Electronics > Beginners
12V PC PWM 4 wire fan datasheets?
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paulca:
Has anyone seen or have a link to a fully detailed datasheet for 4 wire DC brushless fans as used in PCs?

I'm trying to understand their PWM and Tact requirements to pick a PWM controller to control a bunch of them?

I can't even work out if the PWM and Tact are 5V or 12V, just can't find much information on them at all.
aheid:
Here's the official (as far as I can tell) specification for the 4-wire PWM fans: www.formfactors.org/developer/specs/4_wire_pwm_spec.pdf

edit: just to expand a bit, the tach signal is 12V open-drain/collector, two pulses per revolution. The PWM is 5V, 25kHz and needs to be driven low.
paulca:
I had spotted that before, but was unsure if it was just someone's spec for their fans or a standards spec for all fans.

Anyway my electronics brain is not fully functional today.  When they say open-drain on the PWM that means when I leave it floating the pull up in the fan gives a 5V high and the fan spins at 100%.  If however I connect the PWM pin to ground the fan receives a low.... right? 

So LED Drivers, if I can find one that will do 25khz would make good PWM controllers.

Actually I'm finding it difficult to find PWM controllers that will do 8 outputs at 25khz.
Gyro:
No, that's the standard spec.

Yes, leaving the PWM unconnected (with fan internal pullup) runs the fan at full speed. It's a simplicity and safety feature.

Instead of an LED driver, you can do it cheaper and easier with a 555 timer circuit - or if you want something ready made, get an ebay servo tested (555 based) and mod the frequency if necessary. Stick a transistor and base resistor on the output if you want open collector.

If you want to drive 8, then a PIC or Arduino would be easier, you could then use the tach signals to get feedback of actual rotation speeds (if you want to match them to prevent audible beating).

paulca:
Yes I started considering an ATMega328p, I have a thread on that, but it looks like the PWM analogue outs would not work (they are current sources, not sinks and I don't know if you can have PWM sinking) and so I would need to do bit banging in a loop anyway.

For the purposes I might actually go for the ESP8266 to be able to provide an open, easy, API via REST rather than messing around with serial over USB and the like which needs fairly low level programming client side.

I'm hoping to control 6 or 8 fans, with 6 or 8 tach inputs and a number of 1 wire temp probes.

I found this:
https://datasheets.maximintegrated.com/en/ds/MAX31790.pdf

While it's an awkward package to solder it has what I need.  The odd thing is that it's tach inputs are all +6V max, so that means all the tach inputs have to go via a level shifter mosfet or just a divider as they are 12V from the fans.  Seems odd, but maybe MAXIM didn't really have much choice and allowing 12V tach inputs made the design more complex.
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