Monday, March 07, 2005

The Polyphaser PLDO

The Polyphaser PLDO-120US15A Inline Impulse Suppressor is designed to protect equipment power circuits from large voltage surges. I am using it in my ham station in connection with a "single point ground" system to protect against lightning surges. The Polyphaser catalog does not give design details, but I wanted to know just how my equipment is going to be protected. Here is what I've found to date. (Click for more detail on photo.)



PLDO-120US15A Photo


The unit is solidly built. I traced the schematic: (Click for more detail.)



PLDO-120US15A Schematic


Most of the protection comes from MOV devices that begin to fire above 130 V. The inductors, by rough estimate, are about 1.2 ┬ÁH. That converts to 15 ohms of series reactance at 1 MHz. If the second stage MOVs are in conduction, the inductor should provide a significant voltage drop. The series resonance of the two inductors and the capacitor is at about 400 kHz.

I am puzzling over the handling of the system grounds. The AC ground connection naturally goes to the PLDO case, which will bolt onto my local ground plane - where all the surge arresting gear is to be mounted. The AC ground is carried back about 30 feet to the AC service entrance. From that point to a solid earth connection (water pipe entrance) is another 40 feet or so. [I believe this is according to code. The situation is complicated by the location of the house on granite ledge, which rules out a driven ground rod.]

These ground paths go through the crawl space under much of the house. The ham shack antenna feed entrance and ground plane are on an exterior wall, with a possible earth ground only a few feet away. On the one hand, we shouldn't have the radio ground system with a separate ground from the AC, but on the other hand, that AC system "ground" is rather inductive and lengthy compared to a direct ground near the radio room.
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