x2 iMac PSUs to power a SFF?

deadmanwalking

Minimal Tinkerer
Original poster
Dec 1, 2019
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SFX PSUs are ridiculously expensive in my country (USD$200 and more), if I try to import one, the mere shipment cost will be too expensive to even consider it.
After looking around I came with the idea of get an iMac PSU to power the PC, I can get them NEW for cheap and shipment is very reasonable, adding the benefits of smaller size compared even to a SFX PSU, neither need a DC/DC converter, like using a external brick.
I'm planning to use a Apple iMac 21.5" 2009-2011 A1311 205W PSU 614-0444 661-5299 that is enought for the 65w CPU and probably a low power 80w GPU, but if in the future I want to upgrade to a more power demanding CPU/GPU, the 205w can be not enought (besides, I'm sure not all the 205w can be delivered to the 12v connector).
I know I probably can use a ADD2PSU to sync 2 PSUs, 1 feeding the CPU, the other the GPU but I'm not completely convinced about the ADD2PSU...
So the question come here: can I get x2 PSU just wired togheter to feed a single CPU/GPU?
Thanks in advance for the advice and comentaries.
 

Choidebu

SFF Guru
Aug 16, 2017
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Short answer is Yes, you can definitely do that, but you need to find out how your particular PSUs work.

If it's like a dumb brick - single voltage, on all the time, then you can get a dc-atx unit to your cpu and the other connected to your gpu, preferably with a load switch (what add2psu essentially is), but some cards are even fine without it.

If it's an intelligent one with on/off control and standby voltages then it's a bit more wiring work but you don't need anything else provided all voltage rails are there.

Sorry I don't have time to google up your apple psu, maybe others can help.
 

deadmanwalking

Minimal Tinkerer
Original poster
Dec 1, 2019
4
0
Short answer is Yes, you can definitely do that, but you need to find out how your particular PSUs work.

If it's like a dumb brick - single voltage, on all the time, then you can get a dc-atx unit to your cpu and the other connected to your gpu, preferably with a load switch (what add2psu essentially is), but some cards are even fine without it.

If it's an intelligent one with on/off control and standby voltages then it's a bit more wiring work but you don't need anything else provided all voltage rails are there.

Sorry I don't have time to google up your apple psu, maybe others can help.
Thanks for your answer, the 614-0444 PSU pinout diagram are aviable, and it have standby and soft power on/off functions, can I just join a couple of them in parallel like if they are AA batteries to reach the needed wattage?

Are you brazilian?
No, but I believe it's not relevant...
 

Choidebu

SFF Guru
Aug 16, 2017
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1,070
can I just join a couple of them in parallel like if they are AA batter
NO. Series, yes, not parallel.

Just use one to power the gpu, and tbe other for everything else. Connect both psu's ground, replicate ps_on to reach both psu.
 

deadmanwalking

Minimal Tinkerer
Original poster
Dec 1, 2019
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Just curiosity, bro... I'm brazilian myself and would be nice to discuss about the market in here with someone that found iMacs PSUs. nvm.
*Maybe* tou can get them from broken iMacs, the A1224 is a very comon old model, often worth more as spares than as a working unit, I fix several PSUs from that model by a easy and cheap capacitor replacement. You can buy a working one, take the PSU and sell back as spares, or buy a broken one, fix the PSU (if it's the broken part) and sell the pieces. If you get a controller for the LCD, now you get a fancy monitor and a PSU. The disadvantage will be the high initial cost, even if in the end could be "free" by the sell of the unused parts.
NO. Series, yes, not parallel.

Just use one to power the gpu, and tbe other for everything else. Connect both psu's ground, replicate ps_on to reach both psu.
I thought you couldn't connect PSUs in series (+12v of PSU1 to -12v of PSU2, feed the component with -12v from PSU1 and +12v from PSU2), if it could be done it feed 24v instead of 12v, maybe I'm wrong...
 

Choidebu

SFF Guru
Aug 16, 2017
1,096
1,070
Yes you can. And it's not -12V, it's V-, essentially 0V. 12V to -12V would be 24V. And yes it'd feed 24V, max current would follow weakest one.

The problem with running parallel PSU is that (1) Both PSU should have proper back current protection (2) There's no telling which one will supply more current - it depends on multiple things such as output impedance etc, so say you run a 300W load across two 200W PSU, one will most definitely surge 200W peak while the other cruising only serving 100W. You do this long enough that one PSU would fail quick.
 

deadmanwalking

Minimal Tinkerer
Original poster
Dec 1, 2019
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I found something that likely can work here:


Need some soldering and I still have to read about the topic more to understand completely the system.

Too I was thinking as alternative about something like the PCI-E risers used in mining rigs to separate the signal from the power so I can use 1 PSU to power the CPU and the second PSU to power the GPU, but these risers are only 1x, and to max out the GPU I need at least 8x bandwidth. Do anyone know a 8x or 16x external powered PCI-E riser? thanks
 

Thehack

Shrink Way Wielder
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Mar 6, 2016
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I found something that likely can work here:


Need some soldering and I still have to read about the topic more to understand completely the system.

Too I was thinking as alternative about something like the PCI-E risers used in mining rigs to separate the signal from the power so I can use 1 PSU to power the CPU and the second PSU to power the GPU, but these risers are only 1x, and to max out the GPU I need at least 8x bandwidth. Do anyone know a 8x or 16x external powered PCI-E riser? thanks
That's not a very good system. At low currents (wattage of your system), it's hard to drive up the temperature to force the two to a balance mode. The other issue is you need the two power supplies to be near voltage for a passive system to work. If the two power supply lack voltage adjustments and their voltage differs too much you'll get incorrect current flow.

The schematic you provided is an "dumb" load balance circuit that requires higher temperature to increase resistance to encourage load balance. This solution won't work unless you've already calculated the heat dissipation in your exact use case and choose the correct mosfet that will be matched to load balance. Too high spec of a mosfet will mean it won't heat up enough to make a dent in load balancing.