diode across vcc & ground on fan header...

[bigjoncoop]

hey my fellow DiYers,

 I have a PCB that was missing the components for an optional fan header that was not implemented from the factory. I installed the missing transistor and completed the signal path. now the circuit is working.
 
but right next to the fan header Is a missing diode across Vcc and Ground...  the fan circuit Works without it obviously and I'm assuming it is there so when the circuit turns the fan off, it doesnt backfeed voltage into the circuit while it still spinning for a few seconds... correct?
 
should I install a diode or is it not a huge deal? and if so will any schottky blocking diode work? for instance a  1n5819 or 1n4007    If not what's the minimum value diode i should use?

this is a 12v circuit and its a 40mm fan.

thx

[bobbydazzler]

I think you should install a diode - the fan can still be spinning fast when the power goes out and therefore has a bit of energy that can go the wrong way.  1n4007 should be fine.

[Siwastaja]

That is not what the diode is helping against.

BLDC fans have internal half-bridge driver for each phase (typically 2 or 3 phases), these switches have implicit or explicit diodes for freewheeling the current stored in the winding inductance.

Just a simple MOSFET, either on the ground or Vcc side, is often used to PWM these fans. I do have a habit of adding a freewheeling diode though but the purpose is to freewheel the (fairly small) amount of energy stored in the parasitic indutance of the wiring. It may indeed be completely unnecessary. I would suggest probing for any voltage peaks between the fan output pin and ground with an oscilloscope.