PowerPoint ® Presentation Chapter 4 The Simple Circuit and Ohm’s Law Conductors • Switches • Switch Characteristics • Loads • Overcurrent • Overcurrent Protection Devices • Voltage and Current Measurements • DC Voltage Measurements • DC Current Measurements • Ohm’s Law • Determining Current • Determining Voltage • Determining Resistance • Determining Power
Alternating Current CircuitsDirect Current CircuitsOhm's Law
Citation preview
PowerPoint PresentationConductors • Switches • Switch
Characteristics • Loads • Overcurrent • Overcurrent Protection
Devices • Voltage and Current Measurements • DC Voltage
Measurements • DC Current Measurements • Ohm’s Law • Determining
Current • Determining Voltage • Determining Resistance •
Determining Power
Chapter 4 — The Simple Circuit and Ohm’s Law
An electrical circuit consists of a voltage source, insulated
conductors, a load, a switch, and a fuse.
4-*
An electrical circuit is an assemblage of conductors and electrical
devices through which electrons flow. See Figure 4-1. Electrical
circuits consist of a complete path for the flow of electrons
between two or more points. The most fundamental electrical circuit
is a simple circuit in which a single energy source supplies
current to a single load through electrical conductors.
Chapter 4 — The Simple Circuit and Ohm’s Law
In a schematic or wiring diagram, conductors are shown as lines.
Conductors that are connected often use a dot to indicate the
connection.
4-*
A conductor is a material that has a low electrical resistance and
permits electrons to move through it easily. Conductors are
generally a single wire, a group of wires, or another material
suitable for carrying electric current. Most single conductors are
enclosed in an insulated cover to protect the conductor, increase
safety, and meet electrical code requirements. Some single
conductors, such as ground wires, may be bare. The schematic symbol
for a conductor is a line connecting two devices in an electrical
circuit. This symbol is the same for insulated or bare conductors.
See Figure 4-2.
Chapter 4 — The Simple Circuit and Ohm’s Law
Switches are control devices and are used to close and open
circuits safely.
4-*
A switch is a mechanical, electronic, or solid-state electrical
device that is used to start, stop, or redirect the flow of
electrons in an electrical circuit. Switches are added to a circuit
as a control device. See Figure 4-3. Turning a lamp ON and OFF by
using a switch is safer and more convenient than connecting and
disconnecting a conductor. The switch is connected in series with
the voltage source and load. There is only one current path in the
circuit, so when the switch is open, current does not flow to the
lamp.
Chapter 4 — The Simple Circuit and Ohm’s Law
The position of the contacts, number of poles, number of throws,
and type of break are used to describe switch contacts.
4-*
Most switches are rated for the maximum current and maximum voltage
they can safely handle. Large switches may also be rated in
horsepower. All switches use contacts to start or stop the flow of
electrons in a circuit. A contact is the conducting part of a
switch that operates with another conducting part to make or break
a circuit. The position of the contacts (normally open or normally
closed), number of poles (single-pole, double-pole), number of
throws (single-throw, double-throw), and type of break
(single-break, double-break) are used to describe switch contacts.
See Figure 4-4.
Chapter 4 — The Simple Circuit and Ohm’s Law
Switches are available in many shapes and are often designated
according to their use.
4-*
Switches are available in many shapes. Switches can be activated
manually, mechanically, or automatically. Once activated, the
switch changes the position of the contacts. The contacts are used
to start and stop the flow of electrons in a circuit. See Figure
4-5.
Chapter 4 — The Simple Circuit and Ohm’s Law
Common lighting circuit switches include two-way, three-way, and
four-way switches.
4-*
Common lighting circuit switches include two-way, three-way, and
four-way switches. The switch used to control a lamp depends on the
number of different locations the lamp must be controlled from. See
Figure 4-6.
Chapter 4 — The Simple Circuit and Ohm’s Law
Rotary switches are used to connect multiple positions to a single
pole.
4-*
Another type of switch is the rotary switch (wafer). A rotary
switch is a switch that has one or more poles that can be connected
to several positions. See Figure 4-7. A rotary switch is typically
used to switch between several signal sources. For example, a
headphone user can choose between listening to a radio station, an
audio cassette, a compact disc, or a digital video disc (DVD). More
than one wafer controlled by a single shaft may be stacked to
switch multiple circuits.
Chapter 4 — The Simple Circuit and Ohm’s Law
Loads convert electrical energy to another form of energy such as
motion, light, heat, or sound.
4-*
A load is a device that converts electrical energy to motion,
light, heat, or sound. Common loads include motors (electrical
energy to motion), lamps (electrical energy to light), heating
elements (electrical energy to heat), and speakers (electrical
energy to sound). See Figure 4-8.
Chapter 4 — The Simple Circuit and Ohm’s Law
A short circuit has a resistance that is lower than the normal
circuit resistance.
4-*
A short circuit is any circuit in which current takes a shortcut
around the normal path of current flow. A circuit may contain a
partial short that causes an increased electron flow (overcurrent)
or a dead short. A partial short may or may not cause damage
depending on the ratings of the circuit components. A dead short
may develop that completely removes the resistance of the load from
the circuit. See Figure 4-9.
Chapter 4 — The Simple Circuit and Ohm’s Law
Overcurrent protection devices such as fuses are used to protect a
circuit from a short circuit or overcurrent that can cause circuit
damage.
4-*
An overcurrent protection device must be used to provide protection
from short circuits and overloads to prevent the possible loss of
property or life. An overcurrent protection device (OCPD) is a fuse
or circuit breaker used to provide overcurrent protection in a
circuit. Fuses and circuit breakers are OCPDs designed to
automatically stop the flow of current in a circuit that has a
short circuit or that is overloaded. See Figure 4-10.
Chapter 4 — The Simple Circuit and Ohm’s Law
Cartridge and plug fuses may be surrounded with glass or encased in
a composite material to suppress an arc or flame.
4-*
Fuse designs include cartridge and plug fuses. Fuses are available
in various sizes and shapes. See Figure 4-11. Glass cartridge fuses
may have a single wire or flat conductor as the fuse element. The
flat conductor has less conducting area in the middle where the
fuse element melts. A glass delayed action cartridge fuse opens
only when the current is greater than its rating for a
predetermined amount of time. Delayed action cartridge fuses are
used for loads that have an initial current surge when power is
applied. Some glass fuses have pigtails attached to them so that
they can be soldered into a circuit. Glass fuses allow the
observation of the condition of the fuse element without using an
ohmmeter.
Chapter 4 — The Simple Circuit and Ohm’s Law
A circuit breaker is an overcurrent protective device that does not
need to be replaced each time the circuit current rating is
exceeded. Circuit breakers may be thermally or magnetically
operated.
4-*
A circuit breaker is an overcurrent protective device with a
mechanical mechanism that manually or automatically opens a circuit
when a short circuit or overload occurs. See Figure 4-12. Like
fuses, circuit breakers are connected in series with circuit
conductors. A circuit breaker opens and prevents current from
flowing in a circuit when the current exceeds the rating of the
circuit breaker. Circuit breakers contain a spring loaded
electrical contact that opens the circuit. The spring opens and
closes the contacts with a fast snap action. A circuit breaker does
not have to be replaced each time the current rating is exceeded.
Circuit breakers have replaced fuses in many applications. Circuit
breakers have voltage, amperage, and interrupting ratings similar
to fuses.
Chapter 4 — The Simple Circuit and Ohm’s Law
Thermal circuit breakers use a bimetallic strip attached to a latch
mechanism to open the circuit when a short circuit or overload
occurs.
4-*
Circuit breakers may use thermal or magnetic methods to open a
circuit when a short circuit or overload condition occurs. Thermal
circuit breakers use a bimetallic strip attached to a latch
mechanism. The bimetallic strip is made of two dissimilar metals
that expand at different rates when heated. The bimetallic strip
bends when heated and opens the contacts. See Figure 4-13. The
bimetallic strip may be heated directly by circuit current or
indirectly by the rise in temperature caused by an increase in
circuit current.
Chapter 4 — The Simple Circuit and Ohm’s Law
Magnetic circuit breakers use an electromagnet coil and armature to
open the circuit when a short circuit or overload occurs.
4-*
A magnetic circuit breaker uses an electromagnet coil and armature.
In a magnetic circuit breaker, circuit current passes through the
coil, producing a magnetic field. See Figure 4-14. Normal circuit
current does not affect the armature. However, when circuit current
creates a magnetic force that exceeds the spring force on the
armature and the friction of the latching mechanism, the armature
is pulled to the electromagnet coil. The spring-loaded contact arm
is released, breaking the current flow to the load instantaneously.
Magnetic circuit breakers can be manually reset immediately.
Chapter 4 — The Simple Circuit and Ohm’s Law
Thermal overload relay contacts open when the current level is
exceeded for a given period of time. The temperature rise in the
metal frame of the motor is used to heat the bimetallic
strip.
4-*
Thermal overload relays operate by opening a bimetallic strip upon
a rise in temperature. Most thermal overload relays can be reset
using a pushbutton. Automatic-reset thermal overload relays do not
have a spring action that requires manual resetting of the
contacts. The contacts open when the current level is exceeded for
a given period of time. When the bimetallic strip cools, the
contacts snap closed, and the circuit is automatically energized.
Automatic-reset thermal overload relays are often used on motors
but can be a safety hazard. To protect personnel and property,
automatic-reset thermal overload relays should only be used on
circuits that must be kept running. See Figure 4-15.
Chapter 4 — The Simple Circuit and Ohm’s Law
In a standard motor control circuit, a relay coil controls a set of
normally open contacts and a set of normally closed overload relay
contacts.
4-*
An overload relay may be incorporated into a standard start/stop
motor control circuit. In a standard motor control circuit, a motor
starter or contactor coil controls a set of normally open contacts,
and a set of normally closed overload relay contacts. See Figure
4-16.
Chapter 4 — The Simple Circuit and Ohm’s Law
DC voltage measurements using a digital multimeter are taken by
connecting the black test lead to the negative polarity test point
and the red test lead to the positive polarity test point.
4-*
DC voltage is measured with a DMM using a standard procedure.
Always exercise caution when taking any circuit measurement. See
Figure 4-17.
Chapter 4 — The Simple Circuit and Ohm’s Law
DC voltage is measured with an analog meter using standard
procedures.
4-*
Voltage measurements can be made with an analog meter in a similar
manner as with a digital multimeter.
DC voltage is measured with an analog meter
using the following standard procedure. Always refer
to the instruction manual before using any meter.
See Figure 4-18.
Chapter 4 — The Simple Circuit and Ohm’s Law
To measure current flow through a component, a meter must be
connected so that the total electron flow is through the meter
circuit.
4-*
To measure current flow through a component, a meter must be
connected so that the total electron flow is through the meter
circuit. This means that the meter must be connected in series with
the component so that only one path for electron flow exists.
Meters set to measure current must have a very low resistance that
does not substantially change the value of the current in the
circuit. Direct current is measured with a DMM using the following
standard procedure. Always turn the power to a circuit OFF before
taking any measurements. See Figure 4-19.
Chapter 4 — The Simple Circuit and Ohm’s Law
DC current is measured with an analog multimeter using standard
procedures.
4-*
Current measurements can also be taken using an analog multimeter.
Direct current is measured with an analog multimeter using the
following standard procedure. Always turn the power to the circuit
OFF before taking any measurements. See Figure 4-20.
Chapter 4 — The Simple Circuit and Ohm’s Law
Clamp-on ammeters measure current by measuring the strength of the
magnetic field around a single conductor.
4-*
Clamp-on ammeters measure current in a circuit by measuring the
strength of the magnetic field around a single conductor. Care
should be taken to ensure that the meter does not pick up stray
magnetic fields. Whenever possible, conductors under test should be
separated from other surrounding conductors by a few inches. If
this is not possible, several readings at different locations along
the same conductor should be taken. Direct current is measured with
a clamp-on ammeter or a DMM with a clamp on current probe accessory
using the following standard procedure. See Figure 4-21.
Chapter 4 — The Simple Circuit and Ohm’s Law
Ohm’s law is the relationship between voltage, current, and
resistance in an electrical circuit.
4-*
Ohm’s law is the relationship between voltage (V), current (I), and
resistance (R) in an electrical circuit. Using Ohm’s law, any value
in this relationship can be found when the other two are known. The
relationship between voltage, current, and resistance may be seen
best in pie chart form. See Figure 4-22.
Chapter 4 — The Simple Circuit and Ohm’s Law
Current in a circuit increases with an increase in voltage and
decreases with an increase in resistance.
4-*
According to Ohm’s law, if the resistance in a circuit is held
constant and the voltage varied, the current can be determined for
each value of voltage. The voltage/current curve is linear, which
means that a specific change in voltage causes a specific change in
current. A resistor is referred to as a linear load because of this
straight line curve. See Figure 4-23.
Chapter 4 — The Simple Circuit and Ohm’s Law
Voltage in a circuit increases with an increase in current and
increases with an increase in resistance.
4-*
According to Ohm’s law, if the resistance in a circuit is held
constant and the current varied, the voltage can be determined for
each value of current. The current/voltage curve is linear, which
means that the voltage drop across a resistor is directly
proportional to the current flowing through it. See Figure
4-24.
Chapter 4 — The Simple Circuit and Ohm’s Law
Resistance in a circuit increases with an increase in voltage and
decreases with an increase in current.
4-*
According to Ohm’s law, if the current in a circuit is held
constant at 2 A and the voltage across it varied from 0 V to 6 V,
the resistance value can be determined for each value of voltage.
The voltage/resistance curve is linear, which means that with a
constant current flow through a resistor, the value of the resistor
must be increased to increase the voltage drop across it. See
Figure 4-25.
Chapter 4 — The Simple Circuit and Ohm’s Law
The power formula is the relationship between power, voltage, and
current in an electrical circuit.
4-*
The power formula is the relationship between power, voltage, and
current in an electrical circuit. The power formula is often
referred to as Watt’s law. Any value in this relationship may be
found when the other two values are known. The relationship between
power, voltage, and current may be seen best in pie chart form. See
Figure 4-26.
Chapter 4 — The Simple Circuit and Ohm’s Law
Power in an electrical circuit is calculated by multiplying current
by voltage.
4-*
Power is directly related to voltage and current. Current flow
through an electrical component either generates energy (a power
source) or dissipates energy (a resistance, such as a lamp). Power
is the rate at which energy is generated or consumed. The power
supplied to a circuit must be consumed; therefore, power
consumption must be equal to the power dissipated by a circuit. If
the voltage in a circuit is varied from 0 V to 6 V and the
resistance is held constant at 2 , the current in the circuit is
dependent on the value of the applied voltage. In this circuit, the
current varies from
0 A to 3 A as calculated by Ohm’s law. Power is calculated by
multiplying current times voltage. The voltage/current/power curve
is nonlinear. See Figure
4-27.
Chapter 4 — The Simple Circuit and Ohm’s Law
Power in an electrical circuit calculated by multiplying current
squared by resistance.
4-*
If the voltage in a circuit is fixed at 6 V, and resistance is
varied from 0 to 6 , current also changes. In this circuit, the
resistance/current/power curve is nonlinear. See Figure 4-28. Note
that with voltage applied, and the resistance at 0 , the current
becomes undefined. The power consumed at this point is 0 W. Only
resistance can consume power in an electrical circuit. Since no
resistance is present, no power is consumed.
Chapter 4 — The Simple Circuit and Ohm’s Law
Power in an electrical circuit can be calculated by dividing
voltage squared by resistance.
4-*