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EDITORIAL.
Welcome to our July issue
of DC Supply.
This is our eleventh issue
of "DC Supply", the last issue before our
first aniversary. We would like to thank
each and every one of your
for your support and look forward to
helping you over the coming year.
Following on from last months article, DC-DC
Converters, we look at
DC-AC inverters in this issue, designed to
produce AC from a DC source.
Our article on Linear Regulators looks at
one of the oldest methods of
generating a smooth DC supply. The linear
regulator is still the most
popular
control methods used in AC-DC generation due to its
inherent
low
noise characteristics coupled with excellent regulation. We look
at
the
control method and compare the linear supply to its
switch-mode
counterpart.
We hope
you enjoy this months issue.
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Editor: Alan Fidler.
Alan is the owner and manager of CBC
Design, a leading battery management company
based in the UK. He has worked in the
industry for over nineteen years and has designed
charging equipment and battery monitors for some of the world
largest companies.
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ARTICLE: DC-AC
Inverters: Alan Fidler.
The
DC-AC inverter is a solid state unit designed to convert DC power
from a
battery
or power supply into AC power, similar to that of a standard
domestic supply.
Utilising switching technology, the dc
supply is chopped and fed to the primary
of an
iron core transformer. The secondary output is monitored and
controlled to
ensure
that the ac output from the inverter lies within prescribed limits,
usually
+ or -
10% of nominal.
Unfortunately, there is a functional
parameters that will vary from one model
to the
other, specifically, the quality of the output waveform. A domestic
110 or 230V
50 hz
supply is sinusoidal in nature and of a reasonably smooth quality.
Inverter outputs
can be
sinusoidal but are more often than not, trapezium. In layman's
terms, the latter
is a
rough approximation of a sinusoidal waveform and cannot be used on
equipment
that
requires a good clean AC supply such as a laptop computer for
example. They are
however
perfectly suitable for lighting or heating
applications.
In
deciding then which type of inverter to use, the first factor that
must be considered
is the
quality of the output supply. For hifi, tv or computers, an inverter
with a clean
output
supply is preferred. This prevents the equipment from overheating
due to the
rather
choppy signal supplied by the trapezium models.
The next
consideration is output power. This is calculated by dividing
the operating
power of a specific item by its supply
voltage to find its current requirement. Add
together
the
total of all the items you want to run from the inverter and you
have a current rating
for your
inverter.
The
third consideration is input power to the inverter. A 10A 230V
inverter will require a
little
over 208 amps at 12VDC or just over 104 amps at 24VDC so you will
need a hefty
set of
batteries to run a 2.3KW inverter for any length of time. For 1 hour
of operation,
a 12V
battery pack will need to have an amp hour rating of around 270AH
allowing for
losses.
The 24V battery pack will have to be sized at 130AH or so. The amp
hour required
for a
number of hours can be calculated from the
above.
I am
sure the above figures look quite staggering to some of you who
use inverters in
their
marine or industrial applications but this highlights the
biggest problem with DC-
AC
conversion. To generate 2.3KW at 230V, 2.5KW allowing for losses,
needs to be
supplied
to the input of the inverter. At 12VDC, this is approx 208 amps
hence the
large
battery size required. If you use inverters on boats or campervans,
the battery
can be
easily discharged if the AC supply is used for more than a few
minutes at a
time.
Cost
wise, an inverter with a sinusoidal output will be more expensive
than an inverter
with a
trapezium output. The techniques required to generate a clean AC
supply from a
DC
source are both expensive and difficult hence the cost difference.
Don't be surprised
if this
difference exceeds 100%, you get what you pay for and smoothing
costs!
There
are a huge number of inverter manufacturers from which to choose
from and it
pays to
shop around to get the best deal. Talk to as many suppliers as
possible and
don't
forget to ask them about the quality and type of output waveform. If
you use
an
inappropriate version, you may damage the equipment you connect to
it so you
must get
it right the first time.
Lastly,
make sure the cables that you intend using to connect up your
inverter
are
sized appropriately. The dc cables will need to be very large to
handle to
current required by the inverters input
stage.
______________________________________________________________________
______________________________________________________________________
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______________________________________________________________________
ARTICLE: Linear Regulators. Author: Alan
Fidler.
Linear regulators are without doubt the
most widely used anywhere in the industrial
world. Renowned for their low noise
high regulation characteristics, they are particularly
well suited to low power dc supply
applications.
Unlike a switch mode supply which is
described in an earlier issue, linear
regulators
work in a less efficient but inherently
smoother manner. The input supply is
derived
from an isolating transformer, rectified,
smoothed and fed to a regulating
transistor.
The transistor base voltage is maintained
at approx 0.6V above the output voltage
required thus regulating the transistor
emitter voltage.
Unlike the switch mode supply which is
inherently noisy, the linear output is
very
smooth with ripple levels of a few tens of
millivolts. Switching a supply generates
noise so the linear output is smoother than
a switch mode system because
of
the absence of any
switching.
The downside of linear regulation is
dissipation. A transistor with a 20VDC input
and
a
12V output at 1 amp will generate 8 watts of heat. The wasted power
is self evident
and the linear supply is therefore larger
than its switch mode equivalent due to the
extra
heat sinking required and much less
efficient too. A switch mode psu will have a
typical
efficiency of 87% or higher against the
50-60% efficiency of a linear model.
At low
power levels, efficiency is barely worth consideration though, which
is why the linear
supply
is still popular in many PSU and battery charging applications.
Indeed, millions of linear power supplies are sold every year
throughout the world and dispite the hype, switch mode
supplies cannot compete with linears at the low power end of the
market due to the costs of high frequency
components.
If you
are looking for a low power dc supply with good regulation at a
reasonable cost, a
linear
based PSU will certainly meet your requirements.
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______________________________________________________________________
CLASSIFIED ADS:
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______________________________________________________________________
COMPETITION:
Subscribe to our ezine and you
will be automatically entered into our competition where you can win
a FREE Nicad "AA" battery pack.
Simply send your email
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subject line.
______________________________________________________________________
READERS QUESTIONS.
Questions from Dawn
Sheppard!
Question 1.
What does PSU stand
for?
PSU is the abbreviated term
for Power Supply Unit.
Question 2.
What is
a Thyristor?.
A
thyristor conducts in one direction only at a given period
of
time. In
laymans terms it is a one way valve that can be
turned
on and
off at will.
______________________________________________________________________
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