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Old PC PSU Fan issue.

Discussion in 'Troubleshooting and Repair' started by Alex Scott, Apr 5, 2015.

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  1. Alex Scott

    Alex Scott

    Jul 3, 2014
    Hey guys

    A semi-electronics noob here. I recently turned an old 350 Watt Dell ATX PC power supply into a multi voltage bench power supply for testing basic circuits. I opened up the machine and cut some wires and ran the 3.3v, 5.5v, 12v and ground wires to some terminal posts, and it all works fine! I also installed a switch so I can leave it plugged into the mains constantly. The voltages are all correct and it does indeed provide power (only tested on basic LED circuit so far, nothing that actually consumes a large amount of energy.)

    Anyway, in my naivety I have some how stopped the fan working. I assume this is a normal 12V DC PC fan, and I do not want my power supply to overheat. The 2 wires on the fan are all hooked up straight to the board, but I'm guessing there was a connector on the 20 pin plug that goes into the motherboard that controlled the fan that I have severed? Anyway, how do I apply a load to the fan to get it running again? It worked perfectly fine before I messed about with it.

  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi Alex

    just reroute the fan wires directly to a 12V source :)
  3. Gryd3


    Jun 25, 2014
    As Dave said, just hook it up to 12V and call it done :)

    The 20 or 24 pin plug on a power supply does not carry power or signal of any sort for the fan 'inside' the power supply module.
    Look around and make sure you didn't damage the board.
    Inspect the wires from the fan to make sure they are not damaged.
    Unsolder or cut the wires to the fan and connect it to 12V to make sure the fan itself actually still works. (At this point you can leave it there, or troubleshoot the board)

    You probably already know that there is an 'enable' pin on the 20/24 pin plug that needs to be tied to ground to turn the power supply 'on'. The fan will not operate until the PSU is operating. Also note, that some power supplies have an additional 'sense' wire that is connected to the 3.3V wire (I think). This sense wire may be connected 'at' the plug you cut off, or internally on the board. Do you recall if any of the pins from the 20/24 pin header had more than one wire in it?
  4. Alex Scott

    Alex Scott

    Jul 3, 2014

    Thanks for your info, I don't think this particular PSU was that complex. I had a spare ground and 12v wire inside the PSU that I couldn't connect to the ring terminals I installed (Terminal was too small) and just connected them straight to the wires from the fan after cutting them to the board. Didn't realise it was so easy.

    Question, if I wanted to add a potentiometer (I think that's the right thing I'd need?) onto the side of the case to control the speed of the fan, what resistance value would I need? Or how would I work it out?
  5. Gryd3


    Jun 25, 2014
    Power supplies are pretty simple once you understand that they are all built to a specific standard. On rare occasion, you may find the power supply turns off by itself if you try to draw a decent load from it. This is a 'fault protection' and can be fixed by putting a high wattage resistor between a 5V line and ground. (To trick the PSU into thinking a computer is actually using it. Otherwise it things something has gone wrong).
    Other than that, the 'enable' pin being tied to ground is the signal to turn it on, and the rest of the pins are going to be a mix of 3.3V, 5V and 12V (With some negative voltages as well;) but no data pins)

    Now, as far as fan speed is concerned... you can on rare occasion use a potentiometer, but you will run into a snag.
    I'm sure by now you have realized that resistors come in 1/4 Watt, or 1/2 Watt... heck even 10 Watt ratings. This is the maximum 'power' that can be wasted or dissipated by the resistor.
    Potentiometers on the other hand will rarely if ever state what they are rated for in wattage... Simply put, they are not meant to waste or dissipate power and attempting to do so will damage the potentiometer. They are typically only ever used to manipulate 'signal' level voltages, or for providing reference voltages (at very small fractions of an amp).
    Why the details? Well, to control the speed of then fan, you need to control the voltage going to it. The thought of using a potentiometer is very common, but what gets overlooked is where the extra voltage goes... Let's pretend for a moment that the fan will always draw 100mA (This is to make the math and explanation a little easier)
    If you want to run the fan at 50%, that means that half of the normal voltage must be diverted from the fan, which would end up across the potentiometer. Now, using V = IR (Voltage = Current * Resistance) and P = IV (Power = Current * Voltage), we can determine that the potentiometer will be dissipating 100mA * 6V = 600mW of power. The potentiometer is very very unlikely to be rated at higher than maybe 1/8th of a Watt... so when you try to make it dissipate 5 times the power it's meant to, it will either sizzle a little or may actually start a little flame.
    Now... some people have success doing this anyway, but that relies on two things: Luck, and Luck. As you adjust the speed, the power being dissipated will change, and can actually reach some very high levels.

    So, fan speed done right. You need to control the fan speed with one of two methods:
    Automotive industry method : Using a multi-position switch, simply select between Ω ratings of high power resistors. Low, Med, High is pretty much all you get :p
    Electronic control method : Using PWM (Pulse Width-Modulation) you can control the speed of an electric motor as small as a vibrate motor in your phone or a large DC motor in an Electric Vehicle. The basics involve a circuit that toggles on/off (Ideally, the circuit should never be halfway powered on... either all on or all off... that keeps it running cool and efficient). Now if you toggle on/off fast enough, it pretty much averaged out to be only half the voltage anyway. (Electric drills to this... and you can sometimes hear a small whine as you begin to depress the trigger). By having the circuit stay on a little longer than staying off, you end up speeding up your fan. Now, you can buy PWM controllers pre-made, or you can build one yourself. The path you take here is up to you.
    If you don't want to be that fancy, you can use a Double-Throw switch to toggle your fan between the 12V line and 5V line :p
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