Today's Message Index:
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1. 08:02 AM - Re: Potential for Overvoltage Issue (M Wilson)
2. 02:52 PM - Re: OVM14 MkIII, rev P1 (user9253)
3. 06:13 PM - Re: OVM14 MkIII, rev P1 (Eric Page)
4. 07:26 PM - Re: OVM14 MkIII, rev P1 (user9253)
Message 1
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| Subject: | Re: Potential for Overvoltage Issue |
Hi Bob,
Thanks for your response.=C2- But, remember in this case we are dealing w
ith a user modified alternator, not factory.=C2- The two failure events y
ou list are linked in a chain so their probabilities can't be considered in
dependently.=C2- In this case, if the screw with it's insulating sleeve w
ere to back out exposing it's threads the ring terminal or brush lug would
only have to move 0.020" to make contact and cause a uncontrolled overvolta
ge event.=C2- The probability of that much movement in a high vibration e
nvironment is almost guaranteed.=C2-=C2-I do agree that making that eve
nt extremely improbable so that it doesn't need to be considered is the bes
t approach.=C2- I believe I had put Loctite on that screw.=C2- I think
the next time I'm in there I will put a blob of JB-Weld over the screw and
terminal, potting the area, as a belt and suspenders to make sure that it n
ever moves.
On Saturday, September 23, 2023 at 05:25:33 PM CDT, Robert L. Nuckolls,
III <nuckolls.bob@aeroelectric.com> wrote:
At 03:39 PM 9/22/2023, you wrote:
I've been luring on this listfor years and this is my first post.
I've recently completed my plane (Tailwind) and have if flying.
I used the previous version of the OVM crowbar circuit and I havemodified a
n automotive alternator to use an external Ford-type voltageregulator.
All is working well.=C2- But, in going through a FMEA in my mind I haveid
entified a potential issue:=C2- I modified the alternator using themethod
in Kitplanes, November 2019.
In this method one of the brush terminals is isolated with insulatingwasher
s and an insulating sleeve.=C2- In my case I have grounded theother brush
to case ground.=C2- The issue is that if ever theinsulating washer or sl
eeve were breached the B--lead would directly feedthe field through the bru
sh and an overvoltage would result that thecrowbar could not shut down. Tur
ning off the master switch woulddisconnect the battery but the run away alt
ernator would still be feedingthe bus.
I post this in case anyone else has this setup.
Yes they do . . . along with about every single
engine airplane since 1968 or so.
FMEA also takes into account probability of
any particular event. Many potentially catastrophic
events in aviation are statistically or demonstrably
so small as to be ignored in process of crafting
a plan-b, or redesigning to remove potential for
the fault entirely.
Number one failure of a wiring pathway is
disconnect. Wire breaks, stud nut is works loose,
wire pulls out of terminal crimp, etc. Another
root cause for failure is short to ground due
to compromised wire insulation (chaffing or
mechanical damage). The probability of an
wire becoming disconnected followed by a
low-impedance, re-positioning of that wire
to a robust power source is really TWO
failures happening simultaneously.
Original structural failure followed
by misadventurous 'wandering' of that
same wire to a hazardous source of
power.
This combination of events is so remote
that it has not been treated as potentially
hazardous in hundreds of thousands of airplanes
from Super Cubs to Gulfstream V's
You are to be commended for considering
the scenario and asking the question.
Be assured that the question has been
considered and discounted countless times
before.
=C2- Bob . . .
=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2
-=C2-=C2-=C2-=C2-=C2-////
=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2-=C2
-=C2-=C2-=C2-=C2-(o o)
=C2-=C2- ===========o00o=(_)=o00o====
=====
=C2-=C2- < Go ahead, make my day . . .=C2-=C2- >
=C2-=C2- < show me where I'm wrong.=C2-=C2-=C2-=C2-=C2->
=C2-=C2- ====================
============
=C2-
=C2-=C2- In the interest of creative evolution
=C2-=C2- of the-best-we-know-how-to-do based
=C2-=C2- on physics and good practice.
Message 2
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| Subject: | Re: OVM14 MkIII, rev P1 |
Eric, how about this circuit for an over-voltage indicator light?
I added a Schottky diode to prevent R12 current from turning on the BT169D SCR.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=511519#511519
Attachments:
http://forums.matronics.com//files/ov_indicator_186.png
Message 3
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| Subject: | Re: OVM14 MkIII, rev P1 |
The output of the LM293 comparator is a bit strange. It can "sink" current (draw
current out of the circuit, to ground), but it can't "source" any current (supply
current to the circuit). It's a transistor that can either be on ("low,"
pulling anything connected to the output down to ground), or off ("high," doing
nothing). When the output is low (again, transistor on), the current that
flows through resistor R12 is bled off to ground and can't do anything in the
circuit. When the output is high (transistor off), R12 can "pull up" the output
node to the supply voltage and the current from R12 turns on the crowbar
SCR.
The diode that you added will block the comparator's output transistor from being
able to bleed R12 current out of the circuit, so the crowbar SCR will be permanently
on. Likewise, it will prevent any current from R12 from reaching the
SCR in the LED circuit, so it will never turn on. In this application, you
can think of the diode as an electrical check valve; its symbol tells you which
way current can flow (with the arrow).
To do what you want, the diode should be removed and both SCRs' gates should be
connected to the node that's common to the comparator's output and the bottom
of R12. When the output goes high, current from R12 will turn on both SCRs,
crowbarring the field breaker and turning on the LED.
Each SCR's gate will only absorb as much current as it needs to turn on. Bob specified
a 750-ohm resistor because the crowbar SCR is a high power device and
it wants a stout gate current to trigger. The SCR you selected for the LED requires
so little current that it will trigger just fine from the same pull-up
resistor as the bigger one.
-Eric
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=511520#511520
Message 4
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| Subject: | Re: OVM14 MkIII, rev P1 |
Thanks Eric for explaining how the LM293 works. I had it all wrong.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=511521#511521
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