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Old 23 July 2019, 10:48   #5
Daedalus
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Join Date: Jun 2009
Location: Dublin, then Glasgow
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Grey and purple are both separate signals on a standard ATX supply, and both will typically be 5V when the PSU is operating normally. Purple will be 5V even when the PSU is off, and grey will be 5V when it is on and stable (or just when it is on on cheap PSUs).

Most PSUs these days expect the main load on the 12V rail, with the 3.3V rail derived from the 5V rail and thus sharing its load requirements. The sense wire thing is still a potential issue on any rail. These outputs in particular may need a dummy load, and may also need to be attached together with other outputs of the same voltage, i.e. all 3.3V outputs connected together, all 5V outputs connected together, and all 12V outputs connected together.

Conversely, if the PSU has split outputs (common enough in modern high-end PSUs), it might not like them being connected together as they're designed to operate separately. The outputs will be marked something like 12V1 and 12V2 in the PSU specs in this case. This might explain why it works when disconnected, but not when connected.

Finally, a good PSU won't turn on if it detects a short on any rail. If the A4000 is working fine, then that's not the case here, but some issue with the connectors perhaps could result in a short on one rail.
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