On Wed, 28 Oct 2009 18:29:55 -0400, wrote:

On Wed, 28 Oct 2009 14:13:56 -0700 (PDT),
wrote:

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
Gotta be careful of the "usually"
The definitive test is coil resistance. Most starter solenoids are
around less than16 ohms. There are quire a few insulated base 4
terminal "starter" solenoids. Echlin/Napa ST41, ST56, ST67, ST71,
ST73, ST75, ST77, ST83, ST94, ST96, for starters. Mostly used in Mopar
and AMC applications.

The St56 is 3 terminal unit designed for "ground enable" apps

Most constant duty solenoids run 16-30 ohms - and there are a fair
number of THEM with gounded bases.(st35, ST87, ST97

That's exactly what I need, a constant duty soleniod with a grounded
base. St35/ST87/ST97.. I'll try Napa tomorrow. You say the ST35 is
large, as in physically large? if so, I'll try the other ones. ST87
etc.. I've got 3 contactors, and I checked the coil resistance in all
of them, and the darn things are all right around 15ohms. Not enough
either way to make a definitive classification. Bob Nuckols book says
15 or below are intermittent duty, and the higher resistance ones are
continuous duty. The brand new one I got from Spruce, sold as a master
contactor is right around 15. I wish these thing would measure more
one way or the other to make it more obvious what they are.
Rich


The 4 terminal constant duty units a ST36, ST80, ST85,

The large ST35 and ST36 are the only continuous duty units rated at
14-16 ohms. All the smaller ones are 16-25 ohms.

The ST80 and ST85 are the common ones - the 85 differeing from the 80
only in having copper contacts (much to be preferred)