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CPU Fan wiring ??

BruceW

Well Known Member
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.
 
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.
 
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.
 
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?
 
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.
 
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.
 
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.
 
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.
 
Plus one,always on.

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.
 
Fans are dirt cheap compared to avionics. So….operating under the assumption that even a little extra cooling is good for slightly improving the MTBF of avionics, I run my fans full time. They’re on a pullable breaker if there’s ever a reason to turn them off.
 
This is my thinking also. I'm putting the fans on a dimmer pot (PWM) so i can control them for both purposes.

You just gave me an idea. I have a spare dimmer pot that I was unsure of what to use for. My dimmer pots have integrated SPST switch (Digikey PN:
CT3047-ND). I can use the SPST switch for fan power (to make sure it is off) and the POT to control the speed, or not worry about the switch and use the PWM function when Pot is at 0-ohms to tell system to shut off. Eliminates one switch.

I use the Pot w/ SPST switch just because I like the feeling of the click when you take it all the way off. Right now I am not using the integrated switch.
 
Thanks for all the posts.

After further review . . . . . , I am inclined to KISS this item and direct wire to the avionics master and have it constant ON.
Seems like there are more reasons to have the fan on and little reason to turn it off.
The ES-00056 fan is $9.55 on Vans so I am not too worried about wearing them out. Buy a few for the shelf and replace as needed.
Plus, simplifies the panel switch row.

Thanks again.
 
Thanks for all the posts.

After further review . . . . . , I am inclined to KISS this item and direct wire to the avionics master and have it constant ON.
Seems like there are more reasons to have the fan on and little reason to turn it off.
The ES-00056 fan is $9.55 on Vans so I am not too worried about wearing them out. Buy a few for the shelf and replace as needed.
Plus, simplifies the panel switch row.

Thanks again.

How hard are they to replace? Mine are a hassle.
 
Assumption on my part . . . .

Fans are on the foreskin of the tip up canopy.
Looks like easy access when open.
 
Mine are on a speed control but once I found a good speed it has never been changed. And yes, good for winter air circulation and summer avionics cooling, and a couple of times as a reminder to turn the master off as well ...

Good stuff
 
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