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The PSU has 2 12V rails listed in its specs on the side, however both show continuity between them when I tested it with my multimeter. I then tested continuity between the positive from the fan and 12V1 and it also showed continuity. The fan is rated at 0.7A. This is a higher load than the 22 Ohm 10W resistor I was originally going to use (0.55A). Should this provide enough of a load? Would it be necessary to put a load on one rail or the other if they are both connected internally anyway?

When I started it up with no other load connected, the grey wire came to +5V which I believe means power is OK. Is this a good sign that it likely doesn't need an extra load?

Finally, should I also put a dummy load on the +5V rail?

Extra Info:

I was able to dig up these specs, although they aren't from the manufacturer so it's hard to tell how accurate they are. They do suggest that extra load might be required though.

Min Load:

  • +3.3 - 0.3 A
  • +5V - 0.5 A
  • +12V1 - 1A
  • +12V2 - 1A

Although given that both 12V rails seem to be connected I'm not sure if that means 1A each or just 1A for both.

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    \$\begingroup\$ Should this provide enough of a load? Impossible to answer until you provide the datasheet of the PSU which might not even list that. The PSU probably meets some ATX specification, the minimum load might be listed there. It does not mean that the supply needs a certain minimum load. It can be that the supply will work fine without a load but there is no guarantee for how long it will work like that. \$\endgroup\$ – Bimpelrekkie Jul 12 '17 at 10:11
  • \$\begingroup\$ I realise that without full specifications it's impossible to say definitely, I was hoping for more of a "likely or not likely" to be OK, and also whether the Power OK signal would be a good indicator of that. \$\endgroup\$ – Shaun Bouckaert Jul 12 '17 at 10:14
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    \$\begingroup\$ I'm confused after just the first two sentences. You have a power supply with two 12 V outputs. OK so far. Then you test that both have "continuity". Continuity to what? What does it even mean for a power supply to have continuity? Then in the next sentence you're talking about continuity between the fan we're supposed to know about buy you haven't introduced, and "the 12 V". I thought there were two 12 V outputs. Giving up here and closing as unclear. \$\endgroup\$ – Olin Lathrop Jul 12 '17 at 11:06
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    \$\begingroup\$ "Is "closing" the question as unclear really fair without just asking for clarification?" Yes. We are all volunteers here. Often we do ask for clarification. Sometimes people respond, sometimes they don't. The close process must be started immediately so that the bad question doesn't cause more wasted time in case it is not fixed. Don't like that? There's a simple solution: Don't post crap. There is no excuse for the verbal diarrhea you wasted our time with in the first two sentences. If you can't be bothered to write comprehensibly, then we don't care about your problem either. \$\endgroup\$ – Olin Lathrop Jul 12 '17 at 12:25
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    \$\begingroup\$ Bad questions don't waste time, there's no metric of open questions that's somehow bad for the site. Asking for clarification is perfectly reasonable, killing off someone's question because you think it isn't good enough isn't. I'm your position I would have helped with wording the question better instead of just telling the person they've posted crap. \$\endgroup\$ – Shaun Bouckaert Jul 12 '17 at 12:49
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The problem is that most of the output voltages are generated directly from different windings of the same transformer, so it is not possible to regulate them independently.

If you only load the +12V output the controller will increase power to the transformer to compensate for voltage drop in the rectifiers etc., but this causes voltage on the other unloaded outputs to increase. If the +5V output exceeds its maximum permitted voltage the crowbar will trip and shut the PSU down.

Loading the +12V output down more will only make it worse. To prevent the +5V output from rising too high you need to draw a reasonable amount of current from it, typically 2-3A.

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  • \$\begingroup\$ Most of the sources I've read seemed to imply that more modern PSUs (ATX12V 2.x) only required a load on the 12V to be stable, although I'm reading that adding a load to the 5V rail can also help raise the 12V rail if it is slightly low, which it may have been last time I checked. 2 - 3A seems high compared to most of the sources I'm reading though. This guy is recommending a 10 Ohm which would be .5A and another suggested a 4.7 ohm which would still only be 1A \$\endgroup\$ – Shaun Bouckaert Jul 12 '17 at 10:26
  • \$\begingroup\$ You may be right. PCs are constantly evolving and it's hard to keep up with the changing specs. An 'ATX12V' PSU might regulate the 12V more and therefore require a higher load on it. Now you have some specs which suggest that if you have an ATX 12V PSU (not the same as plain ATX) you might need to draw 2A from +12V as well as 0.5A from +5V and 0.3A from +3.3V. I suggest trying different loads while monitoring the the output voltages, to see what your PSU needs. \$\endgroup\$ – Bruce Abbott Jul 12 '17 at 16:35
  • \$\begingroup\$ ATX12V has been the standard since the early 2000s/the Pentium 4 era so I'm sorry if I'd been confusing or misleading referring to it as ATX as this is fairly common amongst the IT crowd. \$\endgroup\$ – Shaun Bouckaert Jul 12 '17 at 23:10

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