Hi All,

I am currently designing a single system with a bunch of fans but 2 individual PSUs and motherboards in there.

The fans must be powered by both of the ATX PSUs in the system (well, whichever one happens to be on, or if both are on, one of them)

This is because if one system goes off (and thus, its PSU powers down all except the +5vSB rails) I still need all of the fans in the system to function for the remaining system.

I presume I cannot just run both PSUs to the fans directly, as I am not sure the +12v rails are protected by diodes? (I may be able to find out from the mft, or I could just try it, but they are server class PSUs and very expensive so dont want to just "risk" it)

I am sure there is a simple logic setup that could be done with relays but my brain is not capable of working it out due to my general amateur level of electronics experience.

Can anyone enlighten me how to achieve this easily?

Thanks
Mark

 

You just need two diodes like this:

 

Thanks Albert, I had considered that (hence saying I don't know if the PSUs themselves have diodes inside).
The voltage will be 12v at around 19A. Will the voltage drop and heat production of the diodes not be considerable?
What sort of diode would one look for?

 

Thanks Albert, I had considered that (hence saying I don't know if the PSUs themselves have diodes inside).
The voltage will be 12v at around 19A. Will the voltage drop and heat production of the diodes not be considerable?
What sort of diode would one look for?

I thought that you only wanted the fans to work whichever supply was on. If that is the case. then the above circuit is the way to go, with the diodes feeding the fans not the rest of the system. The fans will need a lot less than 19A and they won't mind only getting 11V instead of 12V.

 

Thanks again Albert. To clarify there are 16 fans 1.2A each so 19A max draw - which may well be for sustained periods of weeks or months.
I want them to work whichever supply is on and I guess the above circuit somewhat allows the load to be shared amongst the connector/s I end up using.
I have been considering an alternative as an automotive 5 pin relay, using the 12v from 1 PSU to act as the control wire.
This would mean that 1 PSU would always power the fans, and there would be a momentary loss of power on switching - but nothing that would be noticeable to the system, the fans would barely have a chance to spin down as it switched over.
Is there any reason this is a bad idea?

 

Is there any reason this is a bad idea?

No, that would work fine.