Pardon me if I'm jumping into a discussion a little late (and a dollar
short), but there might be some confusion on how and why relays are designed
for intermittent duty. As an example, take a standard master
relay. It is rated for continuous duty and something like 80
amps. That means the coil won't overheat if left on forever and that
the contacts also won't overheat when passing 80 amps and that the contacts
won't weld when interrupting 80 amps. It is often the limitations when
opening the circuit that determines the rating. Current will try to
keep flowing and if the contacts don't separate rapidly and with enough
force the inevitable arc could create enough heat to weld the contacts and
prevent opening. If it is desired to design a relay in the same
package (for cost, size and weight reasons) that can be used intermittently
to pass a much higher current the design parameters are different.
More force is required to increase the rate at which the contacts are opened
so a higher force spring is installed. Then more force is required to
close the contacts, so fewer turns of heavier wire are used in the coil
(that's right, FEWER turns, but that's another discussion). The result
is that the relay can now successfully switch maybe 300 amps, but the coil
will overheat after a few minutes of operation. That's the difference
between a master and a starter relay. And I have never, ever heard of
using a relay as the "weak link" in the system.
Gary
ps: My Lancair list emails now come all messed up (see
below). Why is that? Am I doing something wrong?
The
only caveat with that fix, Fred, is that intermittent relays are
ther=
e
for
a purpose. They are supposed to be the weak link in a circuit where
=
a
continuous
stuck relay could result in greater problems than a failure.=20
Does
that protective function matter in this circuit?=0D
Should
be considered.=0D
John=0D