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76papa(at)dishmail.net
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 Posted: Sat Oct 31, 2009 11:41 am Post subject: Aircraft electric question |
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I know I could go to aeroelectric with this but they tend to talk too
technical for me. My question is simple and maybe even stupid: When
wiring for the systems that are controlled by switches in the cockpit
such as lighting, flaps, pitot heat, do you run the full amperage
through the 20A switches or should the switches trip relays to handle
the larger current?
Not that I'm doing the wiring now but it's something I've wondered
about for a long time.
Al
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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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| Posted: Sat Oct 31, 2009 1:03 pm Post subject: Aircraft electric question |
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10/31/2009
Hello Al, You wrote: "When wiring for the systems that are controlled by
switches in the cockpit
such as lighting, flaps, pitot heat, do you run the full amperage through
the 20A switches or should the switches trip relays to handle the larger
current?"
If the switch is capable enough to handle the load it is simpler, and with
less parts count, to control the items directly with the switch. So you
start out with the switch rating and the current draw (including surge) of
the item to be controlled.
If you reach the point where the switch rating is inadequate then it is time
to start thinking in terms of relays. Of course the wire guage and circuit
protection device (fuses or circuit breakers) must also be appropriately
sized.
I know that there is a tendency among builders to shy away from the
electrical aspects of the building for a long time. It has been my
experience that once a builder gets his feet wet in this arena he almost
invariable realizes that this electrical stuff is fun, educational, and very
satisfying to work with.
If you don't already have a copy of Bob Nuckoll's book "The Aeroelectrical
Connection", -- see here:
https://matronics.com/aeroelectric/Catalog/pub/pub.html#P-Book
you should buy one or borrow one locally. You may rebel at first look and
say that this is far more than I need to know or want to know about this
subject -- fair enough, then treat it as reference material. It is not a
cook book per se, but that book plus the folks on the matronics
aeroelectric-list will do more towards keeping your electrical system on
track than any two other sources in the world.
I am going to shoot this over to the folks on the aeroelectric-list and see
how they react.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
PS: Please let me display one personal bias here. There are various magic
electrical boxes available commercially. The fundamental theme seems to run
along the line of "if you buy our box then all of your electrical problems
will be solved and you don't really need to learn anything or do anything
except just connect it up." Frequently the customers who go that route wind
up concluding that the magic box does not do exactly what they want it to
and they fish around for some sort of modification.
=================================================
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galinhdz(at)gmail.com
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| Posted: Sat Oct 31, 2009 3:22 pm Post subject: Aircraft electric question |
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Al; It is actually fairly simple.
1 - Figure out what you want each switch to control. This is known as a "circuit" in techspeack. This will also tell you how many total switches you need so you can figure out where to put them on your panel. Make them easily accessible while in flight. A typical set of switches can be:
1 - Avionics (Easy to exceed the max rating if not careful)
2 - Autopilot
3 - Strobe Lights
4 - Position Lights
5 - Landing Lights (Easy to exceed the max rating if not careful)
6 - Taxi Lights (Easy to exceed the max rating if not careful)
7 - Aircraft Beacon
8 - Instrument/Panel Lights
9 - Pitot Heat (Easy to exceed the max rating if not careful)
10 - Fuel Pump
2 - Find out what the total current draw for each switch will be by adding the current draw/pull of each "thing" you want the switch to control. Add a "fudge" factor of about 25% to this amount.
(EXAMPLE: Max current for the autopilot is 5A. Max current for the GPS is 5A. Max current for the Altitude Hold is 5A. Total for all 'things" controlled by this particular switch is 15A. Add another 25% (4A) as the "fudge" factor. 15 + 4 = 19A. You are good to go with one 20A switch for all 3 of these "things".)
3 - If the total DC current draw (including the "fudge factor") is more than the rating of the switch then you need to use more than one switch to control those "things".
(Example: Max current for the Landing Light is 9A. Max current for the Taxi Light is 9A. Total for all "things" controlled by this particular switch is 18A. Add another 25% (4.5A) as the "fudge" factor. 18 + 4.5 = 22.5A. Too much current for a 20A switch.)
Some pieces of equipment require their own switch as per the manufacturer. An example of this is that GARMIN requires a separate switch and circuit breakers for their 430/530 line of NAV/COM/GPS. If you are going to install this as per their instructions, you have to take this into account. BUT, you are building an experimental so you don't absolutley have to.
Don't forget how much the wires can handle. The wires you use should handle 50% more than the switch can. You want the Circuit Breaker or the Switch to fail before the wires do.
(Example: For a 20A switch the factor = 10A. The wire needs to handle 20A + 10A = 30A.)
You use relays when you want to use a particular switch (for whatever reason) but the circuit exceeds the switch capability. Avoid using relays unless you absolutly have to. The relay becomes just another thing that can fail. Remember KIS = Keep It Simple.
(Example: Max current for a particular starter motor is 80A. The switch you want to use is rated for 20A. You have to use a 100A relay, yes you need to add the 25% "fudge" factor on the relay circuit too.)
Different books recommend "fudge" factors ranging from 10% to 100%. But if you check the differences at our current loads, it is not that big.
I hope this helps.
Galin
N819PR
On Sat, Oct 31, 2009 at 1:39 PM, Alfred Rosa <76papa(at)dishmail.net (76papa(at)dishmail.net)> wrote:
[quote]--> KIS-List message posted by: Alfred Rosa <76papa(at)dishmail.net (76papa(at)dishmail.net)>
I know I could go to aeroelectric with this but they tend to talk too technical for me. My question is simple and maybe even stupid: When wiring for the systems that are controlled by switches in the cockpit such as lighting, flaps, pitot heat, do you run the full amperage through the 20A switches or should the switches trip relays to handle the larger current?
Not that I'm doing the wiring now but it's something I've wonde :
-List" target="_blank">http://www.matronics.com/Navigator?KIS-Listh
a href="http://www.matronics.com/contribution" target="_blank">http://www.matronics.com/contribution
===========
[b]
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76papa(at)dishmail.net
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| Posted: Sat Oct 31, 2009 5:54 pm Post subject: Aircraft electric question |
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Thanks, I knew you guys could explain it simple enough for me to
catch on quick. I'd be interested in seeing what aerolectrics says
about this also.
Al
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sstearns2(at)yahoo.com
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| Posted: Sat Oct 31, 2009 8:51 pm Post subject: Aircraft electric question |
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I didn't use relays for anything. 20 amp switches should be fine for everything in a light airplane. The highest current switch I have in pitot heat and it pulls 8-10 amps.
I'll second what someone else said about doing you own electrical and panel. I knew next to nothing about wiring when I started aside from Mark K showing me how to solder years ago. The electric was the most fun and rewarding part of the project. The key to it is being organized. It's basically detailed book keeping. I used amp circular connectors and love them. Forget termial blocks.
I can send the spreadsheet that lays out my electrical if anyone wants it
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