Chapter 3
Signal conditioning systems
Basic electrical circuits
คณะวิศวกรรมศาสตร
มหาวิทยลัยธรรมศาสตร
Measurement and Instrumentation
Voltage
Voltage (E) is a term for electric potential difference between
2 nodes in the electric circuit. It is also a measure of the energy of
electricity,
l t i it specifically,
ifi ll it isi the
th energy per unitit charges.
h
The
Th unitit off
measuring the electric potential in SI unit is “volt”
Current
VB
VA
Power supply
1
Chapter 3
Signal conditioning systems
Electric current
Electric current (I) is the rate of flow of electric charge
through a medium. The unit of measuring the electric current in
SI unit is “ampere”
ampere
Q [coulombs]
t [sec]
I [ampere] =
Q
I = dQ
dt
[A]
Source: www.kpsec.freeuk.com/electron.htm
Electrical energy and power
Electrical energy (W) is the energy that an electric charge is
required for moving through a conductor due to the applied
voltage The unit of electrical energy in SI unit is “joule”
voltage.
joule
W
= E [volts] x Q [coulombs]
[J]
Power (P) represents the rate of change of the energy, with time
and can be determined by
P = dW
dt
= IxV
[W]
2
Chapter 3
Signal conditioning systems
AC and DC currents
Alternating Current (AC) is a specific type of electric current in which the
direction of the current's flow is reversed, or alternated, on a regular basis.
I, E
0 sin
t
or
0
220 Vac 50 Hz
Direct Current (DC) is no different electrically from alternating current
except for the fact that it flows in the same direction at all times.
I, E
0
t
Resistor
Resistor is a two-terminal passive electronic component that
implements electrical resistance as a circuit element. The unit of measuring
the electrical resistance (R) is “ohm: Ω”
Ohm’s law: When a voltage V is applied across the
terminals of a resistor, a current I will flow through the
resistor in direct proportion to that voltage.
R=E/I
[Ω]
The power P dissipated by a resistor is calculated as:
P = I x E = I 2R [W]
Symbol
3
Chapter 3
Signal conditioning systems
Series and parallel resistors
Series configuration
1
2
Parallel configuration
1
1
1
1
2
1
Capacitor
Capacitor consists of a pair of conductors separated by a
dielectric (insulator) and is used for storing electric charge when applied
the voltage across the conductors. The capacitor is characterized by a
constant called, capacitance (C), measured in farads and is defined as
C = Q / E [F]
or
The energy stored by a capacitor is
WC = ½ C E2
Symbol
4
Chapter 3
Signal conditioning systems
Series and parallel capacitors
Series configuration
1
1
1
1
1
2
1
2
Parallel configuration
Inductor
Inductor is a conducting wire shaped as a coil and can store
energy in magnetic field created by the electric current passing through
it. The inductor is characterized by a constant called, inductance (L),
( )
measured in henries and is defined as
/
The energy stored by an inductor is
WL = ½ L I2
Symbol
5
Chapter 3
Signal conditioning systems
Series and parallel inductors
Series configuration
1
2
Parallel configuration
1
1
1
1
2
1
Impedance
Electrical impedance, Z extends the concept of resistance to
AC circuits, describing not only the relative amplitudes of the voltage and
current, but also the relative phases. It is a measure of the total
opposition that a circuit presents to alternating current and is defined as
In general, the impedance is written, in Cartesian form, as
Z = R+iX
where R
X
=
=
Resistance
Reactance
[Ω]
|Z|
[Ω]
θ
X = XL - XC
R
6
Chapter 3
Signal conditioning systems
Pure conductive reactance
1
2
Voltage lags current by 90
Pure inductive reactance
2
where
f
Voltage leads current by 90
= frequency of signal [Hz]
R
θ
|Z|
|Z|
XC
XL
θ
R
XC > XL
Impedance of RLC circuits
XL > XC
Diode
Semiconductor diode is a two-terminal electronic
component that allows electricity to flow in only one
direction. The two terminals are the anode (P) and
cathode (N).
(N) The diode operation depends on the
Symbol
polarity of the applied voltage:
1. Current flow is permitted when the diode is forward biased
2. Current flow is prohibited when the diode is backward biased
I=0
I
7
Chapter 3
Signal conditioning systems
Kirchhoff’s laws
y Current law
“The sum of currents flowing into any node in the circuit is equal to the sum of
currents flowing out of that node.”
0 1
4
3 2
y Voltage law
“The directed sum of the electrical ppotential differences ((voltage)
g ) around anyy
closed circuit is zero.”
0 1
2
3
4
0
8