[TowerTalk] Relay Problems

[dan hearn]

Before retirement I had access to a very unusual book written by a
company which produces various alloys for electrical contacts. As I
recall, power relay contacts are made of materials which can easily
oxidize. In their intended application, there is sufficient voltage
present to punch thru the oxide layer and make a small "spot weld" type
conductive path. When we attempt to use them with small, sub microvolt
level received signals, contact is problematical. Gold, Rhodium and some
other precious metals are used in a flash coating for contacts in small
signal applications. I believe the book said there are sometimes even
problems with these.
  It is desirable in any case to have the relay mechanical design such
that there is a sliding action as the contacts close to penetrate oxide
or any other insulating material on the contacts.
  I used some of the Potter Brumfield PRD series power relays for
several years on a 4 SQ. system in Texas with no problems. I believe the
lack of problems was due to the semi floating spring loaded contact
design which allows them to slide around on closure. I believe these
relays are available from Granger and many radio supply houses. These
are pretty large. It is possible to buy vacuum relays for about twice
the cost of the PRD jobs. I used some previously on the 4 SQ but
lightening surges welded the contacts on one of them and I replaced with
the PRD's, needing the vac's for another project. Hope this information
is of use. 

73, Dan, N5AR

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