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Will putting a resistor between a PC's fan header pins convince the BIOS the fan is on? Will it catch fire? Do I even need the resistor?

How can I convince the BIOS the fan is on without a fan?

Edit: maybe I can power a ATTINY off the fan power header to send pulses of resistance to the tachometer header?

winny
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cubesareneat
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  • Can't you just go into the BIOS setup and go to the fan/monitor area and pick `Ignore` on that fan? Usually, you can. – doug65536 Sep 14 '22 at 04:19
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    Sounds a bit like [you and me have the same problem](https://superuser.com/questions/1688935/how-do-i-explain-to-my-msi-bios-that-i-dont-have-a-pump-fan-anymore-and-thats). – Philipp Sep 14 '22 at 10:12
  • lol at least you have options in your bios @philipp maybe try taking out the cmos in your's. reset it, I imagine the mobo default dosen't included the pump fan maybe. mine has like 5 settings lol, its an old computer. – cubesareneat Sep 14 '22 at 15:23

4 Answers4

19

Connect the 3rd pin from another fan in the same PC to the 'dummy' header. It will think there is a fan in each.

jp314
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  • " If the board uses PWM on the power and expects a proportional change on the tach signal to ensure the fan is working, this also won't work." AUTHOR = {Passerby (https://electronics.stackexchange.com/users/17178/passerby)}, URL = {https://electronics.stackexchange.com/q/634640} i like this idea, but i want zero fans – cubesareneat Sep 14 '22 at 15:06
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If the pc measures the simple load at the fan pins, then a resistor would work. Size it for 100 mA at 12 V (assuming it's a 12 V 2 pin fan header) and get an appropriately sized wattage. But now you've added 1.2 W of waste heat in your case. If it looks for stall currents or anything more complex a resistor may not work. Do not in any case short out the power and ground. A resistor or load is needed.

If it uses basic TACH signal monitoring on a 3 pin header, then you can make a fake tach signal using a basic microcontroller or 555 timer. See https://www.techidiots.net/notes/fake-fan-sensor. If the board uses PWM on the power and expects a proportional change on the tach signal to ensure the fan is working, this also won't work.

If it's a 4 pin with PWM and turns on and off as needed then it's more complicated than it's worth.

winny
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Passerby
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7

It depends on how the motherboard/BIOS determines how the fan is on. Most I've seen are 3rd wire tachometer affairs, so your resistor would not work in that case. Is this for a fanless PC setup? You should be able to disable whatever warning or protective feature you want to spoof in BIOS itself.

vir
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    its an old pc, if it dies it dies, going to google tachometer (it's a 3pin fan). the bios had like 5 options, none to turn of warnings. i want it too boot headlessly and pressing f1 to ignore warnings wont work. i found this https://electronics.stackexchange.com/q/8295/321803 – cubesareneat Sep 12 '22 at 21:51
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    The tachometer signal is usually an open-collector (i.e. normally floating and then more or less connected to ground) 2 or 4 pulses per revolution output. You could spoof it with a suitable oscillator and output transistor but it's probably easier to just get a fan and plug it in. – vir Sep 12 '22 at 22:00
  • a AT-tiny85 as a oscillator and output transistor? i will see where google takes this – cubesareneat Sep 12 '22 at 22:05
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If it's a 3-pin connector, it will almost certainly be using the tachometer pin to sense that the fan is present and working. Simply detecting the current through the fan is not reliable, as a fan might stall and draw significant current even though it isn't spinning.

The tachometer signal is generated using a hall-effect sensor inside the fan. As the fan spins, the magnetic field passing the sensor causes a pulse. In the case of a PC, the standard interface works by having the hall-effect sensor effectively short the sense pin to ground each time the fan completes a revolution, with the motherboard pulling the sense pin up to 12V using a resistor. For example:

schematic

simulate this circuit – Schematic created using CircuitLab

To trick the motherboard into thinking the fan is spinning, you would need to fake the hall sensor pulses, e.g. by having a microcontroller or square wave oscillator drive the gate of a MOSFET placed between the SENSE and GND pins.

Here's an example implementation with a 555 timer.

Schematic

It simulates a fan speed of around 800rpm. You'll need an small N-channel FET with a Vgs(max) and Vds(max) of >12V.

Polynomial
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