Patâs Electronics Lecture

8/3/2019 Pat s Electronics Lecture

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Pats Electronics Lecture

basics


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Water Analogy (helped me)

Does somekind of work

Watersource

Pressure pusheswater through pipe

Flow

(Water drain= return)

Laptag MILL andSWEATSHOP

Currentreturn

Current flow

Does usefulwork

Battery


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Water Analogy, contd

Water Pressure(the higher the pressurethe more water flows)

Voltage(The higher the voltagethe more current flows)

Water Flow Rate(e.g. gallons per

minute)

Current(which is actually charge

flow rate: 1 coulomb/sec= 1 ampere)


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Work

In both pictures, potential energy isconverted to work, where

Work =

Kinetic energy

Heat

Some other kind of potential energy

Physics note: Total Energy is conserved


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Whats Happening

Electric charges can flow in conductors

Like charges repel

Unlike charges attract

Battery

+ + + +

- - - - -

Because of the chemistryinside the battery, there is

a voltage set up acrossthe terminals


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Whats Happening


Like charges repel


Battery

- -- -

- - -- - - -

+ + ++ + +

+ ++ +

If we connect wires, theyalso become charged up


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Whats Happening


Like charges repel


Battery

+ charges

+ charges


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Whats really happening

Electrons are flowing out of bottom ofbattery, around to the top

Since they are negative, the direction ofthe current flow (by convention) isopposite their physical movement

It is MUCH EASIER to think of positivecharges flowing, even though they areslightly fictitious


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Typical Elements of a circuit

Wires

Voltage Sources

Electronic Components Resistors

Capacitors

Inductors Modular circuits (e.g. amplifiers)

Occasionally diodes and transistors


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Wires

These are good conductors, withpractically unimpeded flow of current

Electrons in metal form a kind of plasma

Any flowing current creates a magneticfield (which btw can be used to measurethe current)

Size is measured by AWG, American

Wire Gauge, since the 1850s


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Interesting note on AWG

The gauge number is similar to decibelmeasurement for sound

20 steps in AWG is (almost) a factor of 10in wire diameter

For instance, #1 AWG wire is ~ 10x thediameter of #20 AWG

We typically use #20 to #24 for circuits


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Voltage

(the Electrical version of pressure)

Measured with a meter, if time variation isslow enough

Measured with a scope and typically ascope probe if fast time variation

Hazards: HIGH VOLTAGE CAN KILL YOU

(actually its the current through your heart)


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Pressure is not exactly Voltage

One difference: voltage is alwaysmeasured between two points (e.g. ameter has a common probe and a

measurement probe. The reason for this goes back to the

attraction of charges,

Still a very good analogy, though

meterVoltsFieldElectric distance

in voltagechange


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Water flow is not exactly Electricalcurrent

Water can flow even when there is not an(obvious) return path


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2 hazards we will encounter

1: DO NOT USE A SCOPE OR METERTO 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


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2d hazard: Death

High voltages in our lab can kill you.

Best case scenario: you accidentally touch a high voltage terminal, and currentstarts to flow through your arm. If this current is much larger than your nerveimpulses, you can no longer pull your arm away, because your muscles dont

receive the command. It hurts. You begin to think about how dumb you were tohave one hand resting on ground while you poked around with the other one.Next, some guy who also didnt listen grabs onto you to try to pull you away.

Current flows through him, too, so he is useless. Finally someone who paidattention 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.


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High Current

This can also be dangerous:

wires can heat up, and cause fires.

Circuit elements (wires) can literally explode ifa lot of energy is dumped into them quickly

More subtly, interrupting a high current cangive a high-voltage transient!!! Of all the

hazards, this is the only one I personally hadexperience with that actually did kill a guy.

(We will get to the reason for this.)


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Resistors Resistors impede the flow of electrical

current

Like a pin-hole for water flow

Watersource

Constriction in piperesists the water flow need more pressure to get the same flow pressure after the constriction is lower

High pressure

Lower pressure

Similarly, there is a voltage drop across a resistor whencurrent flows throu h it.


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Resistors

Symbol

Measured in ohms:

A resistance of 1 ohm will let 1 Amp of currentflow for a voltage drop of 1 Volt (across theresistor).

Ampere

Volt11


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Ohms Law

IRV

RVI

I

V

R


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Computing resistance

Resistance

Where (rho) is the resistivity of the material

L is the length

A is the area

A

LR

length L

Area A

=


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Some Resistivities

Material Resistivity

Copper m 8107.1

Carbon m 5102

Silicon m 600

Water m 5108.1

Glass m 1010

Teflon m 2210


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Resistor Marking

Color Code

First 2 bands = digits

3d band = power of 10 4th band = tolerance: gold 5%, silver 10%, none 20%

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

01

2

3

4

5

6

78

9


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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 = 2etc.


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From http://www.token.com.tw/resistor/image/color-code.jpg


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Simple Circuit Diagrams 1

1 Voltage Source (e.g. battery)

1 resistor

Given a 9 V battery, and a 1000 ohm resistor, whatcurrent will flow?


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Resistors in series:


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Resistors in parallel:


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Convenient formulas:

Series resistors:

Parallel resistors:

21 RRRtotal

21

21

RR

RRRtotal

Note: it may help to think about the construction of a resistor


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Another circuit


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think about what happens in this

arrangement:

Watersource

High pressure

Lower pressure


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What about this one?

Hint: symmetry helps


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Other useful components

Inductors

Capacitors

Diodes Integrated Circuits (e.g. RF amplifier)

MOSFETs

Occasionally transistors Rarely vacuum tubes


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Electrical Power

Power is rate of dissipation of energy

Also rate of getting work done

CurrentVoltageP

Energy is conserved, so if we are notstoring any energy:

Power in = Power out + heat dissipatedas losses


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AC Voltage, Current

AC stands for alternatingcurrent

Nevertheless people still talk

about AC current coming outof the wall.

The voltage alternates: if youhad a really fast meter, youwould see the polarityreversing 60 times a second*

* Or just use an oscilloscope, BUT DONT HOOK IT UP DIRECTLY


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Water analogy:

2 buckets on a see-saw

Watersource/sink

Watersource/sink


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Water analogy:

2 buckets on a see-saw

Watersource/sink

Watersource/sink


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Why AC?

See War of Currents on wikipedia Edison wanted DC

Tesla wanted AC

No good way to transform DC to a differentvoltage (at least in 1900) Transmission requires high current

Must generate near point of load

AC can be transformed up to high voltage, lowcurrent, for transmission, then back to saferlevels (110 V) near point of load


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AC Outlet: 110 V (rms)

Ground

Low side,or neutral

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


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AC Voltage Measurement

Level is quoted as

Peak-to-peak (least ambiguous)

Peak

RMS = root mean square, which is the average valueof the square of the voltage. This is what a typicalhandheld voltmeter reads on the AC setting.

110 V is the RMS value, peak is around 160 V,or 2110


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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

p

s

in

out

N

N

V

V

primary

winding

secondary

winding


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Transformers, conts

ps NVV primary

secondary

#

#

turns

turnsN

ps IN

I1

where

ssppppin IVI

NNVIVP

1

Also

Note: Power is conserved:


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Capacitors

Symbols:

Let AC through, but not DC; another way of saying thisis that they tend to keep the voltage across themconstant

Have an impedance (not a resistance because theydont dissipate any power)

CfZ

2

1||


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2 conductors separated by a physical space

Capacitor construction

dAC 0 d

A

C, in Farads, is a measure ofhow much charge can bestored for a given voltage

meterFarad120 108.8


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Water Model

Water balloons in a sealed oil-filledenclosure:


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Water Model



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Water Model



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Water Model



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Capacitors, contd

Often the gap is filled with a dielectric materialto increase the capacitance; using an insulatoralso allows the gap to shrink, d0, but voltagestays 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, orelectrolytic advantage is higher capacitance

Ugly fact that we will not worry about: mostdielectrics change their value as they are biasedto higher voltages!


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Inductors

Symbol

Let DC through, but not AC; another way of saying thisis that it tends to keep the current flowing through it ata constant level

Have an impedance (not a resistance because theydont dissipate any power)

LfZ 2||


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Inductor Construction

Any coil of wire

Sometimes iron is added to

increase the magneticstored energy, whichincreases the inductance


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Inductance

N

A

turns#

Area

Length

ANL

2

0

Why N2?

Current flowing through the windings produces a magneticfield; more turns produces more field, proportional to thenumber of turns in a given length.

Each turn then picks up voltage from the changing magneticfield; with the turns in series the voltage adds, so the total is

proportional to the total number of turns.


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Example circuit

Initially the switch is open, so no current is flowing

Close the switch: the inductor tends to keep thesame current flowing, which is zero.

Eventually the inductor looks like a wire, so thecurrent is given by Ohms law: I= VR


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Water analog: heavy paddle-wheel

Flow

2. Paddle-wheel has heavy flywheelattached so it is hard to spin up,but once it is spinning it tends to

keep going

Valve

1. Once valve isopened, paddle-wheelbegins to spin

3. Eventually the paddle-wheel gets up to speed, and the flow

is limited by the resistance in the line

A th i it th d f hi h


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Another circuit: the dangers of highcurrent

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 initialvalue, then the switch is opened: what happens?

Inductor tries to keep the same current flowing, so

initially V = 100x the battery voltage


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Generating high pressure due to current flow

Flow

Suppose valve is initially closed

Valve

Paddle-wheel is spinning slowly

Then we open the valve for some amount of time, letting theflow build up (paddle-wheel spins faster)


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Generating high pressure due to current flow

Flow

Then valve is closed again

Valve

Paddle-wheel spins up

Flow throughthis leg stops

Flow transfers to

this leg

The pressure ahead of the resistance goes up, since thepaddle-wheel keeps spinning; eventually slows down to

steady state


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Diodes

Symbol:

Function: only let current flow one direction

Convert AC to DC useful for power supplies,

detecting radio signals,

Pos Neg


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Water Analog of a diode

A flap inside a pipe

flow: no flow:


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SOLDERING

Solder works by forming a solution of the metalsbeing joined in the liquid solder.

So the solder needs to be hot enough to flow,BUT

Too much heat traveling up the leads willdestroy semiconductors!

The work pieces rather than the soldering ironmust melt the solder

When done, the two conductors being solderedshould look wetted


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Solder wire

Has flux inside. Flux is a wax-like goothat has a few percent acid, for cleaningthe oxide layer from wires being soldered.For plumbing, the same thing happens except the flux is usually applied separately.

And you cant use lead solder anymore. And usually a torch is used instead of an iron.


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Soldering Hints

Liquid solder conducts heat better than a dry tip,so it helps to put a dab of solder onto the tipbefore soldering. The associated flux can alsohelp clean up the tip.

It helps to tin the leads being solderedindividually before actually trying to solder themtogether.

The smoke comes from burning flux, not lead,but still probably not healthy to breathe it in.

Dont hold solder in mouth.


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Soldering Irons - experience

Temperature regulated ones are crucial

Tips are special if you decide that youwant a sharper tip, you can sand the tip

down to a point, but it will dissolve a littlebit each time you use it and disappearbefore too long.


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Solder joint cross section

From http://www.emeraldinsight.com/fig/2170250306001.png


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Making a Circuit Board

1. Generate a layout, using some kind of PCB software. There areprograms that are free but that I know very little about (we use a badbut 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.
http://www.cadsoftusa.com/http://www.lis.inpg.fr/realise_au_lis/kicad/http://www.lis.inpg.fr/realise_au_lis/kicad/http://www.cadsoftusa.com/


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Circuit Board, contd

2. Use laser printer to print layout (also calledartwork) 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.


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Circuit Board, etching

5. Expose in UV box for 5 minutes. The UV goesthrough the clear parts of the transparency, and doessomething to the photoresist.

6. Soak board in developer this washes off theexposed photoresist. (Dilute the developer solution 1part developer to 10 parts water.)

7. Rinse developer off using water8. Etch in Ferric Chloride solution. The

photoresist that is still on the copper prevents the

copper from being etched, at least for a while. Etchingusually completes in 15-45 minutes, depending on howold the solution is. You never know, so you need tokeep an eye on the progress.


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Circuit board fab cautions and notes:

The ferric chloride solution will irritate your skinafter a few minutes, so a little is OK butgenerally you should rinse it off.

It will also eat holes in your clothes, if it gets on

them and dries there. ( mysterious little holesnext time you wear them) There is an aquarium heater and a bubbler in

the ferric chloride tank, to help speed things upremember to turn it off.

Dont pourferric chloride down the copper drainpipes.

Patâs Electronics Lecture

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