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dave.saylor.aircrafters(a
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 Posted: Wed Jun 09, 2010 4:14 pm Post subject: Ground power cart protection |
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Is there a simple circuit to install on a ground power cart that will shut off a 24V source when connected to a 12V system? Something like "under-voltage crowbar protection"?
Dave Saylor
AirCrafters
140 Aviation Way
Watsonville, CA 95076
831-722-9141 Shop
831-750-0284 Cell
[quote][b]
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nuckolls.bob(at)aeroelect
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| Posted: Wed Jun 09, 2010 9:30 pm Post subject: Ground power cart protection |
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At 07:12 PM 6/9/2010, you wrote:
| Quote: | Is there a simple circuit to install on a ground power cart that
will shut off a 24V source when connected to a 12V
system? Something like "under-voltage crowbar protection"?
|
Hi Dave, long time no hear!
This will probably work for you . . .
http://www.aeroelectric.com/articles/grndpwr.pdf
Bob . . .
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dave.saylor.aircrafters(a
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| Posted: Thu Jun 10, 2010 6:52 am Post subject: Ground power cart protection |
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Hi Bob,
That's along the right lines, but what I'm trying to accomplish is to make my 24V APU cart a little safer by having it shut off if it's connected to a 12V system. If every plane had this "real" APU circuit, we could just plug 24V into anything and let the system take care of itself. Unfortunately, a lot of the planes we see aren't so well thought out. It's just a matter of time until something gets damaged.
Any other ideas? I've tried labeling the cart, posting warnings, educating everyone who uses the cart...I think I need a hardware solution.
When 24V goes into a 12V battery, what would happen on the cart side of the system that could be measured to set off an alarm, trip a breaker, etc? Seems tricky because sometimes a 24V battery drops low enough to seem like 12V, but it happily soaks up the input. Then you don't want the alarm. I don't know enough about how a battery behaves to know what to look for.
Dave Saylor
AirCrafters
140 Aviation Way
Watsonville, CA 95076
831-722-9141 Shop
831-750-0284 Cell
On Wed, Jun 9, 2010 at 10:31 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:
[quote] --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>
At 07:12 PM 6/9/2010, you wrote:
| Quote: | Is there a simple circuit to install on a ground power cart that will shut off a 24V source when connected to a 12V system? Something like "under-voltage crowbar protection"?
|
Hi Dave, long time no hear!
This will probably work for you . . .
http://www.aeroelectric.com/articles/grndpwr.pdf
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Thu Jun 10, 2010 11:19 am Post subject: Ground power cart protection |
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At 09:43 AM 6/10/2010, you wrote:
| Quote: | Hi Bob,
That's along the right lines, but what I'm trying to accomplish is
to make my 24V APU cart a little safer by having it shut off if it's
connected to a 12V system. If every plane had this "real" APU
circuit, we could just plug 24V into anything and let the system
take care of itself. Unfortunately, a lot of the planes we see
aren't so well thought out. It's just a matter of time until
something gets damaged.
Any other ideas? I've tried labeling the cart, posting warnings,
educating everyone who uses the cart...I think I need a hardware solution.
|
Opps . . . NOW I understand . . . I'll need to think
about it some . . .
| Quote: | When 24V goes into a 12V battery, what would happen on the cart side
of the system that could be measured to set off an alarm, trip a
breaker, etc? Seems tricky because sometimes a 24V battery drops
low enough to seem like 12V, but it happily soaks up the
input. Then you don't want the alarm. I don't know enough about
how a battery behaves to know what to look for.
|
Yup, that's the phenomenon to understand and THEN
to design hardware that operates reliably based on
that understanding.
Obviously, the initial connection current demands
of 24 volt cart connected to a 12 volt battery
are high and do not fall off quickly (unless the
ship's battery is in REALLY bad shape). There's
also a concern for whether or not the ground power
cart can be connected to the ship's battery and
the connection evaluated BEFORE any ship's hardware
is powered up.
This illustrates my preference for (1) having
a ground power contactor that (2) feeds ground
power directly to the battery upstream of the
battery contactor.
Unfortunately, when you walk up to an airplane
with a ground power connector on it, there's NO
standard way to be sure that they're all wired
this way . . . in fact, most small aircraft have
NO ground power contactor.
MY airplane's ground power contactor would be
placarded for the requested ground power voltage,
but placards do run against the grain of builders
who spent a LOT of time on a slick paint job.
At first blush, I don't see a totally automatic
way to resolve the question . . . but you can
probably do some things to reduce risk. For
example, you could add a "test" contactor in
series with the power cart output cable and
place a power resistor across the contacts.
Something on the order of 1/2 ohm seems appropriate.
A voltmeter or sensing module downstream of
this resistor can test behavior of the ship's
electrical system when connected to 24 volt
source with a high source impedance. If it's
a 24 volt airplane, the downstream side would
quickly rise above 18 volts (a few seconds)
even if the ship's battery were super-flat.
Now, this assumes that the ship's battery is
on line before ground power is applied. Some
airplanes put ground power to the ship's bus
and if the battery is really dead, dead, dead,
you can't even get the battery contactor
to close. The idea would be to short out the
series resistor after it was deduced that
it was REALLY a 24 volt aircraft.
I'm not seeing a process that relieves the pilot
and ground power cart operator of understanding
reacting to what can be observed. There's just
a whole lot of variables. I'll think about it
some more. If anyone on the list has some ideas,
I would be useful to hear them.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Fri Jun 11, 2010 5:57 am Post subject: Ground power cart protection |
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| Quote: | Any other ideas? I've tried labeling the cart, posting warnings,
educating everyone who uses the cart...I think I need a hardware solution.
|
Dave,
I've pondered your question without much success.
There are too many variables. The MOST important
variables include whether or not a good battery is
connected to the system getting ground power -AND-
how many electro-whizzies are energized on the
system at the time ground power is applied.
I think you're stuck with the same functional
problem that gave rise to a host of flight
line ops mistakes that run the gamut from
fueling of piston airplanes with Jet-A down to
waving a departing aircraft to the taxi ramp while his
nose wheel chocks are still in place.
I think it's that "situational awareness" thingy
we pilots are so fond of talking about. Its
value extends far outside the cockpit.
Bob . . .
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