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Pat’s Electronics Lecture
basics
Water Analogy (helped me…)
Current flow
Water
source
Pressure pushes
water through pipe
Battery
Does useful
“work”
Laptag MILL and
SWEATSHOP
Flow
Current
return
Does some
kind of work
(Water drain
= “return”)
Water Analogy, cont’d
Water Pressure
(the higher the pressure
the more water flows)
 Voltage
(The higher the voltage
the more current flows)
Water Flow Rate  Current
(e.g. gallons per
minute)
(which is actually charge
flow rate: 1 coulomb/sec
= 1 ampere)
“Work”
• In both pictures, potential energy is
converted to “work”, where
• Work =
– Kinetic energy
– Heat
– Some other kind of potential energy
• Physics note: Total Energy is conserved
What’s Happening
• Electric charges can flow in conductors
• Like charges repel
• Unlike charges attract
++++
Battery
-----
Because of the chemistry
inside the battery, there is
a voltage set up across
the terminals
What’s Happening
• Electric charges can flow in conductors
• Like charges repel
• Unlike charges attract
+++
+
+ +
+ +
+ +
Battery
- - - -
- - -
If we connect wires, they
also become charged up
What’s Happening
• Electric charges can flow in conductors
• Like charges repel
• Unlike charges attract
+ charges
Battery
+ charges
What’s really happening
• Electrons are flowing out of bottom of
battery, around to the top
• Since they are negative, the direction of
the current flow (by convention) is
opposite their physical movement
• It is MUCH EASIER to think of positive
charges flowing, even though they are
slightly fictitious
Typical Elements of a circuit
• Wires
• Voltage Sources
• Electronic Components
– Resistors
– Capacitors
– Inductors
– Modular circuits (e.g. amplifiers)
– Occasionally diodes and transistors
Wires
• These are good conductors, with
practically unimpeded flow of current
• Electrons in metal form a kind of plasma
• Any flowing current creates a magnetic
field (which btw can be used to measure
the current)
• Size is measured by “AWG”, American
Wire Gauge, since the 1850s
Interesting note on AWG
• The gauge number is similar to decibel
measurement for sound
• 20 steps in AWG is (almost) a factor of 10
in wire diameter
• For instance, #1 AWG wire is ~ 10x the
diameter of #20 AWG
• We typically use #20 to #24 for circuits
Voltage
• (the Electrical version of pressure)
• Measured with a meter, if time variation is
slow enough
• Measured with a scope and typically a
scope probe if fast time variation
• Hazards:
– HIGH VOLTAGE CAN KILL YOU
– (actually it’s the current through your heart…)
Pressure is not exactly Voltage
• One difference: voltage is always
measured between two points (e.g. a
meter has a “common” probe and a
measurement probe.
• The reason for this goes back to the
attraction of charges,
change in voltage
Volts
Electric Field 

distance
meter
• Still a very good analogy, though
Water flow is not exactly Electrical
current
• Water can flow even when there is not an
(obvious) return path
2 hazards we will encounter
• 1: DO NOT USE A SCOPE OR METER
TO MEASURE THE AC LINE VOLTAGE!!!
(what is AC voltage? We will cover this)
WHY?
• THE METER CAN LITERALLY EXPLODE
• You might kill a $10,000 scope
► ► ►Use a “Wiggy” instead
2d hazard: Death
• High voltages in our lab can kill you.
Best case scenario: you accidentally touch a high voltage terminal, and current
starts to flow through your arm. If this current is much larger than your nerve
impulses, you can no longer pull your arm away, because your muscles don’t
receive the command. It hurts. You begin to think about how dumb you were to
have one hand resting on ground while you poked around with the other one.
Next, some guy who also didn’t listen grabs onto you to try to pull you away.
Current flows through him, too, so he is useless. Finally someone who paid
attention to this lecture finds a non-conducting hook and saves both victims.
Worst case: sufficient current finds its way through your heart to stop it, too.
High Current
• This can also be dangerous:
– wires can heat up, and cause fires.
– Circuit elements (wires) can literally explode if
a lot of energy is dumped into them quickly
– More subtly, interrupting a high current can
give a high-voltage transient!!! Of all the
hazards, this is the only one I personally had
experience with that actually did kill a guy.
(We will get to the reason for this.)
Resistors
• Resistors impede the flow of electrical
current
• Like a pin-hole for water flow
Water
source
High pressure
Lower pressure
Constriction in pipe
resists the water flow 
 need more pressure to get the same flow
 pressure after the constriction is lower
Similarly, there is a voltage drop across a resistor when
current flows through it.
Resistors
• Symbol
• Measured in ohms:
Volt
1  1
Ampere
A resistance of 1 ohm will let 1 Amp of current
flow for a voltage drop of 1 Volt (across the
resistor).
Ohm’s Law
V  IR
V
I 
R
V
R
I
Computing resistance
• Resistance
R
L
A
Where ρ (rho) is the “resistivity” of the material
L is the length
A is the area
Area A
=
length L
Some Resistivities
Material
Copper
Carbon
Silicon
Water
Glass
Resistivity
 1.7  10 8 m
 2  10 5 m
 600 m
 1.8  105 m
 1010 m
Teflon
 10 22 m
Resistor Marking
• Color Code
• First 2 bands = digits
• 3d band = power of 10
•
4th band = tolerance: gold 5%, silver 10%, none 20%
0
1
2
3
4
• E.g. brown black red is
= 1 0 00
= (a one followed by a zero followed by 2 zeros)
Other Notes:
3d band = gold: divide by 10
3d band = silver: divide by 100
5
6
7
8
9
Remember
•
•
•
•
•
Black = 0 (no color)
White = 9 (all colors)
Grey is close to white, so make it 8
Brown = ? Might as well be 1
The rest correspond to the spectrum
– ROYGBV
(You may have heard of this guy: Roy G. Biv)
Red = 2…etc.
From http://www.token.com.tw/resistor/image/color-code.jpg
Simple Circuit Diagrams 1
• 1 Voltage Source (e.g. battery)
• 1 resistor
Given a 9 V battery, and a 1000 ohm resistor, what
current will flow?
Simple Circuit Diagrams 2
• Resistors in series:
Simple Circuit Diagrams 3
• Resistors in parallel:
Convenient formulas:
• Series resistors:
Rtotal  R1  R2
• Parallel resistors:
R1 R2
Rtotal 
R1  R2
Note: it may help to think about the construction of a resistor
Another circuit
…think about what happens in this
arrangement:
Water
source
High pressure
Lower pressure
What about this one?
Hint: symmetry helps
Other useful components
•
•
•
•
•
•
•
Inductors
Capacitors
Diodes
Integrated Circuits (e.g. RF amplifier)
MOSFETs
Occasionally transistors
Rarely vacuum tubes
Electrical Power
• Power is rate of dissipation of energy
• Also rate of getting work done
P  Voltage  Current
• Energy is conserved, so if we are not
storing any energy:
Power in = Power out + heat dissipated
as losses
AC Voltage, Current
• AC stands for alternating
current
• Nevertheless people still talk
about “AC current” coming out
of the wall.
• The voltage alternates: if you
had a really fast meter, you
would see the polarity
reversing 60 times a second*
* Or just use an oscilloscope, BUT DON”T HOOK IT UP DIRECTLY
Water analogy:
• 2 buckets on a see-saw
Water
source/sink
Water
source/sink
Water analogy:
• 2 buckets on a see-saw
Water
source/sink
Water
source/sink
Why AC?
• See “War of Currents” on wikipedia
– Edison wanted DC
– Tesla wanted AC
• No good way to transform DC to a different
voltage (at least in 1900)
– Transmission requires high current
– Must generate near point of load
• AC can be transformed up to high voltage, low
current, for transmission, then back to safer
levels (110 V) near point of load
AC Outlet: 110 V (rms)
Low side,
or neutral
Ground
High side,
or line
In an AC line cord, standard colors are: Green for ground, White for neutral, and Black for line
NOTE: in most AC wiring, BLACK is the hot, or high voltage, side
AC Voltage Measurement
Level is quoted as
– Peak-to-peak (least ambiguous)
– Peak
– RMS = root mean square, which is the average value
of the square of the voltage. This is what a typical
handheld voltmeter reads on the AC setting.
• 110 V is the RMS value, peak is around 160 V, or
 110 2
Transformer
• 2 sets of windings, with their magnetic fields coupled.
• Use iron to channel the field from one set to another
• Step up or down the voltage according to the turns
ratio
Vout N s

Vin N p
“primary”
winding
“secondary”
winding
Transformers, cont’s
Vs  NV p
Also
where
N
# turnssecondary
# turnsprimary
1
Is  I p
N
Note: Power is conserved:
1 
Pin  V p I p  NV p  I p   Vs I s
N 
Capacitors
• Symbols:
• Let AC through, but not DC; another way of saying this
is that they tend to keep the voltage across them
constant
• Have an impedance (not a resistance because they
don’t dissipate any power)
1
| Z |
2 f C
Capacitor construction
2 conductors separated by a physical space
C
0 A
d
d
A
 0  8.8 10
12
Farad
meter
C, in Farads, is a measure of
how much charge can be
stored for a given voltage
Water Model
• Water balloons in a sealed oil-filled
enclosure:
Water Model
• Water balloons in a sealed oil-filled
enclosure:
Water Model
• Water balloons in a sealed oil-filled
enclosure:
Water Model
• Water balloons in a sealed oil-filled
enclosure:
Capacitors, cont’d
• Often the gap is filled with a “dielectric” material
to increase the capacitance; using an insulator
also allows the gap to shrink, d  0, but voltage
stays the same without breakdown.
• All dielectrics have a safe operating voltage,
which is given as the voltage rating
• Sometimes the dielectric can only be charged in
one direction: the capacitor is polarized, or
electrolytic – advantage is higher capacitance
• Ugly fact that we will not worry about: most
dielectrics change their value as they are biased
to higher voltages!
Inductors
• Symbol
• Let DC through, but not AC; another way of saying this
is that it tends to keep the current flowing through it at
a constant level
• Have an impedance (not a resistance because they
don’t dissipate any power)
| Z | 2 f L
Inductor Construction
• Any coil of wire
• Sometimes iron is added to
increase the magnetic
stored energy, which
increases the inductance
Inductance
Length  
Area  A
# turns  N
L
0 N 2 A

2
Why N ?
• Current flowing through the windings produces a magnetic
field; more turns produces more field, proportional to the
number of turns in a given length.
• Each turn then picks up voltage from the changing magnetic
field; with the turns “in series” the voltage adds, so the total is
proportional to the total number of turns.
Example circuit
• Initially the switch is open, so no current is flowing
• Close the switch: the inductor tends to keep the
same current flowing, which is zero.
• Eventually the inductor looks like a wire, so the
current is given by Ohm’s law: I = V ÷ R
Water analog: heavy paddle-wheel
1. Once valve is
opened, paddle-wheel
begins to spin
2. Paddle-wheel has heavy flywheel
attached – so it is hard to spin up,
but once it is spinning it tends to
keep going
Valve
Flow
3. Eventually the paddle-wheel gets up to speed, and the flow
is limited by the resistance in the line
Another circuit: the dangers of high
current
• Initially the switch is open, so some current flows,
such that I = V ÷ R
• Close the switch: current starts to increase
• Suppose the current builds up to 100x its initial
value, then the switch is opened: what happens?
• Inductor tries to keep the same current flowing, so
initially V = 100x the battery voltage
Generating high pressure due to current flow
Suppose valve is initially closed
Paddle-wheel is spinning slowly
Flow
Valve
Then we open the valve for some amount of time, letting the
flow build up (paddle-wheel spins faster)
Generating high pressure due to current flow
Then valve is closed again…
Paddle-wheel spins up
Flow
Flow through
this leg stops
Flow transfers to
this leg
Valve
The pressure ahead of the resistance goes up, since the
paddle-wheel keeps spinning; eventually slows down to
“steady state”
Diodes
• Symbol:
Pos
Neg
• Function: only let current flow one direction
• Convert AC to DC – useful for power supplies,
detecting radio signals, …
Water Analog of a diode
• A flap inside a pipe
flow:
no flow:
SOLDERING
• Solder works by forming a solution of the metals
being joined in the liquid solder.
• So the solder needs to be hot enough to flow,
BUT
• Too much heat traveling up the leads will destroy
semiconductors!
• The work pieces rather than the soldering iron
must melt the solder
• When done, the two conductors being soldered
should look “wetted”
Solder wire
Has “flux” inside. Flux is a wax-like goo
that has a few percent acid, for cleaning
the oxide layer from wires being soldered.
For plumbing, the same thing happens except the flux is usually applied separately.
And you can’t use lead solder anymore. And usually a torch is used instead of an iron.
Soldering Hints
• Liquid solder conducts heat better than a dry tip,
so it helps to put a dab of solder onto the tip
before soldering. The associated flux can also
help clean up the tip.
• It helps to “tin” the leads being soldered
individually before actually trying to solder them
together.
• The smoke comes from burning flux, not lead,
but still probably not healthy to breathe it in.
• Don’t hold solder in mouth.
Soldering Irons - experience
• Temperature regulated ones are crucial
• Tips are special – if you decide that you
want a sharper tip, you can sand the tip
down to a point, but it will dissolve a little
bit each time you use it and disappear
before too long.
Solder joint cross section
From http://www.emeraldinsight.com/fig/2170250306001.png
Making a Circuit Board
1.
Generate a layout, using some kind of PCB software. There are
programs that are free but that I know very little about (we use a bad
but expensive tool, which is not even sold anymore):
– Eagle, from http://www.cadsoftusa.com/
– Kicad, from http://www.lis.inpg.fr/realise_au_lis/kicad/
Top
Bottom
For our process, we generate a “positive” image: colored parts
(which print as black) will be copper, white parts no copper.
Circuit Board, cont’d
2. Use laser printer to print layout (also called
artwork) on a transparency
3. Align top and bottom, and tape them together.
4. Slip a pre-sensitized board between them.
Top transparency
Bottom transparency
Circuit board, has copper on both sides,
covered with “photo-resist”.
Circuit Board, etching
5. Expose in UV box for 5 minutes. The UV goes
through the clear parts of the transparency, and does
something to the photoresist.
6. Soak board in developer – this washes off the
exposed photoresist. (Dilute the developer solution 1
part developer to 10 parts water.)
7. Rinse developer off using water
8. Etch in Ferric Chloride solution. The
photoresist that is still on the copper prevents the
copper from being etched, at least for a while. Etching
usually completes in 15-45 minutes, depending on how
old the solution is. You never know, so you need to keep
an eye on the progress.
Circuit board fab cautions and notes:
• The ferric chloride solution will irritate your skin
after a few minutes, so a little is OK but
generally you should rinse it off.
• It will also eat holes in your clothes, if it gets on
them and dries there. ( mysterious little holes
next time you wear them)
• There is an aquarium heater and a bubbler in
the ferric chloride tank, to help speed things up –
remember to turn it off.
• Don’t pour ferric chloride down the copper drain
pipes.