[Amps] Automotive Relays

[Larry Benko]

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
>
>
>   
>> I did a survey about 15 years ago on CQ-Contest (then the only reflector
>> available).  IIRC, there were essentially no complaints about the
>> Ameritron RCS-8 or the TopTen remote relay box.  I used two TopTens for
>> maybe 10 years, often operating into >3:1 SWRs at >1200 watts (for
>> example, running an 80m array cut for CW up in the DX phone window) and
>> never had any relay problems.  Currently using an RCS-10, which can have
>> up to 7 relays in the RF path.  Will be interesting to see how it holds 
>> up.
>>
>> Right now my Array Solutions Stackmatch has a bad relay (dirty?).
>> Symptom is occasional high SWR and a lack of received signal on the
>> lower antenna in my tribander stack.  If I send a little high power RF
>> through it things are then fine.
>>
>> 73, Pete N4ZR
>>
>>     
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