[R-390] DC Fan Noise

[Tim Shoppa]

> 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.