Completely new to electronics and I'm learning by trying to replace a bad PSU fan (bad bearing but no power problems to the fan). The original fan was 12 V, 0.5 A. I bought a rinky dink fan that was too small from Microcenter that was also 12 V and the amperage is not marked. I put it in there and this one worked but sounded like it was going a million miles an hour so I opted to buy some nice fans off of Amazon.

The new, nice fans run at 12 V, 0.12 A, however, no matter what combination I try (I tried all six) the new fans will not turn on. I retested with the crappy Microcenter fan and it still worked fine.

If I understand it correctly, the amperage being lower than the original means that the fan would have to provide more resistance if it's a 0.12A fan. Is it possible that the fan has some circuitry in place which prevents it from running because the amperage is too high? (V=IR?)

Original Fan

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New Fan


PSU Three Wire Plug

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Testing the Fan (Update 1)

I talked to a buddy who was an avionics technician - he pointed out the obvious. You can test the fan with a 9 V battery - shown below, it works. I have already tested the connections many, many, times so I'm confident it's connected. He pointed out that in the case of things like iDRAC (out of band management for servers for those unfamiliar) it modulates the voltage dependent on the load. It is possible this fan has an operating range of say, 6-12 V whereas on boot, the computer may modulate the voltage down below some lower bound in which case the fan won't turn on. Next experiment: Use a 1.5 V battery.

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Update 2

I believe my friend's point has been proven - 1.5 V and the fan does nothing.

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Update 3

I've spent all night on this mostly out of curiosity trying pretty much every combination under the sun. I used a multimeter to check the connection and confirmed that only with positive to pin #2 and negative to pin #3 is there any registration so I can quite confidently say connection isn't the problem. What's absolutely odd is that even when off there is a constant voltage of ~0.4 V. When you turn it on it fluctuates and stabilizes at 0.2 V.

If you fire it up with the ohm meter you can watch it do some sort of probe at the beginning before stabilizing at around 6 ohms (even though the fan isn't working).

For whatever reason though, even though the circuit is complete, the board never provides more voltage. I confirmed this wasn't due to some passive power saving by turning on a very graphic intensive game that always has this thing screaming. It's obvious something is wrong because the CPU has downclocked. I noticed this behavior previously when testing several days ago - if either fan isn't spinning the CPU gets downclocked regardless of PSU temperature. Everything will start and run, but it will absolutely crawl. The test is easy to reproduce (if crude) stick a screwdriver in there to stop the fan from moving and watch how slow the computer goes. Then shutdown, let it spin freely, and it runs just fine.

Update 4

Ladies and gents if nothing else we're learning. I know that both the fan from Amazon and from Microcenter both will turn on with a voltage between 3 - 4.5 V. 1 and 2 AA batteries in series did not work but three does so it's somewhere in between those values.

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Update 5

Ok, so what I did was take two Molex connectors from another computer, checked the voltage to make sure it was an even 12 V, then plugged those into the two PSU fans of the computer under test. Then I plugged the tach from those two PSU fans into the PSU of the computer under test. I have established that if the tach is registering, the computer under test runs like a champ. As soon as you remove the tach signal from either fan - the CPU downclocks.

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  • \$\begingroup\$ Are you connecting these in series or parallel (correct)? \$\endgroup\$
    – vir
    Apr 7, 2022 at 22:32
  • \$\begingroup\$ It's the slot on a computer PSU that says FAN1 and FAN2. I'm not really sure how the circuitry works. \$\endgroup\$ Apr 7, 2022 at 22:36
  • \$\begingroup\$ Make and model number? \$\endgroup\$
    – Passerby
    Apr 7, 2022 at 22:36
  • 3
    \$\begingroup\$ If your fan works on a separated system and your power supply works with other fans the first thing that comes to mind is that the cable is not connected properly. This can be bad electric connected, solder failures etc. Or the pins does not match in the cable connection. Check continuity and if the vcc is connected to vcc, gnd to gnd and so on. As you fan is rated below your original fan, this should work without problems, electrically. The problem is that low current in fan mean less temperature dissipation, as a general rule, of course. \$\endgroup\$
    – Gimenez
    Apr 7, 2022 at 23:45
  • 1
    \$\begingroup\$ A 12V fan will not work on 1.5V. On 9, 7, maaayybe 5V. But not 1.5V. Since it works with just 2 pins, disconnect the third pin and try it with your psu. Make sure the fan pinout matches the original fan, i.e. use a multimeter to check which one is V+ and which is ground. \$\endgroup\$
    – Passerby
    Apr 8, 2022 at 0:55

1 Answer 1


The original fan is a DC fan meaning that it has positive negative and a tach/signal cable. This means the PSU detects the tach signal as 0 and then supplies power as needed for that tach signal/thermal needs of the PSU and adjusts voltage to control fan speed.

The fans you have probably have either a incompatible tach signal or are lf PWM fans which work very differently on how they are configured to run in a setup. Which would make sense if they are "nicer more expenive fans". PWM is nice if you have LED's on the fan etc as the fan always gets full power but the fan speed isn't controlled via voltage as such.

If you hook up an LED fan in DC mode on a motherboard you will notice the fan speed and the LED intensity will scale together, in PWM mode the LEDs will maintain correct brightness.


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