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AC-DC power supply conversion - transformer selection

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Junior Member level 1
Jun 27, 2006
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transformer selection

I am designing one mini project.It requires AC-DC conversion as similar similar one used in the following university project.
In this one, i have to select one transformer (power transformer) for my project.
Application: AC-DC conversion. this transformer will be used to de-couple DC part from AC part.After this transformer section, it will be rectified using full-wave rectifier ( yet to be decided).I am new to this transformer selection.What are the parameters to be considered for the transformer? Advance thanks for your support...

transformer selection rectifier

The photo given in the link is a capacitor based design capable of delivering maximum of 50 ma of current. You may use any step down transformor for the said purpose. As in India, you'll get various brands with different qualities. You may also have a choice between torroidal and regular laminated transformors for better performance.
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exceeding amperage rating on dc transformer


The criteria for selecting mains transformers is as follows:-
1) Primary Voltage - I assume you know what your local supplies are!
2) Rectifier type - half-wave (one diode), full-wave (two diodes, centre-tapped transformer) or bridge rectifier (four diodes or a ready-made unit and a single untapped winding). Minature transformers are usually intended for full-wave 2-diode operation but for larger designs a bridge rectifier is more efficent.
3) Power rating of transformer. Most small transformers can be treated for calculation purposes as 75% efficent, so the VA rating needs to be:-
Vsec x Isec x 1.25. for example, let us assume you want to design a 12V 1A d.c. output power supply using a bridge rectifier and a three-terminal I.C. of the 7812 type, you would proceed as follows:-

D.C. Power out = 12 x 1 = 12VA.
For bridge rectification the r.m.s. current rating needs to be 1.414 times the D.C. rating, so we need a 1.5A transformer secondary.
The smoothed D.C. voltage from the rectifier is approx. 1.25 times the r.m.s. rating of the transformer for most small hobby projects so we calculate the required voltage as follows:-
Vsec(rms) = (Regulated DC + Regulator Drop + Ripple voltage + Rectifier Drop)/1.25
For a 7812 the minimum recommended input is 17.5V so, assuming 2.5V ripple and ordinary silicon diode rectifiers:-
Vsec(rms) = 17.5 + 2.5 + 1.4/1.25 = 21.4/1.25 =18V approx.
Choose a 18V 1.5A transformer, therefore VA needed is 27VA minimum.
Allowing for losses, a transformer rated at 18V r.m.s and 30VA or more would be adequate!!!
Can I also refer you to Duncan's PSU Designer on his Tube Amp site, by the way - this freeware tool illustrates the example I've given!!!
Hope this helps!!!
Chris Williams
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power supply (1a transformer)

Hi Chris,
Thanks for your valuable guidance.It helped me a lot to understand this AC-DC practical implelmentation much better. but it's exceeding my buget drastically :| . So i am considering alternative implementation topologies. one of the topology, which i am considering is the one which is used in ACme (that university project).But i am wary of that one ( in my circuit project, DC load ( ~<1A@5V ) and AC line may have current from 15A to even 100 A.) what are the pros and cons of that topology (i was worring about isolation thing; so i opted for transformer apporach earlier ) ? any other low-cost topology for this application requirment?
Advance thanks,

dc power supply 2-rectifiers

nramesh said:
( in my circuit project, DC load ( ~<1A(at)5V ) and AC line may have current from 15A to even 100 A.) what are the pros and cons of that topology (i was worring about isolation thing; so i opted for transformer apporach earlier ) ? any other low-cost topology for this application requirment?
It seems you are confusing the current ratings. ~<1A(at)5V is no big deal. Simple googling may bring many solutions to chose from.

power suply using 1a transformer

Let me set things straight..
This is what my idea
i am basically electronics guy with more exposure towards digital side.I visited one factory and happened to be present in one of the operational issue .. Many machines are being used with wide range of current consumption (1A to 110A at 240Vac). cost of power drawn seems to be more than it should be (some manual approximate calculation and difference is too high).. so i got idea to measure current of each equipment and summing up it. I came across this university project. but this wireless is not an option here and doing workaround it which involves human involvement.
This small piece of circuit should measure the current consumption (and i decided how to do it).But that measurement section requires DC supply of 3V/5v (with maximum of 1A [yes; there are ways to reduce this one also]) for its operation.
So here this AC-DC conversion comes here. So I have to use the same AC supply line for DC voltage generation.
My concern/requirement is -
1) how to tackle transients (too many power -cuts and power surges) from spoiling this DC section
2) Safety of the usage
3) Very low cost

Yes; googling provided some valuable exposure to some 'good practices to PSU designers’.. But looking at that university project circuit, i am worried of like 'something is missed-out?!?!!’
here back to main business....
What is the low-cost (but no-compromise on safety) solution for connecting that (high current) AC line to my rectifier section? If possible, can you please provide (likely) circuit .
Transformer isolation is not viable option for me..
Looking forward for your valuable guidance..
Thanks in advance...

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