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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
