Repair guides for the MacBook Air, Apple's current line of consumer laptops.

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4 wired fan 5V 0.32A, which resistor to use for 3V?

Hi everyone,

Because of a problem, my macbook air fan is runing constantly at 100% (because the temp sensor is broken).

Let's say I want it to run at 50% permanently, which resistor should I use and where should I solder it.

I have 4 cables, from left to right:

Black - Grey - Green - Brown/Orange

This is what I've measured (assuming black one is ground)

Black - Grey: 5V

Black - Green: 3.3V

Black - Brown: 5V

What's on the fan:

5V 0.32A


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Thanks in advance for your help.


EDIT Picture :

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you may use three diode series connection ,then series connection in the Black - Grey: 5V circuit and voltage will drop 2.1 volts down to 2.9v . it will solve your problem .

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benwake, using Ohm's Law I = V/R for your project you would have to take how much voltage you want it to drop and then how much current you want it to have going through it and plug it into ohms law. Vdrop / Icircuit= Rseries 2/.32=6.25ohm Or you could just create a Voltage divider circuit. Dropping down from 5v to 3v is a 2v drop, gives a resistor ratio of 2:3 (drop 2 volts down to 3v), or 0.66:1,round down to 0.6:1. You could use 12ohm & 20ohm, 120ohm & 200ohm, 1.2kohm & 2kohm, 12kohm & 20kohm, 120kohm & 200kohm. Any of those in a voltage divider circuit would give you 3V.

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and the calculation for your voltage divider, with the values of the before mentioned resistor (R1=lower value)




Hope this helps, good luck


benwake, try to connect the two resistors as on the 2nd attached image. This way you stay with the 2:3 ratio. The calculation above should give you the proper voltage. It may also solve the other issue.

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Thanks for your answer.

I tried the divider, and I have strange results.

The fan wants to spin, but doesn't get enough current/voltage to spin.

When I put my multimeter on it, I also get strange results.

Not only the +5V has changed - orange cable, also the +5V grey cable has changed.

Check the picture I added above.

Both resistors are the same (0.6k)


Just checked and the way you have it connected the equation should be Vout=(Vin*0.6)/(0.6+0.6)





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How about a software solution to reduce your fan speed ?

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You cannot lower the fan speed with this kind of software :-/ But thanks for the answer :-)


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Sorry I do not know much about mac products but on a pc fans with 4 wires, one will be supply, one is a signal, one is ground and the other is what's called pulsed width modulation which is used to give you a wide range of fan speed as opposed to 3 wire which only gives you two settings.

I am also going to assume you used DVOM to measure this and not an oscilloscope. Either way the green would be the PWM wire.

The way PWM works is it counts the amount of +5v-0v in a given time period so unless your DVOM has a frequency counter you can not measure this accurately. Also you cant change PWM with a simple resistor so you can try the grey or brown wire but I wouldn't bet on it.

How do you know the temperature sensor isn't working?

Sorry this such a long answer for so little result and let me know if it works with the other wires.

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Thanks for your answer, this is the link about the temp sensor:

Temp sensor connector broken, where to solder? HF/E1, see picture

Do you know how to reduce the fan speed?

With a resistor?



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diode are nonlinear devices ,through voltage poor reduce voltage ,no comply with ohm law

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