Question Second PSU turning off

Oct 7, 2021
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Hi guys,

I'm trying to install a second PSU. I took an Antec 500W PSU off of a working computer.
I'm trying to test the setup before making the actual build.
I jumped the right pin on the 24-pin connector (green+black) so that it'll work as soon as I turn it on.
So far everything is fine (the PSU fan is spinning).
I even measured the output from the pins with a multimeter.
My problem is that if I connect a device that draws power from the PSU, the PSU shuts itself down (fan ceases to work).
Anyone has any idea as to why this might be happening?
 
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More information is needed. What is the device? Is it a full build? If so, what are the full specs? Also, what are the specs of the "working computer"? How old is the 500W Antec PSU, and what's its model number?
 
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Oct 7, 2021
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I'm trying to install a second PSU to a modified Mac Pro so that it can run the GPU installed at full capacity (an AMD Radeon RX Vega 56).
The GPU is now running under "economy" mode and I can't reinstall the disks I have because the original PSU can't handle the load.
I want to add a second PSU and connect it directly to the GPU.
The PSU is an Antec NeoPower 500W and has a few of years on it.
Not too many though, as I used it to replace a bad PSU in an older Mac Pro but I ended up buying a new Mac Pro a few years later.
 
Ah. Understood. Then your question is outside my field of knowledge, but hopefully others will chime in. If your Mac Pro is modern, it honestly wouldn't surprise me if Apple enacted a process to prevent your project from being successful.
 
Oct 7, 2021
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Ah. Understood. Then your question is outside my field of knowledge, but hopefully others will chime in. If your Mac Pro is modern, it honestly wouldn't surprise me if Apple enacted a process to prevent your project from being successful.
First, it's feasible as you can even find step-by-step instructions by people who've done this.
Second, the exact mod I'm attempting is irrelevant to my question since the PSU's odd behavior comes even before putting it inside a computer.
I also tried it with a PC, same behavior.
In my opinion the PSU is not faulty, I think there's something I'm missing, I just don't know what.
Thanks anyway for trying to help.
 
Hi guys,

I'm trying to install a second PSU. I took an Antec 500W PSU off of a working computer.
I'm trying to test the setup before making the actual build.
I jumped the right pin on the 24-pin connector (green+black) so that it'll work as soon as I turn it on.
So far everything is fine (the PSU fan is spinning).
I even measured the output from the pins with a multimeter.
My problem is that if I connect a device that draws power from the PSU, the PSU shuts itself down (fan ceases to work).
Anyone has any idea as to why this might be happening?
Welll I can say that Antec PSU is based on older design 5v 12v group regulated.. the issue with this design is.. PSU can't handle cross rail load.. so if you put 50% load on 12v and 10% load of 5v and 3.3v PSU will shut down.. because these PSUs faces bad voltage regulation.. under cross rail load... Then PSU protection trips the PSU.. I willl say get s PSU with labeling LLC resonance converters + DC- DC converter design... They handles cross rail load amazingly..
 
Oct 7, 2021
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Thanks arif1752002m4a1. Well, if I understand your explanation correctly then I don't think I'm putting a heavy load on the 12V.
Anyway, I had this PSU lying around and I wasn't planning on buying a new one.
Your explanation does really makes sense, but it looks a bit odd to me that this PSU can handle an old Mac Pro flawlessly but fail when a single hardware piece is connected to it.
 

DSzymborski

Titan
Moderator
Thanks arif1752002m4a1. Well, if I understand your explanation correctly then I don't think I'm putting a heavy load on the 12V.
Anyway, I had this PSU lying around and I wasn't planning on buying a new one.
Your explanation does really makes sense, but it looks a bit odd to me that this PSU can handle an old Mac Pro flawlessly but fail when a single hardware piece is connected to it.
If you're using this with a GPU, you're putting a heavy load on the +12V. A PCIE power connection doesn't even have +3.3V or +5V wires. And these very old PSUs, which were basically designed to deal with 1990s PCs, have a lot trouble with unbalanced loads like this. Basically, as soon as you're dealing with post-Pentium III hardware and a GPU that needs supplementary power, you ought to not touch a group-regulated PSU.
 
Oct 7, 2021
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I know that a GPU is heavy load (that's the reason for a second PSU in the first place), but I think it gave way even when I connected a wire with nothing connected to it. I'm not sure about this, I guess I'll have to test this again.
 

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