On Oct 28, 2:30 pm, rich wrote:
My homebuilt's master contactor is going bad. Sometimes when I turn it
on it doesn't make connection. It's got 1700 hours on it, so I'd just
as soon replace it. But the way the builder wired it, he's has
positive power from the battery going through the master switch to the
small terminal on the contactor. (cole-Hersey type) But the master
contactors, such as Spruce sells, are set up to actuate with ground
power going to the small terminal. They also have plastic around their
mounting feet so their case doesn't make ground. A starter contactor
would work perfectly with the way the plane is wired. I just wonder,
are starter contactors made to withstand continous use, like a master
contactor does? And how can one tell the difference in the two, they
look identical? And if not, can the master/continuous duty type be
made to work with postitive power to the small terminal?

The builder wired that contactor to make it safe. If you have
it set up so that battery power feeds though the cabin to the master
switch, you have a fire point since there will be no fuse in that line
from the battery. A fuse can fail with age and let you down. Cessna
uses the hot-terminal idea, and grounds the contactor's other terminal
through the master so that there's at least some resistance in the
line, limiting the current.

A master contactor is a continuous-duty solenoid. A starter
contactor is an intermittent-duty solenoid, and usually has its coil
wired internally to ground so that it wouldn't work as a hot-terminal
master anyway.

Dan