[Elecraft] HPS-1a Gamma Hybrid,Communications Power Supply

[Paul Christensen]

> output resistance to pull the output below the clamping level.  Don't know 
> what fuse you are using: say it's an ordinary "fast" blow 20A, then you 
> will need >40A for 30s to blow it by which time the transient has long 
> gone.  The clamping voltage also depends on the current drawn and could be 
> as high as 21.2V according to the spec. I would have to draw a load line 
> to find out how that would perform with the supply but that would be only 
> static conditions.

Agree with all your statements.   I tested the circuit ahead of time with a 
variable switching supply and a 25A fast-acting fuse (a value I am currently 
using with the K3).  At about 15.5V the diode clamped, the fuse blew and the 
circuit opened up as intended.  The diode remained good after that fault 
event.  Now, how fast did the fuse blow?  According to the info I've 
gathered from the Littelfuse and Bussman 3AG fuse datasheets, not fast 
enough to ensure 100% protection.   Also agree that some switching designs 
will limit current ahead of the zener device and that may preclude the fuse 
blowing.  I don't particularly care as I still want that fuse there for 
safety purposes.  Again, all this may be trivial in the grand scheme of 
things if the Gamma OVP design actually works.

But consider this: as long as the diode is clamping, even if we're waiting 
for the fuse to blow or the current of the supply to self-limit, the diode 
should still be performing a reasonably fast protection function.  OTOH, if 
the diode fails while we're waiting for the fuse to open or the circuit to 
self-limit, all bets are off on the usefulness of the diode.

A fast-acting circuit breaker may be better -- or not.  I need to research 
the available types of CBs a bit more, including the Airpax types.

Paul, W9AC