, or photodiodes, are typically used as ON/OFF devices, measuring devices, or limited power
sources.
Transistors
A transistor is a semiconductor device used
for switching and amplification. The two
types of transistor construction are PNP and
NPN. See Figure 9. A PNP transistor is a
transistor with a thin layer of N-type material between two layers of P-type material.
An NPN transistor is a transistor with a thin
layer of P-type material between two layers
of N-type material. The thickness of the
center layer is typically about 0.001″.
Transistors are three-terminal devices.
The terminals are the emitter (E), base (B),
and collector (C). In any transistor, the baseemitter junction must always be forward
biased and the base-collector junction must
always be reverse biased. For an NPN transistor, the external voltage bias is connected
so that the positive terminal connects to the
P-type material in the base.
A bipolar junction transistor (BJT) is a
transistor in which the amount of current
between the base and emitter controls the
actions of the transistor. The action of a
BJT is such that a small current between the
base and emitter results in a large current
between the emitter and collector when a
DC power supply is used to bias the transistor. This results in the transistor acting
as an amplifier. A field-effect transistor
(FET) is a transistor in which output current is controlled by the voltage across the
input. The voltage between the two areas
of P-type material controls the actions of
the transistor.
Silicon-Controlled Rectifiers. A silicon-controlled rectifier (SCR) is a four-layer (NPNP),
three-electrode semiconductor device with
a very low resistance when conducting and
a very high resistance in the OFF state. The
three electrodes of an SCR are the anode, cathode, and gate. The gate serves as the control
point for the SCR. An SCR has a characteristic
forward breakover voltage.
An SCR does not conduct significant
current, even when forward biased, unless
the anode voltage equals or exceeds the forward breakover voltage. When the forward
breakover voltage at the anode is reached,
the SCR switches ON and becomes highly
Principles of Electricity
conductive. Once an SCR starts conducting,
it continues to conduct until the current drops
below the holding current, at which point the
SCR switches OFF.
Triacs. A triac is a three-electrode control
device used to switch AC circuits. A triac
is triggered into conduction in both directions by a gate signal in a manner similar to
the action of an SCR. Triacs generally have
relatively low current capabilities compared
to SCRs and are usually limited to less than
50 amps. A triac can be considered to consist
of two NPN switches sandwiched together
on a single wafer of N-type material. A triac
remains OFF until the gate is triggered.
A separate trigger circuit pulses the gate
and turns the triac ON, allowing current to
flow. The trigger circuit eventually returns
the gate voltage to zero. This causes the
triac to turn OFF. The trigger circuit can be
designed to produce a pulse that varies at any
point in the positive or negative half cycle.
Therefore, the average current supplied to
the load can vary.
Transistor Symbols
C
C
N
B
B
P
N
E
E
NPN Symbol
C
C
P
B
B
N
P
E
E
PNP Symbol
Figure 9. A transistor is a semiconductor device used for switching and
amplification.
KEY TERMS
•
disconnect: A switch that disengages the supply of electric power from a circuit.
•
overcurrent: A condition that exists in an electrical circuit when the normal load current is exceeded.
•
overcurrent protection device (OCPD): A circuit breaker or fuse that provides protection to a circuit
when the current exceeds the designed safety limits.
•
circuit breaker: An overcurrent protection device with a mechanism that opens a circuit when a
predetermined amount of current has been exceeded.
•
fuse: An overcurrent protection device with a fusible link that melts and opens a circuit when an
overload condition exists.
•
Kirchoff’s voltage law: A law stating that the sum of the voltage drops in a series circuit is equal to
the sum of the applied voltages.
•
Kirchoff’s current law: A law stating that the sum of the currents entering a junction point in an
electrical circuit is equal to the sum of the currents leaving the junction point.
•
current: A flow of charge passing between two points in a circuit.
•
electron current flow: Current flow from negative to positive.
•
conventional current flow: Current flow from positive to negative.
•
alternating current (AC): The flow of electricity that reverses direction on a regular basis.
9
INSTRUMENTATION
10
•
cycle: One complete positive and negative alternation of a waveform.
•
direct current (DC): The flow of electricity in one direction through a circuit.
•
resistance: The property of an electrical circuit that opposes current flow.
•
ohm: The resistance of a conductor in which a voltage of 1 V causes a current flow of 1 A.
•
insulator: A material that has a very high resistance.
•
series circuit: A circuit that has two or more loads connected so that there is only one path for current flow.
•
parallel circuit: A circuit that has two or more components connected so that there is more than one
path for current flow.
•
variable resistor: A resistor that can change its resistance to current flow.
•
thermistor: A temperature-sensitive resistor consisting of solid-state semiconductors made from
sintered metal oxides and lead wires, hermetically sealed in glass.
•
capacitance: The ability of an electrical device to store electric charge.
•
capacitor: A circuit component that possesses capacitance.
•
dielectric: An insulating material between the conductors of a capacitor.
•
magnet: A device that attracts iron and steel because of the molecular alignment of its material.
•
electromagnetic field: A force that causes interactions between magnets and electrical current in a
conductor.
•
ferromagnetic material: A material, such as soft iron, that is easily magnetized.
•
magnetic flux line: An invisible line of force that makes up a magnetic field.
•
induction: The process of causing electrons to align to create a magnetic or electrical force.
•
inductance: The property of an electrical circuit or device that resists a change in current due to
energy stored in a magnetic field.
•
inductor: An electrical device that possesses inductance.
•
reactance: The property of a circuit or device that opposes a change in the flow of current.
•
capacitive reactance: A capacitor’s resistance to alternating current.
•
inductive reactance: An inductor’s resistance to alternating current.
•
impedance: The total opposition of any combination of resistance, inductive reactance, and capacitive reactance offered to the flow of alternating current.
•
doping: The process by which the crystal structure of a semiconductor is altered by adding trace
amounts of other elements.
•
diode: A semiconductor device that allows current to pass through in only one direction.
•
forward bias: The condition where a voltage is applied to a diode in such a way as to allow the diode
to conduct easily.
•
reverse bias: The condition where a voltage is applied to a diode in such a way as to cause the
device to act as an insulator.
•
light-emitting diode (LED): A diode that produces light through the use of semiconductor materials.
•
PNP transistor: A transistor with a thin layer of N-type material between two layers of P-type material.
Principles of Electricity
•
NPN transistor: A transistor with a thin layer of P-type material between two layers of N-type material.
•
bipolar junction transistor (BJT): A transistor in which the amount of current between the base and
emitter controls the actions of the transistor.
•
silicon-controlled rectifier (SCR): A four-layer (NPNP), three-electrode semiconductor device with
a very low resistance when conducting and a very high resistance in the OFF state.
•
triac: A three-electrode control device used to switch AC circuits.
11