Your cup shall runneth over.

I suspect "nothing good" is right, but also keen to hear from anyone who knows.

Both power connectors should go to the same traces and voltage regulators. Nothing bad will happen if both power supplies are working properly at the same voltage/ripple. If the power supplies are too far out of spec with each other, one can potentially drag the other down and exceed the AT/ATX specifications, which impacts system stability.

Interesting! So in theory you could set it up like that as a redundancy/fallback option.

I think the most problems would occur if AT was backfeeding the ATX when it went into standby, but that's a WAG. Could get some funky current loops though if one had levels a ways off from the other say 4.75 vs 5.4 or 11.5 vs 12.5.

My guess is nothing good. Power supplies are never exactly matched unless calibrated at the factory for dual PSU systems like servers, and even then they are typically run in split rails or used redundantly anyways.

As I recall what will likely happen is that one PSU will be lower voltage than the other and that the higher voltage one will "Win out" and take on all the load making the other PSU worthless, or in some cases if the difference is large enough it can backfeed and kill something.

Wouldn't recommend, but if you don't care about the wellbeing of the board, either PSU, or yourself, go ahead and see how it goes

There are methods used to parallel two DC supplies.
https://techweb.rohm.com/knowledge/dcdc/dcdc- … dcdc-ldo01/9235

Which is to say it can be done but isn't necessarily as straight forward as you hope.

gdjacobs wrote on 2021-06-07, 01:45:

There are methods used to parallel two DC supplies.
https://techweb.rohm.com/knowledge/dcdc/dcdc- … dcdc-ldo01/9235

Which is to say it can be done but isn't necessarily as straight forward as you hope.

And in more high end environments there are diode modules like this: https://www.phoenixcontact.com/online/portal/ … af-cbb2f106c22e

On a client site, the control network is set up with gear like this for redundancy. A given network switch is ultimately powered from two different 11kV substations located a mile apart. Pretty cool.

Pierre32 wrote on 2021-06-07, 02:07:

gdjacobs wrote on 2021-06-07, 01:45:

There are methods used to parallel two DC supplies.
https://techweb.rohm.com/knowledge/dcdc/dcdc- … dcdc-ldo01/9235

Which is to say it can be done but isn't necessarily as straight forward as you hope.

And in more high end environments there are diode modules like this: https://www.phoenixcontact.com/online/portal/ … af-cbb2f106c22e

On a client site, the control network is set up with gear like this for redundancy. A given network switch is ultimately powered from two different 11kV substations located a mile apart. Pretty cool.

Indeed, although redundant switchover mechanisms is a little different than ballasted supplies. The overlap isn't 100%.

Pretty sure the motherboards that have both AT and ATX connectors have a warning in the manual to only use one or the other.