CPU Fan wiring ??

[BruceW]

Is the CPU fan wired as constant ON with avionics bus, or activated with panel switch?
Some -14 panel examples have a 'defrost fan' switch, some dont.
Doing some panel planning and wondering if a switch is needed.
Thanks.

[Kyle Boatright]

Originally Posted by BruceW

Is the CPU fan wired as constant ON with avionics bus, or activated with panel switch?
Some -14 panel examples have a 'defrost fan' switch, some dont.
Doing some panel planning and wondering if a switch is needed.
Thanks.

Your choice. Mine are on a switch.

[BOHICA]

I have worked in the PC industry for 22 years. One of our biggest failure items are fans. The bearings just wear out. Minimizing the operational time is the best you can do. So, turn on when you need to defrost. Having it just run means it will fail and according to Murphy's Law of the universe it will fail when you need it the most. Adding run time to something when you don't need it (hot summer day) makes no sense.

I have a defrost fan control in my panel.

[BruceW]

Thanks for the replies. That's two votes for switch vs. constant ON. (where I was going)

But wait, aren't these fans also used for avionics cooling?
So you WOULD want them on for hot summer days, right?
(and defrost on cold winter days)
Am I missing?

[jaradtke]

It’s a belt and suspenders thing. The avionic don’t require the additional fans. The plans don’t even call out to holes in the glareshield. The holes alone provide for convective cooling. The fans give you the opportunity to move more air.

[lr172]

I did an SPDT swith ON-OFF-ON. One position sends full voltage for defrost and the other sends around 8-9 volts for cooling. This is all preference. I leave it off in the winter to preserve bearing life.

[BOHICA]

Originally Posted by BruceW

Thanks for the replies. That's two votes for switch vs. constant ON. (where I was going)

But wait, aren't these fans also used for avionics cooling?
So you WOULD want them on for hot summer days, right?
(and defrost on cold winter days)
Am I missing?

Well the avionics fan would be best to have on always. With Electronics on a cold day things may not start (oscillators don't like to start). The fan is useful for moving warmer air to the electronics. On hotter days the fan of course help cooling. Just experience from designing Rugged Laptops and Tablets that operate from -20°C to 45°C (-4F to 113F). But this means as I am sure other will attest the fans on Avionics will fail eventually (Usually bearings).

Defost give it a switch. Avionics always on with bus.

[georgemohr]

Originally Posted by BruceW

Thanks for the replies. That's two votes for switch vs. constant ON. (where I was going)

But wait, aren't these fans also used for avionics cooling?
So you WOULD want them on for hot summer days, right?
(and defrost on cold winter days)
Am I missing?

This is my thinking also. I'm putting the fans on a dimmer pot (PWM) so i can control them for both purposes.

[rocketman1988]

PWM dimmer for me; I used two 120mm Noctua fans in my -10...

[N427EF]

defrost in winter and cooling avionics in summer.
As an aside, you can hear the fans when your headset is off and it might one day remind you to turn the master off.