Can anyone give a link to a site which covers modifying an ATX PS 12 volt
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.

--
*Ever stop to think and forget to start again?

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
Can anyone give a link to a site which covers modifying an ATX PS 12 volt
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.

--
*Ever stop to think and forget to start again?

Dave Plowman London SW
To e-mail, change noise into sound.


I'm pretty sure that RadCom did an article on doing this not so long back.
With licenses now available on the backs of cornflake packets, I guess it
would have to be pretty simple. I used to keep all my RadComs, but took to
chucking them out over the last couple of years. I'll have a look anyway, as
I still tend to keep them until there's a pile, then lob 'em. If it wasn't
in RadCom, it was in Elektor.

Jacking these PSUs up a couple of volts is not usually that hard. There's
often an 'adjustable zener' IC on the primary side, which is fed from the
secondary side via an opto coupler. This zener serves as the reference to
set the PWM for the desired output voltage. Altering that voltage is just a
case of changing one resistor to alter the reference voltage.

Arfa

Arfa Daily wrote:

Jacking these PSUs up a couple of volts is not usually that hard. There's
often an 'adjustable zener' IC on the primary side, which is fed from the
secondary side via an opto coupler. This zener serves as the reference to
set the PWM for the desired output voltage. Altering that voltage is just a
case of changing one resistor to alter the reference voltage.


Another resource.

http://www.pavouk.org/hw/en_atxps.html

The above schematic could be frigged about with by selecting a new value
for R25.

Typical 13.8v project.

http://www.webx.dk/oz2cpu/radios/psu-pc1.htm

--
Adrian C

In article ,
Adrian C wrote:
Arfa Daily wrote:

Jacking these PSUs up a couple of volts is not usually that hard.
There's often an 'adjustable zener' IC on the primary side, which is
fed from the secondary side via an opto coupler. This zener serves as
the reference to set the PWM for the desired output voltage. Altering
that voltage is just a case of changing one resistor to alter the
reference voltage.


Another resource.

http://www.pavouk.org/hw/en_atxps.html

The above schematic could be frigged about with by selecting a new value
for R25.

Typical 13.8v project.

http://www.webx.dk/oz2cpu/radios/psu-pc1.htm

The one I have to hand is a bit more modern and uses an all in one SG6105
supervisor IC.

--
*Acupuncture is a jab well done*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
Adrian C wrote:
Arfa Daily wrote:

Jacking these PSUs up a couple of volts is not usually that hard.
There's often an 'adjustable zener' IC on the primary side, which is
fed from the secondary side via an opto coupler. This zener serves as
the reference to set the PWM for the desired output voltage. Altering
that voltage is just a case of changing one resistor to alter the
reference voltage.


Another resource.

http://www.pavouk.org/hw/en_atxps.html

The above schematic could be frigged about with by selecting a new value
for R25.

Typical 13.8v project.

http://www.webx.dk/oz2cpu/radios/psu-pc1.htm

The one I have to hand is a bit more modern and uses an all in one SG6105
supervisor IC.

--
*Acupuncture is a jab well done*

Dave Plowman London SW
To e-mail, change noise into sound.

There are reference and feedback inputs at pins 11,12,13 &14 on that IC, and
if you examine the internal schematics, you will see that behind those pins
are the 'adjustable zeners' which are often external devices on PSUs which
use a slightly less sophisticated management IC. I expect that if you played
with the resistors attached to those pins, you could alter the output
voltage of the supply. One thing to note is that this IC also has under and
over-voltage lockouts, and if the feature has been implemented on your PSU,
then cranking the 12v up to 13.8v, may result in an unwanted shutdown, so
you might need to disable this feature.

Just as a matter of interest, what are you hoping to run from this PSU ?
Requiring 13.8v would suggest that it is something designed for mobile use.
However, most such devices will happily run from 12v, as this may be all the
vehicle voltage that's available, if the engine isn't running.

Arfa

In article ,
Arfa Daily wrote:
Just as a matter of interest, what are you hoping to run from this PSU ?
Requiring 13.8v would suggest that it is something designed for mobile
use. However, most such devices will happily run from 12v, as this may
be all the vehicle voltage that's available, if the engine isn't
running.

It is indeed. However I'd like the correct nominal voltage for prolonged
soak testing.

--
*The most wasted day of all is one in which we have not laughed.*

Dave Plowman London SW
To e-mail, change noise into sound.

In message , "Dave Plowman (News)"
writes
Can anyone give a link to a site which covers modifying an ATX PS 12 volt
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.

Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take you
into the realms of voltage dividers and the TL431.

Basically, the PSU will regulate the 5v or 3.3v rail and all the other
rails are set by the transformer ratios. You need to alter the voltage
divider for the 5v or 3.3v rail and all the other rails will change
proportionally (with the exception of the +5v SB as this is not
generated by the main switcher and maybe the -12 and -5v as these are
often provided by 79xx linear regulators). Don't forget, you may need
to add some load to the 5v and/or 3.3v rails to make the PSU stable and
don't mess around with it unless you're sure the main reservoir
capacitors are discharged.
--
Clint Sharp

In article ,
Clint Sharp wrote:
Can anyone give a link to a site which covers modifying an ATX PS 12
volt rail to 13.8v in reasonably simple terms? I Googled for a couple
of hours last night and found answers from the bizarre to ridiculous.
The +12v output is rated at 20 amps and I only need 8 max.

Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take you
into the realms of voltage dividers and the TL431.

Ain't got one of those.

--
*The only difference between a rut and a grave is the depth.

Dave Plowman London SW
To e-mail, change noise into sound.

On Tue, 04 Aug 2009 08:23:07 +0100, "Dave Plowman (News)"
wrote:

Can anyone give a link to a site which covers modifying an ATX PS 12 volt
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.
Without trying to be negative about the project, ATX power supplies
are 'jelly bean' items. The only common points between different
brands and models are the case dimensions, the pinouts of the output
leads, and the AC in connector.

You are going to have to continue what you have started. Analyse the
circuit, locate the voltage regulator and over-voltage protection and
adjust the output voltage by changing the appropriate resistors.

Depending on the availability of a suitable power supply, your
electronics skills, and time available, you might be better off taking
a simpler approach. This supply would be much easier to work on.
http://www.mpja.com/prodinfo.asp?number=17095+PS

PlainBill

On Tue, 04 Aug 2009 08:23:07 +0100, "Dave Plowman (News)"
put finger to keyboard and composed:

Can anyone give a link to a site which covers modifying an ATX PS 12 volt
rail to 13.8v in reasonably simple terms? I Googled for a couple of hours
last night and found answers from the bizarre to ridiculous. The +12v
output is rated at 20 amps and I only need 8 max.

I've done this with an AT PSU. All I did was to modify the voltage
sensing resistors at the feedback pin of the error amp. In my case the
PSU regulated by sensing a weighted average of the +5V and +12V
supplies, so I removed the +5V sense resistor and reprogrammed the
+12V resistive divider. You may also have to modify the overvoltage
sense circuitry, in my case a zener diode. You could also remove the
components associated with the other rails and then fudge the POK
circuit. I also added an aluminium clad dummy load resistor and
mounted it in the fan's air stream.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

In article ,
wrote:
Can anyone give a link to a site which covers modifying an ATX PS 12
volt rail to 13.8v in reasonably simple terms? I Googled for a couple
of hours last night and found answers from the bizarre to ridiculous.
The +12v output is rated at 20 amps and I only need 8 max.

Without trying to be negative about the project, ATX power supplies
are 'jelly bean' items. The only common points between different
brands and models are the case dimensions, the pinouts of the output
leads, and the AC in connector.

I've certainly found that out. ;-)

You are going to have to continue what you have started. Analyse the
circuit, locate the voltage regulator and over-voltage protection and
adjust the output voltage by changing the appropriate resistors.

I'm not going to waste much time on it - given the cost of new units that
do what I want.

Depending on the availability of a suitable power supply, your
electronics skills, and time available, you might be better off taking
a simpler approach. This supply would be much easier to work on.
http://www.mpja.com/prodinfo.asp?number=17095+PS

But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
lot, if I decide to spend some cash.

--
*The most wasted day of all is one in which we have not laughed.*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
Clint Sharp wrote:
Can anyone give a link to a site which covers modifying an ATX PS 12
volt rail to 13.8v in reasonably simple terms? I Googled for a couple
of hours last night and found answers from the bizarre to ridiculous.
The +12v output is rated at 20 amps and I only need 8 max.

Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take you
into the realms of voltage dividers and the TL431.

Ain't got one of those.


Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ... If you're not understanding the principles of how
these things regulate, then I don't think you are going to get too far with
this project.

Arfa

On Tue, 04 Aug 2009 13:46:35 +0100, "Dave Plowman (News)"
put finger to keyboard and composed:

In article ,
Adrian C wrote:
Arfa Daily wrote:

Jacking these PSUs up a couple of volts is not usually that hard.
There's often an 'adjustable zener' IC on the primary side, which is
fed from the secondary side via an opto coupler. This zener serves as
the reference to set the PWM for the desired output voltage. Altering
that voltage is just a case of changing one resistor to alter the
reference voltage.


Another resource.

http://www.pavouk.org/hw/en_atxps.html

The above schematic could be frigged about with by selecting a new value
for R25.

Typical 13.8v project.

http://www.webx.dk/oz2cpu/radios/psu-pc1.htm

The one I have to hand is a bit more modern and uses an all in one SG6105
supervisor IC.

Datasheet for SG6105:
http://www.datasheetarchive.com/pdf-...DSA-104265.pdf

There is an application circuit on page 8.

It seems to me that your +13.8V modification would be very easy.

Just remove the 5V sense resistor from the SG6105's IN pin. Then
reprogram the 12V and SS resistors so that the voltage at the IN pin
is equal to +2.5V when the 12V rail is at +13.8V.

Gnd o-- 2K5 --o-- 11K3 --o +13.8V
|
|
IN

Remove the connections to the V33, V5, and V12 over/undervoltage sense
pins and then construct a potential divider (see below) with the
+13.8V output rail at the top end, and taps at +12V, +5V, and +3.3V
connected to each of the sense pins. In this way you would be
monitoring only the +13.8V rail and faking the others.


Gnd o-- 3K3 --o-- 1K7 --o-- 7K --o-- 1K8 --o +13.8V
| | |
| | |
V33 V5 V12

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

On Wed, 5 Aug 2009 01:37:49 +0100, "Arfa Daily"
put finger to keyboard and composed:


"Dave Plowman (News)" wrote in message
...
In article ,
Clint Sharp wrote:
Can anyone give a link to a site which covers modifying an ATX PS 12
volt rail to 13.8v in reasonably simple terms? I Googled for a couple
of hours last night and found answers from the bizarre to ridiculous.
The +12v output is rated at 20 amps and I only need 8 max.

Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take you
into the realms of voltage dividers and the TL431.

Ain't got one of those.


Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ... If you're not understanding the principles of how
these things regulate, then I don't think you are going to get too far with
this project.

Arfa

AISI, the main rails are regulated via the error amp at pins 16, 17,
and 18. The OP would need to remove/modify the resistors at the IN
pin.

The TL431's at the FB2 and FB1 pins regulate the +3.3V and +5VSB
supplies. I don't believe they need to be touched. I suspect, though,
that the +3.3V regulator will be working flat out to bring the output
down, if indeed it can.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

In article ,
Arfa Daily wrote:
Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take
you into the realms of voltage dividers and the TL431.

Ain't got one of those.


Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ...

And it's a TL431? If you mean a shunt regulator that would be ok.

If you're not understanding the principles of how these things
regulate, then I don't think you are going to get too far with this
project.

I doubt I'm alone in that. Try Googling on 'modify ATX 13.8v' and see just
how many different approaches there are.
Which is why I asked here.

--
*Of course I'm against sin; I'm against anything that I'm too old to enjoy.

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Jeff Liebermann wrote:
But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
lot, if I decide to spend some cash.

The most popular official voltage is apparently 13.5VDC, not 13.6VDC.

13.8 is the usual voltage on a car with the engine running and things
stabilised. But of course most equipment will tolerate a wide range and
still work.

--
*Gun Control: Use both hands.

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Franc Zabkar wrote:
AISI, the main rails are regulated via the error amp at pins 16, 17,
and 18. The OP would need to remove/modify the resistors at the IN
pin.

The TL431's at the FB2 and FB1 pins regulate the +3.3V and +5VSB
supplies. I don't believe they need to be touched. I suspect, though,
that the +3.3V regulator will be working flat out to bring the output
down, if indeed it can.

Google gave this hit:-

http://www.chirio.com/atx_14_volt.htm

Which says to use 1N4448 in the 3.3, 5 and 12v error sensing rails, which
I did.
No difference. Tried changing the resistor feeding 'IN' with a pot and
altering that from the fixed value cause shut down.

--
*If I throw a stick, will you leave?

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Arfa Daily wrote:
Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take
you into the realms of voltage dividers and the TL431.

Ain't got one of those.


Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ...

Right - having read the spec more carefully it does indeed state it
contains two of these. So please accept my apologies.

If you're not understanding the principles of
how these things regulate, then I don't think you are going to get too
far with this project.

Well, if I were an expert on them I'd hardly need to ask here.
It's not for some life support mechanism, you know.

--
*Bigamy is having one wife too many - monogamy is the same

Dave Plowman London SW
To e-mail, change noise into sound.

"Meat Plow" wrote in message
...
On Wed, 05 Aug 2009 10:11:03 +0100, "Dave Plowman (News)"
wrote:

In article ,
Jeff Liebermann wrote:
But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
lot, if I decide to spend some cash.

The most popular official voltage is apparently 13.5VDC, not 13.6VDC.

13.8 is the usual voltage on a car with the engine running and things
stabilised. But of course most equipment will tolerate a wide range and
still work.

I like 13.7.

14.0 volts is the best. You will lose a tenth or so in the leads and
connectors

On Wed, 05 Aug 2009 00:13:54 +0100, "Dave Plowman (News)"
wrote:

In article ,
wrote:
Can anyone give a link to a site which covers modifying an ATX PS 12
volt rail to 13.8v in reasonably simple terms? I Googled for a couple
of hours last night and found answers from the bizarre to ridiculous.
The +12v output is rated at 20 amps and I only need 8 max.

Without trying to be negative about the project, ATX power supplies
are 'jelly bean' items. The only common points between different
brands and models are the case dimensions, the pinouts of the output
leads, and the AC in connector.

I've certainly found that out. ;-)

You are going to have to continue what you have started. Analyse the
circuit, locate the voltage regulator and over-voltage protection and
adjust the output voltage by changing the appropriate resistors.

I'm not going to waste much time on it - given the cost of new units that
do what I want.

Depending on the availability of a suitable power supply, your
electronics skills, and time available, you might be better off taking
a simpler approach. This supply would be much easier to work on.
http://www.mpja.com/prodinfo.asp?number=17095+PS

But with respect that's 12 volts. I want 13.8. Plenty on Ebay for not a
lot, if I decide to spend some cash.

Glad to hear it. It's always better to start off with something
designed for the job IF the price is reasonable.

PlainBill

"Dave Plowman (News)" wrote in message
...
In article ,
Arfa Daily wrote:
Ahh, grasshopper, to modify the 12v rail first you must find the
voltage reference for the 5 v or 3.3 v rail, your journey will take
you into the realms of voltage dividers and the TL431.

Ain't got one of those.


Yes it has, Dave. With that particular supervisory IC, it's behind the
reference and feedback pins, and not an external discrete device, as I
previously explained ...

Right - having read the spec more carefully it does indeed state it
contains two of these. So please accept my apologies.

If you're not understanding the principles of
how these things regulate, then I don't think you are going to get too
far with this project.

Well, if I were an expert on them I'd hardly need to ask here.
It's not for some life support mechanism, you know.

--
*Bigamy is having one wife too many - monogamy is the same

Dave Plowman London SW
To e-mail, change noise into sound.

I didn't intend any offence, Dave - probably could have picked my words
better. All I was trying to imply, is that switch mode power supplies are a
bit of a black art, as you have discovered, and unless you are working with
them all the time, and thus understand the subtleties of some of the
circuitry and regulation techniques which they employ, then this project is
quite likely to tie you up in knots. I must admit that I didn't look too
closely at the application schematic to see how exactly they went about
regulating the thing, but when I saw the two undedicated adjustable zeners
in there, I assumed that they were probably making use of at least one of
them in the primary feedback loop. It is a common technique to use an
(usually external) adjustable zener to achieve main rail set voltage and
regulation.

Franc is probably right in this case with his proposal of altering the
resistors on the IN pin. You may well find that when you put the pot in, you
actually had the desired effect of increasing or decreasing the output
voltage, but then fell foul of the under / over voltage lockout feature,
which I think I said that you might have to disable. If you put a digital
meter on the output, and watch very closely, you might see the voltage start
to alter up and down, before it trips out.

Arfa

In article ,
Arfa Daily wrote:
Well, if I were an expert on them I'd hardly need to ask here.
It's not for some life support mechanism, you know.

I didn't intend any offence, Dave - probably could have picked my words
better.

No offence taken - it is Usenet, after all. ;-)

All I was trying to imply, is that switch mode power supplies are a bit
of a black art, as you have discovered, and unless you are working with
them all the time, and thus understand the subtleties of some of the
circuitry and regulation techniques which they employ, then this project
is quite likely to tie you up in knots.

Which is why I'm happy to experiment and hopefully learn.

I must admit that I didn't look
too closely at the application schematic to see how exactly they went
about regulating the thing, but when I saw the two undedicated
adjustable zeners in there, I assumed that they were probably making
use of at least one of them in the primary feedback loop. It is a
common technique to use an (usually external) adjustable zener to
achieve main rail set voltage and regulation.

Snag is my actual circuit differs considerably from either of the examples
given in the spec sheet - including a sprinkling of external zenners. But
it's beyond my patience/available time to trace it all out properly.

Franc is probably right in this case with his proposal of altering the
resistors on the IN pin. You may well find that when you put the pot in,
you actually had the desired effect of increasing or decreasing the
output voltage, but then fell foul of the under / over voltage lockout
feature, which I think I said that you might have to disable.

The network between actual outputs and sensor inputs is far from simple -
the spec sheet just shows a direct connection.

If you put a digital meter on the output, and watch very closely, you
might see the voltage start to alter up and down, before it trips out.

Indeed.

--
*I can see your point, but I still think you're full of ****.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
Arfa Daily wrote:
Just as a matter of interest, what are you hoping to run from this PSU ?
Requiring 13.8v would suggest that it is something designed for mobile
use. However, most such devices will happily run from 12v, as this may
be all the vehicle voltage that's available, if the engine isn't
running.

It is indeed. However I'd like the correct nominal voltage for prolonged
soak testing.

Why wouldn't you soak test at worst case numbers rather than nominal?

Certainly depends on the power supply, but there needs to be enough flux
in the transformer to make the outputs work.
If the 5V is the regulated output...or 3.3...or whatever,
and there's no load, there won't be enough flux to make the 12V work at
any significant load.
You're gonna need to redesign the circuit to sense the 12V output
so you can get the current you need. The rest is just resistor ratios.

But wait...if there's no load on the other outputs, you run the risk
of transients charging the caps till they explode.

And depending on the physical location on the circuit board you
pick to connect your new components, you may make significant changes
to the transient response. With high-currents, one point on a trace
ain't the same as another. You can compensate the feedback loop,
but load transient induced spikes can/will cause you grief if your
layout doesn't consider it. Building the transient test fixture
to test your design is non-trivial.

There's an interesting ratio. How much you are saving on the power
supply (including costs of
design/debug/labor/testing/documentation/training)
divided by the cost of blowing up all the stuff connected to it.

And what do you do when it breaks...or you need another one
and the original PS product is no longer available?

Few people understand the subtleties of power supply design.
It's a black hole (art) that sucks in everything around it.

Go buy a real honkin' power supply and use a slew of small circuit
breakers to power your individual test stations.
I don't know your burdened design labor rate, but modifying
PC power supplies is often false economy.

Your time might be better spent
simulating the automotive power
environment for your tests.

littelfuse appnote an9312 is a place to start.

Are we having fun yet?
mike

In article ,
spamme0 wrote:
It is indeed. However I'd like the correct nominal voltage for
prolonged soak testing.

Why wouldn't you soak test at worst case numbers rather than nominal?

Worst case for soak testing wouldn't be lower than nominal.

--
*Eagles may soar, but weasels don't get sucked into jet engines *

Dave Plowman London SW
To e-mail, change noise into sound.