Converting a ATX PS to 13.8v

[bw]

"Meat Plow" <meat@petitmorte.net> wrote in message
news:318l0t.8t5.17.2@news.alt.net...

On Wed, 05 Aug 2009 10:11:03 +0100, "Dave Plowman (News)"
dave@davenoise.co.uk>wrote:

In article <mfmh751qj1nok16bf66rrn3verf8kfblgk@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> 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)"
<dave@davenoise.co.uk> wrote:

In article <ngvg75p1o3peqscfjrkhb839p9p7pldqs4@4ax.com>,
PlainBill47@yahoo.com> 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

 

[Meat Plow]

On Wed, 5 Aug 2009 14:11:57 -0500, "bw" <bwegher@hotmail.com>wrote:

"Meat Plow" <meat@petitmorte.net> wrote in message
news:318l0t.8t5.17.2@news.alt.net...
On Wed, 05 Aug 2009 10:11:03 +0100, "Dave Plowman (News)"
dave@davenoise.co.uk>wrote:

In article <mfmh751qj1nok16bf66rrn3verf8kfblgk@4ax.com>,
Jeff Liebermann <jeffl@cruzio.com> 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

There's a joke in here somewhere.

 

[Arfa Daily]

"Dave Plowman (News)" <dave@davenoise.co.uk> wrote in message
news:5086135f1fdave@davenoise.co.uk...

In article <zv4em.164608$tU4.75310@newsfe19.ams2>,
Arfa Daily <arfa.daily@ntlworld.com> 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 dave@davenoise.co.uk 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

 

[Dave Plowman (News)]

In article <nkqem.157257$qz1.147671@newsfe12.ams2>,
Arfa Daily <arfa.daily@ntlworld.com> 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 shit.

Dave Plowman dave@davenoise.co.uk London SW
To e-mail, change noise into sound.

 

[spamme0]

Dave Plowman (News) wrote:

In article <3BWdm.125758$GX7.29427@newsfe23.ams2>,
Arfa Daily <arfa.daily@ntlworld.com> 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

 

[Dave Plowman (News)]

In article <h5rdr1$36g$1@news.eternal-september.org>,
spamme0 <spamme0@netscape.net> 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 dave@davenoise.co.uk London SW
To e-mail, change noise into sound.