Sounds like a job for a 555 timer
Any fan that "needs to warm up" is not one that I want keeping my parts cool. That's a sure way to eventually lose more than just the fan.
1) TIP120 is too big. Not saying it won't work... but it's like using a sledgehammer to push in a thumbtack. To pull a signal line low, you just need a small signal generic device, like a 2N2222. The FANs RPM sender usually have an "open collector" NPN device that PULLS the signal LOW when the RPM magnetic sensor engages.2) Seriously... my advice is to get the right fan and install that.Any fan that "needs to warm up" is not one that I want keeping my parts cool. That's a sure way to eventually lose more than just the fan.
it runs a bit too slow according to the psu's build in safety.
Quoteit runs a bit too slow according to the psu's build in safety.So basically that is your problem. You need to either change what your PSU expects or change the signal into it.You can change the frequency of your fan's output by using a phase locked loop. It is simplest to double the frequency but by the creative use of dividers you can get any ratio os frequency increase you like.A simpler alternative might be to have a frequency to voltage converter, then an amplifier, then a voltage to frequency converter.
In more recent flavors of BIOS you can set the lower threshold RPM alarm level and so solve your problem without even opening the computer case. More "dated" versions of BIOS typically included an option to suppress the alarm (ignore PSU fan PRM).
because the frequency on the RPM line to it will then be too high.
Quotebecause the frequency on the RPM line to it will then be too high.I don't understand this. The frequency on the RPM line comes from the motor, it is not a command but a measurement. The motor speed is controlled only by voltage on the fan.
Too low or too high at a given voltage, and it starts beeping.
QuoteToo low or too high at a given voltage, and it starts beeping.Yes and your problem is the fan you use gives too low a frequency for any given voltage. That is why I suggested using a PLL to modify the frequency you fan gives and so fool the PSU into thinking things are as it expects.