ATX power-on circuit, help?

[yonnie]

I'm building a special application and need some info. Can someone provide a drawing of how the power-on circuitry works? I already know stuff like the green wire connects to the black wire. What I need to know is how much sink current is required to pull the green wire to ground and if it's an inductive load or not.

I also need to know about the typical power-on button. Is it normally open or normally closed? Is it a momentary? Does it connect two pins together, does it ground a pin?

The board I'm working with has a 3-pin connector, next to a 4 pin Molex (same as what's on a HDD). This 3-pin connector is labeled: 1: +5v_SB 2: Ground 3: PS_ON. Other info with connector is that it connects to an ATX power-supply.

Could somebody please share a drawing and specs on this?

 

[Mark R]

The PSU will power on when PS_ON# is connected to ground. PS_ON# is a standard TTL input with an interal pull-up to +5V_SB. The maximum pull-up current that the external switch will need to sink is 1.6 mA.

The power-on button is a momentary NO button. The switch connects 2 pins on the motherboard together (one may be ground, but this is not specified and may vary from board to board).

There is no standard 3-pin connector on an ATX PSU. However, all 3 signals are present on the main 24 pin Molex connector; the pin out for the primary connector is readily available via a web search.

 

[yonnie]

This board doesn't have a 24-pin Molex, just a 4-pin for power (+12 and +5 and ground), that's all. The manual says the 'atx pwr-on' (a 3-pin) goes to the ATX power supply. Having never seen a corresponding 3-pin plug on an ATX power-supply, I was trying to figure out what's needed to make something work, that could use this feature. The link below will take you to a spec page of board.
www.tri-m.com/products/iei/files/specs/nova8522g2_spec.pdf

The board is going to run a machine, so I really need a way to power-up and power-down a mains contactor to isolate the entire machine from AC power. I was thinking I could employ this circuit, at least for a controlled power-off. Emergency Power Off (EPO) is a different issue.

What do you know about the +5vsb to the little motherboard? Is it required to power the latching circuit that keeps a standard motherboard ON? I'm thinking maybe I can build a relay latch using this circuit to hold the computer on till it shuts itself off after normal shutdown house-keeping.

Any thoughts?

 

[Colt45]

I've got a similar board around, but I haven't used it in "ATX" mode. (which is a big misnomer here, but whatever...)

AFAIK, you just give the board 5VSB (say an amp or so, always on), and then it can turn itself on and off (via the PWR_ON pin), then there should be another 2 pin header somewhere, for a normal "power on switch", which doesn't power it on as such, but tells the board to drop PWR_ON and turn itself on. So you should be able to use a normal ATX PSU with it, of course you'll need to rig a mechanical adaptor.

The alternative of course is AT mode, in which the board is dumb, and you manually cut the mains connection to the PS. The only benefit to ATX is that the board can shut itself off. (and turn itself on, in case of wake-on-( LAN | key | etc )). I can't really see that being required in an industrial setting, but i might be missing something

In either case, if you're running the contactor off the computer PSU (say 12V rail), the machine will be powered up before the computer has booted, which seems like bad mojo to me, if it controls anything crucial... I'd prefer something like lpt -> transistor -> contactor, so you can turn the machine on/off in software, once it's already booted and functional. Throw a kill switch on the mains... You could have the computer stay up during a kill scenario that way.

 

[yonnie]

Almost diagonally opposite from the atx pwron is the front panel connector. That connector has two pins labeled button1 and button2, from MarkR's description I'm guessing button1&2 are really one button with two pins. Now that I have a better picture of how this works I can get to work on designing an interface. the computer board tells another computer how to run the machine and there is also a hardware interlock bd, so at no time can the machine do something prior to the computer booting up and a person to tell it what to do.

How about controlled power-downs after main AC power has been removed as in the EPO switch being thrown or a drunk driver hits a power pole? Any trick ideas on coming up with just enough power to keep the computer on long enough to shut down?

 

[alkalinetaupehat]

Going kinda basic, but integrate a battery backup which is charged off a PS rail?

Obviously the amount of energy that would need to be stored is dependent on how long the computer takes to complete a formal shut down plus some extra juice in case of a stubborn app or other cosmic forces of the universe.

Actual integration could be done by having the backup in-line between the power supply and motherboard and function as an automatic failover.

 

[yonnie]

I wouldn't need much power, just a few minutes. Do you know if anyone makes batteries and chargers that can be connected to small motherboards? All I need is a +5 and +12 source.