At 10:48 3/28/2002 -0800, you wrote:
>I believe that one of the most important factors involved in small
>signal switching has not been addressed. That is the mechanical nature
>of the relay contact assembly. Some relays are designed so that the
>contacts merely open and close. Others are designed so that the contacts
>slide or "wipe" as they close. You can see this action if you slowly
>close the armature with your finger and watch the contacts. Large
>contacts do not solve the problem, in fact the relays used in
>transceivers usually have small contacts. They are frequently in the
>form of small bars operating at right angles to each other. These are
>surprisingly reliable. These designs have fairly large PSI force to push
>thru the oxides or other contaminents on the contacts.
Hi,
This is right. The wipe of the contacts is important to the breaking of
the oxide coat. One should consider also the function of the relay as to
whether it is used in a load or non-load break situation. Since most of
the relays in antenna switching are non-load break the contacts need to be
"wiped" each time they are used. When relays are not used the contacts
will oxidize and the contact resistance will go up. One of the best
things to do is to operate the relays often to insure this cleaning action.
This is also true with circuit breakers that tend to sit for long periods of
time not being switched. I try to turn my circuit breakers both here at the
plant and at home on and off at least a couple of times a year. Other
examples I can think of are switches, pushbuttons, and organ keyboards
with crossbar wires. These are generally silver plated and need to be
used often. Mine has sat for a while and some notes don't play. (Don't
let your mind wonder here with smart remarks...... :-> HI.........)
> I was at a trade show once where a manufacturer of precious metal
>alloys for contacts was exhibiting. There was a book on contact design
>on their table. The salesman said it was written for internal use by the
>company"s designers. I talked him out of a copy and took it with me. The
>book talked about problems with small signal switching reliability as
>well as power switching, etc. They discussed Gold, Rhodium and other
>metal contacts. Even Gold was not totally reliable in this application.
>In fact there was no material that would guarantee total small signal
>reliability.They talked about bleeding a small current thru the contact
>via a large resistor or RFC to help punch thru the contaminent film.
We use bleeder resisters here at work and in some control circuits for
this exact reason.
> When I retired, I did not take the book with me and, sadly, I have
>forgotten the name of the company. I have used several types of relays
>in 4 sq arrays and other phased arrays with very few problems except for
>lightening induced damage. I usually parallel 2 sets of contacts and
>look for a design that wipes the contacts.
As I am somewhat of a packrat, I have collected some books over the years and
on of them is "Relay Engineering" that was given out in the early days by
Struthers-Dunn. I was fortunate to get a number of the chief engineer's
books when he died.
One interesting part in the book it talks about current carrying capacity and
in part says:
"The current carrying capacity remains constant regardless of whether the power
through the contacts is a-c or d-c or regardless of the voltage."
Have to remember here they are talking strictly current carrying. This is
assuming
no breaking of contacts and not considering insulating voltage breakdown.
>73, Dan, N5AR
I apologize if I went off topic a little as I didn't save the original post so
I don't know what it said.
Hope this helps.
73 Phil W9IXX
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Philip Florig e-mail:pflorig@ieee.org
PO Box 44 (or):W9IXX@arrl.net
Worth, IL. 60482 amateur radio:W9IXX
USA also:J3X-T30,31,32,33P
team member:AH1A-J3A
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Metro DX Club club call:W9TY
Oak Forest, IL. qsl via W9IXX
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