Electrical questions

[AK]

1) Should aircraft use switches for both positive and negative (SPXX
vs. DPXX switches) or just the positive (especially in application
from point 2 below) ? I was planning on switching just the positive
only and have the negative side permanently connected; both batteries
and negative of each circuit joined together.
2) I am about to separate radio and transponder into circuit 1 and
rest of the instruments into circuit 2. I will have two batteries. I
am planning to use two switches; one for battery 1 and one for battery
2. Each of the switches should be able to switch a battery to either
circuit 1 or circuit 2. I would also have another switch that could
connect circuit 1 and circuit 2 (they need to be connected while
flipping the battery switches to ensure constant power supply). I
would also need a couple of diodes to make sure one battery does not
drain the other battery and that I don't get overload in any circuit.
The question is what diodes should I use?
3) Does anyone have another solution for battery switching maybe
simpler one?

[JS]

Way too complicated.
If both batteries are the same type, capacity and age - preferably
new - install fuses on each battery and run them in parallel to one
master switch. MUCH simpler, and you don't run one battery down before
switching to the other.
Have owned two gliders using this scheme with no problems. One with
two 12V 12A/H, one with two 12V 20A/H. The house runs on twelve 12V
220A/H sealed batteries in (4)series/(3)parallel configuration to give
48 Volts to the inverter. They seem quite happy after more than 6
years of service.
Jim

On Feb 26, 4:59 pm, AK > wrote:

> 3) Does anyone have another solution for battery switching maybe
> simpler one?

[Gary Emerson]

AK wrote:
> 1) Should aircraft use switches for both positive and negative (SPXX
> vs. DPXX switches) or just the positive (especially in application
> from point 2 below) ? I was planning on switching just the positive
> only and have the negative side permanently connected; both batteries
> and negative of each circuit joined together.
> 2) I am about to separate radio and transponder into circuit 1 and
> rest of the instruments into circuit 2. I will have two batteries. I
> am planning to use two switches; one for battery 1 and one for battery
> 2. Each of the switches should be able to switch a battery to either
> circuit 1 or circuit 2. I would also have another switch that could
> connect circuit 1 and circuit 2 (they need to be connected while
> flipping the battery switches to ensure constant power supply). I
> would also need a couple of diodes to make sure one battery does not
> drain the other battery and that I don't get overload in any circuit.
> The question is what diodes should I use?
> 3) Does anyone have another solution for battery switching maybe
> simpler one?

Regarding the Diodes, take a peak at this...

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=497-2738-5-ND

At 1/2 an amp in the forward direction I've measured the voltage drop to
be right at 0.3V So far this is the lowest I've found. Rated for 25V
and up to 10A so it should be very robust in the standard glider 12V
arrangement.

I personally don't use switches (other than a master), but have both
batteries in parallel and diodes in line with both batteries so I'm
always getting power from the best battery.

[Darryl Ramm]

You only want to switch the positive. Negative should be common/
ground. How just you run the positive wiring and common "bus" varies
- and the bus us usually configured as a common attach point/star
topography.

If you are after reduced complexity, which always seems a good idea,
why start with running two electrical buses? Start with the absolute
simplest approach unless there is a hard reason it won't work. Can you
put everything on one bus and run from that? Then you are down to an A/
Off/B switch for the battery.

The only real reasons for splitting loads are 1. if you have
relatively high loads running off smaller batteries and want to
improve overall battery life (e.g. you want to be on a different load/
capacity discharge curve) 2. You want a logger or other critical
device on a dedicated battery and don't want to risk even transient
hiccups while switching batteries for that device under normal use
(this is usually a much weaker reason, and most loggers etc. handle
transient switching fine, or you can work around this with make before
break switches of use separate on/off switches for both batteries and
just leave both on temporarily while switching batteries or have a
large capacitor across the logger, etc.).

With two buses and two batteries where are you instrumenting the
voltages so you can see what is going on? Separate meters on each bus?
or meters built into instruments on each bus?

I'm not sure I follow why you need a switch to tie both buses together
during switching to avoid transients or why diodes are needed (yes
they can be, if you want to tie the batteries together but not if you
don't, they are certainly not needed during transient switching).
There are just not huge current flow between charged and discharged
batteries - just make sure you use properly sized master switches. If
you are thinking diodes you are probably meaning Schottky diodes for
their low forward voltage drop.

The are lots of places this discussion could go, and many recent
discussions here cover relevant things, like wiring standards, wire
types, crimp vs. solder connections, switch ratings, fuses vs.
breakers, etc. etc. But based on the opening questions, I'd start with
asking do you really want to do this yourself? Do you have an A&P who
is going to look over your shoulder and help you out? Even with an
experimental ship, and even if not strictly needed I'd encourage you
to find an A&P with some experience with gliders and talk with them
and get them involved. Also draw a detailed circuit diagram, including
breakers and fuses, wire gauges, etc. and have them look it over.
There is just a very large number of things to consider. Personally I
have a strong background in electronics and I still get a A&P to look
over my shoulder, and if needed sign off anything I do with my toys.


Darryl



On Feb 26, 4:59 pm, AK > wrote:
> 1) Should aircraft use switches for both positive and negative (SPXX
> vs. DPXX switches) or just the positive (especially in application
> from point 2 below) ? I was planning on switching just the positive
> only and have the negative side permanently connected; both batteries
> and negative of each circuit joined together.
> 2) I am about to separate radio and transponder into circuit 1 and
> rest of the instruments into circuit 2. I will have two batteries. I
> am planning to use two switches; one for battery 1 and one for battery
> 2. Each of the switches should be able to switch a battery to either
> circuit 1 or circuit 2. I would also have another switch that could
> connect circuit 1 and circuit 2 (they need to be connected while
> flipping the battery switches to ensure constant power supply). I
> would also need a couple of diodes to make sure one battery does not
> drain the other battery and that I don't get overload in any circuit.
> The question is what diodes should I use?
> 3) Does anyone have another solution for battery switching maybe
> simpler one?