Master contactor question

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?

On Wed, 28 Oct 2009 15:30:30 -0500, 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?


Check the coil resistance. Constant duty contactors are higher
resistance, so they draw less wower and heat up less than starter
solenoids.

On Wed, 28 Oct 2009 16:28:04 -0400, wrote:

On Wed, 28 Oct 2009 15:30:30 -0500, 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?


Check the coil resistance. Constant duty contactors are higher
resistance, so they draw less wower and heat up less than starter
solenoids.

Does the coil resistance change much as the contactor wears out?
Somehow, the contactor coil seems to get weak over time, or the
contacts inside are dirty or both. My master contactor would click,
but not turn everyhing on until I cycled it a couple times.
One thing I've discovered is, on new contactors, the coil resistance
on continuous duty vs. intermittent duty isn't much different. Just
barely enough to make a determination of what type they are.
I've got one new intermittent one reading 14.7 ohms, and a continuous
one brand new from ACS reading 15 ohms, then another continuous one
that reads 17.3 ohms. I wish there was a more difinitive reading
between the two types. They are so close, it's hard to make the call
using that method.

On Oct 29, 10:18 pm, rich wrote:

Does the coil resistance change much as the contactor wears out?
Somehow, the contactor coil seems to get weak over time, or the
contacts inside are dirty or both. My master contactor would click,
but not turn everyhing on until I cycled it a couple times.

It's the contacts inside that get oxidized or burned, not a
weakening coil. They're usually copper, and copper's oxide isn't a
good conductor so they will heat up and oxidize even faster. The
opening of the contacts creates a spark and burns them, adding more
resistance. Clicking it on and off will sometimes result in contact,
but the contactor is basically shot.

Dan

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

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

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)

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)

On Oct 28, 7:13 pm, rich wrote:
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.


At 15 ohms and 14 volts, it will draw close to an amp. That's 13 or
14 watts, and I'd expect it to get hot.

Dan

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

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)


Nice post!

Brian W

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

Isn't there a difference in resistance betwen the active state and the
passive? I feel some people have determined coil resistance by measuring
with a cheapo multimemer - this uses a small battery, the ensueing
measurement will reflect the inactive state. Better would be to activate
with a battery (or better still, a stable 12V reference) and measure the
current.