[Elecraft] Power Supply (WAS: New amp)

[Tom Hammond]

Not to take issue with Ron's comments... they are all appropriate, however 
one thing I think which may have been missed, or at least glossed over, is 
the fact the Astron supplies include a very good FOLDBACK CURRENT LIMITING 
circuit which, if directly shorted (e.g. HOT lead directly to GND) will 
instantly reduce the output current (and the voltage) to nearly zero with 
little or no damage to the supply, the attached equipment, OR to the 
cabling (or user).

This having been said, as with just about any P/S which included this type 
of current limiting, if only a partial short (e.g. low resistance, but not 
quite a dead short) appears across the output, the P/S will attempt to 
provide as much current as it can.. until it reaches that 'trip point' 
internally set at which it then thinks it sees a dead short and reverts the 
foldback current limiting. So, if you place a really heavy load across such 
a p/s, it WILL try to honor your request, so to speak.

All too often, I've (accidently) shorted together the "+" and "-" leads of 
my Astron VS35M (35A) supply. The only thing I hear is a momentary 'tick' 
when the wires touch, and the I see the voltage and current drop to zero... 
removing the short brings everything back to normal.

This is definitely NOT something you'd want to do with a P/S you weren't 
ABSOLUTELY CERTAIN has foldback current limiting. For, as Ron stated so 
eloquently, BAD THINGS CAN (AND WILL) HAPPEN!!!

73,

Tom Hammond   N0SS

At 12:51 PM 8/18/2005, Ron D'Eau Claire wrote:
>David wrote:
>Thanks everyone I have the answers I need and I am now planning to purchase
>an rs50m for my shack
>
>-----------------------
>
>A few things to consider:
>
>1) A 20 amp power supply will run the K2/100 with current to spare. My
>K2/100 runs 16 to 18 amps from my Astron 20M.
>
>2) Large supplies (even the 20 am, but especially one like a 50 amp) can be
>very dangerous! While you won't get a shock from 13 vdc, they require all
>the care you'd exercise working with an automobile battery. Shorts can cause
>instant fires. Hot, molten metal spraying, splattering and dribbling all
>over the place from a short can burn lots of holes in things, including your
>own body and eyes. Shorts tend to become instantly welded in place and if
>it's not good enough to cause the crowbar circuit to trip, the power supply
>will keep dumping current into the circuit. An RS 50 supply will dump 50
>amps into a 0.3 ohm short nonstop. Fuse everything and be careful just the
>same.
>
>3) You might get more amps for your buck by buying two or three smaller
>supplies than one big supply, and thereby be less likely to experience a
>total failure if a supply dies on you.
>
>Ron AC7AC
>
>
>
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