[R-390] DC Fan Noise

[n4buq at knology.net]

The reason the IF deck is involved is this project is a replacement for the 
ballast tube.  It is intended as a plug-n-play substitution for the ballast 
with the added advantage of having a small fan.  The fan cools the dropping 
resistors as well as anything else in its path.  If it's going to be noisy, 
though (these are new fans), then I may just have to forego the fan idea.  :
(

Barry - N4BUQ

> > I've been working on a project that will power a small fan on the IF
> deck.  
> > I'm using the 26.2VAC supply through a single diode to a 24VDC fan. 
> This 
> > yields about 11.5VDC (at least that's what my Fluke meter shows).  I 
> > realize this is half-wave DC so 11.5 is an "averaging" value.
> > When I hooked it up last night, my Grundig Majestic nearby picked up
> noise 
> > from the fan motor.
> 
> I think this is inevitable when you drive a commutating motor (unless
> your 24VDC fan is decades old, it is probably solid-state commutation).
> 
> I don't think it has a lot to do with the pulsating half-sine drive.
> 
> An entirely different 24VDC fan may be much better or much worse,
> hard to say without trying.
> 
> If you can, try to find a small AC fan. 24VAC units do exist but are not
> real common. It's vaguely possible that 24VAC may work to slowly
> turn a 120VAC fan, but even if it'll keep it turning it probably won't
> start.
> 
> Big AC fans (I'm thinking of 10" or 14" rotrons etc.) work OK without
> making a lot
> of wind noise if you use 120VAC to drive them and a dropping resistor.
> 
> A little odd that you're aiming it at the IF deck. Yeah, there's a lot
> of
> tubes in a small space there. I honestly do not know if crumbling foam
> in the mech filters is accelerated or not by "typical" heat on the IF
> deck.
> 
> Tim.
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