pat’s electronics lecture basics. water analogy ( helped me… ) does some kind of work water...

Pat’s Electronics Lecture

basics

Water Analogy (helped me…)

Does some kind of work

Water source

Pressure pushes water through pipe

Flow

(Water drain = “return”)

Laptag MILL and SWEATSHOP

Current return

Current flow

Does useful “work”

Battery

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(e.g. gallons per minute)

Current (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




Battery

- - - - - - - - - - -

+ + ++ + + + + + +

If we connect wires, they also become charged up

What’s Happening




Battery

+ charges

+ 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,

• Still a very good analogy, though

meter

VoltsFieldElectric

distance

in voltage change

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

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 when current flows through it.

Resistors• Symbol

• Measured in ohms:

A resistance of 1 ohm will let 1 Amp of current flow for a voltage drop of 1 Volt (across the resistor).

Ampere

Volt11

Ohm’s Law

IRV

R

VI

I

VR

Computing resistance

• Resistance

Where ρ (rho) is the “resistivity” of the material

L is the length

A is the area

A

LR

length L

Area A

=

Some Resistivities

Material Resistivity Copper m 8107.1 Carbon m 5102 Silicon m600

Water m 5108.1

Glass m 1010

Teflon m 2210

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

0

1

2

3

4

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?


• Resistors in series:


• Resistors in parallel:

Convenient formulas:

• Series resistors:

• Parallel resistors:

21 RRRtotal

21

21

RR

RRRtotal

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

Current Voltage P

• 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

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)

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

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 2110

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

Transformers, cont’s

ps NVV primary

secondary

#

#

turns

turnsN

ps IN

I1

where

ssppppin IVIN

NVIVP

1

Also

Note: Power is conserved:

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)

CfZ

2

1||

2 conductors separated by a physical space

Capacitor construction

d

AC 0

d

A

C, in Farads, is a measure of how much charge can be stored for a given voltage

meter

Farad120 108.8

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)

LfZ 2||

Inductor Construction

• Any coil of wire

• Sometimes iron is added to increase the magnetic stored energy, which increases the inductance

Inductance

N

A

turns#

Area

Length

AN

L2

0

Why N2 ?

• 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

Flow

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

1. Once valve is opened, paddle-wheel begins to spin

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

Flow

Suppose valve is initially closed

Valve

Paddle-wheel is spinning slowly

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

Flow

Then valve is closed again…

Valve

Paddle-wheel spins up

Flow through this leg stops

Flow transfers to this leg

The pressure ahead of the resistance goes up, since the paddle-wheel keeps spinning; eventually slows down to “steady state”

Diodes

• Symbol:

• Function: only let current flow one direction• Convert AC to DC – useful for power supplies,

detecting radio signals, …

Pos Neg

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.

http://www.cadsoftusa.com/

http://www.lis.inpg.fr/realise_au_lis/kicad/



Circuit Board, cont’d2. 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 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. 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 coppercopper drain pipes.

pat’s electronics lecture basics. water analogy ( helped me… ) does some kind of work water...

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