I hope this is not off topic here, but I have a PC with a 900w ATX PSU (Be Quiet! Dark Power Pro 8 900W). I have it set up in my PC with motherboard, GPU and case fans connected to it. I`m now connecting some more case fans, which seem to cause some problem.

The PSU has 5 12V connections specifically for case fans. Each case-fan output I use to drive some fans. I cant make out from the sticker on the back for which amperage those outputs are rated, but I assume it to be sufficient for several case fans.

I have one set of 5 50mm fans (Revoltec Airguard, rated 0.08 A at 12V). I first connect them together in parallel with some 'jumper' wires. I then connect them to the one 12V case fan output that was still free. When I switch on the system it continuously tries to switch on but seems to lose power and then tries to switch on again. This does not occur when I disconnect these fans that I just added. This behaviour really puzzles me, I cant imagine these small fans draw too much power.

What could be possible causes for this behaviour?

Things I`m thinking about are: Am I connecting these fans incorrectly? Could connecting 5 fans in parallel cause some kind of surge at boot? Could one of the fans be shorting out? I dont know how to approach this.

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  • 1
    Often with ATX supplies you need to draw a minimum amount of current from one of the rails. If you're not doing this it may be shutting down. Do you have a manual for the power supply? – Colin__s Aug 15 '17 at 8:15
  • Well how do you approach a problem? You try out different things and see what happens. If you don't connect the fans, does it work? If you connect 1, does it work? If you connect 2, does it work? Maybe you see a pattern there. And this question would be better suited to Superuser or something like that. – Arsenal Aug 15 '17 at 8:15
  • does it boot without all the fans connected? Does it boot with 1 fan connected? Two? you need to try combinations - it may well be that each of those connectors only supports one fan... – Solar Mike Aug 15 '17 at 8:17
  • You should mention the exact model of the PSU and/or a link to its specifications or datasheet. If you have only connected the fans (no PC motherboard etc.) then you're not loading the PSU enough and it can indeed switch off. But you need a motherboard connected to make an ATX PSU switch on so you probably have something connected. Does the system power on properly with no fans connected ? It is safe to try that for a few seconds. The 12 V for the fans is often directly connected to the main 12 V of the supply so it can provide enough current. – Bimpelrekkie Aug 15 '17 at 8:42
  • Some motherboards won't boot unless they have a fan connected to one of the fan connectors on the board as they monitor them to make sure they're working. – Finbarr Aug 15 '17 at 9:15

When I switch on the system it continuously tries to switch on but seems to lose power and then tries to switch on again.

Many power supplies display that behaviour when they detect a short-circuit; the short-circuit protection circuitry shuts down the PSU to prevent damage.

I have one set of 5 50mm fans (Revoltec Airguard, rated 0.08 A at 12V). I first connect them together in parallel with some 'jumper' wires. I then connect them to the one 12V case fan output that was still free. ... This does not occur when I disconnect these fans that I just added.

In that context, I suspect you wired something incorrectly, or maybe one of the fans is defective.

Connect one of those 5 fans, and check if the system will start without problems. Repeat for the 4 others. This should determine which (if any) of the fans are faulty.

If that works, try with just two fans in parallel. Fewer wires is fewer chances for mistakes. If that works, add a third. And a fourth, etc.

Finally, editing your question to include a picture of your wiring could help us shed some light on the issue.

Could connecting 5 fans in parallel cause some kind of surge at boot?

Yes, electrical motors (which fans are) with no mitigation measures draw a much larger current on startup (or when stalled), from several times the nominal current up to maybe 10 times.

But hey, 0.08 * 10 * 5 is still only 4 amps, and I suspect the PSU should be able to handle that.

You could start the system without the fans connected, and then connecting the 5 parallel fans in one go when the system is already running.

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