Circuit
In electronics, a circuit is a path between two or
more points along which an electrical current can be carried. (A circuit breaker is a device that interrupts the path
when necessary to protect other devices attached to the circuit - for example,
in case of a power surge.)
A virtual circuit, sometimes called a logical
circuit, is a path between two or more points that seems like a fixed
physical path, but actually is one path out of many possible physical paths
that can be arranged. A permanent virtual circuit(PVC) is a
virtual circuit that provides a guaranteed connection between two or more
points when needed without having to reserve or commit to a specific physical
path in advance. This allows many companies to share a common pool of circuits.
This approach is used in a frame relay network and offers a committed set of resources to a
telephone company customer at a lower price than if the customer leases their
own circuits. A switched virtual circuit (SVC) is
similar to a permanent virtual circuit, but allows users to dial in to the
network of virtual circuits.
Types Of Electrical
Circuits
There are basicly three
types of basic electronic circuits a Series Circuit, Parallel Circuit, and
Series Parallel Circuit this is a combination of both series and parallel
circuits.
Series
Circuit: A series circuit has
more than one resistor (anything that uses electricity to do work) and gets its
name from only having one path for the charges to move along. Charges must move
in "series" first going to one resistor then the next. If one of the
items in the circuit is broken then no charge will move through the circuit
because there is only one path. There is no alternative route
Equivalent Resistance and Current
Charge flows together through the external circuit at a rate
that is everywhere the same. The current is no greater at one location as it is
at another location. The actual amount of current varies inversely with the
amount of overall resistance. There is a clear relationship between the
resistance of the individual resistors and the overall resistance of the
collection of resistors. As far as the battery that is pumping the charge is
concerned, the presence of two 6-Ω ;resistors in series would be equivalent to
having one 12-Ω resistor in the circuit. The presence of three 6-Ω resistors in
series would be equivalent to having one 18-Ω resistor in the circuit. And the
presence of four 6-Ω resistors in series would be equivalent to having one 24-Ω
resistor in the circuit.
This is the concept of equivalent resistance.
The equivalent
resistance of a circuit is the amount of resistance that a single
resistor would need in order to equal the overall effect of the collection of
resistors that are present in the circuit. For series circuits, the
mathematical formula for computing the equivalent resistance (Req)
is
Req =
R1 +
R2 +
R3 +
...
where R1, R2, and R3 are the resistance values of the
individual resistors that are connected in series.
Parallel
Circuit: A parallel circuit has
more than one resistor (anything that uses electricity to do work) and gets its
name from having multiple (parallel) paths to move along . Charges can move
through any of several paths. If one of the items in the circuit is broken then
no charge will move through that path, but other paths will continue to have
charges flow through them. Parallel circuits are found in most household
electrical wiring.
Current
The rate at which charge flows through a circuit is
known as the current.
Charge does NOT pile up and begin to accumulate at any given location such that
the current at one location is more than at other locations. Charge does NOT
become used up by resistors in such a manner that there is less current at one
location compared to another. In a parallel circuit, charge divides up into separate branches such that
there can be more current in one branch than there is in another. Nonetheless,
when taken as a whole, the total amount of current in all the branches when
added together is the same as the amount of current at locations outside the
branches. The rule that current
is everywhere the same still
works, only with a twist. The current outside the branches is the same as the
sum of the current in the individual branches. It is still the same amount of
current, only split up into more than one pathway.
In equation form, this principle can
be written as
Itotal = I1 + I2 +
I3 + ...
where Itotal is the
total amount of current outside the branches (and in the battery) and I1,
I2, and I3represent the current in the individual
branches of the circuit.
Throughout this unit, there has been an extensive reliance
upon the analogy between charge flow and water flow. Once more, we will return
to the analogy to illustrate how the sum of the current values in the branches
is equal to the amount outside of the branches. The flow of charge in wires is
analogous to the flow of water in pipes. Consider the diagrams below in which
the flow of water in pipes becomes divided into separate branches. At each node (branching location), the water takes
two or more separate pathways. The rate at which water flows into the node
(measured in gallons per minute) will be equal to the sum of the flow rates in
the individual branches beyond the node. Similarly, when two or more branches
feed into a node, the rate at which water flows out of the node will be equal
to the sum of the flow rates in the individual branches that feed into the
node.
The same principle of flow division applies to electric
circuits. The rate at which charge flows into a
node is equal to the sum of the flow rates in the individual
branches beyond the node. This is illustrated in the examples shown below. In
the examples a new circuit symbol is introduced - the letter A enclosed within
a circle. This is the symbol for an ammeter - a device used to measure the
current at a specific point. An ammeter is capable of measuring the current
while offering negligible resistance to the flow of charge.
Diagram A displays two resistors in
parallel with nodes at point A and point B. Charge flows into point A at a rate
of 6 amps and divides into two pathways - one through resistor 1 and the other
through resistor 2. The current in the branch with resistor 1 is 2 amps and the
current in the branch with resistor 2 is 4 amps. After these two branches meet
again at point B to form a single line, the current again becomes 6 amps. Thus
we see the principle that the current outside the branches is equal to the sum
of the current in the individual branches holds true.
Itotal =
I1 + I2
6 amps = 2 amps + 4 amps
Diagram
B above may be slightly more complicated with its three resistors placed in
parallel. Four nodes are identified on the diagram and labeled A, B, C and D.
Charge flows into point A at a rate of 12 amps and divides into two pathways -
one passing through resistor 1 and the other heading towards point B (and
resistors 2 and 3). The 12 amps of current is divided into a 2 amp pathway
(through resistor 1) and a 10 amp pathway (heading toward point B). At point B,
there is further division of the flow into two pathways - one through resistor
2 and the other through resistor 3. The current of 10 amps approaching point B
is divided into a 6-amp pathway (through resistor 2) and a 4-amp pathway
(through resistor 3). Thus, it is seen that the current values in the three
branches are 2 amps, 6 amps and 4 amps and that the sum of the current values
in the individual branches is equal to the current outside the branches.
Itotal =
I1 + I2 + I3
12 amps = 2 amps + 6 amps + 4 amps
A flow analysis at points C and D
can also be conducted and it is observed that the sum of the flow rates heading
into these points is equal to the flow rate that is found immediately beyond
these points.
Series Parallel Circuit: The
type of circuit is a combination of both series and parallel. Electric current
travels through both circuits.
With simple series circuits, all components are connected
end-to-end to form only one path for electrons to flow through the circuit:
With simple parallel circuits, all components are connected
between the same two sets of electrically common points, creating multiple
paths for electrons to flow from one end of the battery to the other:
With each of these two basic circuit configurations, we have
specific sets of rules describing voltage, current, and resistance
relationships.
- Series
Circuits:
- Voltage
drops add to equal total voltage.
- All
components share the same (equal) current.
- Resistances
add to equal total resistance.
- Parallel
Circuits:
- All
components share the same (equal) voltage.
- Branch
currents add to equal total current.
- Resistances
diminish to equal total resistance.
However, if circuit components are series-connected in some
parts and parallel in others, we won’t be able to apply a single set
of rules to every part of that circuit. Instead, we will have to identify which
parts of that circuit are series and which parts are parallel, then selectively
apply series and parallel rules as necessary to determine what is happening.
Take the following circuit, for instance:
This circuit is neither simple series nor simple
parallel. Rather, it contains elements of both. The current exits the bottom of
the battery, splits up to travel through R3 and R4,
rejoins, then splits up again to travel through R1 and R2,
then rejoins again to return to the top of the battery. There exists more than
one path for current to travel (not series), yet there are more than two sets
of electrically common points in the circuit (not parallel).
Because the circuit is a combination of both series and
parallel, we cannot apply the rules for voltage, current, and resistance
“across the table” to begin analysis like we could when the circuits were one
way or the other. For instance, if the above circuit were simple series, we
could just add up R1 through R4 to arrive at a
total resistance, solve for total current, and then solve for all voltage
drops. Likewise, if the above circuit were simple parallel, we could just solve
for branch currents, add up branch currents to figure the total current, and
then calculate total resistance from total voltage and total current. However,
this circuit’s solution will be more complex.
The table will still help us manage the different values for
series-parallel combination circuits, but we’ll have to be careful how and
where we apply the different rules for series and parallel. Ohm’s Law, of
course, still works just the same for determining values within a vertical
column in the table.
If we are able to identify which parts of the circuit are
series and which parts are parallel, we can analyze it in stages, approaching
each part one at a time, using the appropriate rules to determine the
relationships of voltage, current, and resistance. The rest of this chapter
will be devoted to showing you techniques for doing this.
By Hitesh Kushwah (Ex.Branch and Hariom kushwah (EC.
Branch) Final year student
