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nuckolls.bob(at)aeroelect
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 Posted: Sun Feb 05, 2023 12:48 pm Post subject: Master Relay Typical Resistance (long Story) |
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At 09:48 AM 2/2/2023, you wrote:
| Quote: | | Is the master contactor physically hot after extended use? Might hold a clue? |
A typical battery master contactor (Metal, 'whisky' barrel')
will start out cold with a current draw on the order
of 0.8A and drop to somewhere around 0.6A after it warms
up . . . and warm up it will.
0.6A at 14.5V is right at 9 Watts. In time, this
will raise the device temperature to something
on the order of 170F in still, room-temperature
air. It will probably not be nearly that warm in
your airplane. Hence, continuous operation current
will be a bit HIGHER than 0.6A.
Nonetheless, a NORMALLY operating contactor will
be too hot to touch. This is a GOOD thing. Your
contactor IS mounted cap-down and has an itty-
bitty drain hole in the cap? Warm ops go to drying
out the contactor's innards thus extending its
service life.
If voltage drop across the contactor's power path
is over 100 mV or so . . . and especially if it
VARIES with vibrational stimulus (rap it with the
handle of a screwdriver), the contacts are AGED
and the device should be replaced. This basket
carries a LOT of the systems 'eggs' but they're cheap.
You'll spend more $time$ fussing over its diagnosis
than the thing costs.
When in doubt, replace it.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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Radioflyer
Joined: 04 Feb 2009
Posts: 43
Location: Boston
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| Posted: Thu Feb 16, 2023 9:31 am Post subject: Re: Master Relay Typical Resistance (long Story) |
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Bob N. and others...Thanks for your comments. I did replace the high milliOhm battery contactor with a new Cole-Hersee unit. I also replaced for good measure the battery master switch since the original contactor was not fitted with a suppression diode.
Overall, it made no difference to my 15+ overvoltage and occasional alternator dropouts. Further tracing of Voltages indicated that the Voltage Regulator was getting same V as present directly at the battery and yet not "regulating" so I replaced the Voltage regulator. Problem solved (for now)!
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