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12v 20a power supply

Ashley Williams

Sep 21, 2011
1
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Sep 21, 2011
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1
Hi guys

Got to make a project for college ive decided i want to make a power supply. I want a 230v input and a 12v output (13.8v) capable of supplying 15 to 20 Amps. Ive had a look around the web for a schematic but im struggling to trust some of the drawings. Want to keep it as simple as possible. But with a steady DC supply.

Any help or advice is greatly apprechiated.

Cheers Ashley.
 

davelectronic

Dec 13, 2010
1,087
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Dec 13, 2010
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1,087
Bench top power supply

Hi Ashley.
Welcome to the forum from me, i build a far few power supply's, and modify others.
I think there is plenty of good projects on the net, most can be trusted, a break down of the circuit in question, if you choose an already established design will soon tell if its any good.

There are a few threads on the forum active already, but i fully understand you wanting your own brief for a power supply.

So i expect other members will join your thread, and have some ideas, as i said my source of schematics is the internet, and written designs, searching the web will produce a wealth of projects, unless you want to try your own design, but for this your need to post some psu requirements etc.
Dave. :)
 

davenn

Moderator
Sep 5, 2009
14,016
Joined
Sep 5, 2009
Messages
14,016
Hi guys

Got to make a project for college ive decided i want to make a power supply. I want a 230v input and a 12v output (13.8v) capable of supplying 15 to 20 Amps. Ive had a look around the web for a schematic but im struggling to trust some of the drawings. Want to keep it as simple as possible. But with a steady DC supply.

Any help or advice is greatly apprechiated.

Cheers Ashley.

Hi Ashley
welcome

show us the links to some of the ones you have found and then us guys can let you know which would be the most suitable, or if there are errors in the circuit diagrams etc

cheers
Dave
 

Digital_Angel_316

Oct 1, 2011
41
Joined
Oct 1, 2011
Messages
41
Ashley,

Is this a required class project in fulfillment of graduation requirements or is it a hobby
project? If the former, please consider the following part of the post. If the latter, ignore
the following other than for reference.

University Projects should involve a process -- see referenced thread at the end of this
post for more ranting. A power supply 'design' must come from a power supply
specification, and should include the considerations below (Why 12V and not 5V or 5V
AND 12V for example?). The general approach to a design project again being posted
below. In addition, often a 'proposal' for a project would be drafted that would include
much research, market analysis and tradeoff discussion that leads to a functional
specification.

Some Power Supply Design Considerations / Specification Parameters:

Input Voltage Requirements and Tolerances

Input specifications refer to what the power supply requires for its electrical power
input--in other words, what it wants to see coming from the wall, or from your UPS.
Most electrical input specifications will be provided as a range, because while the
power supply may require 115 V as input it of course doesn't need exactly 115 V. The
range of acceptable values on a specification are sometimes called the specification's
tolerances.

Input Voltage Range: For example: "85 to 135 V AC" and "170 to 270 V AC".

Voltage Selection: If the power supply supports both 115 V and 230 V nominal
voltage, does it automatically select between them, or is there a manual switch?

Frequency: Acceptable frequency of input power (50 Hz, 60 Hz, or 50 and 60 Hz).
Alternately, a range of acceptable frequencies (for example, 48-62 Hz). Most power
supplies can handle both nominal 50 Hz and 60 Hz input.

Power Factor:
The power factor that the power supply presents as a load to the utility
power line. Normal power supplies will be in the 60% to 70% range (0.6 to 0.7). Power-
factor-corrected supplies will have a number like "0.99".

Electrical Characteristics

The electrical characteristics of the power supply describe the quality of the power
supply's outputs, and its ability to handle special situations such as disruptions or
disturbances to its input power, or variations in the loads the power supply drives.

Hold (or Hold-up) Time: Probably the most important electrical characteristic, this is
the amount of time the power supply will keep producing its output, if it loses its input.
A typical figure is about 20 milliseconds (the energy-storing components within the
power supply are what allow this number to exceed zero.) This value indicates the
length of a blackout that the power supply may be able to tolerate before dropping the
Power Good signal.

Load Regulation: Sometimes called voltage load regulation. This specification refers
to the ability of the power supply to control the output voltage level as the load on the
power supply increases or decreases. The voltage of a DC power source tends to
decrease as its load increases, and vice-versa. Better power supplies do a better job of
smoothing out these variations. Load regulation is usually expressed as a "+/-"
percentage value for each of the voltages the power supply delivers. 3% to 5% are
typical; 1% is quite good.

Line Regulation: The complement of load regulation, this parameter describes the
ability of the power supply to control its output levels as the level of the AC input
voltage varies from its minimum acceptable level to its maximum acceptable level.
Again, a value for each output level is usually specified as a "+/-" percentage. +/- 1% to
2% is typical.

Ripple:
Also sometimes called "AC Ripple" or "Periodic and Random Deviation
(PARD)" or simply "Noise". The power supply of course produces DC outputs from AC
input. However, the output isn't "pure" DC. There will be some AC components in each
signal, some of which are conveyed through from the input signal, and some of which
are picked up from the components in the power supply. Typically these values are
very small, and most power supplies will keep them within the specification for the
power supply form factor. Ripple values are usually given in terms of millivolts,
peak-to-peak (mVp-p).

Transient Response: As shown in the diagram here, a switching power supply uses a
closed feedback loop to allow measurements of the output of the supply to control the
way the supply is operating. This is analogous to how a thermometer and thermostat
work together to control the temperature of a house. As mentioned in the description of
load regulation above, the output voltage of a signal varies as the load on it varies. In
particular, when the load is drastically changed--either increased or decreased a great
deal, suddenly--the voltage level may shift drastically. Such a sudden change is called
a transient. If one of the voltages is under heavy load from several demanding
components and suddenly all but one stops drawing current, the voltage to the
remaining current may temporarily surge. This is called a voltage overshoot.

Transient response measures how quickly and effectively the power supply can adjust
to these sudden changes. For example, a specification might be: "+5V output returns
to within 5% in less than 1ms for 20% load change." What this means is the following:
"if the output is at a certain level (call it V1) and the current load on that signal either
increases or decreases by up to 20%, the voltage on that output will return to a value
within 5% of V1 within 1 millisecond". Obviously, faster responses closer to the original
voltage are best.

Peak Inrush Current / Input Surge Current: The absolute maximum amount of
current that the power supply will draw in the moment after it is initially turned on. This
is sometimes used to indicate how much "shock" the power supply is subjected to
when it is turned on. Lower values are better.

Overvoltage Protection: In addition to specifying a normal maximum voltage level,
good power supplies will include protection against the output voltage exceeding a
certain critical level. If for some reason the voltage lines goes above a certain value, the
power supply will shut down that output. The number is usually expressed as a "trip
point voltage" (for example, +6.25 V for the +5 V line) or a percentage (which for a trip
voltage of +6.25 V would be 125%). The specification will also say what the power
supply does when an overvoltage is detected; usually, it will reset.

Overcurrent Protection: If the power supply's outputs exceed their maximum ratings,
some power supplies will detect this condition and reset the unit. The supply will
specify what percentage over the maximum rating for each voltage output will cause
this to occur.

Output Specifications

The most important specifications that you will find listed for most power supplies are
probably those that relate to its output signals. The reason for this is pretty obvious:
providing the output voltages are what the power supply exists to do. Carefully check
over all the output specifications for any power supply that you are considering.

Some manufacturers will list separately the values for each of the specifications shown
below. Other manufacturers may provide a table that shows all of the relevant output
statistics (and sometimes, some of the electrical characteristics of each voltage level
at the same time). There's really no difference, other than how the information is
presented.

Output Rating (Watts): The nominal, total, maximum output of the power supply in
watts. This is actually sometimes not even supplied in the specification sheet; the
name of the power supply will usually have a number in it that is supposed to represent
this value, and sometimes even does.

Output Current Ratings (Maximum Load By Voltage):
The maximum amount of
current provided by the power supply at the specified voltage level.

Minimum Current Ratings (Minimum Load Requirement By Voltage): The
minimum amount of current that must be drawn by loads, for each voltage level it
provides, in order for it to function properly.

Peak Output: The amount of current that the voltage specified can supply for a limited
amount of time. Ideally, specify not only the peak output current but the amount of
time the supply is rated to sustain that peak. For example, the continuous maximum
for 5V may be 20 A, the peak level 24 A, and the peak level may be sustainable for 10
seconds.

Output Voltage Range: For each output voltage, the range that the power supply
guarantees its output to be within. Power supplies can't say that they will produce, for
example, exactly +5.000 V. There's a range, and that's not a problem since systems
are designed with this in mind. Generally speaking, the smaller the range the better.
Either specific numbers will be provide (e.g., +4.8 V to +5.2 V) or a "+/-" percentage
will be given (which would be +/- 4% to result in a range of +4.8 to +5.2 on a +5
voltage.)

Efficiency: What percentage of the total energy supplied to the power supply is
converted to usable form by the power supply and conveyed to the load. Typical
numbers for power supplies may be ~ 60% to 85%; the other 15% to 40% is wasted as
heat. Clearly, the more efficient the power supply, the better! Not only will you save
electricity, you will ensure that the power supply runs cooler at the same time, making
the supply's components last longer and the system work better overall. Efficiency is
probably more important for supplies that provide a lot of power, since the percentages
equate to larger numbers.

Power Good Delay:
The typical time from when the power is applied to the supply,
until the Power Good signal is asserted. The specification may also specify a minimum
and maximum time.

Auto Restart: If the power supply supports automatically restarting the system after an
AC power failure, this must be mentioned in the specifications.

(excerpted and edited from - http://www.pcguide.com/ref/power/sup/spec-c.html)

SEE THE POST IN THIS FORUM ON ANOTHER THE THREAD REGARDING DESIGN APPROACH FOR PROJECTS:


I would start the project with a Requirements Specification, your professor
(teacher?) should require it. Detail the features and functions that are
required. A rough draft outline may be just a page or two. Follow up when
you have had a peer review with others on your team to finalize at least a draft
copy. Make a table of Frequencies, Times, User Interactions, Outputs and
Processing. It may contain a simple block diagram and flow chart of the
requirements (a processor is not a requirement, it is an implementation detail).
You should also develop a simple SCHEDULE and BUDGET outline. This should
be presented to your professor as a key milestone of your work effort and each
subsequent design phase should also go through peer review and higher level
(professor, boss, client, customer) review and acceptance.

The next step is to develop a Design Specification. This will take the
REQUIREMENTS and relate them to an implementable design.
It is not until
this step that you concern yourself with choosing components. The DESIGN
implementation is inferred from the REQUIREMENTS.


From there you can begin to think about design implementation details. What
input devices, what processor, what output mechanisms (switches,
Ucontrollers, speakers, printers, recording media, packaging, layout etc.)

Finish up with a Design Disclosure document. This will describe the theory of
operation, have a block diagram and flow chart and walk through the
operation. Much of it can be pulled from the Requirements and Design
Specifications if done properly.
This will also include a Schematic, Code
listings Bill of Materials, and perhaps operating instructions (to save you from
writing a 'Users Manual' which some might also require depending on the
project scope)

The PROCESS is just as (more) important than the implementation. You could
do it on a PC, in a cell phone app or a dedicated device you design and build.
The requirements will drive these sorts of selections and involve Research and
Tradeoffs.


Sorry, I took out all the fun and haphazard approach. It is a real discipline to
walk through the design process. In the end it saves a lot of time and money
from false starts, misunderstood 'requires and desires' and provides greater
satisfaction to the designer (or design team) and most imortantly to the
'customer' / end-user.


Time and cost constraints are a part of the tradeoff process and the above
mentioned process is not completely linear / sequential. You may already
have a favored platform or development tools, or expertise for example that
leads you to 'require' certain implementation details -- same concept with
budgetary aspects.

Best wishes and Godspeed. Get back to us when you are further into the
project.
 

shiekh

Oct 11, 2010
77
Joined
Oct 11, 2010
Messages
77
Want it simple, use an iron core transformer, rectifier and smoothing capacitors; heavy, expensive and inefficient.

Want to go modern, use a switching mode approach where the mains is rectified, switched at high frequency to allow the use of a small efficient and cheap transformer. Much more complicated, but cheaper, lighter and efficient.
 

davenn

Moderator
Sep 5, 2009
14,016
Joined
Sep 5, 2009
Messages
14,016
Want it simple, use an iron core transformer, rectifier and smoothing capacitors; heavy, expensive and inefficient.

Want to go modern, use a switching mode approach where the mains is rectified, switched at high frequency to allow the use of a small efficient and cheap transformer. Much more complicated, but cheaper, lighter and efficient.


You do realise that this thread is some 4 months old ?? ;)

cheers
Dave
 
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