Member Avatar for syx

I have connected (temporarely) my old 200W AT PSU in my computer to power everything except mobo and everything is powering up normally, i have some questions!

My power ok (AT PSU) isn't connected to anything, to what i connect it?
12V 5V ground??

I want to put a relay on the switch of the at power supply so the ATX PSU will start it automatically, how much amps goes in that switch? i want to get a relay big enouph!

BTW i have a common ground and my second PSU in in my 2 upper CD drive bay, yes it fit... tight but it fit! Looks kind of weird!

THX all!!! :oops:

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Short summary and safe way forward for ’s dual‑supply idea, with notes tied to earlier replies by , and .

A few key rules before anything else: the AT supply’s mains (AC) switch must be driven by a mains‑rated contact, only the hot/line conductor should be switched, and both supplies must share a solid common chassis/ground connection to avoid floating returns or signal problems (the concern raised by is valid). The “Power Good” / PWR_OK line is a signalling output on a PSU, not a power feed for other devices — if the AT PSU is not supplying the motherboard it can be left unconnected, but understand what that line does if the AT supply ever is asked to drive logic.

Relay choice and placement

  • Use a mechanical relay or contactor whose contact rating comfortably exceeds the AT PSU’s expected line current plus startup/inrush. For a 200 W PSU on 120 VAC the steady current is modest (~2 A), but inrush and connected motor/transformer loads can spike. Pick a relay rated 10–20 A at mains voltage (ideally 250 VAC/10 A or better) and with AC ratings shown on the datasheet.
  • Use the relay’s contacts in series with the AT PSU’s hot (live) input or replace the AT supply’s switch with the relay contacts. Do not try to switch both hot and chassis ground.

How to control the relay safely (simple, reliable option)

  • Drive a low‑current 5 V or 12 V relay coil from the ATX main rail that only appears when the ATX supply is fully on (NOT from +5VSB, which is always present). That way the AT PSU will only be switched on after the ATX main rails are live.
  • Fit a flyback diode across the relay coil and, if the coil current exceeds what the motherboard header can source, use a small NPN transistor or MOSFET as a driver with the coil powered from the ATX rail and the transistor switching coil‑ground. Keep grounds common.

Small illustrative wiring (conceptual)

ATX +5V main  -----+----> Relay coil (+)
                   | 
                 [relay coil]
                   |
ATX GND  ----------+----> Relay coil (-) via driver + flyback diode

Relay contacts (NO) in series with AT PSU mains hot (Line) -> AT PSU input

Practical cautions and testing

  • Always work with the unit unplugged when wiring mains; use properly insulated terminals and a fuse on the AT PSU mains feed.
  • Test the control circuit first with the AT PSU disconnected (verify the relay only pulls in after the ATX main +5 V appears). Measure with a meter or clamp meter before connecting drives.
  • Avoid paralleling DC rails between supplies. Distribute loads (motherboard/CPU on ATX, drives on AT) and tie grounds at a single chassis point to minimise loops.

As hinted, a physically small relay will often “work,” but choosing a contact rating with headroom and adding coil suppression and proper isolation is what keeps the case safe and reliable.

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Member Avatar for TallCool1

My power ok (AT PSU) isn't connected to anything, to what do I connect it?

Don't worry. That's a "feedback" connection -- you have no use for it.

I want to put a relay on the switch of the AT power supply so the ATX PSU will start it automatically, how much amps goes in that switch? i want to get a relay big enough!

6 amps should be enough, but 10-amp relays are still relatively cheap.

Member Avatar for syx

fIRST ANSWER, FIRST GOOD INFORMATION, I LOVE YOU MAN!

thX!!!!!

Member Avatar for Lynn

I am curious is the reason for using two power supplies to get enough
power to supply the motherboard seperately? Using the second one to supply the all the other things that are normally connected to the primary power supply?
This may not work due to grounds being connected by the various hookup cables to the motherboard. I would personaly not recommend doing this.
As for the relay ATX supplies can be turned on with the mouse or keyboard through the setup in Bios. As for the AT supply it can be turned on by the power switch that is normally mounted on the front of the case.
I reserve the right to be wrong, these are my thoughts, HTH. ;)

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