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