[Elecraft] K3: Added protection for RS-232 port

[Jim Brown]

On Tue, 20 Jul 2010 07:48:24 -0400, Tom W8JI wrote:

> MOV's might not help or might not be 
>*useful* on all lines, but adding them would certainly not hurt 
anything.

The issue with MOVs and other shunt mode suppressors is that they divert 
the strike into whatever ground they are connected to. If all grounds 
are properly bonded, as Tom, Joe, and Paul have described, that's fine. 
One MAJOR bonding error is the Pin 1 problem. But if the grounds are NOT 
properly bonded, a big part of the strike potential can exist between 
equipment that is grounded at different points, and when that equipment 
has low voltage connections between it, that high potential causes 
destructive equipment failure. This is the mechanism that causes 
failures with three mode suppression that Paul is talking about. A 
primary function of bonding all the grounds is to minimize the potential 
throughout the building (and between equipment) in the event of a 
strike. 

An important way of thinking about this is that the function of an MOV 
(or other shunt suppressor) on a signal or control line is to limit the 
size of the DIFFERENTIAL voltage. When we mount a typical coax 
suppressor on a copper ground window, the outside of the coax is 
grounded, which minimizes the common mode voltage of the strike, and the 
differential signal is limited by the suppressor. 

Things get more complicated with a balanced line, like a rotor cable or 
the control lines to a SteppIR, because both sides of the line need 
MOVs, and the two MOVs on a line may not conduct equally with a strike. 
That gets even more complicated with multiple balanced circuits feeding 
a SteppIR. :) 

Yet another problem with shunt mode suppression on signal and control 
lines (like RS232) is that the suppressor adds capacitance, and the 
speed/distance of serial lines is limited by the capacitance of the 
circuit. That capacitive limit is another reason that CAT5 makes 
excellent serial cable -- it's capacitance is quite small compared to 
other cables, typically about 12pF/foot. We've used this in the pro 
audio world to extend the useful range of a simple RS232 circuit to 
several hundred feet. 

73, Jim K9YC