basic circuits, power supplies, transistors

8/7/2019 Basic Circuits, Power Supplies, Transistors

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Basic Circuits, Power Supplies,Transistors,


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Basic Circuit Analysis

What we wont do:

most things: RC, RLC, filters, detailed analysis

What we will do:

set out basic relations

look at a few examples of fundamental

importance (mostly resistive circuits)

look at diodes, voltage regulation, transistors

discuss impedances (cable, output, etc.)

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Example: Voltage divider

Voltage dividers are a classic way to set a

voltage

Works on the principle that all charge

flowing through the first resistor goes

through the second

so VR-value

provided any load at output is negligible:

otherwise some current goes there too

So Vout = V(R2/(R1 + R2))

R2 here is a variable resistor, or

potentiometer, or pot

typically three terminals: R12 is fixed, tap

slides along to vary R13 and R23, though

R13 + R23 = R12 always

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1

2

3

R1

R2

V

Vout


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Real Batteries: Output Impedance

A power supply (battery) is characterized by a voltage

(V) and an output impedance (R)

sometimes called source impedance

Hooking up to load: Rload, we form a voltage divider,

so that the voltage applied by the battery terminal is

actually Vout = V(Rload/(R+Rload))

thus the smaller R is, the stiffer the power supply

when Vout sags with higher load current, we call this

droop

Example: If 10.0 V power supply droops by 1% (0.1 V)

when loaded to 1 Amp (10 load):

internal resistance is 0.1

called output impedance or source impedance

may vary with load, though (not a real resistor)

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V

R

D-cell example: 6Aout of 1.5 V batteryindicates 0.25 outputimpedance


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The block diagram of the basic powersupply. Most power supplies are made

up of four basic sections: aTRANSFORMER, a RECTIFIER, aFILTER, and a REGULATOR.


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The transformer serves two primarypurposes: (1) to step up or step down

the input line voltage to the desiredlevel and (2) to couple this voltage tothe rectifier section.


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Power Supplies and Regulation A power supply typically starts with a transformer

to knock down the 470V peak-to-peak (220 V AC) to something

reasonable/manageable

We will be using a center-tap transformer

(A B) = (winding ratio)(A B)

when A > B, so is A > B

geometry of center tap (CT) guarantees it is midwaybetween A and B

(frequently tie this to ground so that A = B)

note that secondary side floats: no ground reference built-in

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A

B

A

CT

B

AC input AC output


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Diodes

Diodes are essentially one-way current

gates

Symbolized by:

Current vs. voltage graphs:

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V

I

V

I

V

I

V

I

0.6 V

plain resistor diode idealized diode WAY idealized diode

no current flows current flows

the direction thearrow points in thediode symbol is thedirection that currentwill flow

acts just like a wire(will support arbitrarycurrent) provided thatvoltage is positive


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Diode Makeup

Diodes are made of semiconductors (usually silicon)

Essentially a stack ofp-doped and n-doped silicon to form ap-n junction

doping means deliberate impurities that contribute extra electrons (n-

doped) or holes for electrons (p-doped)

Transistors are n-p-n orp-n-p arrangements of semiconductors

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p-type n-type


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LEDs: Light-Emitting Diodes Main difference is material is more exotic than silicon used in ordinary

diodes/transistors

typically 2-volt drop instead of 0.6 V drop

When electron flows through LED, loses energy by emitting a photon of light

rather than vibrating lattice (heat)

LED efficiency is 30% (compare to incandescent bulb at 10%)

Must supply current-limiting resistor in series: figure on 2 V drop across LED; aim for 110 mA of current

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Getting DC back out of AC AC provides a means for us to distribute electrical power, but most

devices actually wantDC

bulbs, toasters, heaters, fans dont care: plug straight in

sophisticated devices care because they have diodes and

transistors that require a certain polarity

rather than oscillating polarity derived from AC

this is why battery orientation matters in most electronics

Use diodes to rectify AC signal

Simplest (half-wave) rectifier uses one diode:

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AC source load

input voltage

voltage seen by loaddiode only conductswhen input voltage is positive


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Doing Better: Full-wave Diode Bridge

The diode in the rectifying circuit simply prevented the negative swing of

voltage from conducting

but this wastes half the available cycle

also very irregular (bumpy): far from a good DC source

By using four diodes, you can recover the negative swing:

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A

C

B

D

AC source

load

input voltage

voltage seen by load

B & C conduct

A & D conduct


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Full-Wave Dual-Supply By grounding the center tap, we have two opposite

AC sources the diode bridge now presents + and voltages relative

to ground each can be separately smoothed/regulated cutting out diodes A and D makes a half-wave rectifier

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A

C

B

D

AC source

+ load

load

voltages seen by loads

can buy pre-packaged diode bridges


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Smoothing out the Bumps Still a bumpy ride, but we can smooth this out with a

capacitor

capacitors have capacity for storing charge acts like a reservoir to supply current during low spots

voltage regulator smoothes out remaining ripple

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A

C

B

D

AC source

load

capacitor


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Regulating the Voltage

The unregulated, ripply voltage may not be at thevalue you want depends on transformer, etc. suppose you want 15.0 V

You coulduse a voltage divider to set the voltage But it would droop under load

output impedanceR1 || R2 need to have very small R1, R2to make stiff the divider will draw a lot of current perhaps straining the source power expended in divider >> power in load

Not a real solution

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1

2

3

R1

R2

Vin

Vout

Rload


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The Zener Regulator

Zener diodes break down at some reverse

voltage

can buy at specific breakdown voltages

as long as somecurrent goes through zener, itll

work

good for rough regulation

Conditions for working:

lets maintain some minimal current, Iz through

zener (say a few mA)

then (VinVout)/R1 = Iz + Vout/Rload sets the

requirement on R1

because presumably all else is known

if load current increases too much, zener shuts

off (node drops below breakdown) and you just

have a voltage divider with the load

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R1

Z

Vin

Vout = Vz

Rload

zener voltage

high slope is what makes thezener a decent voltage regulator


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Voltage Regulator IC Can trim down ripply voltage to

precise, rock-steady value

Now things get complicated!

We are now in the realm of

integrated circuits (ICs)

ICs are whole circuits in small

packages

ICs contain resistors, capacitors,

diodes, transistors, etc.

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note zeners


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Voltage Regulators The most common voltage regulators are the

LM78XX (+ voltages) and LM79XX ( voltages) XX represents the voltage

7815 is +15; 7915 is 15; 7805 is +5, etc

typically needs input > 3 volts above output (reg.)voltage

A versatile regulator is the LM317 (+) or LM337 () 1.237 V output

Vout = 1.25(1+R2/R1) + IadjR2 Up to 1.5 A picture at right can go to 25 V datasheetcatalog.com for details

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beware that housing is not always ground


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Transistors Transistors are versatile, highly non-linear

devices Two frequent modes of operation:

amplifiers/buffers switches

Two main flavors: npn (more common) or pnp, describing doping

structure

Also many varieties: bipolar junction transistors (BJTs) such as npn,

pnp field effect transistors (FETs): n-channel and p-

channel metal-oxide-semiconductor FETs (MOSFETs)

Well just hit the essentials of the BJT here20

B

C

E

B

E

C

npn pnp


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Diode and Transistor Testing

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Step by step instructions:

The instructions are given primarily for an NPN transistor asthese are the most common types in use. The variations areshown for PNP varieties.

Set the meter to its ohms range - any range should do, but

the middle ohms range if several are available is probablybest.

Connect the base terminal of the transistor to the terminalmarked positive (usually colored red) on the multimeter

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Connect the terminal marked negative or common(usually colored black) to the collector and measurethe resistance. It should read open circuit (there shouldbe a deflection for a PNP transistor).

With the terminal marked negative still connected to

the base, repeat the measurement with the positiveterminal connected to the emitter. The reading shouldagain read open circuit (the multimeter should deflectfor a PNP transistor).

Winter 2007 UCSD: Physics 121; 2007 23


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Winter 2007 24


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Base emitter must

haveRESISTANCE

Base-collector alsohaveRESISTANCE

Emitter-collectorhasRESISTANCE


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Now reverse the connection to the base of the transistor,this time connecting the negative or common (black)terminal of the analogue test meter to the base of the

transistor.Connect the terminal marked positive, first to the collectorand measure the resistance. Then take it to the emitter. Inboth cases the meter should deflect (indicate open circuitfor

a PNP transistor).It is next necessary to connect the meter negative orcommon to the collector and meter positive to the emitter.Check that the meter reads open circuit. (The meter shouldread open circuit for both NPN and PNP types.

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Now reverse the connections so that themeter negative or common is connected tothe emitter and meter positive to the

collector. Check again that the meter readsopen circuit.

If the transistor passes all the tests then it isbasically functional and all the junctions areintact.

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Improved Zener Regulator

By adding a transistor to the zener regulator

from before, we no longer have to worry asmuch about the current being pulled away

from the zener to the load

the base current is small

Rload effectively lookstimes bigger

real current supplied through transistor

Can often find zeners at 5.6 V, 9.6 V, 12.6 V,

15.6 V, etc. because drop from base to

emitter is about 0.6 V

so transistor-buffered Vreg comes out to 5.0,

9.0, etc. Iz varies less in this arrangement, so the

regulated voltage is steadier

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Vreg

Rload

Vz

Vin

Rz

Z

Vin


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Designing PCB

cut your COPPERCLAD BOARD to

desired size

Clean by soap andwater

a pencil eraser may

also help

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Put a masking tapeon the copper sideof the board, enough

to fully cover thesurface

using the carbonpaper transfer your

design to themasking tape

Use a cutter toremove excess

masking tape fromyour design.

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Etching

Material andEquipment

Ferric Chloridepowder

distill or plain water

glove

glass, plastic,wooden rod or old

chopstick long container for

the etchant

a boarder containerfor boiling hot waterbelow

3 litre of boilingwater

drilling machine

plastic string

developed PCB

board container with water

for washing

detergent

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Ferric Chloride is useto etch away coppersurface on the PCBboard. It is a verytoxin chemical and is

harmful to theenvironment. Pleasehandle and disposethe chemical waste

with care. It is darkyellowish in colorand can stain yourclothing.

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Remember to wearprotective gloves

while handling

FeCl3. Chemical is

toxin and will cause

skin irritation Wash

skin with running

water immediatelywhen in contact

with skin.

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Stronger FeCl3 solution enables etching

process to be faster.

When design PCB board, it may be a good idea

to fill up with regions of copper. This is to

minimise the area of copper surface to be

etched away. With less copper to etched, it

will also means that the solution can beeffectively use to etch more PCB board



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Etchant mixture

The solvent compositefor making the etchantconsist of about 1 unitof Ferric ChlorideFeCl3 is to 3 unit of

water or about 1 unitof AmmoniumPersulphate is to 5unit of water.

Stir the mixture untilFeCl3 is fully dissolvedwith the water.

Winter 2007 35


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Board preparation for etching

Drill a small hole onthe PCB board so

that a string can be

secure to the

board. The string is

use to position or

pull out the PCB in

the toxin solution.

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or

A scotch tape can be

used to secure the

string to the PCB

board.

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Immerse the PCBboard slowly intothe FeCl3 solution.Agitate the PCB by

tilting the containerto and fro gently,until the unwantedcopper layer are

properly etchedaway, leaving onlythe required regionon the PCB.

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The process maytake 15 - 60

minutes to

complete. Process

duration will

depends on the

concentration,

temperature of theetchant solution.

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After theunnecessarycopper clad isremoved, use a

hand drill fordrilling componentholes.

1/16 for diodes andwires and 1/32 forresistors andcapacitor.



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The soldering irontip transfers thermal

energy from theheater to the solderconnection. In mostsoldering iron tips,the base metal iscopper or somecopper alloy becauseof its excellentthermal conductivity.

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basics of Soldering.


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A tip's conductivity

determines how

fast thermal energycan be sent from

the heater to the

connection.

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Right Amount of Solder

a) Minimum amount ofsolder

b) Optimalc) Excessive solder


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What is the first component?

1. Resistors2. Capacitors

3. Diodes

4. Transistors5. ICs

6. Connecting wires

7. Heat sink

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