Basic Knowledge of
Electricity and Electronics
for Embedded Systems´
Programmers
1
Elementary Electricity Concepts



Ohm´s Law:
•
•
Current = Electromotive Force / Resistance
=> I = E/R
•
NOTE: An ideal voltage source (E) is considered to have
ZERO internal resistance, that is, for an analysis of
resistance accross its terminals it is a short circuit.
E = 12
Volts
Series Circuit and Kirchoff´s Voltage Law
The Three Rules of a Series Circuit:
•
•
•

I=?
Current has the same value at any point within a
series circuit
The values of the resistances of individual
components add up to the total circuit resistance.
Rt = R1 + R2 + R3
Voltage drops across the individual component
resistances add up to the total applied voltage.
E = V1 + V2 + V3
R=2
KΩ
V1=?
E = 12 V
R1 = 2 K
I=?
R 2= 2 K
R3 = 2 K
V3=?
Resistive Divider – Class Exercise
2
Elementary Electricity Concepts

Parallel Circuits and Kirchoff´s Current Law
•

A parallel Circuit provides two or more paths or
branches for circuit current.
The Three Rules of a Parallel Circuit:
•
•
•
The same voltage is applied across each individual
branch
The total current is equal to the sum of the
individual branch currents.
E
The equivalent (or effective) resistance is equal to 12 V
the applied voltage divided by the total current,
and this value is always less than the smallest
resistance contained in any one branch.
I total
R1
3Ω
R2
4Ω
R3
6Ω
I1
I2
I3
I total = I1 + I2 + I3
Req = E / I total
1/Req = 1/R1 +1/R2 +1/R3
3
DC and AC Voltage and Current

DC voltage/Current
• Batteries
• Electronic Equipment Power Supplies
convert AC voltage/current to DC
voltage/current
• Usually Low Power Low Voltage Circuits
• Current Flows in Only One Direction

V
t
AC Voltage/Current
• Provided by Electricity Companies
• Normal Illumination, Medium and High Power
Equipment are usually driven by AC
Voltage/Current
• Current flows cyclically in both directions of
the circuit.
• Vrms= 0.707 * Vpeak (0.707 * 179 V peak =
127Vrms)
• frequency(f) = 1/T => 60 Hz
• Phase -> 0 degrees to 360 degrees
+V
V peak = 179V
t
-V
T
4
Capacitor
1

Capacitor
E
S R=1K
2
10 V
C
1uF
Vc
• Opposes changes in Voltage Between
Its terminals.
• Stores Electric Charge
• Can be used to minimize (filter) the
ripple of analog signals
• Can be used to supply high
instantaneous current to circuits
• Together with a Resistor can be used to
delay signals
Switch in 1
Switch in 2
Vc
T = RC (Time to reach 63.2 % of
Final Value During Charge) or 36.8%
of Initial Value During Discharge
T – Seconds; R – Ohms; C - Farads
5
Inductor

Inductor
• Opposes Changes in Current accross
its terminals.
• Made by Winding a Conductor
around a core (magnetic material or
insulated material)
Vr
1
E
I
S
2
10 V
R=1K
L
1uH
• Inductance measured in Henries
Switch in 1
• Used in Transformers to Isolate one
Power Source from another.
• Used in Transformers to Convert
from one voltage Range to Another
and in Tuned LC Communication
circuits.
Switch in 2
Vr
T = L/R (Time to reach 63.2 % of
Final Value) or 36.8% of Initial Value
T – Seconds; R – Ohms; L - Henries
6
Class Exercise -1


Draw a scheme with a resistor and a push button switch to reset a
microcontroller. Consider a time constant of 20 ms (the time
required to reach 63% of the maximum voltage) for a resistor of
100 Kohms. Calculate the capacitor required.
•
In scheme 1 the signal is normally at high (3 volts) and goes to zero volts when
the push button key is pressed (RESET AT LOW)
•
In scheme 2 the signal is normally at zero volts and goes to 3 volts when the
push button key is pressed. (RESET AT HIGH)
Draw a scheme with an ON-OFF switch (without central pole) that
will indicate to the microcontroller that when in position A, that pin
will be at zero volts (LOW) and when in position B it will be at 3
volts (HIGH).
7
Reactance and Impedance

• The opposition to AC voltage changes
offered by a capacitance (Xc or capacitive
reactance measured in ohms)
• Xc = 1 / (2π f C) ; f = frequency (Hz); C =
Capacitance in Farads

C
Xc
R
θ
Xc
(R2+Xc2)
Z=
Inductive Reactance
• The opposition to AC current changes
offered by an inductor (Xl or inductive
reactance measured in ohms)
• Xl = 2π f L ; f = frequency (Hz); L =
Inductance in Henries

R
Capacitive Reactance
R=8Ω
Z=
(R2+Xl2)
Xl=5Ω
R
θ
Xl
Impedance and Vectorial Addition
• The combined opposition to current
changes by resistance, inductance and
capacitance is called impedance and is
represented by the symbol Z.
R, L, C circuit:
Z=
(R2+(Xl-Xc)2)
8
How can an AC Signal “Cross” a Capacitor?
+ 5V
Vin = AC Signal –> f = 1 MHz
4V
C = 1 uFarad
R1
Xc = 1 / (2π f C)
Xc = 1/ (2 x 3.14 x 1 x
0
106
x1x
C
2V
0
10-6)
Xc = 1/ (2x3.14) ohms
Vin
Xc
R2
Vout
Xc = 0.16 ohms
Xc = ALMOST A SHORT CIRCUIT !
THUS, An AC signal can “cross” (or
be propagated across) a capacitor,
provided the frequency of the AC
signal is high enough to make the
capacitor impedance very low.
On the other hand, the capacitor
blocks DC signals (or very low
frequency signals)
9
Triple Phase Transformers

Transformers in Delta
Organisation
• Used to convert and Isolate Very
High Voltage (from Power Plants) to
High Voltage in City Distribution

Transformers in Star
Organisation
• Used to Convert and Isolate from
High Voltage to Medium Voltage
F1
F2
F3
F1
N
F2
F3
10
Power Supplies, No-Breaks, Stabilizers

Power Supplies
• Convert AC Voltage to DC Voltage
• Convert 127V AC to +5V, +12V, -12V, -5V DC

No-Breaks
• When there is AC Voltage, they Charge a Battery
• When there is no AC Voltage, The DC Voltage of the Battery is
Converted to AC Voltage to feed the Computer
• A computer normally needs a Stabilizer and a No-Break.

Stabilizers
• Guarantee that AC Voltage Supplied to a Computer is kept
within Certain Limits to Avoid Damaging the Computer

Note:
Common computer No-Breaks do not generate electricity from the
battery when there is external power. They just generate
electricity when there is no external power. Therefore, if the
external power varies its amplitude considerably, it is not good
for the computer. Due to this, a stabilizer is needed before the
No-Break.
11
Switches, Relays, Solenoids, Power Relays
Push
Button
Take to Class:
Off
On
1

Switches
1
1
2
C
• Some Types

Relays, Solenoids
• AC (127V) or DC Driven
(12V, 5V, etc) – The diode in
parallel prevents a high
peak voltage accross R
when the circuit is opened.

Central
2 Pole
Three-Phase Power
Relays or Solenoids
12V
DC
2
Same Types
with Multiple
Contacts
Neutral
127V AC
(Phase)
LAMP or
Equipment
R
F1 or
F2 or
F3
Neutral
F1
Return F1
F2
Return F2
F3
Return F312
Debouncing

When a switch (any type) changes
state (on -> off or off -> on), it presents
a mechanical bouncing which
generates a signal similar to the one
shown at the right.
5V
R
5V
1
C
S
Microcontroller
2

The resistor R is needed because the
signal S can not be left “floating” in na
undefined state when the switch
changes from state 1 to 2.

Without debouncing the signal can
generate several interrupts (or status
changes) corresponding to just one
action.


Debouncing consists in “Filtering” the
signal S so that a proper operation of
the switch action is sensed.
Debouncing can be done in hardware
of software
Δt
Read Key (should
be stable)
Δt
Techniques that can be used:
-If status loop: after first status change,
program timer and after elapsed time
read key status.
-If Interrupt: on first interrupt program
timer which will interrupt after elapsed
time. Then read key status.
13
Logic Circuits using Switches and Relays
14
Grounding

Grounding Reference
•

Safety Ground
•

Two Equipments(circuits) can communicate
either using commom mode signals or
differential signals. The most common are
common mode signals. In this case a
common reference wire is established and
voltages are related to that common wire,
which is usually termed Ground wire.
Safety ground is a wire employed to connect
metalic parts of the equipment to the earth´s
ground. This is done to prevent accidents by
persons touching metalic parts with voltage
levels (due to defective parts)
Neutral Wire
•
Comes from the street transformer (where is
earth grounded) but can not be used for
safety ground. A separate safety ground is
required.
Equip.
A
Signals
Common
(ground)
Equip.
B
Hot Wire
Neutral
Safety
Ground
Equip.
Signals
Equip.
B
A
Common
(ground)
Safety
ground
15
Volt – Ohm - Meter (VOM) or Multimeter
16
V.O.M Measurements
17
Some Common Electronic Devices

Diode
•

V
Vz
Used to establish a certain reference voltage in a circuit
section, almost independently of the reverse current flow
across it. Reverse voltage depends on the type of zener
diode. (direct voltage drop is 0.65 V)
XTAL
Crystal
•

Used to allow current in just one direction, to rectify AC
waveforms. When current flows in the correct direction,
voltage drop across it is approximately 0.65V, almost
independently of the current. When voltage is reversed, it
acts like an enormous resistor. A LED is also a diode but
the voltage drop across its terminals is usually 1.5 Volts,
instead of 0.65 Volts.
Zener Diode
•

Current flow
Used to provide highly accurate clock oscillators
Light Dependant Resistor (LDR)
•
Resistance varies according to light
A
B
18
Diode Voltage x Currrent Characteristics
19
Class Exercise -2

Draw the output waveform for the circuit below
0
Sinusoidal
Vout ?
Generator
20
Class Exercise -3

Common LEDs (Light Emmitting Diodes) can be turned “ON” with a
current between 5mA and 10mA.

Microcontrollers Can Have Currents in Output pins between 2mA and
20ma when in “LOW” Level (IOL) and currents between 2mA and 10mA
when in “HIGH” level (IOH). Note that when the output pin of the
microcontroller is at 0 volts level the current flows into the pin and when it
is at 3 volts, the current flows out of the pin.

In order to protect the Microcontroller to not exceed its output current and
also to protect the LED to not exceed the current across its terminals, a
resistor in series with the LED is used.

Draw a scheme to connect a LED to an output pin of a microcontroller,
turning the LED “ON” when the output pin is at logic “0” and turning it
“ON” when the output pin is at logic “1”.

Calculate the resistor to limit the current across the LED to 5 mA.
Consider that the power supply is 3 Volts. Remember that there is a
voltage drop across the LED when the current is flowing in the correct
direction.
21
Remember:
Ohm’s Law
V  IR
Kirchoff’s Voltage Law – The
V  0
Kirchoff’s Current Law – The
I IN   I OUT
algebraic sum of all voltage rises and voltage drops
around a closed loop must equal zero
algebraic sum of all currents entering and leaving a
node must equal zero
Series Equivalent
Parallel Equivalent
REQ  R1  R2 ... RN
1
1
1
1


...
REQ R1 R2
RN
22
Mesh Analysis
R4
• First, determine the number of closed loops and
then draw the direction of the currents around the
loops.
300
R1
• Second, identify the plus and minus voltage on
each resistor, according to the currents flowing
across them.
• Third, traverse the loop associating a plus or
minus sign, depending on the the entrance sign,
when traversing the loop.
100
20V
I1
I3
R2
500
R3
100
R5
200
I2
Loop1: - 20 + 100 (I1-I3) + 500 (I1-I2) = 0
Loop2: 500 (I2-I1) + 100 (I2-I3) + 200 (I2) = 0
Loop3: 300 (I3) + 100 (I3-I2) + 100 (I3-I1) = 0
System with 3 equations – Solve using determinants
I1=85.7mA ; I2 = 57.14mA; I3 = 28.57mA
23
Thevenin´s Theorem
Thevenin´s Theorem: Any two terminal circuit can be
15
4
replaced by a resistance, equal to the resistance measured
across the two terminals, in series with a voltage source,
equal to the open circuit voltage accross the two terminals. 25V
A
2
B
What For ??: => The Key objective is to find a simple
circuit that is equivalent to a more complex one. For
example: Obtain an equivalent circuit between terminals A
and B that is simpler than the circuit on the right:
Rth 
A
Vth
Step1 : Find Vth. Because the terminals are open, there will
be no current flowing through the 4 ohms resistor, and
therefore, no voltage drop accross it. So, the open circuit
voltage (Vth) will be equal the voltage across the 10 
resistor. Thus, by the voltage divider rule:
B
15
4
2
A
Rth 
B
Vth = 10V.
10 
Step2 : Find Rth (the resistance seen looking into terminals
A and B). Replacing the voltage source with its ideal
10V
internal resistance (zero ohms), we find that:
A
Rth = 4 + 15  // 2 = 10 
B
24
TRANSITOR - PNP
Emitter
Current
Base
Collector
Bipolar Transistor -PNP
Emitter
p
Base
n
n
Emitter
Current
Base
p
Collector
Collector
Ie = Ic + Ib
Bipolar Transistor - NPN
Collector
Collector
n
Base
p
p
Base
n
Emitter
Emitter
Ie = Ic + Ib
Bipolar Transistor Construction
• NPN / PNP Block Diagrams
Emitter
Collector
N
Base
P
N
Emitter
Collector
P
N
P
Base
28
Bipolar Transistor Biasing (NPN)
FB
Emitter
-
N
RB
P
N
Collector
+
Base +
29
Bipolar Transistor Biasing (PNP)
FB RB
Emitter P
+
N
Collector
P
-
Base +
30
Transistor Characteristic Curve
90 uA
80 uA
70 uA
IC
60 uA
Saturation
IB
Q-Point
50 uA
40 uA
30 uA
20 uA
10 uA
0 uA
Cutoff
VCE
31
Class ‘A’ Amplifier Curve
90 uA
80 uA
70 uA
IC
IB
60 uA
Saturation
50 uA
40 uA
30 uA
20 uA
Q-Point
10 uA
0 uA
Cutoff
VCE
32
Class ‘B’ Amplifier Curve
90 uA
IC
IB
80 uA
70 uA
60 uA
Saturation
50 uA
40 uA
30 uA
20 uA
Q-Point
10 uA
0 uA
Cutoff
VCE
33
Analyzing The Transistor Operation Region:
Conduction, Saturation or Cutt-Off

Conduction Region –For any transistor to conduct, two things must occur.
 The emitter - base PN junction must be forward biased.
 The base - collector PN junction must be reverse biased.
 In this region the transistor presents an amplification in current (gain)
or “ß”, which relates collector current (Ic) and base current (Ib)
according to the following equation:
 Ic = ß Ib
(Equation 1)
 It is also known that Ie = Ic + Ib
(Equation 2)
 Then: Ie = (ß Ib) + Ib => Ie = (ß + 1) Ib
(Equation 3)

Cut-Off Region – When Ic is almost zero and the absolute value of the
Base-Emitter voltage is lower than 0.65 V (for silicon transistors). When the
transistor reaches this region, the equations above are no longer valid.

Saturation Region – When Vce is almost zero and the absolute value of the
Base-Emitter voltage is larger than 0,65 V (for silicon transistors). When the
transistor reaches this region, the equations above are no longer valid.
34
Bipolar Transistor Operation (PNP)
•90% of the current carriers passes through
the reverse biased base - collector PN junction
and enter the collector of the transistor.
•10% of the current carriers exit transistor
through the base.
•The opposite is true for a NPN transistor.
35
Amplifier Operation
• The transistor below is biased such that there is a
degree of forward bias on the base - emitter PN
junction.
• Any input received will change the magnitude of
forward bias & the amount of current flow through
the transistor.
RC
+
0
Input Signal
+VCC
+
Q1
0
RB
Output Signal
36
Amplifier Electric Switch Operation
•When the input signal is large enough, the transistor
can be driven into saturation & cutoff which will make
the transistor act as an electronic switch.
•Saturation - The region of transistor operation where a
further increase in the input signal causes no further
increase in the output signal.
•Cutoff - Region of transistor operation where the input
signal is reduced to a point where minimum transistor
biasing cannot be maintained => the transistor is no
37
longer biased to conduct. (no current flows)
Amplifier Electric Switch Operation
–Transistor Q-point
•Quiescent point : region of transistor operation where
the biasing on the transistor causes operation / output
with no input signal applied.
–The biasing on the transistor determines the amount of time
an output signal is developed.
–Transistor Characteristic Curve
•This curve displays all values of IC and VCE for a
given circuit. It is curve is based on the level of DC
biasing that is provided to the transistor prior to the
application of an input signal.
–The values of the circuit resistors, and VCC will determine
the location of the Q-point.
38
Some Common Electronic Devices

Bipolar
Transistor
(NPN)
e.g.: BC237,
BC 337, BC 547,
BC 557, 2N2222
(NPN and PNP)

MOS
Transistor
(N type)
Source
Drain
Gate
39
Determining Saturation, Cutoff or Conduction
Determine the operation Region of the circuit below:
R1
Rc
10K
100K
Ic
C
+ 10V
B
E
Ib Vbe
Ic = ß x Ib = 100 Ib
R1
100K
Rc
10K
B
Ic
C
E
Vce=?
Ie
Assume Transistor Current
Gain = ß = 100 and Vbe =
0.65V
Ie = Ic + Ib
Simplified
Model
Ib Vbe
+ 10V
Vce=?
Ie
Calculating Ib:
10 V = Vbe V + Ib x 100 x 103 
(10 – 0.65) V = Ib x 100 x 103  => Ib = .0935 mA
Thus Ic = 100 x Ib = 9.35 mA
10 V = Vce + Ic x 10 x 103  => Vce = - 83.5 V < 0V !!
In reality Vce does not go negative because when it is
almost zero (saturation), the current gain (ß) does not
apply any more and Vce sets at 0 volts.
Thus the transistor is SATURATED
40
Building Logic Gates with Transistors &
Resistors
41
Transformers
A transformer is a passive electronics
component and consists of a pair of wire
coils coupled together with an iron core.
The input coil is called the primary coil
and the output coil is called the secondary
coil.
Transformers
Transformer Voltages
150
100
Volts
50
Primary
0
0
-50
-100
-150
1
2
3
4
5
6
7
8
Seconday
Filters
Plot of Transformer Voltages: Left side, primary; Right side,
secondary
14
12
Volts
10
8
Series1
6
4
2
0
0
2
4
6
8
10
Power Supply
Unrectified
15
10
Unrectified
0
Rectified
15
0
1
2
3
4
5
6
7
8
10
-5
Effect of Filter
0
0
1
2
3
4
5
6
7
8
-5
-15
-10
10
8
Rectifier
6
4
2
0
0
0
-15
Transformer
12
20
10
0
Volts
-10
Volts
5
Volts
Volts
5
2
4
6
8
Filter
1
2
3
4
5
6
7
8
Full-Wave Rectifier
bridge
Volts
Rectified
20
10
0
0
2
4
6
8
46
Full-Wave Rectifier
Volts
Rectified
20
10
0
0
2
4
6
8
47
Power Supply Capacitor Filter and Regulator
12V
Vem dos
Diodos
Retificadores IN
REGULADOR
5V
+ 5V DC
7805
OUT
TERRA
48
Interface Circuits with the Real World

Switches and Relays – already shown

Keyboard with No Logic

Keyboard with Logic

Light Emitting Diodes (LEDs)

Seven Segment Displays – can be driven
by resistors connected to normal logic –
check current

Liquid Crystal Displays – 20 or 40 columns
by 2 to 4 rows – some have serial interface
49
Example 1 – Driving a 7-Segment Display
“Common Anode”
+5V
v+
v+
Is
8 x 180 R
abcdefg
abcdefg
Is = (5V – 1.5V) / 180R = 20ma
50
Example 1 – Driving a 7-Segment Display
“Common Anode”
+5V
10K
It
It
10K
v+
v+
Id
8 x 180 R
abcdefg
abcdefg
Id = (5V – 1.5V) / 180R = 20ma
51
Interface Circuits with the Real World
O.C. Inverter (7406)

Transistor-Transistor Logic (TTL) (5
volts) to other voltages Converters:
7406, 7407 (open collector)
Max 232
+12v

0v
TTL (5 volts) to RS232C (serial port ->
+12V to –12V) converters: MAX 232, +5v
1489, 1488
0v

TTL common mode to RS422
Differential mode – Higher speed than
RS232 and higher noise immunity
-12v
I
52
Interface Circuits with the Real World

R
Digital to Analog (D/A)
E
Converter
8/12/16-bit Data G
Analog Signal
I
S
T
Clock E
R

Analog to Digital (A/D)
Converter
D/A
Analog
Signal
A/D
Clock
R
E
Digital
G
Data
I
S
T
E
R
53
Programmable Logic Arrays (PLAs and PALs)

F= (A.B) + (A.C.D) + (D.E) + (A.F) + (B.E.F)
A
B
A
C
D
D
E
A
F
CI-1

F
B
E
F
CI2
CI-3
CI4
O3 = I0.I2./I3 + I3.I5 + /I2.I4 + /I4.I5 + /I5.I1
54
Field Programmable Gate Arrays (FPGAs)
55
FLASH Memory
Flash memory is a form of EEPROM that allows multiple memory locations to be erased or written
in one programming operation. Normal EEPROM only allows one location at a time to be
erased or written, meaning that flash can operate at higher effective speeds when the systems
using it read and write to different locations at the same time. All types of flash memory and
EEPROM wear out after a certain number of erase operations.
Flash memory is made in two forms: NOR flash and NAND flash. The names refer to the type of
logic gate used in each storage cell.
NOR flash was the first type to be developed, invented by Intel in 1988. It has long erase and write
times, but has a full address/data (memory) interface that allows random access to any
location. This makes it suitable for storage of program code that needs to be infrequently
updated, as in digital cameras and PDAs. Its endurance is 10,000 to 100,000 erase cycles.
NOR-based flash is the basis of early flash-based removable media; Compact Flash and
SmartMedia are both based on it.
NAND flash from Toshiba followed in 1989. It has faster erase and write times, higher density, and
lower cost per bit than NOR flash, and ten times the endurance. However its I/O interface
allows only sequential access to data. This makes it suitable for mass-storage devices such as
PC cards and various memory cards, and somewhat less useful for computer memory. NANDbased flash has led to several much smaller removable media formats, MMC, Secure Digital
and Memory Stick.
Flash memory forms the core of the removable USB interface storage devices known as keydrives.
NOR Flash uses hot electron injection for writing and tunnel release for erasing. NAND Flash uses
tunnel injection for writing and tunnel release for erasing. Flash memory is erased through a
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mechanism called Fowler-Nordheim tunneling - a quantum mechanical tunneling process.