On 06/19/2010 09:53 AM, Tony Renshaw wrote:
> Maybe I can use my Ball of String skills to use
> such a cable to pull the (+) blade from the fuse, and all will be
> well, for not a lot of cash.
Great idea! You could also convert a standard relay into a mechanical
switch. Just replace the coil by your bowden cable. It then also saves
the holding current.
> What amperage fuse should I use for a
> 914 with a normal starter??
Wait a minute! You should take the starter out of the equation completely.
The starter will have its own circuitry. You put the starter soleonid as
closely as possible to the battery, and connect it directly to the
battery, not to your master switch/relay. The starter cables will then
only carry current during starting. So it is not going to be an issue
during a forced landing.
It is not normal practice to fuse the starter circuit. Starting current
is very high anyway (over 100 Amps) and the cables are designed to
handle this current. The starter cables are most likely to stand a full
short cut without bursting into flames, and it won't take long before
the battery is exhausted in such a situation anyway.
Also, don't run the starting current through your master relay. There is
no need to do this.
Your master relay then only carries the current to your master bus,
which isn't very high.
I used car-style relays for this. They are much lighter, need less
holding current, and are these days pretty reliable.
About the fuse for the master bus: This should be clearly higher than
the total load of your system, plus some extra for surge spikes. So,
just add up the required current for your strobes, radio (while
transmitting), flap drive motor, fuel pumps, transponder, etc. together.
If you want to be more precise, just measure the current all these
appliances take, and apply at least a 50% safety margin for the cable.
The cable from the battery to your master bus should be able to handle
this current easily, don't take any savings here. Then use a fuse with a
rating just below the cable rating. So, you will likely end up with a
fuse of 30 to 40 Amps.
The same with your alternator. Wire it to your bus, not to your battery.
So your bus can be fed by the battery and/or the alternator. If one
fails (including the relay) you will still have the other. The
alternator should be fused as well, take the same rating here as for
your battery fuse, or preferably, a somewhat lower rating. If there is a
shortcut in the alternator itself, the battery will feed the alternator
to speed up its destruction, and you want the fuse to blow at the
alternator's end, not at the battery's end.
(The master bus is theoretically capable of supplying the summed up
current from both your battery and alternator, keep this in mind. That's
one of the reasons I recommend connecting the battery and alternator
together exactly at the point where you are going to divide the current
to all the individual appliances.)
Of course, from your master bus, you will divide the power over the
individual items in your ship. They should be fused individually, with
again the same rules concerning wire diameter and fuse. Here you should
not use heavier wire than needed. Remember that the fuse is there to
protect the wire. Under no circumstances should you use a higher rated
fuse than the current the wire is designed for (otherwise the wire will
protect the fuse, that is, if there is a short cut the wire will take
fire and the fuse wil stand).
I used CB's for the individual items, but regular fuses for the master
busses. If a master fuse blows, there is something very wrong here, and
you shouldn't try to reset from that condition during flight.
Worth mentioning here is that I have two independant buses, each with
their own alternator and battery. If you are going to use an additional
alternator anyway, I recommend splitting up the bus and battery, for
redundancy and weight savings. The latter because you can use very small
batteries, as there is not a single failure point that requires you to
use the battery to keep the fuel pump going. Alternators are lighter
than batteries.
Frans
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