[Elecraft] Samelex Power Supplies with a K3

[Steve Ellington]

Paul....It makes me wonder if they couldn't just put a decent heat sink on 
those switchers and virtually eliminate the fan or at least make it very 
rare to ever turn on. I realize they want to keep them small and lightweight 
and I guess fans are cheap and light but I sure do hate the racket they 
make!

Steve
N4LQ
----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: <elecraft at mailman.qth.net>
Sent: Sunday, May 23, 2010 8:40 AM
Subject: Re: [Elecraft] Samelex Power Supplies with a K3


>> Finally, I went back to my Aston 35m which has no fan and helps hold down
>> the desk. Ahhh....No noise, No hash....Just nice clean DC.
>
> I've been dealing with the same issues as I transitioned from a 20 
> year-old
> Astron RS-35 to an HP bench supply and an Astron switcher.  In each case,
> both supplies use loud rotational fans.  Recently, I've added fan speed
> control with a PWM controller.  Instead of limiting the fan's speed 
> through
> current alone, a pulse-width-modulator controls speed by producing short
> pulses of voltage.  Speed is then controlled by the duty-cycle of the
> circuit's pulse train.
>
> After trying several attempts to smoothly control rotational speed with
> series resistors and voltage-dividers, I found several low-cost PWM kits 
> (~
> USD $20) available that accurately control speed down to slow levels 
> without
> stalling.  In the case of the SS-20, I created two threshold steps, one to
> engage when the heatsink temperature rises above a pre-determined amount.
> However, after adding the PWM controller and letting the idle speed rest
> where no noise is heard, enough air flows to keep the higher speed from
> kicking in, except when I key down into a load for more than a minute.
>
> Paul, W9AC
>
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