Debating on cooling fan installation - Page 1
Forum section: electronics / beginners

author: DarkBabylon
author_rank: Contributor
timestamp: June 24, 2016, 03:55:13 pm
content: Hello, I have been working on hacking together a DC power supply out of a computer PSU. So far so good, already have most of it put together however we already know these things tend to get hot, and due to lack of room inside the unit I had to replace the fan for a Scythe Big Shuriken 2.It is rated at 12 volts with 0.39 amps input current, while the original fan was 12 volt 0.35 amps.Several questions:Should it be installed in such a way that it takes hot air out or inserts cool air in? Regardless there should be air flow between the fan and one of the walls of the unit.The second question is that I have 3 potential wires to hook the fan to:-A +12 volt able to deliver up to 17 amps (two rails)-A -12 volt with up to 0.8 amps-A 12 volt difference inside the PSU, presumably able to deliver at least 0.35 amps.I'd rather not use the one inside the PSU specifically made for the original fan, but I would like to use either the -12 volt rail or the +12 volt rail.Is it possible to connect a -12 volt rail instead of the +12 volt? (obviously with the ground of the fan hooked to the -12 volt) And is it likely that I would fry the unit with any of the 3? It is impossible for me to test it at 12 volts for the moment as I don't have anything that can deliver such an amount of electric tension.

author: Signal32
author_rank: Frequent Contributor
timestamp: June 24, 2016, 05:31:28 pm
content: Install it the same way the previous one was installed - almost all PSU's that I've seen blow hot air out.The 12v inside the PSU is very likely connected to the +12v external supply, perhaps with some extra filtering. Check continuity. This is what I would use. Also it might try to detect if a fan is installed (and will not power on if it doesn't detect it). Plus maybe it's temperature controlled so that would also be a benefit.Yes you can hook up -12v by using ground as positive.No, you won't fry the fan unless the PSU is damaged or you hook the fan up between -12v and +12v

author: System Error Message
author_rank: Frequent Contributor
timestamp: June 24, 2016, 05:33:12 pm
content: use the +12V supply. You could technically use the -12V supply if the fan could spin the other way but i wouldnt bet it could. You want +12V to 0V and not +6V to -6V.

author: timb
author_rank: Super Contributor
timestamp: June 25, 2016, 01:43:37 am
content: Quote from: System Error Message on June 24, 2016, 05:33:12 pmYou could hook the fan's "+" wire to GND (0V) and hook the "-" wire to -12V. That would deliver +12V to it.+12V is +12V, no matter if you go 0V to -12V, +12V to 0V or +6V to -6V.(The only catch to this would be if it was a 3-wire fan with an RPM output. In that case you'd have to power it from +12V to 0V if the circuit it was being plugged into expected a ground referenced signal.)

author: newbrain
author_rank: Super Contributor
timestamp: June 25, 2016, 09:49:49 am
content: Quote from: Signal32 on June 24, 2016, 05:31:28 pmGood advice.Quote from: Signal32 on June 24, 2016, 05:31:28 pmReality check: of the three PSUs in front of me (EVGA 750W G2, Enermax Liberty 400W, OCZ Fatal1ty 550W) exactly none extracts air with the fan.The reason is quite clear, in a normal ATX case the PSU is on the top, so the fan ends up on the bottom face of the PSU.Using it to blow air towards the PSU inside is helping the natural convection instead of fighting it.There might still be some lower quality PSU with a small fan on the back panel extracting air, but given the replacement the OP is choosing, this should not be case here.