PSU 3.3V Mod with a twist no go :(

[Tarkan]

Hi guys

I decided to have a go for the 3.3V mod on my PSU to get more juice for my Vmem mod. I have a Zalman400 PSU . I didnt actually want to chop the wires on the PSU directly, so instead, i thought if i used an ATX extension cable and do the actual mod on the extension might be ok. I used a 5K Cermet type pot on 3.3V sense wire, middle leg to one of the grounds side leg to the sense line itself ( both returning back to PSU ) After the mod i wanted to try it on a cheapo 250W PSU. Set the pot to max resistance , fired it up and started lowering the resistance. I dont get no joy whatsoever . Voltage stays at 3.32 almost all the way when i adjust the pot. When it gets to around 100ohm it actually starts lowering the voltage, with the final voltage being around 1.4V . I have actually tried this with another cheapo with similar spec with almost identical results. Can someone please tell me if this because it will not work with an extension cable like this ( tot i was being clever ) Any suggestions and comments appreciated.

[Tarkan]

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[Tarkan]

one more pic

[Hell-Fire]

Keep in mind that this will not work if the actual ATX bundle off the psu does not include the sense wires as well.

If it would work at all.

Also, although the sense wires need not go back into the actual plug for the mods to work, completely severing the Ground from the plug this way could cause some problems as well.

Out of morbid curiousity, try redoing the mod with a fixed resistor connected between the cut sense wires, then the VR middle leg connected to the top of the fixed resistor leg and the outter leg to ground. Top of the fixed resistor being the leg connected to the sense wire lead that goes to the psu and not the plug.

Worth a shot since you are already working on this.

[Tarkan]

Thanx for your input HF.

There is 3.3V sense wire also present on the ATX block coming from the PSU. I have also tried this on yet another 300W PSU and it burnt the pots twice, when the resistance was set at low levels. I will have a go at adding the fixed resistor. What size would you recommend?

On a side note i did lift the cover off my Zalman 400A to see if adjustable pots inside. Didnt spot anything. Does anyone know for a fact whether or not Zalman 400A have pots inside or not? Im thinking maybe they are hidden under heatsinks etc, i didnt want to mess with it too much.

Thanx in advance.:)

[STEvil]

This does not work because the 3.3 sense wire comming from the PSU joins at the ATX Header.. which means you are not modifying anything.

The modification must be performed before the sense and +3.3 meet at the ATX header.

If your Zalman PSU has "FSP" in the model then it may have potentiometers, but if you cant see them right off the bat its not likely they are there... they usually stand out on a riser card for the FSP based PSU's as far as I have seen.

I would also add that fixed resistor (30-50 ohms).. I need to redo my antec 400w smartpower with a larger resistor since I think 10 ohms was not enough and my pot cant handle it.. may explain 3.45v wall ;)

[Hell-Fire]

You're burning trimmers b/c the low resistance yields a higher current.

STEvil's advice is the best way to go.

[Tarkan]

Thanks for spotting that one and pointing out STevil. I guess im gonna have to get my hands dirty and do it on the PSU itself this time, with the addition of the fixed resistor :D