Starting up an ATX psu without a motherboard.

Capt'n Midnight

One idea I had a while back was to have an AT and an ATX PSU in a tower case. The AT psu would power the drives and fans and be left switched on, the power cable coming out from the ATX PSU so both get turned on and off at the same time. This would mean that the ATX PSU would only have to power the Motherboard - two points of failure but both PSU's under less stress.

Anyway with an AT psu, as long as there is a small amount of current being drawn form the +5 supply (eg: a fan) you can simply switch it on and get loads of amps from the +12 too.

How do you power on an ATX Psu ? Because they don't need on/off switches

Is it as simple as connecting +5 from pin 9 (5VSB) to pin 14 (PS-On) ?
Or are there other considerations

http://www.dvss.com/dvss/pinouts/#power

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."

tuxy

why?

Capt'n Midnight

Why not - it's a MOD ?

Couple of old PSU's be nice to use them for other things. eg: 12V low voltage lighting systems, Also means I could put a rake of SCSI drives in a case and not worry about the single PSU keeling over. Reusing a lower power P4 Psu with newer CPU's by offloading some of the load to the other PSU

http://www.informit.com/articles/article.asp?p=31105&seqNum=3 I've found more PSU info here but a little general for what I'm looking for - page two has the pinout for the infamous Dell cross wiring from 1996 to 2000 ( I've got one or two of those two , not much use to man nor beast except to repair old Dell's or to MOD )

The Dr00g

A somewhat educated guess would be that the PSU also needs the PW-OK signal from a mobo.

duridian

To power on an ATX psu you only need to ground the PS-ON. One of the grounds to either side of it will do fine if you use a paperclip or something like that. I've done it hundreds of times.

If you want a fully automated setup, ie. power button of pc switches on both psus (you need 2 ATX psus rather than an ATX and an AT) then see this thread where I posted a lot of stuff on twin ATX PSUs when unklerosco was doing this mod last July. All it really requires is a motor car relay and some wire and it works perfectly. I'm not gonna repeat what's already been posted but basically the principle behind it is simple. A relay is just a electrically activated switch. When the PSU connected to the motherboard turns on frist its 12V output causes the relay to switch on the second PSU by grounding its PS-ON.

Capt'n Midnight

Thanks for the info about grounding - thought you'd have to apply +5V to the pin,

Re: the relay - make sure you fit a diode across the coil to get rid of the back emf or the spike could damage the mother board. might be a better idea to use an NPN transistor to do the "shorting" - would be faster than the relay too.
1 K resistor on the base from a 5V signal on the first PSU..

The Dr00g

The Dr00g wrote:

A somewhat educated guess would be that the PSU also needs the PW-OK signal from a mobo.

Or vice-versa.

Gone West

a friend tried something like that (paperclip) with a gigabit board and he blew it, and then blew his chip! just a word of warning, though im sure you dont need it!

duridian

Capt'n Midnight wrote:

Thanks for the info about grounding - thought you'd have to apply +5V to the pin,

Re: the relay - make sure you fit a diode across the coil to get rid of the back emf or the spike could damage the mother board. might be a better idea to use an NPN transistor to do the "shorting" - would be faster than the relay too.
1 K resistor on the base from a 5V signal on the first PSU..

Yes, I thought there might be another even better way to do it, using something more from the world of electronics, than the motor car relay, never actually did a dual psu setup myself but the relay method appeared safer to me than what some sites were suggesting about joining together the PW-ON of the 2 PSUs which was why, as you can see from the other thread I linked, I dissuaded Unklerosco from doing it that way when he was going about doing this mod.

FuzzyLogic wrote:

a friend tried something like that (paperclip) with a gigabit board and he blew it, and then blew his chip! just a word of warning, though im sure you dont need it!

Your point is very valid, I would never recommend trying to ground the PW-ON if the connector is actually plugged into a motherboard at the time, it is only really ìntended for starting a PSU to check the PSU is powering on or as a method of starting a second PSU to power drives, fans ,watercooling or the like.

TacT

Tuxy, shame on you, there is no why, just how -- because you can

On the AT psu, you should just be able to plug it in and flick the switch?
On an ATX psu, I think you need to short the green wire on your right and the black wire to the right of it.

[edit] I knew I put it somewhere

here ya go:

short those two -- plug in, switch on and away you go.

[edit2] I felt like entertaining myself a little bit and just tested that out on an old 350W codegen I have lying around here. It's sitting here on the desk powered, with a wire going between those two pin connectors -- plugged into the wall and on it was, great stuff for testing

just connected my 80mm Vantec Tornado to it for a laugh -- LOL! We have liftoff..

BeatFreak

What tact said, I often short those two pins to when installing watercooling kits. Short them let, the system run and check for leaks. When your happy enough you can give the rest of the machine juice that way if it does leak you don't fry anything

Can't see how anyone would blow anything seeing as the PSU isn't connected to anything except the pump. Well in my case anyway.

Capt'n Midnight

TacT wrote:

On the AT psu, you should just be able to plug it in and flick the switch?

Yes but there must be a small load on 5V or the PSU will shutdown, wven a 12V fan is enough and it's so slow it's nearly silent.

Actually just realised I've an old 3com switch at home that needs +3.3 +5 and +12 ..

TacT

Damn, never even crossed my mind! Keep us posted with regards the AT psu, I haven't even got one of them lying around I could check with unfortunately -- the switch idea sounds good but does it work?

Me and AT psu's = know very little or next to nothing about em