DESIGN OF A SMART EMBEDDED UPS

Digital equipment such as computers, telecommunication systems and instruments use microprocessors that
operate at high frequencies allowing them to carry out millions or even billions of operations per second. A
disturbance in the electrical supply lasting just a few milliseconds can affect thousands or millions of basic
operations. The result may be malfunctioning and loss of data with dangerous or costly consequences (e.g.
loss  of  production).  That  is  why many  loads,  called  sensitive  or  critical  loads,  require  a  supply  that  is
protected. Many manufacturers  of  sensitive  equipment  specify  very  strict  tolerances, much  stricter  than
those  in  the distribution  system  for  the  supply of  their  equipment, one  example being Computer Business
Equipment Manufacturer’s Association  for  computer  equipment against distribution  system disturbances.
The design of  this uninterrupted power supply (UPS)  for personal computer (PC)  is necessitated due  to a
need  for  enhanced  portability  in  the  design  of  personal  computer  desktop  workstations.  Apart  from  its
original functionality as a backup source of power, this design incorporates the unit within the system unit
casing,  thereby  reducing  the  number  of  system  components  available.  Also,  the  embedding  of  this  unit
removes the untidiness of connecting wires and makes the whole computer act like a laptop. Not to be left
out is the choice of a microcontroller as an important part of the circuitry. This has eliminated the weight
and space-consuming components that make up an original design. The singular use of this microcontroller
places the UPS under the class of an advanced technology device.
KEYWORDS
Embedded System, Uninterrupted Power Supply, Personal Computer, Automation, Power Electronics.
۱٫ INTRODUCTION
An  uninterruptible  power  supply,  commonly  called  a  UPS  is  a  device  that  has  the  ability  to
convert  and  control  direct  current  (DC)  energy  to  alternating  current  (AC)  energy.  It  uses  a
conventional battery of 12V rating as the input source and by the action of the inverter circuitry, it
produces an alternating voltage which  is  sent  to  the  load. This particular UPS  is designed  for a
small scale load like a personal computer and hence only a basic power rate  is generated by  the
UPS. Many believe that because an inverter is operating from a nominal 12V battery and it cannot
deliver  as much output  as a normal mains power outlet,  it’s  relatively  safe. This  is not usually
true. Even a low power inverter rated at a mere 60watts has an output which is potentially fatal if 2

you become its load. Such an inverter can have a typical output of 350mA at 230V. This is above
ten (10) times the current level connected to cause fatal fibrillation and stop your heart.
Generally,  uninterrupted  power  supply  (UPS)  can  be  grouped  by  source  or  method  of
functionality.  (۱) By Source: Here we have a voltage  source  (DC)  for  its operation or a current
source (DC). The current source however is used for very high power consumption devices hence
this  design  is  a  voltage  source UPS.  (۲) By  Functionality: Amongst  others  here  is  the  single-
tracked and dual-tract UPS. The single-tract UPS  feeds  the  load continuously  from  the  rectified
DC  supply directly. This  type of UPS  is disadvantaged because a  fault  in  the  rectification stage
leads to a complete system failure. The dual tract acts like the single tract but it has a bypass that
sources from the mains supply. Hence the battery is used only as backup and does not run all the
time  unlike  the  single  track. This design  is  a dual  track methodology. For  an  ideal UPS, basic
functionality is needed (1) Being a backup utility, a UPS must ensure that there is no break in the
power supply at any point in time unless major faults like fuse cuts are experienced. (2) An ideal
UPS must provide the battery with an adequate charge so as to maintain the optimum conversion
rate to AC when needed. (3) It must also ensure overcharge protection to prevent the battery from
being damaged. (4) All forms of surges and undesired waveforms that may emanate from inverted
source  voltage  are  to  be  filtered  and well  suited  to  the  output  level.  (۵) Must  be  sensitive  to
maintain  stability when  the  battery  safe  voltage  is  being  exceeded.  (۶)  It must  also  provide  an
overload protection for the entire unit.
Many  embedded  devices  provide  a  rich  GUI-based  user  experience;  use  file  systems,
multiprocessing,  and  multi-threading;  and  include  networking.  An  operating  system  (OS)  can
provide  these  features  to  support  the  rapid  development  of  application  programs  [۱,  ۲].  In
charging a battery of  the personal computer  (PC), a cheap, unattended, unregulated charger can
destroy  a  battery  by  overcharging  it.  A  temperature  compensated  charger  is  also  highly
recommended  [۳]. Thus most power  supplies have a PWM controller based on  the well-known
TL494  [۴] or equivalent chips  (for  instance KA7500). TL494  features  two error amplifiers, but
most power  supplies only use one of  these. A PWM controller  featuring  two error amplifiers  is
recommended  in some design because one controls the output voltage and  the other controls the
output current.
However,  after  careful  consideration  of  any  existing  design  of  the  UPS  and  some  embedded
systems, this particular design incorporated the following methodology upgrades: (1) The battery
charging unit is basically handled by the micro-controller which detects in split microseconds the
point  at which  the  safe  battery  (voltage  at which  operating  the  battery  to  generate  alternating
voltage  is not  safe)  is being exceeded. This causes  the  system  to  shut down  in order  to prevent
damage to the battery. (2) Also handled by the microcontroller is the overcharge protection. The
controller disengages the battery at full charge voltage. (3) Application software interfaced via the
USB  (Universal Serial Bus)  port  of  the  computer motherboard maintains  a  constant  check  link
between  the  operating  system  and  the  UPS.  (۴)  To  enhance  compactness,  ۲-pole  relays  and
switches are used  to eliminate duplication of components. (5) Very simple and readily available
components are sourced making the device commercially viable.
For  clarity  and  neatness  of  presentation,  the  article  is  outlined  into  five  (۵)  sections. The  First
Section gives a general  introduction of a UPS and  smart embedded  systems. Review of  system
components used for this system design is presented in Section Two. In Section Three, we outline
the  design  and  implementation  procedures.  Section  Four  presents  the  experimental  results  and
discussion  of  the  results.  In  Section  Five, we  conclude  the work with  some  recommendations.
Finally, the references are presented at the end of the paper.

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