ATX Power Supply Dual +12 V Outputs: Fakery Thereof

I wondered if the Thing-O-Matic would benefit from having its two high-current heaters on a separate +12 V supply than the DC Extruder, after finding that the heaters dragged the +12 V output down by nearly half a volt.

A bit of rummaging turned up a suitable ATX supply with a data plate that might justifiably lead one to believe that the supply provides separate +12 V outputs:

Turbolink ATX-CW420W power supply data plate
Turbolink ATX-CW420W power supply data plate

There’s no indication which of the four connectors might use +12V1 and +12V2, but, being that sort of guy, I applied an ohmmeter to the various yellow wires and found they were all exactly 0.0 Ω apart.


So I opened the Warranty Void If Seal Removed top cover and found this situation:

ATX with fake dual 12 V supplies
ATX with fake dual 12 V supplies

Nota bene:

  • All the yellow wires terminate in the same solder blob below the PCB
  • Two incoming wires got neatly spliced together in mid-air, despite having free holes in the PCB

This may not come as much of a shock: they lie…

Perhaps if you spend more money on your supply, it’ll actually live up to the data plate specs. Then, again, perhaps you’ll just be spending more money.

And, if you swap in a fancy supply for the MBI-stock one, it might not make much difference at all. I suspect the various power levels and current capacities have pretty much the same degree of integrity…

6 thoughts on “ATX Power Supply Dual +12 V Outputs: Fakery Thereof

  1. There’s a good writeup on why there “are” two +12V rails (it’s part of the ATX 2 spec) and PC power supplies in general there. Dan’s blend of snark, intelligence, and humour reminds me of yours. Does the Thing-O-Matic pay attention to the “PWR-OK” lead?

    1. Great link.

      Confirms my suspicions that there’s more smoke-and-mirrors than actual substance in those boxes, too. I like the spec:

      a power supply should not only not catch fire or emit “excessive” smoke or “molten material”, but it also should not make a “startling noise”.

      You mean that fap was inadvertent? Zowie!

      Does the Thing-O-Matic pay attention to the “PWR-OK” lead?

      Haven’t a clue, as I didn’t rummage in that part of The Source, but I suspect not. Various stories indicate that when the +12 V line goes toes-up, the Motherboard doesn’t notice: it’s using a separate logic supply, of course.

      If I were doing a from-scratch design, I’d run the Motherboard from the +5 V Standby circuit so it’d be powered whenever the supply was turned on.

      1. Depends; the G3 motherboard runs on the standby 5V. G4 uses a linear regulator from 12V, but can live through a power dip by switching to USB power if the USB is still plugged in. Neither checks the PWR_OK line.

        1. That’d be something to add in the next version: a brownout sufficient to trip PWR_OK ought to bring the proceedings to an immediate, albeit controlled, halt.

          I think the standby supply is supposed to survive such brownouts, but … who knows what’s actually implemented?

    1. no resistance between any of the 12V lines

      We know that one careful measurement is worth a kilo-opinion, but it’s a shame the specifications aren’t more trustworthy.

      Makes you wonder what else isn’t quite up to spec, doesn’t it?

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