There is one quite important thing to note, no matter how one wires the
switches (I used two DPST switches in the Sling - one for the power to the
Enigmas and one to the backup batteries)- that is that the backups will not
charge unless the voltage to them is sufficiently high. Obvious of course,
but where this becomes a problem is if you have a diode between your main
bus and the e-bus. You get a 0.7v drop across it and then there are some
voltage drops inside the Enigma and the backup batt gets a voltage of
between 12.5 -13v.
To solve this we now run the aircraft with the e-bus alt feed permanently
on, which boosts the e-bus voltage back up sufficiently to charge the backup
battery.
Jay
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Kevin
Horton
Sent: 18 November 2009 06:43 AM
To: aeroelectric-list@matronics.com
Subject: Re: Re: EFIS backup battery switch wiring
What are the consequences of failure of both poles in that switch,
given how you will operate the aircraft (day/night and VFR/IFR) and
the backups you have installed? If you are certain that you will be
able to safely get the aircraft on the ground with an acceptable
workload, then the potential single point failure is acceptable.
Kevin Horton
Sent from my iPod
On 2009-11-17, at 10:36 PM, "messydeer" <messydeer@yahoo.com> wrote:
> >
>
> Thanks, E., for bringing up the issue of single point of failure.
> Hadn't thought about that, which makes me ask some questions about
> DPDT vs SPST switches.
>
> Looking at the schematic of a DPDT switch it looks like there are
> two circuits, an upper and a lower. Their terminals are separate but
> contained in the same housing. They also use the same lever and the
> mechanism the lever connects to that makes and brakes the contacts.
>
> If the only way a switch failed was through these shared components,
> then a DPDT would have the same reliability as a SPST, so there
> would be a single point of failure.
>
> And if DPDT's never failed at these shared components, then having
> one DPDT would be just as reliable as having two SPST's, and there
> wouldn't be a single point of failure.
>
> I'm sure the truth lies somewhere between these two extremes. If it
> turns out that a DPDT has 90% of the reliabilty as two SPST's, I
> might stick with just one DPDT. If it has only 20% of the reliabilty
> of two SPST's, I might consider going with two switches.
>
> --------
> Dan
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=273392#273392
>
>
