Today's Message Index:
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1. 08:02 AM - Re: Re: Z8 Ground Power test switch (Bob McCallum)
2. 08:31 AM - Re: Re: Z8 Ground Power test switch (Bob McCallum)
3. 09:33 AM - Re: Z8 Ground Power test switch (jonlaury)
4. 11:15 AM - Re: Re: Z8 Ground Power test switch (Bob McCallum)
5. 04:48 PM - Re: Z8 Ground Power test switch (jonlaury)
6. 10:19 PM - Re: Re: Z8 Ground Power test switch (Bob McCallum)
Message 1
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Subject: | Re: Z8 Ground Power test switch |
Jon;
No.
The problem of frying the alternator isn't caused by closing the contactor,
(which rightly won't close via the grnd power switch if the polarity is
reversed, or as you suggest via the stby alt switch from the main battery
regardless of external battery) it's caused by the fact that the external
battery is directly connected across the aux alternator as soon as the plug
is inserted into the ground power jack. This is a non-issue with the
polarity correct, but it is great cause for concern if the polarity is
reversed. This becomes an even greater issue (potentially explosive) should
the external power be reversed AND the stby alt switch turned on, because
you then have both batteries connected in series and shorted. (24Volts dead
short, potentially thousands of amps depending on the type of batteries
involved) Providing two separate pathways to activate the contactor raises
many of these issues because while you've protected one pathway, the other
pathway (unprotected) can still activate the circuit.
If you follow the electrical pathways represented by the wires in your
latest drawing from your ground power plug, you will find both the plug and
alternator grounded of course and thus connected, but you will also find the
central pin of the plug connected to the ground power contactor, then via
the 40A ANL thru the shunt directly to the alternator. Therefore any
external battery is directly connected across the aux alternator regardless
of the state of the stby alt switch, grnd power switch, or contactor.
One solution is to connect the 40A ANL to the opposite side of the
contactor, but this then raises other issues with being able to isolate the
aux alternator when desired etc. and you still have two independent ways to
activate the contactor in spite of potential reverse polarity/incorrect
voltage etc. The most obvious, simple, safest solution is separate
contactors for ground power and the aux alternator, then all of these issues
disappear, and the overvoltage/reverse polarity circuit as suggested by Joe
works perfectly, but if I remember back far enough this is what this whole
exercise was to avoid in the first place.
So, no, sorry, without keeping it simple, I don't have a satisfactory
solution.
As Bob Nuckolls has always advocated, manipulation of any switch, at any
time, in any combination or configuration, should never have the potential
to create smoke, flames, or any other expensive deleterious consequence,
which is why he tries to educate us all to follow the basic simple ideas
without adding complexities with their potential associated problems.
Bob McC
> -----Original Message-----
> From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-
> server@matronics.com] On Behalf Of jonlaury
> Sent: Saturday, October 09, 2010 1:17 AM
> To: aeroelectric-list@matronics.com
> Subject: AeroElectric-List: Re: Z8 Ground Power test switch
>
>
> Bob,
>
> Thanks for looking this over.
>
> The starter jumper was an errant line that was left over after moving some
things
> around. I removed it and connected the warning lamp circuit to the 'I'
terminal of the
> contactor that receives bus voltage when the contactor is closed.
>
>
> > The diode location you're querying causes the lamp to light only if the
> > ground power is correct polarity and only allows you to close the
contactor
> > if the polarity is correct, but please note the issue above.
>
>
> So wouldn't that provide the needed protection for the alternator? The way
I see it is
> that, if GP polarity is reversed, it only becomes a problem if the Stby
Alt switch is
> closed.
> I confess that I'm at the ragged end of understanding the circuit and the
problems
> that you foresee. Do you see a solution?
>
> John
>
>
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=315186#315186
>
>
>
>
>
>
>
> _-
> ====================================================
> ======
> _-
> ====================================================
> ======
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> ====================================================
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>
>
Message 2
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Subject: | Re: Z8 Ground Power test switch |
Jon;
Maybe I've misunderstood your misunderstanding of the reverse polarity
concern.
When a battery of correct polarity is directly connected to an alternator,
as it is normally in all of our cars and trucks and as it is in the ground
power battery circuit in your electrical drawing, then the rectifier diodes
in the alternator represent an almost infinite resistance to current flow
from the battery to ground through the alternator windings. (an open
circuit) However if the battery is connected in reverse polarity, then the
rectifier diodes represent almost zero resistance to current flow to ground
from the battery through the alternator windings. (A short circuit) This
high current flow supplied by the battery in a "shorted" state will
potentially fry/melt/burn out, the diodes which are not capable of safely
conducting this much current, they are only designed to carry alternator
rated output plus some safety margin. One must remember that a diode is a
one way valve for electricity, conducting in one directing and not in the
other. In normal alternator operation when the voltage generated in the
alternator exceeds the battery voltage sufficiently then current flows from
the alternator into the battery. When the battery voltage exceeds the
voltage generated within the alternator then no current flows.
Reverse the battery polarity and the currents reverse. Alternator represents
a dead short to the battery, not an open circuit as it does with correct
polarity. (The winding resistance does limit current to some extent, but the
battery still sees essentially a short circuit if reversed.)
Does that help??
Bob McC
> -----Original Message-----
> From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-
> server@matronics.com] On Behalf Of jonlaury
> Sent: Saturday, October 09, 2010 1:17 AM
> To: aeroelectric-list@matronics.com
> Subject: AeroElectric-List: Re: Z8 Ground Power test switch
>
>
> Bob,
>
> Thanks for looking this over.
>
> The starter jumper was an errant line that was left over after moving some
things
> around. I removed it and connected the warning lamp circuit to the 'I'
terminal of the
> contactor that receives bus voltage when the contactor is closed.
>
>
> > The diode location you're querying causes the lamp to light only if the
> > ground power is correct polarity and only allows you to close the
contactor
> > if the polarity is correct, but please note the issue above.
>
>
> So wouldn't that provide the needed protection for the alternator? The way
I see it is
> that, if GP polarity is reversed, it only becomes a problem if the Stby
Alt switch is
> closed.
> I confess that I'm at the ragged end of understanding the circuit and the
problems
> that you foresee. Do you see a solution?
>
> John
>
>
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=315186#315186
>
Message 3
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Subject: | Re: Z8 Ground Power test switch |
Bob McC,
> Does that help??
Immensely. And I'm extremely appreciative of your efforts to explain the problem
in detail to my electrically pedestrian mind. But I need to study my drawing
vis a vis your tutorial to fully understand the hazard that you've identified.
At some point, Bob N thought that the shared contactor would work and the diode
location was jockeyed around for reverse polarity/OV protection and I didn't
fully understand then what the issues were. I think I may have complicated
the circuit since then, so I need to retrace steps.
I want to understand this, but adding a contactor may be the most expedient way
of getting down the runway.
Thanks,
John
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=315212#315212
Message 4
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|
Subject: | Re: Z8 Ground Power test switch |
John; (Is it Jon or John??? Got Jon from your e-mail address. Sorry if I
was wrong)
Glad I'm able to shed some light.
There are at least three distinct and separate hazards.
One is the simple reverse polarity battery connected directly to an
alternator. No switches or contactors involved, which will likely damage the
alternator.
Two is the ability to close the grnd power contactor (via the aux alt
switch) with a reversed ground power battery connected, thus creating a
short between the two batteries, in spite of the fact that you have reverse
power protection preventing closing this same contactor using the ground
power switch. One switch thus defeats the protection you have provided for
the other.
Three is the possibility of connecting a 24 volt ground power battery to
your on board battery, (with unpleasant consequences) through the mechanism
described in scenario two above, thus defeating the OV protection built into
the ground power switch circuit by energizing the contactor via the
unprotected aux alt switch. (Once again that second source defeating the
well considered protection provided for the other source)
You cannot take the functional reverse voltage/overvoltage protected
circuitry offered by Joe and randomly add devices parallel to portions of it
without totally changing how that circuit functions, or at least negating
some or all of its features.
Bob N may be correct in that one contactor may suffice, however by
introducing multiple switches which mutually defeat the features of each
other, you are creating scenarios which leave open the possibility of
operator induced failures. The circuitry should take into account that
multiple inappropriately positioned switches CANNOT be allowed to create a
hazardous situation.
As an aside to what you are trying to accomplish, no switching is required
for your ground power present/correct polarity light. If you connect the
light to the positive ground power connection as you've shown then simply
ground the other side through a diode without the switch, then when correct
polarity external power is present the lamp will be illuminated and you will
know it's safe to close the contactor. (It will be brighter if a higher than
normal voltage is present, but your OV protection will prevent closing the
contactor) If you switch on the aux alternator in flight this same light
will also now illuminate through virtue of now being connected to the
aircraft power buss indicating that aux alternator is energized. If you want
to get even more detailed you could add a second light (large red??)
parallel to the first connected to the ground power positive pin and
grounded through a diode but whose polarity is reversed with respect to the
one you've already shown. This would illuminate when reverse polarity is
present on the ground power jack. Green light = correct polarity ground
power present. Red light = reverse polarity ground power present. No lights
= no power present.
If you did this you would still need the diode as shown in the circuit
switching the contactor in order to provide the reverse polarity protection
which prevents you actually connecting the reversed battery.
If you were to eliminate separate ground power and aux alt switches and
instead use one three position switch with one position being ground power
connected, one being off, and the third being aux alt on, you would make
several of these issues go away. (All those caused by having two switches
serving the same purpose.) Further if you obtained the ground for this
switch from the fuse which you currently show providing ground for your grnd
power switch then your OV protection would function with the aux alt as well
as with the ground power. This would only leave the issue of a reverse
ground power battery connected directly to the alternator to be resolved.
That could be addressed by using a two pole contactor whose second set of
contacts would be in series with your 40A ANL and whose first contacts would
be as presently depicted.
Bob McC
> -----Original Message-----
> From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-
> server@matronics.com] On Behalf Of jonlaury
> Sent: Saturday, October 09, 2010 12:30 PM
> To: aeroelectric-list@matronics.com
> Subject: AeroElectric-List: Re: Z8 Ground Power test switch
>
>
> Bob McC,
>
>
> > Does that help??
>
>
> Immensely. And I'm extremely appreciative of your efforts to explain the
problem in
> detail to my electrically pedestrian mind. But I need to study my drawing
vis a vis
> your tutorial to fully understand the hazard that you've identified. At
some point, Bob
> N thought that the shared contactor would work and the diode location was
jockeyed
> around for reverse polarity/OV protection and I didn't fully understand
then what the
> issues were. I think I may have complicated the circuit since then, so I
need to retrace
> steps.
>
> I want to understand this, but adding a contactor may be the most
expedient way of
> getting down the runway.
>
> Thanks,
> John
>
>
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=315212#315212
>
>
>
>
>
>
>
> _-
> ====================================================
> ======
> _-
> ====================================================
> ======
> _-
> ====================================================
> ======
> _-
> ====================================================
> ======
>
>
Message 5
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Subject: | Re: Z8 Ground Power test switch |
Bob,
> (Is it Jon or John??
It's actually John. I've had the email moniker for 20+ years from when ATT would
only allow 8 letters. It was for both me and my wife so we shortened my name
to 'jon' and Laurie's name to 'laury'. When email came into common use, my wife
got her own address and I stuck with this one.
Once again, you're flooding light on my little dilemma. I like your elegant 3 pos
switch (w/appropriate indicator lights) and 2pole contactor idea. Do you know
a source for a 2 pole 12vdc 200-300 amp contactor? I didn't pull anything up
from Google in a quick search.
The simplest solution is to have stand alone contactors. I can use all my existing
switches, wiring, (FW pass throughs are sealed up), and the contactor that's
mounted for the GPJ. I would just get a Bosch type 40/60 amp ice cube relay
w/neg spike suppression (Beuler makes several), for the stby alternator. I
will probably go this route as it is the simplest, smallest and most economical
solution.
Thank you again for trying to keep me from frying myself :) .
John
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=315233#315233
Message 6
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Subject: | Re: Z8 Ground Power test switch |
John;
This manufacturer makes DPST contactors, with many different features and
options available. http://tinyurl.com/287xe2g Click on diagrams tab then
click on the JBD series. This is a DPST 200 Amp contactor available in
either continuous or extended duty and with an enclosed option. This is just
one example, there are many others used in the recreational vehicle
industry, by off-roaders for winch control, by the renewable energy industry
for battery control, as well as in golf carts and industrial lift trucks
etc.
However I can't fault your statement below.
Bob McC
>The simplest solution is to have stand alone contactors. I can use all my
existing
> switches, wiring, (FW pass throughs are sealed up), and the contactor
that's
> mounted for the GPJ. I would just get a Bosch type 40/60 amp ice cube
relay w/neg
> spike suppression (Beuler makes several), for the stby alternator. I will
probably go
> this route as it is the simplest, smallest and most economical solution.
>
> Thank you again for trying to keep me from frying myself :) .
>
> John
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