Simplistic & cheap PWM-to-3pin-fan converter

mikeclueby4

mikeclueby4

mikeclueby4

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Note that this is absolutely as KISS as it gets. Particularily, you can kiss your RPM readings goodbye on anything other than 100% speed.

If you want better, Tealc's http://forums.overclockers.co.uk/showthread.php?t=18421145 is for you.

Note to future readers with 3pin fans & 4pin headers: Before you do anything else, check if your BIOS has a setting for 3pin/4pin control. If it does, you can probably just set it to 3pin and stop reading now.


4pin-pwm-to-3pin-fan.jpg


Things you need
  1. A resistor package suitable for your fan, Zalman's package should handle most everything with its beefy 56 ohm resistor (7v operation)
  2. A nice big N-channel MOSFET. 3 amps or bigger and you don't need extra cooling. Minimum 20V throughput. The max Gate voltage can be between 6--20 volts, but don't go bigger than that or the 5V PWM pin feed becomes too low to open the MOSFET properly.
    My TV repair shop gave me an IRF630 which is complete overkill. I like it.
    Absolutely best is if you can get one rated as logic level input (but still drives plenty of amps)
  3. Steal one connector from your PC chassis.

My version drives my Zalman 9700 CPU fan from 40% PWM up to 100%, resulting in about half of max speed due to the resistor. The action sounds sort of linear-ish, which probably means it isn't. (Sound level increases exponentially with RPMs)

If you like, you can short out the resistor entirely for full range action. Be aware however that your fans will start spinning at 10-15% PWM and that the curve becomes very aggressive. You'll be going fairly high on RPMs at 60% already. This means that standard BIOS fan controls will likely drive high RPMs throughout, so you might want to be able to tune the PWM curve yourself (via e.g. SpeedFan)... unless you simply want high RPMs, of course.


About the RPM monitoring: no, there is no way to get working RPM monitoring with this few components, you can forget it. With 3 (and switching the MOSFET for a P-channel in high-side driver configuration) you might be able to get it working. But don't take my word for it. Perhaps with a 10-100uF capacitor parallel with the fan? Unknown & untested. One definite caveat is that the heatsink goes +12V live.

Caveats:
  • Your BIOS will likely "F1 to continue" you in the face with this adapter on the CPU fan because it can't see RPM readings. Disable the monitoring in the BIOS!
  • Tools like ASUS Fan Xpert will probably hate you too for the same reason. SpeedFan works!

Wins:
  • You can actually turn your CPU fan completely off!
 
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Thanks for the inputs - it's always very appreciated to know there's someone out there willing to doublecheck my think!

I worried about all the things you listed and researched them.

I found answers in http://www.formfactors.org/developer\specs\REV1_2_Public.pdf

PC fans are required to have diodes in them. No back EMF is allowed.

The PWM pin output is specced to source max 5mA, suggesting a minimum 1kohm resistor. My original spec had a 1.3k+3.7k pulldown but that shot me spectacularily in the foot when I ended up trying to drive my gate with 2.5 volts (measured!). Turns out my motherboard has a 2kohm resistor on the PWM pin. Oops. The MOSFET also went hot as heck. Running it the way I do now, it's actually cold to the touch. (And no I didn't check the charts when I picked it up in the shop, REALLY my bad with the pulldown mess.)

You're very right that my beefy MOSFET is not fully open at 5v, but check the power curve and you'll see that it'll pass several amps already at that point so no problem there :-)

But I will cede the general point - a MOSFET with a logic level-rated input is your best bet, especially if it's not as overkill as mine.

You're also fortunately wrong in worrying about the 0 signal. The PWM spec clearly states max 0.8v low.

I have no idea why your initial circuit doesn't work. I've ran 3 different fans on 2 mobos with this now - with and without resistor:
- An old 7000 series non-heatpipe Zalman CPU cooler
- A 9500 zalman CPU cooler
- A Noctua NF-S12B case fan

I also did tries with a (33u cap + 56 ohm) in parallel with the fan. Also works, but the end result is just even more RPMs early in the the PWM% spectrum.

Damn shame that you don't have an oscope - I don't have one either :-(


Edit: Oh and you're right about the MultiSim curves. They look absolutely atrocious. But MultiSim only has simplistic motors, not ones with lots of logic in the hub like any PC fan has.
 
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Aye, I would fully expect a multimeter to average out the voltage over the duty cycle, otherwise it would be completely useless when any kind of harmonics are involved. (Four flickering 7-segment digits, yay)

I measured my own 2.5V above with 100% duty cycle.

I'm not surprised you're getting 5V@100% all the time on your 555 circuit - it doesn't have an internal resistor.
 
I didn't measure. I just did the math: 5V -> X internal -> 1.3k -> gate connector -> 3.7k -> gnd. Measured 2.5V at gate. Gives X=2ksomething.

The IRF730 you had should work?
 
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Tealc said:
I think perhaps that the PWM voltage isn't necessarily always 5v as the fan spec is supposed to pull it up to 5v.
Click to expand...

Hrmmmm I just checked the 1.3 spec (been reading 1.2) and it suddenly says to pull up, yes (whereas 1.2 does not).

Do you have any non-ASUS mobos to measure on (with 100% duty cycle obviously). Mine are all ASUS at the moment.

Iffy work though, pulling up a voltage when you don't know the internal resistance. Unless you go with silly high resistances of course. (Assuming the voltage you have to work with is non-5V obviously. Like in a fan where they have 12v. Unless you start mixing in zeners. Sigh.)

Edit: ******, it seems Intel is up to rev1.61 on PWM, but it's all secret-hushy on their site and needs a password.
 
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Yeah like I said in the schematic, I have the +5V pulse source set up "backwards" with +5V being the normal state and 0V being the pulse. That way I can run it on +5V for 100-300ms at the start to rev up the motor. (Not pulsing during that time makes the sim run MUCH MUCH faster.)

And yes, the -87v on the sim oscope is super awesome. But if you graph wattage in a manual sim you'll see that there's absolutely no amps involved.

As for sim experience.. nahh, I mostly just swear and tinker until I get it to behave.
 
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