DC Supply! Monthly Electrical Ezine

CBC Design (tm) - July 2002 Issue. ISSN 1475-3464
Email: cbc_design@btconnect.com
http://www.cbcdesign.co.uk

"...Maintaining a reliable DC supply."

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IN THIS MONTHS ISSUE

- Editorial
DC-AC Inverters. (Article)
Linear Regulators. (Article)
- Competition - Win FREE battery pack!.
- Readers Questions
- Subscriber Ads

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EDITORIAL. 

We
lcome 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.
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 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:

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Simply send your email address (No free email) to cbc_design@btconnect.com with the words
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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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