Welcome to the September issue of "DC supply".
In this months issue, we look at Switch-Mode Power Supplies in detail, how
and why they work, the pros and cons of using them and typical application in
which they can be utilised.
The second article looks at Nicad batteries and how to get the best
performance from them. It separates fact from fiction and explains exactly
what is meant by the term "Memory" and "Thermal Runaway"
which are two of the most mis-understood terms used to explain premature Nicad
In our October issue we will be publishing a step-by-step guide on how to
prepare and maintain your leisure batteries over the coming winter months.
Between now and then, we invite our subscribers to submit suggestions that
can be included within this guide.
Now onto our competition. Last months winner was Ronald Wood from
the UK. Congratulations to Ron. This month’s competition prize is a free
Nicad battery pack "AA". All our subscribers are entered into the
competition each month automatically so good luck to you all!
We hope you enjoy this month’s issue and welcome any comments, questions,
suggestions or relevant articles you may have.
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 seventeen
years and has designed charging equipment and battery monitors for some of the
world largest companies.
ARTICLE: Switch-Mode Power Supplies. Author: Alan Fidler.
A switch mode power supply is probably the most efficient type of PSU
available. Their compact size, reduced weight and highly efficient
performance contribute to their popularity.
HOW THEY WORK
Switch mode supply function sounds complicated but can be easily explained
using the following analogy: -
If a tap on a water tank has a flow rate of 1 gallon per minute and the tap
is left on for a full 60 seconds, 1 gallon of water has left the tank. If the
tap was switched on for 30 seconds then switched off for the remaining 30 seconds,
half a gallon of water has left the tank. The tap has been operating on a
50/50 duty cycle.
Switch mode supplies work in a similar manner. On a switch mode PSU a DC
supply is connected to a transistor working as a switch. With a 15V supply and a 50/50 duty
cycle, the output voltage would be 7.5VDC. By adjusting the ratio between the
off period and on period, the output voltage can be adjusted and controlled.
This is a technique knows as pulse width modulation.
Of course in practice, the switching occurs thousands of times per second and
smoothing filters are required to turn the square wave output signal created
by the switching into a smooth DC supply.
The efficiency of these psu's is the result of the switching action. When the
transistor is off, no current is flowing and dissipation (generated heat) is
zero. When the transistor is fully on, current is flowing but the volts drop
in the transistor is small so dissipation is low. In fact, most of the heat
is generated during the transistors transition from one state to another and
this is why the transistors are switched VERY quickly indeed, normally in
Most supplies operate at frequencies between 20kHZ and 120KHZ. Unfortunately,
switching any electrical device on and off thousands of times per second
generates noise so special filtering techniques are employed to dampen it
down as much as possible. With the right filters, noise levels of less than
50mV Peak to Peak are perfectly feasible.
Overall, an efficiency of over 85% can be achieved in a supply capable of
delivering several kilowatts making them an ideal supply for the vast
majority of applications including telecommunications; chargers and general
DC supply function.
Another principal to which switch mode lends itself is in the generation of
stable DC supplies from a variable input source. A 12v battery operating over
a 10-15 volt range can be turned into a stable 12V supply that varies by less
than 1%. Supplies of this nature are called DC-DC converters and are used on
electrical appliances that require a voltage critical source.
The input supply on a DC-DC converter is switched at high frequency to the
primary stage of a high frequency transformer. The transformer output voltage
is defined by the ratio of primary (input) to secondary (output) turns on the
transformer. By varying the pulse width, the secondary output voltage is
(rectified using a high speed bridge rectifier and smoothed using capacitors)
If efficiency, size reduction & minimal dissipation are important in your
PSU application, switch mode is an ideal solution
Do you use non-standard batteries that are difficult to obtain through retail
CBC Design can supply a wide range of special application cells for
everything from camcorders & laptop computers to calculators, watches and
Go to http://www.cbcdesign.co.uk/chargers/order.html
and complete the details including the type of battery you require.
ARTICLE: Nicad Batteries: Author: Alan Fidler.
Nicad batteries have been around for many years and yet in all that time,
many misconceptions have arise regarding what they do and how best to
Nicad (Nickel Cadmium) are manufactured in a wide variety of shapes and sizes
and capacities. The range is so wide that the typical "AA" cell is
available in numerous capacities for low to high power applications. It
stands to reason therefore that selecting the right battery is more
complicated than simply selecting a battery of a particular size.
Nicad batteries work by converting chemical energy into electrical energy
when discharged and visa versa when recharged. A single cell has an anode and
a cathode and a number of cells can be series connected to create a battery.
The exact operating principal is quite complicated and a detailed explanation
is beyond the scope of this article.
One thing is for certain; more batteries are destroyed by poor charging
techniques than any other factor. Under charged cells will never perform
to specification whilst over charging cells creates a lot of internal heat
which is equally damaging. A third problem dubbed "Memory" is
sometimes responsible for a lack of performance in Nicads.
Under charging your battery will result in poor performance, i.e. a lack of
capacity (C). In general terms, the capacity of the battery multiplied by 1.2
equals the total recharge current required to bring the battery back to the
fully charged state. A slow recharge at C/10 for 12 hours is generally
recommended. Nicads can be charged more quickly but can sometimes get quite
warm in the process, which leads to another condition called thermal runaway.
Thermal runaway occurs in over charged batteries. The heat within the cell causes
the battery voltage to fall, which results in yet more heat due to the
increasing dissipation within the cell structure. Obviously a fixed recharge current level helps
to limit the effects of this phenomenon but the battery capacity will not be
100% on an overcharged cell. Indeed in extreme cases, the batteries can be so
badly damaged that they fail completely. The best way to avoid this condition
is to not over charge your batteries in the first place
"Memory" is a term that has been given to Nicad batteries which develop
an inherent inability to deliver more current that they are used to
delivering due to repeated use at a fixed duty, or at least that the theory.
The simplest way to avoid this is to discharge the cells completely then
recharge them using the cell manufacturers recommended recharge regime.
In last months issue, I suggested cycling Nicad batteries on a regular basis
to avoid the problems mentioned above. Experience suggests that this is the
best way to keep your batteries in tip-top condition. A number of individuals
have commented that this can lead to premature ageing of the cells. This is
indeed true if you over cycle them so limit your conditioning cycles to once
per month and they should last for several years! With the right care, it is
possible that nicads will continue to work for anything up to 15 years.
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Questions from Duncan Harding! Is DC dangerous?
12 or 24V DC supplies are reasonably safe as long short circuits are avoided.
From 48VDC upwards, DC becomes increasingly unsafe. At 110VDC there is a very
real possibility of electrocution if you come into direct contact with the
Question2! What are cells?
A battery is made up of a number of cells. An AA Nicad is a one-cell battery
whilst a car battery is made up from 6 lead acid cells connected in series, i.e.
positive to negative.
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