Paul, WD7S uses a small current flow in the relays of his tuned input board.
You may be able to download his board informatoin from his site and take a look
at what he does.
73
Jim W7RY
> -----Original Message-----
> From: d.cutter@ntlworld.com
> Sent: Sun, 8 Aug 2010 20:14:55 +0100
> To: xxw0qe@comcast.net, amps@contesting.com
> Subject: Re: [Amps] Automotive Relays
>
> 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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