Today's Message Index:
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1. 01:12 AM - Re: Re: Spark suppression diode on starter MOTOR (Bob Verwey)
2. 08:26 AM - Re: Re: Spark suppression diode on starter MOTOR (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Spark suppression diode on starter MOTOR |
..and one should consider duty cycle?
On Mon, 4 Apr 2022 at 23:29, Robert L. Nuckolls, III <
nuckolls.bob@aeroelectric.com> wrote:
>
> Viewed another way, if we used a robust enough switch to control the relay,
> with really fast acting contacts, we might be able to eliminate the
> flyback diode from the relay coil. But the diode is much cheaper/simpler
> than a switch that could handle the arc issue without damage.
>
> Charlie
>
>
> Dead on. The only thing I would expand upon
> is the term 'damage'. 'Wear' is the operative
> term. I recall a demonstration I did many moons
> ago where I switched a 1.5 volt flashlight cell
> across a 1 ohms resistor.
>
> Approx 1+ amps of current flow. I did this with
> a sub-miniature, 'micro' switch with the side
> sanded off so that one might observe the inner
> workings.
>
> While watching the contacts under a microscope
> in a dark room, I could see a tiny arc between
> the spreading contacts. The color of the arc
> was blue . . . HOT. This suggests that for
> even the smallest energy levels, the potential
> for erosion of metal off the contact surfaces
> is not zero; hence wear is not zero either.
>
> Engineering applications data for all switches
> will include predicted SERVICE LIFE values
> for switches under various operating conditions.
> See:
>
> http://aeroelectric.com/articles/Switch_Ratings.pdf
>
> Diodes didn't even start to show up in TC aircraft
> until the late 60's. I was at Cessna when we
> started fabricating diode assemblies to facilitate
> installation in various places about the systems
> including contactor coils.
>
> I remember Gordon Wood (PhD in physics)
> bought an HP peak-reading voltmeter and discovered
> that some contactor coil spikes exceeded 400-500
> volts! The fact that such a potential existed
> was contrary to our pledge of compliance with
> Mil-STD-704 . . . so began a program to mitigate
> our 'transgression'. It had nothing to do with
> life of the battery contactor switch . . .
> that astounding spike that just had to be dealt with.
>
> Don't recall any discussions about ENERGY nor
> how it might have propagated into the system . . .
> but we sure stomped out those spikes. Here's
> a constellation of Cessna diode assemblies
> dating from the 1960's to present day.
>
> http://aeroelectric.com/Pictures/Cessna_Diode_Assy/
>
> Are we not grateful NOT to own a TC aircraft?
>
> Competent design is based on properties of materials,
> management of energy and refinement of process.
> But as we may observe in the photos cited above,
> 'refinement' hasn't really moved in the
> right direction.
>
>
> Bob . . .
>
> Un impeachable logic: George Carlin asked, "If black boxes
> survive crashes, why don't they make the whole airplane
> out of that stuff?"
>
--
Best Regards,
Bob Verwey
082 331 2727
Message 2
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| Subject: | Re: Spark suppression diode on starter MOTOR |
At 03:11 AM 4/5/2022, you wrote:
>..and one should consider duty cycle?
>
>On Mon, 4 Apr 2022 at 23:29, Robert L. Nuckolls, III
><<mailto:nuckolls.bob@aeroelectric.com>nuckolls.bob@aeroelectric.com> wrote:
>>
>>Viewed another way, if we used a robust enough switch to control the relay,
>>with really fast acting contacts, we might be able to eliminate the
>>flyback diode from the relay coil. But the diode is much cheaper/simpler
>>than a switch that could handle the arc issue without damage.
>>
>>Charlie
Absolutely. Service life on switches (and relays) is
predicated on the type of load being managed by the
device. Those loads are divided into resistive (heaters),
inductive (coils and un-terminated transformers),
and lamp (high inrush like incandescent lamps, motors,
capacitive). There can be a huge variance of predicted
service life on the same switch over that range of
loads.
But note: Service lives are generally given in thousands
or tens of thousands of operations. After all, who would
even consider a switch rated for only 500 operations?
Service life numbers are critical to revenue generation
and risks for loss-of-vehicle (and the folks inside).
Hence, folks who design and maintain air transport
or military aircraft tend to migrate to the higher
numbers. It's not only a matter of safety but cost
of operation. The boss is really unhappy when a $million$
machine is down for lost functionality in a $10 switch.
We who fly around in machines with lower criticality
can reduce hazard with failure mode effects analysis
while mitigating expense with owner built and maintained
aircraft.
Back to that duty-cycle thingy. Over the lifetime
of the airplane, how many times will the most exercised
switch be operated? Hmmm . . . battery master? Landing
gear? Trim switch? Last stat I read (many moons ago)
was that the typical light aircraft is flown 50 hrs
a year. How many flight cycles is that? What's
the probability of using up all nine-lives of the
most severely stressed switch in the airplane?
Those are all questions for which we can apply no
specific numbers . . . our designs and utilization
goals are anything but 'average'. But with attention
to simple details and diligent preventative
maintenance, risk for an unhappy day in the cockpit
due to electrical issues is exceeding small compared
to fuel exhaustion, weather, poor mission
planning, etc.
Bottom line is that pleasures to be secured
with the operation of your airplane has much
more to do with being a pilot than with worrying
about diodes on the relays.
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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