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Problem after connecting case fan to 5V from PSU

Discussion in 'General Hardware' started by Derek12, Aug 20, 2011.

  1. Derek12

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    I have installed a old case fan to 5V because it doesn't have a mobo header and with 12V it's very noisy, I had connected to 5V from the power supply and it works fine and quieter but..

    After shutting down the computer, it immediately turns on again. This only happens with the fan connected.

    Anyone knows why this strange behavior with the fan??

    Many thanks :)
     
  2. BraveSoul

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    strange,, have u tried a diff fan,,, or connecting to motherboard's 5v
     
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  3. de.das.dude

    de.das.dude Pro Indian Modder

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    i think the 5V line is on even when standby. or power is off.
    this means that the fan is still "running".

    i guess there must be something in the PSUs circuitry that when the current draw during standby gets over a certain limit (for your case this is due to the fan) the PSU automatically starts. this is probably as a safety measure to make sure the transistors and stuff are cooled.

    its better to use a 1/2watt resistor in series with the fan in connection to a 12V supply.

    if your fans current rating is 'i' in amperes

    then

    solve this equation.
    R=16.8/i

    to get the resistance required in ohms.

    if youre interested who that came out:
    [​IMG]
     
    Last edited: Aug 22, 2011
    Derek12 says thanks.
  4. Jetster

    Jetster

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    Change the power options in the BIOS?
     
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  5. Mussels

    Mussels Moderprator Staff Member

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    thats a weird one, i've changed the wiring on heaps of fans and never seen that happen before. can you show us some pics of how you have it wired in?
     
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  6. de.das.dude

    de.das.dude Pro Indian Modder

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    check my post. its updated!
     
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  7. Mussels

    Mussels Moderprator Staff Member

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    his problem is that the PC turns back on, not the fan still spinning.


    also, the 5V doesnt stay on. there is a 5VSB rail, but he'd hafta do some weird shit to connect that to a fan, as its not on a molex or 3 pin connector.
     
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  8. de.das.dude

    de.das.dude Pro Indian Modder

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    cheap PSU = all are same check the guide in my siggy.

    all rails are interconnected for me.
     
    Derek12 says thanks.
  9. Mussels

    Mussels Moderprator Staff Member

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    true but how can his 5V rail trigger a power on through the sense wires there?


    your electrical... crap is beyond me. not my field.
     
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  10. de.das.dude

    de.das.dude Pro Indian Modder

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    5V is in use.

    the fan takes some power. that means a rise in the current in the circuit and hence transistors and the like.

    more current = more heat
    so if there is a overheating protection, that would trigger the PSU to self start which inturn switches on the cooling.

    this is just my logic. i am not sure. this just seems to justify what is going on in the easiest way. i dont know PSU circuits in detail :p i hated electronics the way it was taught.

    anyhoo he should use my formula to find out the resistance and use that for fan controlling.
     
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  11. Derek12

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    Many thanks to all :)

    @BraveSoul, yeah, if I connect to 12V no problem happens! I don't have another fan to try :(

    @de.das.dude I will try your circuit, I will get a resistor and connect to 12V so it will reduce its speed :)

    @Jetster I checked bios and all seems fine, though the problem only happens with the fan without it it shuts down correctly and stays there.

    @Mussels Sorry I don't have a camera on my own :( I connected it to a red and black terminals on the floppy connector and insulated with electric tape (yes very fudge :eek: ) though I didn't shorted anything by now LOL

    Yeah the problem is veeeery strange doesn't matter if I shutdown via Windows or via the button or from BIOS, the computer shuts down for .5 second, then it gets up and boot again normally and if I stop the fan with the hand before shutting it down, the computer doesn't come up again, so seems that the fan spinning inertia introduce some noise and trigger the PSU or mobo to turn on :confused:
     
  12. newtekie1

    newtekie1 Semi-Retired Folder

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    The fan spinning is likely creating a charge that is somehow signalling the computer to turn back on. That is odd.
     
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    Crunching for Team TPU 50 Million points folded for TPU
  13. scaminatrix

    scaminatrix

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    Could it be something to do with the rig waiting for a 5v wake-up when turned off (like it's waiting to be turned on with the keyboard)? And it's getting it with the residual power from the fan?

    Try going into BIOS and turning off an option that says something like "wake up on 5v" or "5vSB"
     
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  14. de.das.dude

    de.das.dude Pro Indian Modder

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    yes that could be the reason too.
    fans do have caps that can store the charge and release it :D
     
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  15. Derek12

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    Many thanks to all :)

    Yeah that's what I'm thinking that the fan inertia would cause some voltage spikes and make the motherboard to turn on :?:

    Meanwhile I solved it, I used a series resistor like de das dude show and connected to 12V and now the computer is fine while the fan spins at low speed.

    The resistor is 45ohm taken from a broken PSU and the fan voltage is now 8,5 V. the resistor, being very huge (I think it's 2W LOL), warms considerably.

    BTW @scaminatrix, I can't find "wake up on 5v", though there are: "Power On By Keyboard" "PME Event Wake Up" "Power-On by Alarm" and "USB Wake Up from S3" I believe (I will check first) that they are disabled but the latest one.
     
  16. Derek12

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    %&·$·$$%!!

    I ear a pop and the fan stopped but the computer still run OK
    The resistor has blown literally, poured smoke, burned the molex luckily the components haven't suffered damage despite the resistor was NEAR the video card LOOOL and it blackened a bit because of the smoke!

    I will choose a better resistor.


    EDIT: well the computer is artifacting now, I will clean the video card soot, if it still fails I will swear a lot!!!


    Cleaned and new resistor, now the computer is fine and the fan is running and the resistor isn't overheating
     
    Last edited: Aug 24, 2011
  17. de.das.dude

    de.das.dude Pro Indian Modder

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    dude you need at LEAST a 1/2 watt. it better if you first try with higher wattage.

    anyhoo so my formula and ckt is working?
     
    Last edited: Aug 25, 2011
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  18. Derek12

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    Well the resistor used was very big and thick (approx 4cm long and 2.5cm thick) I though that it was 1/2W or greater I think that it failed shorted. The resistor, BTW it was pink with 4 color lines that I can't remember currently.

    The fan sticker doesn't indicate the current and I haven't got a amperimeter (I have a multimeter but no current measure) :(

    I will suppose that this fan is 0.45A (it's a 80mm one) so I will search a 36 Ohm resistor (or approximately one) and big and I will report the result.

    Many thanks :)
     
    Last edited: Aug 25, 2011
  19. scaminatrix

    scaminatrix

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    I would just buy a new fan.
     
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  20. Derek12

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    Maybe, though those I saw on stores are also 3 pin and would run full speed I can't find 4 pin ones with PWM. I can't find fan controllers neither
     
  21. Lazzer408

    Lazzer408

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    Derek12 says thanks.
  22. scaminatrix

    scaminatrix

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    I've got one of these with a 3-pin to molex adapter I can chuck you for price of postage. If you're interested, PM me.
     
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  23. Derek12

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    Good idea, I will pull out a diode from the trash PSU and I will insert it in the circuit :)
    that's the product I want!

    That's what I would like to have, I will try to convince my parents to buy online (they are very reticent to do so :mad: ) if I am successful, I will PM you :)
     
  24. de.das.dude

    de.das.dude Pro Indian Modder

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    diodes are messy and even they go bork! bork!
     
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  25. Derek12

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    I tried a diode from my trash PSU and the fan doesn't spin up, the diode is correctly polarized (I tried reversing the diode just in case) so maybe the diode is bad
     

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