Today's Message Index:
----------------------
1. 09:58 AM - the fusible link question (William Chenoweth)
2. 01:32 PM - Master Relay Typical Resistance (long Story) (Radioflyer)
3. 06:24 PM - Re: Master Relay Typical Resistance (long Story) (user9253)
4. 06:53 PM - Re: fusible link construction (Robert L. Nuckolls, III)
Message 1
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| Subject: | the fusible link question |
To all of you who responded to my fusible link question - thank you very
much. Your comments made sense so with one very low current exception I've
abandoned the DB9 pin idea.
Bill
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| Subject: | Master Relay Typical Resistance (long Story) |
I've been chasing a sporadic in-flight alternator drop out issue in my LongEz.
(No engine starting or dead battery issues.) The alternator was checked out to
be fine and the voltage regulator is fairly new. I recently installed a small
EFIS which shows the system voltage. To my surprise, it showed 14.8-15.3V when
my analog Voltmeter had all along displayed just under 14V. This got me to
trace Voltages from battery to contactors (both Essex 70-111225-5 units!) to bus
to overvoltage relay (Lamar).
With master switch on (some small loads), engine not running:
Battery and input to master relay (not the control terminal) was showing 12.16V,
output of master relay showed .4 to .48 drop depending on gentle taps,
bus voltage at breaker panel measured an additional .2V drop,
input to overvoltage relay was 11.45V dropping to 11.41V.
Ostensibly the input to the Voltage regulator would have been about 11.4V, which
would explain the high Voltage I was observing on the EFIS display while flying.
OK, my theory is that the occasional alternator dropouts (which seem more common
in colder weather, BTW) are due to low voltage feed to the voltage regulator
due primarily to high resistance master relay contacts. This would raise the
alternator output voltage to occasionally surge to 16V, thereby tripping the overvoltage
relay off. Do we concur?
So, out of curiosity what should be the proper pass-through resistance for a master
relay? I used the 4-wire probe technique as suggested by Bob N. on the removed
master relay and measured the resistance average to be about 133 milliOhms.
(In the plane it measured more like 203 mOhms.)
Meanwhile, I'm waiting to receive a new Cole-Hersee #24115 contactor to replace
the old Essex unit. If this doesn't fix the problem, I'll sell the plane.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510091#510091
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| Subject: | Re: Master Relay Typical Resistance (long Story) |
With light aircraft loads, the voltage drop across the battery contactor should
be near zero. One half volt is way too much. If all of the crimps are tight and
the nuts are tight, then the contactor is at fault. Replace it.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510094#510094
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| Subject: | Re: fusible link construction |
At 03:33 PM 1/30/2023, you wrote:
>There are a few places where relatively low current links are used;
>the alternator field circuit comes to mind. You certainly wouldn't
>want to try the little pins for really high current.
>
>Charlie
COTS (commercial off the shelf) fusible link wire
is generally not offered in smaller than 20AWG.
https://tinyurl.com/282kj2w3
This means the SMALLEST wire protected by
COTS FLW would be a 16AWG (4 steps larger
that 20AWG). This condition alone suggests
that fusible links are suggested for relatively
fat wires carrying 15A or more.
Fusible links are the same class of circuit
protection as the ANL/ANN style CURRENT LIMITERS.
While they function like a FUSE (melt to clear
a faulted feeder) they are intended for
protection of DISTRIBUTION feeders, not
specific APPLIANCE feeders.
Note the ratings differences between the
plastic ATO fuse and the MIDI current
limiter.
https://tinyurl.com/2dfpw7qw
A 3A fuse loaded to 4A is
on the feather edge of melting. It might
open in as little as 1 second. A 30A
limiter loaded to 40A may never blow;
and it takes about 100A to open in
in 1 second. They don't mix-n-match.
Fusible links are on the same order of
fusing performance as current limiters.
Probability of opening is probable two
orders of magnitude smaller than probability
of a fuse having to do its job . . . and
how often do you ever need to replace a
fuse in your car? I can recall perhaps
a half dozen instances in 60+ years
of driving.
A further point to consider is that
a d-sub pin is maxed out with 20AWG
wire and 7A . . . use of 'barefoot'
d-sub pins on individual wires should
be limited to low current situations
were space is tight . . . like connecting
a MAC trim servo to the ships trim
system wiring.
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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