booksbybibin

[yogi]

This tutorial is taken from the book
MY EXPERIENCE IN AUTONOMOUS ROBOTICS.
you can download it free from the files
section(FILE-ROBO.PDF) of yahoo group-booksbybibin
http://groups.yahoo.com/group/booksbybibin/

comments about this book on
www.booksbybibin.blogspot.com

VOLTAG REGULATORS
Voltage regulators produce fixed DC output
voltage from variable DC (a small amount of AC on it). Normally we get
fixed output by connecting the voltage regulator at the output of the
filtered DC(see in above diagram). It can also used in circuits to get a low
DC voltage from a high DC voltage (for example we use 7805 to get 5V
from 12V). There are two types of voltage regulators
1. fixed voltage regulators(78xx,79xx)
2. variable voltage regulators(LM317)
In fixed voltage regulators there is another classification
1. +ve voltage regulators
2. -ve voltage regulators
POSITIVE VOLTAGE REGULATORS
This include 78xx voltage regulators. The most commonly used ones are
7805 and 7812. 7805 gives fixed 5V DC voltage if input voltage is in
(7.5V,20V). You may sometimes have questions like, what happens if
input voltage is <7.5 V or some 3V, the answer is that regulation won't be
proper. Suppose if input is 6V then output may be 5V or 4.8V, but there are
some parameters for the voltage regulators like maximum output current
capability, line regulation etc.. , that parameters won't be proper. When I
applied 3.55V input, i got around 3.5V. Remember that electronics
components should be used in the proper voltage and current ratings as
specified in datasheet. You can work without following it, but you won't be
able to get some parameters of the component.

Get datasheet from google by searching '7805 datasheet' or from
www.alldatasheet.com
Next task is to identify the leads of the 7805. So first u have to keep the lead
downward and the writing to your side,see the figure below. You can see
the heat sink above the voltage regulator.(1-input,2-gnd,3-output)




This is the same way of lead identification for all 3 terminal IC's (for eg.
Power transistor).


The above diagram show how to use 7805 voltage regulator. In this you
can see that coupling capacitors are used for good regulation. But there is
no need for it in normal case( I never used these capacitors). But if you are
using 7805 in analog circuit you should use capacitor, otherwise the noise
in the output voltage will be high.The mainly available 78xx IC's are
7805,7809,7812,7815,7824

NEGATIVE VOLTAGE REGULATORS
Mostly available -ve voltage regulators are of 79xx
family. You will use -ve voltage if you use IC741. For IC741 +12v and -12v
will be enough, even though in most circuits we use +15v and -15v. You
can get more information about 7905 from the following link.
http://www.national.com/ds/LM/LM7905.pdf
http://cache.national.com/ds/LM/LM7905.pdf
7805 gives fixed -5V DC voltage if input voltage is in (-7V,-20V)




The mainly available 79xx IC's are 7905,7912
1.5A output current,short circuit protection,ripple rejection are the other
features of 79xx and 78xx IC's


VARIABLE VOLTAGE REGULATORS
Most commonly variable voltage regulator is LM317 although other
variable voltage regulators are available. The advantage of variable voltage
regulator is that you can get a variable voltage supply by just varying the
resistance only.
http://focus.ti.com/docs/prod/folders/print/lm317.html
http://www.national.com/pf/LM/LM317.html
http://www.electronics-lab.com/articles/LM317/




From the above the equation you can see that output voltage is
proportional to R1 and R2. But in the above equation we can neglect Iadj
So Vout=1.25(1+R2/R1). If you put R1=R2=1Kohm Vout=2.5V. LM317 can
be used to drive motor because it can handle output current up to 1.5A.
In some low power devices like image sensor or USB we require 3.3V, in
that circuit we use LM317.In a line follower we introduce some speed
variations for motor for different bendings, you can do it by either using
PWM or using the above circuit.
NOTE:
Remember about the input voltage limitations.
Remember about the heat sink of the voltage regulators before touching
the voltage regulator IC because it will be in the heated state normally .
Your hand will get burned(not big burn,some small) if we touch the heat
sink of the voltage regulator. So first touch the heat sink gently and
confirm it is not heated, then only remove the IC from the breadboard.
If you are driving high power circuits and motors from the output of the
voltage regulator screw an external heat sink to the voltage regulator. Size
of the heat sink depends on the output power driving.

[TirthankarMukherjee]

[yogi]

I think u r talking about adapter available on market, they don't have good regulator inside. Use one 7805 at the output of adapter, then u will be able to maintain constant +5V(voltage depend on load too...) formal normal circuits. If u want good one go for computer SMPS

[ferrywayes]

Hi...

I think if u need constant voltage and you are not getting it it is not the problem of the Integrated circuit.You can replace the existing voltage regulator with a new on or you have to use switch mode power supply.
Hope this will work for you.
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