Industrial Electronics. Fundamentals of Electric Circuits презентация

Chapter 1
Fundamentals of Electric Circuits
Circuit is a current path, it is needed for some electrical equipment by certain
Or elements combined in a certain way up.

Circuit R, I, U, C, L, etc.
Current I
Voltage V
Charge Q=I*t
Resistance resistor R
Capacitance capacitor C
Inductance inductor L

Learning Objectives:
Identify the principal elements of electric circuits:
nodes, loops, meshes, branches, voltage and current sources;
Apply Kirchhoff’s laws to simple electric circuits and derive the basic circuit equations.
Compute the power delivered or absorbed by circuit elements.
Apply the voltage and current divider laws to calculate unknown variables in simple series, parallel, and series-parallel circuits.

§1 Fundamentals of Electric Circuits
The resistor
The inductor
The capacitor
§ 1-1 1. Circuit models（电路模型）
Two active（有源）models
Three passive(无源）models

The voltage independent source
The current independent source
The ideal voltage source:
Independent Sources
V = E – R0 I
V0 = E
Is =
When R0 = 0, V = E, the source is called ideal voltage source.
R0 = 0

V0 = IS R0
IS
R0 = ∞
When R0 = ∞ , I = IS, the source is called ideal current source.
The ideal current source:

(CCVS)
(VCCS)
(CCCS)
(VCVS)
Dependent (Controlled) Sources

2. Network variables (网络变量)
The systems of units (单位制) : International system of units; SI unit
Length [meter] (m) Mass [kilogram] (kg)
Time [second] (s)
(1) Current -- i(t), I ---- (A, mA, μA...)
The net time-rate of transference (传递) of charge is referred to as the flow of current.
(2) Energy (or work) -- w(t) (J)

(4) Power -- p(t) (W, kW, mW)
The time rate of performing work is defined as power.

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3. Reference directions(参考方向)
(1) Reference directions for current
(2) Reference directions for voltage
: Electron: negatively charged
Direction of current: is defined as the flow direction of positive charge.

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(3) Reference conventions (惯例) for two-terminal elements (R, L, C or ... )
The relative (关联) reference directions
The reference arrow for the current variable enters the terminal, which is identified by the plus sign of the voltage reference polarity markings.

(4) The reference for power
p(t)<0 the negative power absorbed by the element, or it is actually generating power and delivering it to some external element.
p(t)>0 the power absorbed by the element.

§1-2 Ohm's law
Ohm's law states that voltage across many types of conducting materials is directly proportional to the current flowing through the material.
The resistor is a passive element that cannot deliver power or store energy.

The current through and voltage across a resistor must both vary with time in the same manner.
We define a "short circuit" as a resistance of zero ohms.
We define a "open circuit" as an infinite resistance.

§1-3 Kirchhoff's laws
A point at which three or more elements have common connection is called a node(节点).
Suppose that we start at one node in a network and move through a simple element to the node at the other…, if no node was encountered more than once, then the set of nodes and elements that we have passed through is defined as a path（路径）.

We define a branch（支路） as a single path in a network, composed of one simple element (or more elements ) and the nodes at each end of that element.
Kirchhoff's current law --KCL
The algebraic sum of all the currents entering any node is zero.
If the node at which we started is the same as the node on which we ended, then the path is a closed path（闭合路径）or a loop（回路）.

The algebraic sum of all the currents leaving a node is zero.
or: The algebraic sum of all the currents entering a node must equal the algebraic sum of all the currents leaving the node.
The three equivalent equations
The algebraic sum of all the currents entering any node is zero.

KCL:
The KCL may be extended to the supernode.

A mesh (网孔）is a loop that does not contain other loops.
Loop1
Loop2
Loop3
3 loops: loop1, loop2, loop3;
2 meshes: loop1, loop2

基尔霍夫电流定律(KCL)
Example:
Solution:
KCL: I1 – I2+ I3 + I4 = 0
Known quantities: I1= 9 A， I2 = –2 A，I4 = 8 A.
Determine the current I3 .
9 –( – 2)+ I3 + 8 = 0
I3 = –19 A

The algebraic sum of the voltage around any closed path in a circuit is zero.
Kirchhoff's voltage law--KVL
KVL:

Voltage divider
Equivalent series resistance
R
i
+
–
§1-4 Series resistors and voltage divider rule

Current divider
Equivalent parallel resistance
R
i
+
–
§1-5 Parallel resistors and current divider rule

Current divider
Equivalent parallel resistance

§1-6 Source transformations
The ideal voltage source and a series resistance comprise a practical voltage source.
A practical current source is defined as an ideal current source in parallel with an internal resistance Rsi.

We shall define two sources as being equivalent if each produces identical current and identical voltage in any loads which is placed across its terminals.
Conditions of equivalence: