[Elecraft] queer behaviour !

[Paul Christensen]

True, Guy.  Looking at the internal structure of the switching HEXFET used 
in the K3, a protecting Zener diode appears between the source and drain 
leads.  With a negative keying voltage, the Zener is actually forward biased 
like a typical silicon switching diode.  So, the negative switching voltage 
by itself is probably not the concern, but rather the resulting current 
through the forward-biased Zener diode when a negative voltage is applied. 
That diode probably only handles a fraction of the normal HEXFET switching 
current, so all bets are off when using vintage amps that use a negative key 
line.

Darn, with the addition of another series HEXFET, a cut of jumper W30, and a 
return from the added HEXFET drain to the outer shell of the KEY OUT jack, 
the K3 could safely switch both key line polarities.

Paul, W9AC


----- Original Message ----- 
From: "Guy Olinger K2AV" <olinger at bellsouth.net>
To: "Paul Christensen" <w9ac at arrl.net>
Cc: "Elecraft" <elecraft at mailman.qth.net>
Sent: Wednesday, August 18, 2010 9:18 AM
Subject: Re: [Elecraft] queer behaviour !


Have seen a K3 with toasted circuitry due to trying to key -150 volts.
 Don't do it.  Damage requiring component replacement is definitely
possible.