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Two PSU's In The Same System

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  • Two PSU's In The Same System

    I recently built out a new case because I wanted to split my cooling loop (one loop for the CPU, and one for the GPU). Since I am currently overclocking a little and want to take that aspect of my system a little closer to the edge, I was a bit concerned (whether justified or not) that my old Thermaltake 485w PSU wouldn't be able to handle the load of an additional 12v pump being added to the system. I immediately decided a second PSU was in order, so I ordered a OCZ 520 Modstream. The most obvious question then came to mind - how do you get both PSU's to turn on with the systems power switch?

    The answer is "use a power relay". I puzzled over exactly which one to get because my first thought was to use a PCB-mounted part. Then, the word "soldering" came up, and since I don't have the soldering gene in my DNA (not to mention my troubled past regarding power tools of any description), I decided that I should look for other options. Do you know how many different compact 12VDC relays are available? My biggest concern was getting one that could support an unknown sustained amperage, and that did not require soldering.

    I was in the auto parts store with the intention of getting my antfreeze (two different colors, but that's related to my two loops, not this relay thing) when I had an epiphany - why not use an automotive relay? They're inexpensive, durable, small, and easy to find. So I checked out their selection, and I found precisely what I needed - a 12VDC/30A four-spade "normally open" relay for just $3! Even better, it required solderless connectors, so no soldering was involved, which also meant that I wouldn't be visiting the emergency room as a result of this mod.

    Before I go further, let's establish some terminology. This mod involves two PSUs, one that connects to your motherboard, and one that doesn't. We'll call the one that does connect to your motherboard the "PRIMARY" PSU. The other one will be called the "SECONDARY" PSU. And so we continue...

    Connecting the pins was easy because the schematic for the relay is engraved right on top of it. So, I made two pigtails.

    The first one is comprised of a 4-pin male molex connector that utilizes just two of the contacts, along with two wires (and the correct male molex pins of course). To get the right voltage, line the new male connector up to an existing female connector off your PSU (or fan , or whetever), and make a mental note of where you need to put your pigtail wires on the connector. You want to use the 12V side of the connector, so the yellow wire and the black ground wire right next to it are the ones in which you're interested. Crimp a molex pin on one end of each wire, and then a solderless connector onto the opposite end of each wire. Insert the molex pins into the correct holes of the molex connector, and connect the solderless connectors to the correct terminals on the relay. Polarity does NOT matter, since all you're doing is supplying power to a coil in the relay that pulls a switch closed (also inside the relay), causing the other two pins on the relay to short together..

    The second pigtail uses part of an ATX extension cable. These come in two flavors - 20-pin and 24-pin. Purchase the one that is compatible with your SECONDARY power supply because it needs to plug into the ATX connector on that PSU (these typically cost about $5). Now, find the end of this extension cable that would actually plug into the motherboard. It will have a locking lever on the center of one of the flat sides of the connector. Take some wire cutters and snip the green wire and an adjacent black wire as close to the female connector as possible. Next, extract any pin AT THE OTHER END OF THE CABLE that is not the green and black ones you cut. This leaves you with a male ATX connector with just two wires. Crimp a solderless connector onto each of the two wires. Finally, push the connectors onto the remaining relay terminals. Once again, polarity doesn't matter.

    Now, you're ready to test your creation


    2) Plug the relay's 4-pin molex connector into an available female connector that's connected to your PRIMARY PSU.

    3) Plug the 20/24-pin ATX connector onto the ATX connector from your SECONDARY PSU.

    4) IMPORTANT STEP - plug at least one item into the SECONDARY PSU. A fan is sufficient.

    5) Make sure your SECONDARY PSU is plugged into a wall outlet, and that the switch is in the ON position.

    6) Turn your machine on with the system power switch. If you did it right, both PSU's should power up at almost exactly the same time.

    Tools that would come in handy

    1) A Molex Pin Extractor

    2) An ATX Pin extractor

    3) A Molex crimper

    4) Wire cutter/stripper


    1) Male molex connector and two pins (you may want to get more than two in case you destroy one in the crimping process)

    2) 12-inch ATX extension cable

    3) Automotive Relay (12VDC/30A with four spade terminals)

    4) Four solderless terminals for 16 to 18-guage wire. I would get a couple more in case you destroy one in the crimping process).

    Notes, Warnings, and Disclaimers

    1) Check your work.

    2) Make sure you correctly identify the pins on the relay. Remember the 12v molex connector pigtail connects to the COIL side of the relay, and the ATX pigtail connects to the SWITCH side of the relay.

    3) Check your work.

    4) You're dealing with electricity. It can kill you. TYhere's no reason to fear it, but you should respect it's ability to put you 6-feet under when it decides you really hosed something up.

    5) Double-check your work.

    6) If you hurt or kill yourself, burn down your house, or worse, ruin perfectly good computer components as a direct or indirect result of performing this mod, I won't be held responsible. Afterall, you made the final decision to "throw the switch", as it were.

    7) If it looks like I've forgotten something, or if you have a question, don't hestitate to bring it to my attention.
    Last edited by Snafu; 06-22-2005, 08:33 PM.

  • #2
    Simply awesome guide Mr. it, very nice.

    I vote for a "Sticky"


    • #3
      Originally posted by RyderOCZ
      I vote for a "Sticky"
      Stuck. Haven't had a chance to read through it but so far it sounds intriguing.

      Profile - Webpage

      You can't live life without living - me

      Nothing is that important that it must be done today as long as you will wake up tomorrow - me


      • #4
        nice but cant you use something like this adaptor ?


        • #5
          I didn't know that part existed (I looked on the net for over a month for a pre-made one) , but yes, I could have. It would have taken quite a while to get to the US from Australia, and my own part cost less to make (I already had the tools I listed) than I would have paid to have something similar shipped from AU.


          • #6
            i think had something like this IIRC ?


            • #7
              I looke on their site as well, and didn't see anything.

              This isn't a "where-else-you-can-get-it" article. It's a "how-to-do-it-yourself" article, so I'd appreciate it if you'd stop your out-of-context pecking.
              Last edited by jsimmons; 06-22-2005, 05:13 PM.


              • #8
                Please let's be nice.

                Pics would be nice to see the relay used and how everything was wired up
                Last edited by Snafu; 06-22-2005, 08:32 PM.

                Profile - Webpage

                You can't live life without living - me

                Nothing is that important that it must be done today as long as you will wake up tomorrow - me


                • #9
                  Well, all I have is a pic of the finished relay assembly since my official photographer was out shopping while I was putting it together.

                  The relay is only about an inch in all dimensions, so it was easy to tuck away out of sight in the case.

                  When I get home from work, I'll post the schematic drawing I made (basically I just used Paint to duplicate the engraving that was on the top of the relay). I also have a drawing of the pin side so you can see the orientation of the pins on *MY* relay. I have no reason to think it would be any different on any other similar relay, because these things typically plug into a car's wiring via a one-piece connector.
                  Last edited by jsimmons; 06-22-2005, 09:34 PM.


                  • #10
                    Once again, nice kind of mod...solderless and inexpensive


                    • #11
                      Just Wondering

                      Is this type of thing Safe?

                      I would think that it would really put a dmg on eletric bill

                      If u had 2 psu would they share the voltage or only as u stated one does one thing other does other.

                      Would this type of thing be benifical if say u dont have a strong enough power supply to keep constanct V12+ to oc your processor etc.

                      Thanks for the info


                      • #12
                        Negative_Energy, it's perfectly safe as long as the relay lasts. I've used this very setup for well over a year now. One thing I did was to ensure that my neccessary components were running on the master PSU and not the slave. If the relay fails, and the slave PSU drops out (which I feel is very unlikely), then I only lose things which won't end up causing damage (like a CPU fan, for instance). I suppose it would be wise to replace the relay every once in a while, but even that might be overkill.

                        Yes, it will pull more power....

                        They don't share the voltage. Each PSU runs only whatever you have plugged into it.

                        This setup will certainly be beneficial in some way. Is it worth it? Probably not. If you are looking to improve the power for overclocking means, the best thing to do is probably buy a good, quality PSU. If you are adding a bunch of hard drives and other stuff, then yeah, a second PSU might be just the thing.


                        • #13
                          So what if i was running 4 raptors in a raid 0 array off the secondary Psu in the middle of a 4 hour gaming session and the secondary Fails? Do you lose data, does it get corrupted?

                          This sounds ok for running non-vital components,but 1 pump and two fans just isn't worth the extra juice...or is it?


                          • #14
                            Yes, of course...the same as if your PSU would fail right now. Running one PSU is always going to be better, simply because you are eliminating a component (the relay) that could potentially fail. If you are ultra paranoid, the best way to do it if you have to run 2 PSU's would be to have both of them plugged in and operated other words, when you boot your PC, you would turn your computer on and then switch on the 2nd PSU manually. Obviously you would have to bridge the second one so that it thinks it's plugged into a motherboard.

                            Why would you have to run 2 PSU's? If you can't afford a larger one, or if you are simply running too many components for the largest PSU you have/can buy. If you have a PC all blinged out with a billion fans/lights/etc.

                            Why do I run 2? Because I felt like it. Because I could. Because my case came with two PSU spaces. Because I'm running a buttload of fans and a buttload of drives. I have mine configured so that all my vital components are ran off my primary, so at least I won't damage anything if the relay fails.

                            Again, if you can afford it, the best route is to buy a larger, quality PSU than to go with two.....



                            • #15
                              This should run just about anything you can stuff inside a computer case....