ATX power supply on AT board

[D.Rider]

I want to install a lower wattage power supply to an AT motherboard
and found this post (below) from way back (2002-04-01). What do I do
with the 3.3v and other stuff I don't need for the board? Will it run
without those connected or loaded? Thanks.

Here's the post:

what is the difference between at and atx power supplies?thanks for your

help

AT power supplies used to come with a hard on/off switch. Most ATX
power
supplies are missing this switch. The main board connector is also
very
different. See below. (You have to use a typewriter font to see it
clearly,
I used Courier.)



STANDARD PC MOTHERBOARD CONNECTORS

Main Board (System PCB) Standard DC Power Connector(s)

O Y
r e B B B B W
a l B l l l l h
n R l l a a a a i R R R
g e o u c c c c t e e e
e d w e k k k k e d d d
____________ ____________
| P8 | | P9 |
------------ ------------
Wire Color Assignment

Black ------------------------------- Ground
Orange -------------------------- Power Good (+5V DC)
Red ---------------------------------- +5 VDC
White --------------------------------- -5 VDC
Yellow ------------------------------- +12 VDC
Blue -------------------------------- -12 VDC

Note!! When installing the P8/P9 connector be sure that the four
black wires are adjacent to each other in the center of the
connector.
The Red and Orange wires are on the outside edge of the
connector.

ATX 20 pin power connector
Pin# Pin#
-----------------------
| 3.3V 11 1* 3.3V |
| -12V 12 2 3.3V |
| COM 13*(Gnd) 3 COM |
| PS-ON 14* 4 +5V |
| COM 15 5 COM |
| COM 16 6 +5V |
| COM 17 7 COM |
| -5V 18 8 PW-OK|
| +5V 19 9 5VSB |
| +5V 20 10 +12V |
--------------------------
* NOTE!! On ATX supplies the power supply on/off functions are
controlled
thru the motherboard connector.
See pins 1, 13 and 14 for 3.3V, GND and power on signal.

Peripheral DC Power Connector

Y
B B e
l l l
R a a l
e c c o
d k k w

Wire Color Assignment

Red ------------------------------- +5 VDC
Black ------------------------------ Ground
Yellow ----------------------------- +12 VDC


The ATX power supply is switched on by connecting pin 14 to pin 13.
This and
more information about PC-connectors can be found in
[email protected]-web.net


Pieter Kralt

 

[Circuit Breaker]

I want to install a lower wattage power supply to an AT motherboard
and found this post (below) from way back (2002-04-01). What do I do
with the 3.3v and other stuff I don't need for the board? Will it run
without those connected or loaded? Thanks.

The information you quoted seems accurate, though I cannot speak for pin
numbers specifically. I have the 13th edition of Scott Mueller's
_Upgrading_and_repairing_PCs_ -- an EXCELLENT resource, if a touch
expensive at around $50-$60 US.

According to this book, most recent power supplies require only 0.00 to
0.30 amps on the 3.3v output. At 3.3v, watts = volts * amps, so we need
about 3.3 * 0.3 or roughly a 1 watt load.

You can easily get that from a small flashlight bulb, one for the 2-cell
flashlights. If you'd like a dimmer solution, try a 1-watt 50-ohm
resistor, or 2 half-watts, or 4 quarter-watts. Radio Smack probably sells
them by 5-packs for a couple dollars. Actually, you probably have plenty
of them already, if you're really wanting to do this project.

A slightly less practical solution, if you need a load, might be to put a
bunch of LEDs in parallel on the output. They run about 20 mA each, even
the really bright ones, and usually require anywhere from 1.8 to 3.7v to
drive them. Here's the catch: You want an LED that is a LOWER voltage
than the supply provides. Stream about 15 of them in parallael, and that
will pull around 300 mA, or the 0.3A required. Just another amusing
misuse of resources, doesn't do anything practical.

Here's a thought: Those LED readouts that show your processor's speed --
hardwire them. They usually have a common anode or cathode, and if your
proc is, say, a 166 MHz, that's 2 LEDs for the 1, 6 LEDs for each 6.
That's 14 LEDs right there, and should draw the appropriate current. A
266 is even better, as you add 3 more LEDs for the 2.

You have options, but they're probably not necessary.


Also, with the ATX supply, you quoted a very good point - the ATX had no
true on-off switch. Actually, they do have a cutoff, but when you turn
that on, you still need to ground the P_ON pin (I believe your quotee said
pins 13 and 14 - these will do. 14 is usually a green wire. 13 will be
black, usually, and pin 3 also is, right across from 13. The idea is to
ground pin 14 - the green wire - regardless.

HIH, good luck, and if you fry anything, IT'S NOT MY FAULT.

 

[John Woodgate]

I read in sci.electronics.design that Circuit Breaker

Learning is the ONLY substitution for EDUCATION!

It's often the only antidote to it!

 

[cpemma]

I want to install a lower wattage power supply to an AT motherboard
and found this post (below) from way back (2002-04-01). What do I do
with the 3.3v and other stuff I don't need for the board? Will it run
without those connected or loaded? Thanks.

An AT board will run fine from an ATX psu without the "other stuff" but the
board connectors are very different. AT is 2 x 6-pin SIL connectors, ATX is
1x20-pin DIL connector. You can buy adaptors which include on/off wires to
the AT power switch.

AT goes:
1 Power good
2 +5v
3 +12v
4 -12v
5 Ground (0v)
6 0v

7 0v
8 0v
9-5v
10 +5v
11 +5v
12 +5v

 

[N. Thornton]

Here's a thought: Those LED readouts that show your processor's speed --
hardwire them. They usually have a common anode or cathode, and if your
proc is, say, a 166 MHz, that's 2 LEDs for the 1, 6 LEDs for each 6.
That's 14 LEDs right there, and should draw the appropriate current. A
266 is even better, as you add 3 more LEDs for the 2.

800 would give you more current Seriously though, a resistor is a
more sensible load.

Using an ATX PSU does seem like unnecessary work though when any
computer hamateur can give you an AT PSU for nothing.


Regards, NT

 

[D.Rider]

I was planning to just cut and wrap the AT plugs onto the ATX supply.

 

[D.Rider]

Thanks.
I have the 4th edition of that book -- long since didn't bother to
keep up with the editions because I didn't need to reference technical
specifics until now

So, to followup, the quoted "PW-OK" (pin 8 of the 20-pin ATX
connector) would be the same as the AT's "Power Good" (+5V DC)? What
is "5VSB" on the ATX connector? And why are there so many wires to do
the same thing -- +5V and com on both AT and ATX? Do I need to keep
these separate (within their purpose of course) from the others of its
like (IOW, there are 4 red +5V wires on the AT supply, so do I care
which +5V wires they would connect to on the ATX supply; how would I
know the diff?)

All very interesting now.
Thanks for the info.

 

[Circuit Breaker]

D.Rider wrote:

[Regarding ATX power supply pinouts]

So, to followup, the quoted "PW-OK" (pin 8 of the 20-pin ATX
connector) would be the same as the AT's "Power Good" (+5V DC)?

I'm not 100% positive that the /voltages/ are the same, but the /purpose/
is.

What is "5VSB" on the ATX connector?

On the 13th edition's pinout table, it calls it "+5VSB (Standby)". I
assume it is a voltage that is provided to the board when the P/S is in
standby mode so that when you push the "power" button, it closes an
electronic switch which somewhere along the line grounds pin 14, thus
turning the P/S on. Don't quote me, I haven't read this anywhere, it just
makes sense to me considering that there is no hard on/off switch on an
ATX supply - only a cutoff.

And why are there so many wires to do the same thing -- +5V and com on
both AT and ATX?

Imagine, if you will, that you have a 1/2 inch diameter water hose and you
need to fill a 50 thousand gallon swimming pool. Your water supply
(whether it be city or well pump) can easily provide much more water than
your dinky little water hose can allow to pass. What do you do? Bring a
water hose from another faucet. You now have two 1/2-inch hoses, and thus
twice as much water flow (assuming the water supply can provide enough
water to support the flow). If you have a third faucet, and the water
supply can support it, too, then you can add it to the mix for 3 times the
water flow of just one faucet.

Same concept: Your P/S can support, say, 100 watts (JUST AN EXAMPLE) of
+5V power. 5V @ 100 Watts, W = V*A; A = W/V: 100/5 = 20, so this is 20
amps. That's a bit much for just one wire (water faucet) to handle, so
you add another - and another - and another, until you have enough wires
to pass all the current that your board needs.

The water/electricity analogy is old and worn out, and even a little bit
inaccurate. However, the basic concept is still the same. The idea is to
provide more paths for electricity to flow from the P/S to the motherboard
without overloading any one wire.

Do I need to keep these separate (within their purpose of course) from
the others of its like (IOW, there are 4 red +5V wires on the AT supply,
so do I care which +5V wires they would connect to on the ATX supply;
how would I know the diff?)

No. All the same. If you care to risk electrical shock (which is a very
real risk), open up the power supply with it unplugged from the mains
outlet and look at where all those like-colored wires connect. Yep. All
at the same point in the circuit. So you see, they're /already/ not
separate, so you need not worry. Just keep the different colors separate.

All very interesting now.
Thanks for the info.

Yes, it is interesting. HIH, your life is in your hands, I had nothing
to do with it. ;-)

 

[Circuit Breaker]

I read in sci.electronics.design that Circuit Breaker


It's often the only antidote to it!

Hhhmmmmmmm..... how very true....

 

[Circuit Breaker]

D.Rider wrote:

[Regarding how to take an ATX supply and fit it to an AT board]

I was planning to just cut and wrap the AT plugs onto the ATX supply.

Hey, before you go slicing, you might be interested in a very handy tool
from Radio Splat that helps you safely disconnect the male plugs from the
plastic shell. It's called a pin extractor, and looks mildly like a
syringe. If you look in the end of the connector, you'll see the tiny
little tabs that hold the pin in the shell. The extractor fits snugly
over these tabs, holding them down, so the plunger can push the pins out
through the other side. No slicing necessary usually.

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=274-223

I believe this one will fit the connectors on your power supply - might
want to check with the clerk before purchase. Stuff like this often isn't
sealed good from the factory, and it's also quite common for customers to
return stuff of this nature for one reason or the other, and when either
case happens, they usually just staple the thing back together, so don't
be shy about asking them if you can check it on a P/S connector from the
shelf to make sure it's what you need.


HIH, I'm not responsible for damage to life or property. ;-)

 

[Howard Henry Schlunder]

in message

Thanks.
I have the 4th edition of that book -- long since didn't bother to
keep up with the editions because I didn't need to reference technical
specifics until now

It sounds like you may have all the essential documentation you need, but
the ATX Specification [
http://www.formfactors.org/developer\specs\atx2_1.pdf ] is somewhat
educational and IMO, interesting as well. The good stuff starts on page 19.

What is "5VSB" on the ATX connector?

It is +5V provided all the time that the power supply is plugged in to the
mains and not physically disconnected by a switch. AT power supplies have
no equivalent and you can leave this wire disconnected, or you can use it as
a handy regulated 5V supply for whatever special purposes you have. It is
designed so that computers can turn themselves on from a keyboard, mouse,
network card, modem, real time clock alarm, or other hardware event. Also,
with newer Advanced Configuration and Power Interface (ACPI) motherboards,
some of them support Suspend to RAM (power state S3), which is to say the
power supply is turned off, but the RAM contents are preserved using this
power line and the computer can wake back up and resume with all
applications still open without rebooting in about five seconds or possibly
less.

Howard Henry Schlunder

 

[Spajky®]

I want to install a lower wattage power supply to an AT motherboard
and found this post (below) from way back (2002-04-01). What do I do
with the 3.3v and other stuff I don't need for the board? Will it run
without those connected or loaded? Thanks.

why so much hassle?
people are throwing away old putters, get free or very cheap an AT one
Psu ... just look arround a bit!

-- Regards, MERRY HOLIDAYS & HAPPY NEW YEAR, SPAJKY ®
& visit my site @ http://www.spajky.vze.com
Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!"
E-mail AntiSpam: remove ##

 

[D.Rider]

Thanks Circuit Breaker (and others). Good idea and info.

However, someone elsewhere has suggested that my efforts may be
pointless... I want to run a lower wattage power supply in the hopes
that it translates into less power consumption (meaning the power
supply doesn't eat so much money from the wall).

It was suggested that no matter what power supply I put in, the power
consumed from the wall outlet will be the same due to the computer
load itself. If I put in a 200 watt supply but it only needs 145
watts, then only 145 watts of money will be paid to the power company.
I thought that a 200 watt power supply will use 200 watts of power.
What is correct?

Thanks again.

D.Rider wrote:

[Regarding how to take an ATX supply and fit it to an AT board]

I was planning to just cut and wrap the AT plugs onto the ATX supply.

Hey, before you go slicing, you might be interested in a very handy tool
from Radio Splat that helps you safely disconnect the male plugs from the
plastic shell. It's called a pin extractor, and looks mildly like a
syringe. If you look in the end of the connector, you'll see the tiny
little tabs that hold the pin in the shell. The extractor fits snugly
over these tabs, holding them down, so the plunger can push the pins out
through the other side. No slicing necessary usually.

http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=274-223

I believe this one will fit the connectors on your power supply - might
want to check with the clerk before purchase. Stuff like this often isn't
sealed good from the factory, and it's also quite common for customers to
return stuff of this nature for one reason or the other, and when either
case happens, they usually just staple the thing back together, so don't
be shy about asking them if you can check it on a P/S connector from the
shelf to make sure it's what you need.


HIH, I'm not responsible for damage to life or property. ;-)

 

[Roger Hamlett]

Thanks Circuit Breaker (and others). Good idea and info.

However, someone elsewhere has suggested that my efforts may be
pointless... I want to run a lower wattage power supply in the hopes
that it translates into less power consumption (meaning the power
supply doesn't eat so much money from the wall).

It was suggested that no matter what power supply I put in, the power
consumed from the wall outlet will be the same due to the computer
load itself. If I put in a 200 watt supply but it only needs 145
watts, then only 145 watts of money will be paid to the power company.
I thought that a 200 watt power supply will use 200 watts of power.
What is correct?

Thanks again.

Not quite either!.
The power drawn from the socket, will be the total power drawn from the
supply, divided by it's 'efficiency'. A large supply (say a 500W unit), will
often give slightly better efficiencies than a smaller unit. However the
difference is small (a typical unit will be over 92% efficient, with some
large units pushing over 95% on 50% load). So if the computer draws 145W,
and the supply is 92% efficient, the power drawn from the line will be
145/0.92 = 157.6W. There is then a seperate issue, that depending on the
power factor of the supply, typical household meters, may give a few percent
error in the amount they charge you. Supply efficiencies will usually fall
at very light loads, or when loaded near to their max.
So the 200W supply does not draw 200W all the time (think of the heat it
would have to generate if it did so, and the load was only 145W).

Best Wishes

 

[N. Thornton]

However, someone elsewhere has suggested that my efforts may be
pointless... I want to run a lower wattage power supply in the hopes
that it translates into less power consumption (meaning the power
supply doesn't eat so much money from the wall).

It was suggested that no matter what power supply I put in, the power
consumed from the wall outlet will be the same due to the computer
load itself. If I put in a 200 watt supply but it only needs 145
watts, then only 145 watts of money will be paid to the power company.
I thought that a 200 watt power supply will use 200 watts of power.
What is correct?

Thanks again.

Jeez, dont ever think of telling us what you want to do 13 posts ago.

 

[D.Rider]

[email protected] (N. Thornton) wrote in message

Jeez, dont ever think of telling us what you want to do 13 posts ago.

Well, so sorry I offended you, N. Thornton; wasn't my intention. I
didn't know that this thread would grow. Usually posts generate only
a few responses. In any case, I learned some about ATX PSU's that
should come in handy later... and I won't be hesitant to open them up
and cut wires if I have to, as I was before.

Obviously the reason that I would have (mistakenly) wanted to install
a lower wattage ATX PSU in an AT computer was to save some money on
power, but now that Roger Hamlett explained it with thoroughness, I
see that I would have been going the wrong direction with the PSU.
Who'd thought that bigger translates to better when it comes to power
consumption? OK, so I didn't. Even a small difference in
efficiencies would translate to some money saved over years of
intended continuous service.

By the way, I had this 145 watt ATX PSU on hand and knew that I only
needed about that much to make this old PC run (which it will do for
24/7/365). So thanks to all for the education -- Bye.