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I wanted to ask the question about two parallel power supplies. I was asked by a friend if these would be a good idea Add2PSU

At first thought it looks very simple and good idea to connect 2 ATX PSUs in parallel, and let them share the load for more power and redundancy (in case of failure in one of them).

The problem is, ATX power supplies can vary quite a bit (more than a few mV) One can be 11.9V one can be 12.6V. That can cause voltage drawn from one power supply to another, which can be even dangerous.

Maybe if there was an inline diode I guess on each rail. The question is about the diode voltage dropdown, and the need to deal with the heat (450W could come over the diode).

I don't really know much about the guts of how ATX power supplies are regulated/output, but since these put out high current I wanted to be safe and ask where people would know

Thanks for any input

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    \$\begingroup\$ I don't know the purpose but if you need more current then paralleling with diodes is not the way to go. Because this OR gate will select the higher one instead of both. You can parallel with small resistors but the dissipation will be a limiting parameter. \$\endgroup\$
    – Rohat Kılıç
    Commented Jan 26, 2017 at 3:51
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    \$\begingroup\$ Possible duplicate of Paralleling power supplies for higher current \$\endgroup\$
    – Voltage Spike ♦
    Commented Jan 26, 2017 at 5:52
  • \$\begingroup\$ I would advise against it but if you want to do it as a learning experience and not as a long-term solution, you can do a dirty hack: Open both power supplies and adjust both to 12.4 V. Load them both to max separately and measure the voltage drop. Add a resistor to the one with the lowest voltage drop to match the one with higher drop at that current. Use two Schottky diodes for ORing. It should work is many cases but I would keep my trusty fire extinguisher nearby the first time I use this contraption. \$\endgroup\$
    – winny
    Commented Jan 26, 2017 at 8:34

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The product you link to does NOT connect 2 power supplies in parallel.

You add a second power supply to pick up particular peripherals, and this product enables that by using the +5/+12 connector from the main supply to turn on the second power supply. You then use the Molex and SATA connectors to power peripherals such as disk drives or graphics cards.

I'd almost say this product uses deceptive advertising since they have you plug in the second power supplies motherboard connector to their board, but I'd guarantee that they do not connect back to the first power supply. If you think about it for more than a moment you will understand that the power would have to flow back to the first power supply over the Molex connector!!

If you watch their videos they carefully do not explain anything other than the first power supply turning on the second. And in their final video they show the 12 V supplies being at different voltage levels....which would be impossible if they were in fact paralleled.

I'd suggest the product is quite safe to use since it's NOT paralleling the power supplies. PS: There was just a couple of years ago many Auxiliary power supplies that worked in exactly the same way, they fitted in a 5.25" disk drive slot. You can't find them anymore....and they were great value.

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  • \$\begingroup\$ Thanks for the info. If there is +12V from PSU#1 on the PCIe slot, and +12V from the PCIe 8 pin connector, would not that be paralleling these? (Most graphics cards pull 12V from both a PCIe slot and also an additional connector \$\endgroup\$
    – Dan
    Commented Jan 26, 2017 at 15:39
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    \$\begingroup\$ The +12 V power from the PCIE slot connector should NOT be powering the same points on a graphics card as the Molex auxiliary power connector. If a card manufacturer did this then it's not compliant. See here: s14.postimage.org/b82dn5cb5/2x4_PCIe_Power_Connector_Pinout.jpg The sense pins validate that the external supply is connected and there is a power limit from the auxiliary connector. The power drawn from the PCIE slot is limited to 25 W. Typically the 4-pin is maxed out at 225 W and the 8-pin at 300 W. For example my HD 4850x2 has both 8-pin and 4-pin power and I use both. \$\endgroup\$
    – Jack Creasey
    Commented Jan 26, 2017 at 17:14
  • \$\begingroup\$ @JackCreasey Just to be completely safe I'm basically asking the same as Dan only rephrasing it. So is it completely safe to power the GPU's extra 8pin connectors from a secondary power supply (I have a GPU with 2x8pin connectors on the top) while the PCIe slot gets the power from the main ATX PSU? (sorry for hijacking an old question and answer but the GPU is quite expensive these days I don't want to damage it) \$\endgroup\$
    – Andrew
    Commented Oct 6, 2017 at 20:38
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    \$\begingroup\$ @Andrew. Yes ....the connectors on a Graphics cards are separate power domains on the board. If you want to convince yourself, use a multimeter to measure continuity (resistance) between the connectors. The ground will obviously connect together, but the 12 V supplies will not. \$\endgroup\$
    – Jack Creasey
    Commented Oct 6, 2017 at 23:59
  • \$\begingroup\$ @JackCreasey Thanks (especially for the quick reply). This info calms my nerves :) \$\endgroup\$
    – Andrew
    Commented Oct 7, 2017 at 0:18
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Generally, most computer power supplies have built in potentiometers that can be used to slightly adjust the voltage on each rail. If you match the voltages up almost perfectly, they should work, but I'd still recommend using two diodes or just getting one more powerful supply. What exactly do you need to power?

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I have been thinking about this a while a go. If it's just to experiment with it, I think it might be possible if you use one the voltage feedback circuits of one of the supplies, and use this one also as feedback for the other.

This will require some hacking.

But before you try, make sure you've checked the inner workings and schematics of a atx power supply. And maybe an answer of someone who has more experience with that.

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