Larry
That's the sort of thing I was looking for. I have a book somewhere written
I think by Schrack and my memory of it is that you can't do both jobs at the
same time. Clearly, amateurs do with several amps of rf on transmit and
disappearingly small current on receive. I like the idea of applying a
small dc current to run through the contacts to *guarantee* they will always
work in the nA range at rf, but I haven't come across it and the evidence is
that it is not often needed.
I'll have a look at those references and try to find that book. Thanks for
boiling it down.
73
David
G3UNA
----- Original Message -----
From: "Larry Benko" <xxw0qe@comcast.net>
To: <amps@contesting.com>
Sent: Sunday, August 08, 2010 5:28 PM
Subject: Re: [Amps] Automotive Relays
> David,
>
> It doesn't! Look at the relay specs closely and you will see that very
> few relays have any gold in the contacts if they are rated above a few
> amps. I spec'd the use of 10's of millions of relays and never really
> liked any of them. There were always compromises but when using a
> million relays per year the manufacturers were very good at helping you
> use the proper relay or would make you the proper relay.
>
> There are 2 basic types of relay contacts:
>
> 1.) Dry type which can handle currents down to the uA level. These
> contacts are either in a vacuum (usually Rhodium) such as reed relays or
> use a contact material having a gold alloy since gold doesn't oxidize
> (much). Unfortunately the upper limit on these relays contacts is
> usually less than 3A.
>
> 2.) Power type relays where the maximum current is several amps but the
> relays have minimum current specs of 10mA, 100mA, or 1A usually
> depending on the contact material. The contact material is almost
> always some silver alloy and the relay contacts operate using
> "micro-disconnection" which means there is some tiny amount of material
> transfer between contacts. Sometimes power relays have gold flashing but
> that is generally for contact stability prior to service and the gold is
> expendable once the relay is used much.
>
> The above is a simplification and and all relay manufacturers would like
> a contact material that would handle all levels of current.
>
> Some relays attempt to reduce the minimum current spec:
>
> 1.) Increasing the coil power. Contacting the contacts with more energy
> helps remove any insulating oxide but reduces the relay life due to heat
> and mechanical stress. Operating the coil with high power briefly fixes
> any initial problems but does not address a contact that is operated for
> a month at a time unless the contact is entirely gas tight (when relay
> is new). Of course everyone wants sensitive coil relays which is the
> opposite of this solution.
> 2.) Bifurcating the contacts. The contact is divided into 2 smaller
> contacts and this is a probability improvement since both contacts are
> unlikely to be problematic at the same time. The down side is that the
> 2 smaller contacts do not handle a much current as the one big contact.
> The telephone industry used billions of bifurcated contact relays over
> the years.
> 3.) Designing a relay where the contacts come together and produce a
> minuscule amount of sliding which cleans the contact. All relays do
> this a tiny amount but by increasing this wiping the minimum current
> handling goes to nearly zero but for a relay that operates today and
> doesn't release for a month this is of less help. Again this wiping
> cuts into the contact life somewhat.
> 4.) Finally designing the circuit to produce an inrush current through
> the contacts when they are first operated or even better at some
> periodic interval. This would not be easy to do at the input to a
> receiver.
>
> There is an interesting relay FAQ from Omron at
> http://www.ia.omron.com/support/faq/4/17.html
>
> Panasonic (Matsushita ) previously known as Aromat and later NAIS has a
> huge amount of info at
> http://pewa.panasonic.com/pcsd/tech_info/tech_info.html
> The document on the power relays explains the advantages/disadvantages
> of several contact materials.
>
> This is probably much more than you wanted to know.
>
> 73,
> Larry, W0QE
>
> David Cutter wrote:
>> That begs a question I've often puzzled over: how does a contact rated
>> at
>> 10A cope with uV level signals, particularly after the gold flash has
>> gone ?
>>
>> David
>> G3UNA
>>
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