[Elecraft] Samelex Power Supplies with a K3

[Paul Christensen]

> 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