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.

Andy

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."

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?

This is a great forum by the way...
Andy

"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.

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.

On Tue, 6 Jun 2006 17:53:45 -0700, "Richard Crowley"
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.

Ben Bradley wrote:

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.

That sounds like a disaster to me, both with creating a big magnetic
field (hummmmmmmm) and the fact that you don't have a common ground.

What's a good idea for a device like this, however, is to use a
separate regulator chip for each output. They can be different voltages
if necessary, and if one device shorts out its power input, it won't
affect the others very much.

There was a gadget that showed at NAMM (and they've been advertising
profusely in the magazines lately) called MPathX which is a
multi-output wall wart eliminator. Probably fancier than what the
original poster was dreaming about, but it uses individual regulators
for each output. The chips are cheap.

Mike Rivers wrote:
Ben Bradley wrote:

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.

That sounds like a disaster to me, both with creating a big magnetic
field (hummmmmmmm) and the fact that you don't have a common ground.

What's a good idea for a device like this, however, is to use a
separate regulator chip for each output. They can be different voltages
if necessary, and if one device shorts out its power input, it won't
affect the others very much.

There was a gadget that showed at NAMM (and they've been advertising
profusely in the magazines lately) called MPathX which is a
multi-output wall wart eliminator. Probably fancier than what the
original poster was dreaming about, but it uses individual regulators
for each output. The chips are cheap.

Right! That Mpathx is really something else! computer controlled
voltage assignment and power analysis is really rather innovative. The
idea is the same I am looking for something a little less complex and
feature laden yet well put together.
So a chassis mount transformer supplying 9vAC and 500mA, rectified,
regulated and smoothed to give 9vDC. And another supplying 12vAC and
300mA, regulated and smoothed is roughly the direction i want to take
for this particular project?

thanks for all your help so far!

andy

andyavast wrote:

So a chassis mount transformer supplying 9vAC and 500mA, rectified,
regulated and smoothed to give 9vDC. And another supplying 12vAC and
300mA, regulated and smoothed is roughly the direction i want to take
for this particular project?

I'd be inclined to use a separate 9V regulator chip for each output
just to be safe. You'll probably want to put about 12V into the
regulators. Don't forget to heat-sink them.

"Mike Rivers" wrote ...

Ben Bradley wrote:

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.

That sounds like a disaster to me, both with creating a big magnetic
field (hummmmmmmm) and the fact that you don't have a common ground.

The lack of common ground is what prevents ground loops
on wall-wart powered devices.

What's a good idea for a device like this, however, is to use a
separate regulator chip for each output. They can be different
voltages
if necessary, and if one device shorts out its power input, it won't
affect the others very much.

Separate regulators would certainly make the PS less
sensitive to interaction between the loads, but it would
not do anything for the potential ground-loop problem.

There was a gadget that showed at NAMM (and they've been advertising
profusely in the magazines lately) called MPathX which is a
multi-output wall wart eliminator. Probably fancier than what the
original poster was dreaming about, but it uses individual regulators
for each output. The chips are cheap.

I think those things actually have separate secondary
transformer windings for each output to fully isolate
them from each other. That would be the ideal solution
for the OP's requirements. Alas, such transformers are
not available off the shelf ("off the peg" for readers on
the right side of the Pond).

"andyavast" wrote ...
So a chassis mount transformer supplying 9vAC and 500mA, rectified,
regulated and smoothed to give 9vDC. And another supplying 12vAC and
300mA, regulated and smoothed is roughly the direction i want to take
for this particular project?

thanks for all your help so far!

What are you powering with this supply? Did you read
the caution about ground problems (both ground loops
and worse if some loads are + ground vs - ground.)?

Theoretically you could do both the 12VAC and 9VDC
with a single transformer, but it depends on WHAT you
are powering with this?????????

Richard Crowley wrote:

The lack of common ground is what prevents ground loops
on wall-wart powered devices.

OK, if you say so. But the fact that things get grounded through other
paths is what makes ground loops. If you have a nice, heavy ground wire
to the chassis of each device and they all go back to a common point,
that can help swamp out a ground loop through cable shields, rack
screws, and other "unplanned" ground connections.

Separate regulators would certainly make the PS less
sensitive to interaction between the loads, but it would
not do anything for the potential ground-loop problem.

The way to solve a ground loop problem is not to have one to begin
with. It's an equipment design problem, not a powering problem.

I think those things actually have separate secondary
transformer windings for each output to fully isolate
them from each other.

I know I've seen some "balanced power" gadgets that have separate
windings for the 120 VAC. But that's not to solve a ground problem,
it's to provide common mode rejection for power line borne noise.

andyavast wrote:
snip
So a chassis mount transformer supplying 9vAC and 500mA, rectified,
regulated and smoothed to give 9vDC. And another supplying 12vAC and
300mA, regulated and smoothed is roughly the direction i want to take
for this particular project?

No. I think the consensus is that a single transformer might be
trouble. Better a box full of small transformers, with separate
rectifiers/filters/regulators with isolated outputs. OTOH, that would
take up almost as much room as a box of wall warts. There might be some
advantage in that you could build it all into a single-space rack case....

jak

thanks for all your help so far!

andy

jakdedert wrote:

I think the consensus is that a single transformer might be
trouble. Better a box full of small transformers

If that's the consensus, then the group should come to its senses.
That's no way to design something like this. Perhaps separating the 12V
and 9V supplies would make a small amount of sense, but there is no
reason to use a separate transformer, or a custom transformer with
multiple secondaries, for each output. It's not necessary for this
application.