Today's Message Index:
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1. 06:29 AM - Re: B lead circuit breaker (racerjerry)
2. 08:28 AM - Re: Library opportunity (stephen092)
3. 09:20 AM - Re: Re: B lead circuit breaker (Robert L. Nuckolls, III)
4. 12:29 PM - Re: Re: B lead circuit breaker (Robert L. Nuckolls, III)
5. 10:42 PM - Re: A brown-out alternative? (Eric Page)
Message 1
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| Subject: | Re: B lead circuit breaker |
Bob, I found an old Cessna service bulletin SE 72-15 that relates to over-sensitivity
of the Cessna over-voltage protection circuit. Basically, they just add
a big electrolytic capacitor (500uf 50v) across the sensor to help absorb short
term voltage spikes and reduce sensitivity. Doesn't cure the problem at it's
source, but it might make some false tripping go away.
And YES, I think crowbar circuits can be kinda' tough on alternator field circuit
breakers and could cause eventual degradation; especially after a long series
of false trips. Brings to mind a U.L. report of a homeowner who habitually
'tested' his residential circuit breakers using a screwdriver. As you might
suspect, the outcome was not good.
SE 72-15 https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=34475
Jerry King
--------
Jerry King
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=483759#483759
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| Subject: | Re: Library opportunity |
I would genuinely need to welcome Greenway for the benefit of the exercise you
gave m today. This is truly an amazing however pay people to write essays for
ourselves to be prepared for any circumstance so much obliged for it.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=483760#483760
Message 3
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| Subject: | Re: B lead circuit breaker |
At 08:28 AM 10/11/2018, you wrote:
>
>Bob, I found an old Cessna service bulletin SE 72-15 that relates to
>over-sensitivity of the Cessna over-voltage protection
>circuit. Basically, they just add a big electrolytic capacitor
>(500uf 50v) across the sensor to help absorb short term voltage
>spikes and reduce sensitivity. Doesn't cure the problem at it's
>source, but it might make some false tripping go away.
Really! Hadn't seen that one. I'd like to get a copy.
The 3-wire 'firecracker' OV module used on the single
engine airplanes is a derivative of a design I proposed
to Cessna back about 1970. I left Cessna in 69 to
work on the Mobilizer hospital patient transporter but
I was still very much in contact with the old gang at
the airplane patch.
A few weeks after I had lunch with one of the guys
in the electrical group and gave him the details, I
was told that the idea had been rejected in favor of
another approach. It was several years later that I
discovered that they had out sourced the design to
an outfit in Emporia KS and that indeed, the original
concept was found acceptable.
Hadn't heard about any nuisance tripping cures.
>And YES, I think crowbar circuits can be kinda' tough on alternator
>field circuit breakers and could cause eventual degradation;
>especially after a long series of false trips. Brings to mind a
>U.L. report of a homeowner who habitually 'tested' his residential
>circuit breakers using a screwdriver. As you might suspect, the
>outcome was not good.
Can't speak to breakers in a home but typical
breakers popular in aircraft are qualified to some
pretty robust requirements not the least of which is
endurance. Here's an exemplar call-out for a Klixon
product.
Emacs!
http://aeroelectric.com/Pictures/Breakers/Klixon_1.jpg
When I first proposed the crowbar shutdown method for alternators
in aircraft, the idea was driven by a Beech procurement spec
that called for any offering of an ov 'relay' be tested under
worst case conditions and withstand 50 simulated ov events
on a test stand.
Reasonably priced relays at that time were hard pressed to open
the field on a 100A, 28v alternator in full-bore runaway
50 times in a row.
Read an article in ELECTRONICS that spoke to crowbar systems
used in computer power supplies and it made sense for our
problem as well. NO SERIES switching except for the breaker
which was only being asked to do what it was designed to do . . .
break a large, RESISTIVE load.
Ability of the system to withstand repeated trips was
not limited by anything I was supplying . . . but by
the ability of the circuit breaker to shoulder the stresses.
A breaker with stellar qualification numbers.
Note: not ALL breakers are so qualified. Here's one
example . . .
http://aeroelectric.com/Pictures/Breakers/Breaker_Failure_1a.jpg
Short answer is that crowbar trips are a 'walk in the
park' for the field supply breaker.
Bob . . .
Message 4
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| Subject: | Re: B lead circuit breaker |
>
>SE
>72-15
>https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=34475
>
>Jerry King
Interesting . . . I wonder what was different in aircraft
after those serial blocks. I 'salted' the idea
into Cessna for the 3-wire firecracker about 1970,
very shortly after I left Cessna. So it was very
early in the deployment of the product. I'm wondering
if they upsized the value of the timing capacitor
in production and used the SB as a 'band aid' for
units already fielded . . .
Emacs!
Purely hypothetical now. It would be cool to see
if any of the gray beards still around would
remember the details . . . I suspect they're
all gone by now. They were well into their
careers . . . I was about 27 at the time.
Thanks for the find Jerry!
Bob . . .
Message 5
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| Subject: | Re: A brown-out alternative? |
Bob Nuckolls wrote:
> I wired it as shown so that in the milliseconds after start switch closure until
the contactor closes, the converter has time to come up and shoulder the load.
That makes perfect sense. My suggestion would have caused the DC/DC converter
to start up as the bus voltage was sagging, losing the time advantage inherent
in your schematic.
Bob Nuckolls wrote:
> I have one of these for testing https://tinyurl.com/ybqnl8fj (https://tinyurl.com/ybqnl8fj)
>
> I would set this critter to 15 volts with a 0.7 volt diode drop to give an e-bus
boost of 14.3 volts or thereabouts. This would be about 2 volts higher than
the open circuit on an SVLA battery.
I saw those too... The 10V minimum input seemed possibly marginal, though you might find that they're happy to work below that level. The converter at this link (https://www.ebay.com/itm/263796566855) claims to operate down to 5V and to output 6A.
That said, Joe's comment about exaggerated Chinese specs is right on. Amperes
in China seem to contain about half as many electrons as amperes in the rest of
the world!
Bob Nuckolls wrote:
> Another feature to be explored is drop out delay in the starter contactor caused
by slow decay of coil current through the catch diode. When the start button
is released, boost drive goes away.
>
> The start button is held until the engine catches and starter current drops dramatically.
Hopefully battery voltage will have returned to a value that supports
appliances vulnerable to CBOT.
If this proved to be a problem, we could craft a system that sequences start events.
The attached schematic is my first pass at it. I used a TLC3704 quad comparator
(for its push-pull outputs), two relays and a handful of jelly-bean
parts. It's a bit of a dog's breakfast, but it should work.
When the start switch is pressed, Q1 turns on, completing a ground path for K1
to energize. The output of comparator U1.1 immediately goes high, which in turn
(through the diode-OR network of D2 and D3) causes the output of comparator
U1.4 to go low, energizing K2 through Q3, which powers the DC/DC boost converter
to support essential avionics. C2 charges through R4, taking about 100mS
to reach 7.6V, whereupon the output of comparator U1.2 goes low, energizing K1
through Q2, which in turn energizes the engine starter contactor. C3 charges
through blocking diode D1, causing the output of comparator U1.3 to go high,
reinforcing the signal at U1.4 pin 10.
When the start switch is released, R1 immdiately turns off Q1, causing K1 to drop
out, cutting off the engine starter contactor. R1 also turns off the output
of U1.1, but the high at U1.4 pin 10 is held by the output of U1.3. C2 discharges
through R4 and the output of U1.1, causing the output of U1.2 to go high
(this has no effect, since Q1 has already turned off K1). C3 discharges through
R5; after about 100mS C3 drops below 4.4V and the output of U1.3 goes low,
causing the output of U1.4 to go high, turning off Q3, K2 and the DC/DC boost
converter.
Critiques welcome!
Eric
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=483776#483776
Attachments:
http://forums.matronics.com//files/engine_start_sequencer_rev_a_126.pdf
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