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Thread: PC/PSU won't start up with 4x 7V Molex fan mod

  1. #1
    Senior Member Ulti's Avatar
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    PC/PSU won't start up with 4x 7V Molex fan mod

    Hi All,

    I got a budget PC case for a friend (1st Player DK D4) and it comes with 4 pretty nice RGB fans but they use molex connectors. I thought it wouldn't be an issue as I can daisy chain them and run them at 7V if they were too loud.

    Now let me tell you, at 12V, they are monstrously loud! There's no way I can give this to my friend in good conscience as things stand so I modded all of them to 7V by switching one of the ground wires to the 5V rail so that it runs the difference between 12V and 5V = 7V.

    I've got a Gigabyte P650B PSU which I believe is a pretty decent PSU and I'm testing it with my B450 motherboard, Ryzen 5 3600 and a GTX 1070. I read online that some PSUs might not like something running on 7V with no other load on the line so I tried plugging in a HDD and SSD but it still fails to turn on with all 4 fans on 7V.

    I've noticed if I only have 1 fan connected via the 7V fan mod, then it will boot fine and I can plug in the remaining 3 after booting but obviously this is a massive hassle.

    Can anyone help? As we're on a budget we don't want to buy a new PSU or new RGB fans as they're quite pricey. I can try other mods though.

    Thanks in advance!
    Last edited by Ulti; 17-09-2020 at 12:28 PM.

  2. #2
    Banhammer in peace PeterB kalniel's Avatar
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    Re: PC/PSU won't start up with 4x 7V Molex fan mod

    You can buy fan resistor cables which might be a better way of doing this. They're normally for 3pin cables but you should be able to mod your 4 pins the same (or buy a 4pin -> 3pin adaptor (which also allows multiple fans off one 4-pin). All very cheap.

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    Re: PC/PSU won't start up with 4x 7V Molex fan mod

    If the suggestion from @kalniel doesn`t help you, you can try soldering 50ohm resistor on the 12v wire and it will quiet them down. I`ve used this method for a lot of old and new loud fans with not much loss of cfm.

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    Ulti (14-09-2020)

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    Senior Member Ulti's Avatar
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    Re: PC/PSU won't start up with 4x 7V Molex fan mod

    Quote Originally Posted by kalniel View Post
    You can buy fan resistor cables which might be a better way of doing this. They're normally for 3pin cables but you should be able to mod your 4 pins the same (or buy a 4pin -> 3pin adaptor (which also allows multiple fans off one 4-pin). All very cheap.
    Quote Originally Posted by stawte View Post
    If the suggestion from @kalniel doesn`t help you, you can try soldering 50ohm resistor on the 12v wire and it will quiet them down. I`ve used this method for a lot of old and new loud fans with not much loss of cfm.
    Thanks both! I've got some 3 pin ULNA (ultra low noise adapters) which I imagine are what you're talking about so I'll do some googling to see how to get these to go with the old school molex fans.

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    Re: PC/PSU won't start up with 4x 7V Molex fan mod

    No worries, but just to be clear, I was talking about the 12v wire of the fan (3 pin ), not the molex 12v wire. Basically making them ULN without the A. )

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  9. #6
    Senior Member Ulti's Avatar
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    Re: PC/PSU won't start up with 4x 7V Molex fan mod

    Just to update, I ended up just finding an old 3 pin fan to molex connector thinking it was an adapter but it didn't seem to do anything so I rewired it to produce 12V to the molex and now I've rewired 3 of the fans to use 12V and I've plugged those into the motherboard.

    I've tested with my motherboard which produces 2A on the 3 pin fan header and it seems to be able to power all 3 fans fine and then I can control the voltage through my motherboard. I'm not sure if my friend's motherboard will have good fan control but I believe he's pairing it with an i7 4790K so the motherboard should be half decent.

    Just as a test I tried plugging in the 3 fans into the ULNA adapter but the resistor got REALLY hot so I took that off quickly.

    The final fan I've left to run directly off molex on 7V as that seems fine.

    Thanks again for the help.

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    Re: PC/PSU won't start up with 4x 7V Molex fan mod

    I`m glad you`ve sorted it now. Good luck!

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