Today's Message Index:
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1. 04:33 AM - Re: Ground power (GLEN MATEJCEK)
2. 06:48 AM - Re: Ground power (user9253)
3. 11:40 AM - Re: Re: Ground power (Robert L. Nuckolls, III)
Message 1
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| Subject: | RE: Ground power |
> Hi Bob-
>
>
> With regard to the indicator LED, my intention was that the polarity
> indicator, and especially the reverse polarity indication, be functional
> regardless of the breaker condition. The choice of a bidirectional LED
> for the indicator was due to simplicity and the fact I've got a stack of
> them on hand. It would be a minor thing to go to discrete indicators.
> As to the reverse polarity diode in the contactor ground line, my thoughts
> are that the spike catching diode on the coil wouldn't be long for this
> world if reverse polarity external power were applied across it, and then
> the contactor would close.
> Thoughts?
>
Message 2
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| Subject: | Re: Ground power |
I like your circuit. It will not pop the breaker in case the aux power polarity
is
reversed. Nothing will happen except that the red light will come on.
But the circuit does not solve the problem of the contactor remaining
energized until the aircraft battery is depleted, that is if the pilot does not
shut off the aux power switch. Ideally the aux contactor would drop out
when the aux power plug is removed.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=507659#507659
Message 3
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| Subject: | Re: Ground power |
At 08:48 AM 8/12/2022, you wrote:
>
>I like your circuit. It will not pop the breaker in case the aux
>power polarity is
>reversed. Nothing will happen except that the red light will come on.
The catch diode across the coil takes care of that.
It prevents the contactor from responding to reverse
power while presenting a 'fault' current to the
breaker/switch.
> Nothing will happen except that the red light will come on.
>But the circuit does not solve the problem of the contactor remaining
>energized until the aircraft battery is depleted, that is if the
>pilot does not
>shut off the aux power switch.
That's what the annunciator is for . . . the light
should be illuminated only when ground power is
being used. If the contactor is 'latched' due
to pilot inattention to ground power controls,
then the light will serve as a reminder that
the switch is not properly positioned.
>Ideally the aux contactor would drop out
>when the aux power plug is removed.
That's ONE of the reasons that the third
'pilot pin' was added to the commercial/military
version of ground power receptacles. The pin
is shorter thus preventing arcing at the
ground power plug pins during mate/demate
in addition to de-energizing the ground
power contactor any time the power cart
is not plugged in.
Incorporation of the Cole-Hersee, ground
vehicle power connector was a compromise
in favor of much lower, simpler cost.
Recall also that many installations of
the C-H connector on aircraft did not
include a pilot operated ground power
contator (as explained in my article
of some years back).
The contactor center pin was simply
hardwired to the ships bus structure.
This is lead to an incident I cited
when departing Kansas City on a very
cold morning. The ground power operator
had his back turned to me while dumping
28v into my 14v airplane.
Lack of pilot control opened a door
for the ov event I experienced. It also
made for erosion of the center pin on
FBO ground power cables AND customer
ground power jacks when the connector
was mated/demated under load.
That incident prompted the article
back in '97. Here's a clip of the suggested
wiring. Note steering/clamp diodes that will
deal with reverse polarity. OV crowbar for
the high voltage issue. Annunciator that
speaks to (1) availability of power at the
connector and (2) mis-positioned switch
after ground power is disconnected. The
annunciator press-to-test feature added
a way to see if ground power was indeed
available before closing the ground power
switch. Finally, adding the contactor and
switch put the whole situation under the
pilot's control.
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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