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Why does power connection from riser have to be all on the same PSU?

If you don't its a possibility you will see your card's send out this magical puff of smoke. That is one of the worst feelings you can have mining is seeing a GPU send up like a sos with smoke.

I get the feeling that some of these powered risers are splicing in the power from the molex or whatever connecter, and not actually disconnecting the pci-e bus 12V from its connection to the motherboard? Which would be a dumb way to go about it for sure. But would likely work in practice for preventing overheating and MB damage if you just have both going back to the same power supply (or particular rail of a PSU). Otherwise, you are paralleling two power supplies and they do not like that one bit. Has anyone worked out the pinouts and tested a cheap riser on a multimeter?

If the riser is designed properly (a big if: I only trust the ones that use something shielded like USB, not ribbon cable, and I don't really trust those) then it should work no differently than having a hard drive or other peripheral powered by a different rail and the I/O is fine.

Well, its better to have it in the same PSU. I tried separate and it just works fine..

It should be on one PSU, noone want asmoke in his room.

Don't do it.

I chained 3 powered risers together and then put them on the second PSU (not powering the motherboard) and fried all of my risers within a minute of heavy load. These risers were not cut. It even managed to fry the risers that were NOT powered!

So the rules I've learned are:

1. Plug all powered risers into the same PSU that feeds the motherboard.

OR

2. Cut the the 12v line of the ribbon cable and plug the powered riser into any PSU you want.



Additionally, I plugged 3 7870s onto my motherboard using 16x-16x unpowered risers and STILL managed to burn one of the risers on a card where the fan was blowing at 85% overnight. It wasn't an instant short like above but it did happen eventually. In my opinion if you are running more than two cards then you must use powered risers on ALL cards. If you have two PSUs then you really should cut the cables. Even if your frame is open if you think one of your cards is going to go 85% or higher for prolonged periods it can burn the riser as well!

Thanks for clarifying how to do it. I still don't get why tough...  :-\

Well that proves it then. Whoever designs a powered riser like that is a moron. cryptoderf, in simple terms: you have 12V coming from the motherboard, which gets it from PSU #1. If you then connect this to the 12V coming from PSU #2 then you have just cross-connected two power supplies. They never put out exactly 12.000 volts, there will be some variation, so current begins to flow between the power supplies. In fact, the current flows in such a way that the voltage difference only gets worse. At least, until something melts and the circuit is broken.

They probably do it that way because they expect everyone to only use a single power supply, and that way it still works as an unpowered riser if you don't connect power.