Actually, magnetical histeresis causes the core to temporary become a
magnet.
In good relays this time is generally a short one and a permanent retention
is only a matter of mechanical problems or arc soldering between realy
contacts.
Concerning the lack of receive and relays this is really caused in most
cases by high resistance in the contacts.
Altought looking strange, which is missing here is the lack of a
micro-arcing between contacts that serves to remove oxidization.
Once I saw a tricky method where a limited DC current was set to flow into
a receiver relay contacts to overcome the oxidization problem.
73,
Mauri I4JMY
----- Original Message -----
From: Tom Rauch <w8ji@contesting.com>
To: <amps@contesting.com>
Sent: Saturday, August 12, 2000 12:03 AM
Subject: [AMPS] Relays hanging up
>
> I've never seen a DC relay hang up from residual magnetism.
>
> The reason is the core material is intentionally a type that does not
> retain magnetism. If it is magnetized, it will simply magnetize again.
>
> The reason most relays hang-up is because they weld into position
> from hot switching.
>
> Lack of receive is more commonly caused by high resistance in
> the contacts, which is generally caused because the contacts are
> high current contacts designed to wipe when normal current
> passes through the contact. Running a 20 ampere relay at nearly
> zero amperes will almost certainly insure the relay will become
> unreliable over a period of time.
>
> That is a constant fight when using conventional high current relays
> to switch receiving lines.
>
>
> 73, Tom W8JI
> w8ji@contesting.com
>
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