i am new, power transformer question

Archiver

hi all

i want to make my own power supply.
the powe supply has three outputs(2 positive, 1 negative).
output voltage is 25V to 30 V @ 3A.
but i do not know what kind of power transformer to choose.
a secondary or multi secondary?
if i use single secondary to get three outputs, can i use 2 bridge
rectifiers(one for positive, the other one for negative)?
in this way, will i get 3A from all outputs?
or is there anyway?
how do i earth the green/yellow wire on the secondary of the
transformer?
how do i ground the ground of 3-prong power cord?
just mount to the chassis?
maybe i am asking to many questions.


thanks

Archiver

First off you need to understand the basic AC transformer. So first on the list
is to buy a transformer....for that I'd go to http://www.allcorp.com/

Lets say we went with a 24 Volt 4 amp model....
http://www.allelectronics.com/cgi-bin/category.cgi?category=790&item=TX-244&type\
=store

One side will have 2 wires...that the AC in....take many precautions on that
side...120AC can kill you!! Best to treat it like an angry snake!
The wires on this side can go either way...it's just a big coil.

On the other side you will have 3 connections. The inner one is called the
Center Tap. If you take a meter and measure the AC from center to either side
you will get 12 volts. What! Not 24! It's a half way point on the coil so it
gives you half of the voltage....tricky huh?! If you connect your meter to the
outer two you will get 24 Volts AC.

Now AC is not going to work for you so we run that through the Bridge
Rectifier....you get pulsed DC from that. So a nice big CAP is put on the
output so we can filter that out. 1000uf+ works...just make sure it's rated for
more then 36 volts. Why not 24V DC?! That 24 is an RMS average of the
voltage....it peaks at 1.44 x 24 = 34.56....better have the head room for
that...

Ok...so now we have 30 something volts....it's filtered and the cap will bring
it up just a bit closer to that 34 volt maximum. I'm not to concerned with all
the math right now....but it's going to be more then 24V.

We take that and run it into a Regulator....it's what really dictates the amps
you can handle. 1 amp regulator can't handle a 3 amp load. You can stack
them...but that's another story. Once it's out of the regulator we put a small
cap on the output to filter a bit and we are done.

If you want Negative voltage that's going to take a bit more.... Take a look at
this kit....the PDF shows all [noparse]:)[/noparse]
http://www.ozitronics.com/docs/k83.pdf

And last but not least....you could just buy a ready made power supply for very
little cash....are you sure you want to build one??

>>> i_drives@y... 11/21/03 02:41PM >>>
hi all

i want to make my own power supply.
the powe supply has three outputs(2 positive, 1 negative).
output voltage is 25V to 30 V @ 3A.
but i do not know what kind of power transformer to choose.
a secondary or multi secondary?
if i use single secondary to get three outputs, can i use 2 bridge
rectifiers(one for positive, the other one for negative)?
in this way, will i get 3A from all outputs?
or is there anyway?
how do i earth the green/yellow wire on the secondary of the
transformer?
how do i ground the ground of 3-prong power cord?
just mount to the chassis?
maybe i am asking to many questions.


thanks



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Archiver

This also falls under the category "If you don't know, then
you shouldn't be doing this".

Power supplies can be tricky, and if you don't know some of
the basics, then 110V can be very dangerous!

There are tons of books out there on designing power supplies,
pick one up and be safe! (er)

-Beau Schwabe

>hi all
>
>i want to make my own power supply.
>the powe supply has three outputs(2 positive, 1 negative).
>output voltage is 25V to 30 V @ 3A.
>but i do not know what kind of power transformer to choose.
>a secondary or multi secondary?
>if i use single secondary to get three outputs, can i use 2 bridge
>rectifiers(one for positive, the other one for negative)?
>in this way, will i get 3A from all outputs?
>or is there anyway?
>how do i earth the green/yellow wire on the secondary of the
>transformer?
>how do i ground the ground of 3-prong power cord?
>just mount to the chassis?
>maybe i am asking to many questions.
>
>
>thanks

Archiver

Hi,
If you go to your local Radio Shack Store they have a great book on building
your own power supply. I don't know the number but it is worth the $7.95 (when
I bought it a few years back). It will answer all your questions.

Alan Bradford
Plasma Technologies
PS; I do not own Radio Shack Stock!

In a message dated 11/21/2003 3:50:09 PM Eastern Standard Time,
i_drives@y... writes:
hi all

I want to make my own power supply.
the powe supply has three outputs(2 positive, 1 negative).
output voltage is 25V to 30 V @ 3A.
but i do not know what kind of power transformer to choose.
a secondary or multi secondary?
if i use single secondary to get three outputs, can i use 2 bridge
rectifiers(one for positive, the other one for negative)?
in this way, will i get 3A from all outputs?
or is there anyway?
how do i earth the green/yellow wire on the secondary of the
transformer?
how do i ground the ground of 3-prong power cord?
just mount to the chassis?
maybe i am asking to many questions.


thanks



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Body of the message will be ignored.


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[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

Reading your comments seems to me you are a bit over your head.
Why do not you just buy a already built Power Supply?
Save your sanity and possible your life.

Archiver

One last follow up to the power supply question;

Here is the Radio Shack Book Number

Title Building Power Supplies
RS # 62-1337

It is a good reference from theory to building Power Supplies.

Alan Bradford
Plasma Technologies


[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

--- In basicstamps@yahoogroups.com, "Albert Catano"
<acatano2002@y...> wrote:
> Reading your comments seems to me you are a bit over your head.
> Why do not you just buy a already built Power Supply?
> Save your sanity and possible your life.


Very good advice.


if I were in learing mode, I'd start with 2 seperate transfromers.
one with a center tap, so I could get the +/- voltage and a second
with 5V output.

then all one needs is a rectifier for each, a capacitor for each, and
voltage regulators and presto, 3 seperate voltages.

If, however, I was in customer mode, I'd buy an old model laptop wall
wart, that has +12, -12, and +5 VDC with one cable and plug output.

Dave