Relay manufactures design the contacts of their relays for specific
applications. Relays generally carry an application rating such as 'Make &
Break Load Switching', 'No Load Switching', 'Make Only Load Switching',
etc. The selection of the proper relay for a specific application is crucial
to the life of the relay and it's performance in the circuit. Relays rated
as 'Make and Break Load Switching' depend upon current flow through BOTH the
normally closed AND normally open contacts in order to maintain a specified
minimum contact resistance over the specified life of the relay. Use of such
a rated relay in a circuit where no current flows through either the NC or
NO contacts can cause contact resistance to increase over time and the relay
will eventually fail to provide circuit continuity through the contacts.
This is sometimes misinterpreted to be a sticking relay armature problem.
Use of such a relay in an QSK amp where "hot switching" is prevented by
circuit design but also no current flows through the NC contacts (on
receive) will eventually cause these symptoms to occur, usually identified
as "no signal or a greatly attenuated signal on receive". Rapping the relay
may cause the symptom to disappear (for awhile) and is often interpreted as
a sticking solenoid versus a contaminated NC contact. A better choice for
such an application would be a relay whose contacts are rated as 'No Load
Switching'.
The HC-1 relay meets these specifications, however, I am not familiar with
the SO6JNB218 and SO6HBA318 relays, but if you have data sheets on them you
should be able to make the proper selection.
73, Dave, K1FK
Fort Kent, ME
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