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stearman456
Joined: 14 Aug 2010
Posts: 49
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 Posted: Wed Jun 08, 2011 7:01 am Post subject: Switch Position within the Circuit |
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On a 12 volt DC system, does it make any difference if the toggle switch is located between the device and ground vs the device and the + bus bar? Wiring my aircraft it's going to be more convenient to make the runs from the ground bus to the cockpit and then to the device and then the the + bus. (There is no grounding via the airframe - it's a dual wire installation with a ground block on the firewall.)
Dan
warbirds(at)shaw.ca
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sprocket(at)vx-aviation.c
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| Posted: Wed Jun 08, 2011 7:52 am Post subject: Switch Position within the Circuit |
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The short answer is yes, it does matter. In the event of an electrical
short circuit (or even when you are servicing), you want to be able to
switch off the circuit. If your unswitched wire is always hot (+12) and you
have a short to ground, which is the most likely scenario, then you can't do
this.
If you need to do ground switching, you can wire in a relay in series with
the +12V feed, with one side of the coil connected to +12V as well. Then,
you can switch the other coil connection to ground to control the circuit.
Vern
--
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stearman456
Joined: 14 Aug 2010
Posts: 49
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| Posted: Wed Jun 08, 2011 8:44 am Post subject: Re: Switch Position within the Circuit |
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Of course! All it would take is a chafed wire. Man, I can be dumb! Thanks for the help - back to the drawing board!
Dan
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BobsV35B(at)aol.com
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| Posted: Wed Jun 08, 2011 9:50 am Post subject: Switch Position within the Circuit |
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Good Afternoon Dan,
As another thought, if putting the switch in the "ground" side is more convenient, go ahead and do it.
Just make sure the circuit breaker is properly located as close as practical to the primary power source and that it is properly sized to protect the wire used.
Happy Skies,
Old Bob
AKA
Bob Siegfried
Stearman N3977A
LL22
In a message dated 6/8/2011 11:55:36 A.M. Central Daylight Time, warbirds(at)shaw.ca writes:
| Quote: | --> AeroElectric-List message posted by: "stearman456" <warbirds(at)shaw.ca>
Of course! All it would take is a chafed wire. Man, I can be dumb. Thanks for the help - back to the drawing board!
Dan
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=342416#342416===============================================
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stearman456
Joined: 14 Aug 2010
Posts: 49
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| Posted: Wed Jun 08, 2011 10:21 am Post subject: Re: Switch Position within the Circuit |
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Hi Bob,
It's not a big deal, just a little less direct routing of the main bundle from the junction box on the backside of the firewall, but not by much. Mine is also a Stearman, so there's lots of room for it.
Dan
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user9253
Joined: 28 Mar 2008
Posts: 1925
Location: Riley TWP Michigan
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| Posted: Thu Jun 09, 2011 5:39 am Post subject: Re: Switch Position within the Circuit |
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The master contactor is an example of switching the negative side of a circuit. There is nothing wrong with doing it that way as long as good workmanship is used. If the aircraft is not wired according to common practices, it will be confusing to future troubleshooters. It is better to follow convention unless there is a compelling reason to be different.
Joe
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Joe Gores |
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mrspudandcompany(at)veriz
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| Posted: Thu Jun 09, 2011 6:14 am Post subject: Switch Position within the Circuit |
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The master contactor is an example of switching the negative side of a
circuit. There is nothing wrong with doing it that way as long as good
workmanship is used. If the aircraft is not wired according to common
practices, it will be confusing to future troubleshooters. It is better to
follow convention unless there is a compelling reason to be different.
Joe
Correct me if I am wrong but, I believe that what joe is
saying is that the switch which controls the master contactor goes to
ground. The master contactor itself is on the positive side very close to
the battery.
Roger
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nuckolls.bob(at)aeroelect
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| Posted: Thu Jun 09, 2011 6:18 am Post subject: Switch Position within the Circuit |
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At 12:44 PM 6/8/2011, you wrote:
| Quote: |
Of course! All it would take is a chafed wire. Man, I can be
dumb. Thanks for the help - back to the drawing board!
Dan
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It is not uncommon to have pull-to-ground control
systems. Our automobiles have them at every door
light switch, hour-meters on many airplanes pull-to-
ground. There are dozens of ground-side switched
circuits in a Beechjet.
You do what becomes the most elegant solution to
your design goals. Of course, all circuits are
protected upstream by fuse or breaker . . . which
is what deals with issue of electrical hazard. But
don't add parts count just to steer away from
ground-side switching.
Bob . . .
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stearman456
Joined: 14 Aug 2010
Posts: 49
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| Posted: Thu Jun 09, 2011 9:43 am Post subject: Re: Switch Position within the Circuit |
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Thanks, Bob. I like to stick with convention but I'll have a good long stare at it tonight with your advice in mind.
I appreciate the help everybody.
Dan
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