View Full Version : Old 400w PSU to secondary/cooling system PSU: tips tricks and questions

11-16-2008, 06:45 AM
warning! PSU's have large capacitors that need to discharge for a certain period of time for the unit to be safe to fully disassemble and/or work on! be careful! I have been advised to unplug it and let it sit for anywhere from 10 minutes to a hour to as much as 2 days! I let mine sit for a half hour based on the advice of someone who claims to work with capacitors waiting 10-15 minutes and I have not been zapped yet (knocks on wood!). The actual amount of time will vary by PSU internal components and wattage. I am in no way responsible for the accuracy of any of this information or any injury's or damage that may result from its use!

Meh, with that negativity out of the way...

Today I turned an old (but good) Antec truepower 400w PSU into a dedicated secondary for my cooling system. I had 14 fans, 2 pumps, crossfired 4850's, and a highly overclocked e8400 all running of of one Zalman 750watt PSU, amazingly it was running the system but just barely. I suspect voltage instability was hitting the video cards when everything got going under load. Not sure but we shall see how the much lighter load of a 750w and a 400w working together helps the CPU and GPU's OC higher.

So how do you go about this? At first I just jumped the green 5v line on the 24 pin connector from the PSU to any one of the black ground wires. I taped it down secure and plugged the PSU into a switched power control center so I could turn it on and off from my desk. It works great and I am sure the rig is much happier with the extra juice.


THIS PART IS IMPORTANT, AS FAR AS I KNOW YOU WANT TO KEEP THE SECOND PSU POWERING NON MOTHERBOARD/CLOCK CYCLE GENERATING COMPONENTS!!! This means no connecting the second PSU to the video cards or using the 24 pin mobo connector from one and the 8 pin CPU plug from another. You want your main 'computer' running on one supply and the other one powering all the fans and pumps and any lighting etc. All the 'dumb' components.

I have read posts that had people using a second PSU for the 6 or 6+2 pci-e video connections. I suppose the video card does some voltage regulation to use these lines with the pci-e power coming through the mobo, the extra pci-e GPU plugs are raw from the PSU and have not passed through the mobo voltage regs so maybe the video card does this? Thats a big maybe.... I really don't know the ins and outs of all that but I do know that keeping the second PSU to the 'dumb' parts is for sure safe. Its basically the equivalent of a big multi output power converter like you charge your cell phone or cordless drill with. Just running as many components as its wattage can handle and you can fit in your setup

So it was nice to have the extra power, but it all was stacked haphazardly around my case and cooling box/case stand. I had to fit the extra PSU into a case stand already stuffed with a 120.3 rad, a Lang D5 pump, and a reservoir. All connected with stiff cheap PVC 1/2 ID tubing that was not playing nice with any routing I tried. I had to do better then this messy arrangement, and I did

I have come up with several tricks to make it much more clean looking, and I am going to share a few of them with you all in this thread. Its one of the nicer things I have done for my rig, I know if any of you try some of this you wont be disappointed.

On to the first tip:

For starters lets go over how to make an LED illuminated switch light up while switching the 5v trip to ground "power on circuit" on the PSU. This is a good trick as the 5v that gets tripped to a ground wire to start the PSU will not run an LED without proper resistors to adjust the load to its voltage. I actually suspect the 5v line that gets shorted to a ground line to start the PSU will not keep the PSU on if there is a drain on it. It would not work for me but I do not have enough knowledge or equipment to see if I can use this voltage and still keep the circuit tripped to the PSU. Who effing stole my multi meter at the last airsoft game? !@#@!#!%@!$!$#%!!!!!!

But I digress from the point.

What you do is take apart the LED switch carefully. When you have it disassembled move the LED's pos and neg wires from connecting to the main power and ground posts on the switch. bend them so they come out to the side and dont contact the switch circuit. Solder narrow gauge flexible wire that will handle a low amp 12v load to the LED wires and drill 2 small holes in the side of the switch near the top left and right corners to run the new power lines out. Glue the wires to the inside of the switch tightly to the inside top. and watch out you don't block the lower part of the housing from reconnecting. Run the new positive and negative for the LED in the switch to whatever you want to run it off, the 3.3v lines on the PSU make LEDs nice and bright with no need for resistors. The LED in my switch was a 12v LED (resistor on the positive post) and needed a 12v rail to light it up.

So how does all that confusing crap work? I hit the military style toggle switch on the side of my case stand and it trips the 5v line from the PSU to a ground wire which is exactly what the start button on the front of your case does. As the PSU powers up it sends power down the wires into the switch that connect to the LED, lighting it up to tell me the switch worked and the system is up and running.


A relay wont work as it needs a power source to trip the 5v line, a switch that powers its LED from a different source then the circuit it throws is the only solution I can figure out. As I could not find one to buy I had to make one :glasses:



I really like being able to run the pumps and fans independent of the computer. I let the cooling system run for a minute after shutting down the comp just to make sure the system is cooled down nicely and ready to sleep And it makes filling a loop so easy now that I can turn the pumps on and off like this.

Pics coming, rig is in pieces right now all over the place, so much being done this weekend. Cutting all the extra wires, mounting in the stand with cables wrapped and routed, making the PSU truly dedicated to the cooling system. Routing all the tubing through bottom of case as well... its a major effort tomorrow to push through before the week starts.

Please comment. I will have the next tip tomorrow night... and pics!

If I got anything wrong please let me know. If you try any of this I am not responsible for any damage/injury. If you try it and like it you owe me a cold beverage :banana:

11-16-2008, 03:30 PM
I use very large capacitors at work and the Warnings that come with it are 5 minutes and then they are safe to work on, these are high voltage capacitors. So if you want to be on the safe side for your PSU 10mins from after you unplug it should be good, they are designed to discharge on their own and fairly quickly.

Great guide in my opinion the only thing I'd do is put a relay in that controls whether I can turn my rig when the other PSU is running so in case kids or someone else comes by doesn't start the computer with no cooling going on. Really like the LED switch idea though it's a nice touch and easy to troubleshoot if you have problems!

11-16-2008, 05:42 PM
Thanks :)

I like relays too! I had some trouble getting a proper MOSFET to switch the heavy load on an airsoft gun and burned out a few right on my desk while testing... quite the excitement for the roommates ;)

I give my PSU about half an hour to discharge, but I had read up to a few days and was not gonna be the one to unwittingly give deadly advice. Its nice to know the real scoop though, thanks!

I was thinking about linking the main computer start to the cooling system but decided against it. It would not be flexible enough to have them just start and stop at the same time. I could make it delay with some resistor/capacitor setup so the cooling system shut off after the computer but then I would have to set up a second power switch for the cooling system to run it alone for testing. I will probably do this for the next case mod. I would like to have a sub-panel of power controls for pumps and fans and the secondary PSU and a master on off that activated any 'on' switches on the sub panel as well as both PSU's (for basic PC operation).

Next tip: Cleaning up the excess wires from a secondary PSU while retaining ability to use all PSU functions later. I will demonstrate how I prepare and mount the PSU into the case stand and make a clean wire harness for it all.

OK I have started the process of cutting all the excess wires I don't want to run. I am cutting the wires about 1.5 inches from their terminations in the PSU. This way I can save the plugs and wires and use crimp connectors to easily return the PSU to its full function. I am leaving a few wires that go to the 24 pin motherboard connector long. Specifically the green 'pwr on' wire, the gray 'pwr good' wire, and the 2 'GND' (ground) wires on either side of the green 'pwr on'. These I will be using so I am cutting them an inch down from the 24 pin connector and wrapping them with a 12v, 5v, and 2 more grounds that will all go to the front panel of the stand. With this bundle I have my power switch and wires for a 4 pin molex to run the fans connected to the front panel speed control.

I cut the 4 pin cpu power plug and some bizarre old motherboard connector this ancient PSU had as well. I am saving all the bits, because I am a pack rat more then I expect to ever use this PSU to power a motherboard again. But I also know that I can use the wire and a plug from an old mobo to make an extension so it has some use still. To be honest I just cant throw stuff away. Its bad when you live in a studio...


So after cutting the wires I want to tape each end of wire in the PSU with a little bit of electrical tape so nothing gets shorted out. Tape the ends up good and then tape them in bundles together according to what they went to or however is covenant to get them out of the way and not resting on capacitors or any other components. I slipped some heat shrink over the tops of the cut wire bundles after I taped them up to hold them stable and out of the way.


Trimmed enough to not get in the way but still enough to use crimp connects if I ever need too (though solder and heat shrink are always better).


I left myself a 4 pin molex line with 2 plugs for a spare line and another 4 pin molex line with 3 plugs to run up into the case and power some fans up there. And I have the the bundle I mentioned consisting of the green and gray wires and 2 ground for them as well as 12v, 5v, and 2 grounds from a molex line. This group I am going to use for the fans, some lights, and the switches. The gray 'pwr good' wire I will use if I want another LED to tell me something I already know :neutral: I checked it and at least on my old PSU it is good for LED's without resistors. I kept it just to make sure something would bug me later i suppose :think: The extra cable wrapped bundle is a molex line without the molex, it will be used for something or just another spare.

Installing the secondary

This is how I wanted to mount the PSU, but it would not fit with the radiator...

So this is how it ended up...

My soldering station :redface:

After wrapping everything up nice with cable sleeving and heat shrink it looks good enough that the old bare metal body now needs a slick paint job to match its cables. Meh, maybe later...

11-18-2008, 06:26 PM
Pic of case stand before start of PSU installation and internal loop routing, it was never designed to hold the extra PSU...


And now with the secondary PSU installed, not exactly the way I wanted it... but its in there and secure.
I had a leak on one of the fittings for the home made res, and went nuts with epoxy as the res is kinda crappy and going to be redone as well. It just does not work without threads the epoxy takes to much torque when changing tubing to stay sealed.

Its a super tight fit with the PSU. Zip ties are keeping the tubing firmly seated and secure. The lower one is not kinked so bad with all the screws tightened down.




I have a few panels to finish, but the secondary power supply project stage one is complete.

Stage 2 is a little setup to make the main PSU power up the secondary when it is turned on, and that will keep the 5v grounded to keep the secondary on even if the 2nd's switch is turned off. The switch will still allow the secondary power supply to function alone, but it will be safe from being turned off when the main system needs cooling.

I will have to design the circuit, gimmie a sec...

09-28-2010, 02:57 PM
pics are dead

09-28-2010, 03:42 PM
pics are dead

So is the thread. ;)