Re: Building a custom power supply

On Tue, 6 Jun 2006 17:53:45 -0700, "Richard Crowley"
<rcrowley@xxxxxxxxx> wrote:

"andyavast" wrote ...

Scott Dorsey wrote:

andyavast wrote:
Hello.
I am looking for informed advice (and opinion) on an electronics
project. I want to build the following:

A power supply with 8no. 9VDC outlets supplying 1200mA (total, not
per
outlet) and 1no. 12VAC outlet supplying 500mA.

the supply is to fit into a hammond aluminium enclosure
approximately
120mm x 60mm x 60mm perhaps a little bigger. My initial (if somewhat
uninformed) thoughts are to use two seperate chassis mount
transformers
with a full-bridge rectifier to convert the AC power to DC for the
9VDC
outlets. Inspiration is what i need. Any ideas would be really
appreciated.


Well, you need to know first of all that if you have a 12V
transformer
producing 12VAC that when you rectify it you will get 12*1.4= 16.8V.

If you are going to regulate it afterward, this is fine. You just
use
one 12V transformer and regulate the 16.8VDC down to 12V and 9V with
7809 and 7812 regulator chips.

There is a good introduction to linear supply design in the ARRL
Radio
Amateur's Handbook.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

thankyou scott! i will look into the literature you suggest.Has anyone
ever designed simple linear power supplies of a similar nature to the
one i wish to build?

Sure, no deep magic involved. Note that Mr. Dorsey
may have mis-read you requerements to want 12V DC
and not 12V AC. A single 12V transformer could source
both 12VAC and 9VDC (after rectification, filtering, and
regulation).

NOTE HOWEVER that if you are trying to power a bunch
of small gadgets with this thing (the kind that would otherwise
use individual wall-warts), you may have a SHOWSTOPPER
problem with the interaction between what each of them
perceives as their own "ground" reference. Unless all
your loads for 9V are identical devices AND you know
the internal wiring (which side is "ground", etc.) you may
not be able to do what you are seeking.

Likewise, using the same transformer for the 12V AC output
is also potentially a problem when you try to create the 9V
DC from the same source.

I agree with all of this, and even if you're powering eight
"identical" devices, it would be preferable to have an isolated power
source for each, to prevent ground loops (which can cause problems
even when they are relatively short).
I'd just put nine appropriately-rated wall-warts into a box,
appropriately mounted and wired.
However, your box dimensions appear too small for nine wall-warts.
The next idea is a smallish toroid transformer with nine windings, and
eight full-wave bridge rectifiers and capacitors.
Did you want regulated DC? Perhaps a single 12VAC winding which
could also power a transformer-isolating switching regulator
(something like LT1072 datasheet page 10) with eight secondaries, each
powering the usual high-frequency half-wave diode and capacitor
filter. If you're powering audio devices (as one might presume since
you're posting on RAP), you will likely need more and better
filtering.


While building this is not really that difficult, making sure
that it will work properly with all you loads is a very
different matter. I would urge significant caution.

.

Relevant Pages

  • Re: Voltage shedding question
    ... in order mostly to learn how to design simple power supplies. ... caps and the regulator. ... For the regulator and transformer I'm using, ... Here's the circuit: ...
    (sci.electronics.basics)
  • Re: Transformer -- rated power
    ... the 330ma is there and very real because that is what my VOM is reading, so WHY does my 500ma transformer feel so cool with the switching driver? ... Its regulates it's output current to the 330ma you are reading but if you read the input current to the regulator it will be much less. ... Forgive the horrendous simplification but essentially the regulator is a power converter and is controlling the current output by increasing the output voltage to whatever is needed in this case about 3 volts which is roughly equal to 1 watt. ...
    (sci.electronics.design)
  • Re: Power Supplies again ;/
    ... >>>frequency compared to switching transformer counterparts. ... >> what about of power? ... then I could use a regulator to vary those two and get more ... > Harder to make, but more efficient, is a switching regulator. ...
    (sci.electronics.design)
  • Re: Replacement transformer
    ... Designing power supplies is a cookbook process. ... The transformer you linked to is vast overkill, ... puts out more than 12 volts. ... taken up by losses, particularly in the regulator. ...
    (uk.rec.audio)
  • Re: where do i get a transformer
    ... You said the outlet works and you tested for power after the ... Did you test for power BEFORE the transformer? ... line fuse or rotted power cord would be my first guess. ...
    (rec.games.pinball)