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05-10-2019, 10:39 AM
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Join Date: Dec 2015
Location: Auburn, AL
Posts: 200
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One switch to power two devices
I want to install a switch that will supply power to two devices (in my case, GNX 375, and GTR 200), in lieu of a separate avionics bus with relay. Together they would typically draw 4.2 amps (9.3 max). Essentially, this would be my "avionics master", being the only two devices I would want powered OFF until after engine start.
Will a normal SPST switch work for this? I am using fuses, so separate wires from the fuse block, then spliced together to connect to switch?
Electrics aren't my forte, so please excuse my ignorance here.
__________________
Steve T.
CFII/MEI/ATP 737,DC-10
RV-7 slow build
AUO
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05-10-2019, 11:04 AM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Quote:
Originally Posted by RV74ME
I want to install a switch that will supply power to two devices (in my case, GNX 375, and GTR 200), in lieu of a separate avionics bus with relay. Together they would typically draw 4.2 amps (9.3 max). Essentially, this would be my "avionics master", being the only two devices I would want powered OFF until after engine start.
Will a normal SPST switch work for this? I am using fuses, so separate wires from the fuse block, then spliced together to connect to switch?
Electrics aren't my forte, so please excuse my ignorance here.
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If you must, at least use a DPST switch.... 
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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05-10-2019, 11:12 AM
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Join Date: Mar 2014
Location: KFNT Central Michigan
Posts: 145
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Short answer, yes... Depends on if the switch itself is rated for those currents.
If not, you can use your SPST switch to turn on/off a relay to handle the current.
Again, this is the short answer. There's a lot more to it concerning electrical system design philosophy.
__________________
-Justin
RV-4 N249P
O-320, Dual P-Mags, Warneke Prop, GRT Sport EX
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05-10-2019, 11:18 AM
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Join Date: Sep 2008
Location: Fayetteville, Georgia
Posts: 215
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I think Gil nailed it. If you must.
You could of course forget the switch and just turn the devices off & on with their own power switches.
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05-10-2019, 11:25 AM
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Join Date: May 2009
Location: Reedley, California
Posts: 85
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If you're going to design an electrical system for an aircraft, or if you just want detailed information regarding switches, an excellent source is, "The AeroElectric Connection" by Bob Nuckolls. It provides simple, easy to understand information into the theory, design, fabrication, etc. of aircraft electrical systems.
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05-10-2019, 11:28 AM
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Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,351
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I use one switch for all my avionics but I use the switch to energize a relay which powers the bus avionics bus. The relay is rated at 40A and
__________________
Mehrdad
N825SM RV7A - IO360M1B - SOLD
N825MS RV14A - IO390 - Flying
Dues paid
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05-10-2019, 11:48 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,298
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Quote:
Originally Posted by RV74ME
I want to install a switch that will supply power to two devices (in my case, GNX 375, and GTR 200), in lieu of a separate avionics bus with relay. Together they would typically draw 4.2 amps (9.3 max). Essentially, this would be my "avionics master", being the only two devices I would want powered OFF until after engine start.
Will a normal SPST switch work for this? I am using fuses, so separate wires from the fuse block, then spliced together to connect to switch?
Electrics aren't my forte, so please excuse my ignorance here.
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If both devices are running off one fuse and the switch is rated at 10+ amps, an SPST switch works. If devices are fed from 2 fuses, then a DPST is required, rated at 5+ amps. With DP switches, the rating is for each pole. It is always best to have a switch with some headroom on the rating though.
Never splice together two separately fused feeds at a switch.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 05-10-2019 at 03:11 PM.
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05-10-2019, 12:19 PM
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Join Date: Dec 2015
Location: Auburn, AL
Posts: 200
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Ok thanks, all for the quick responses. My goal all along is to keep things as simple as possible. I think now I will forgo the switch altogether and just power them on/off individually.
Thanks!
__________________
Steve T.
CFII/MEI/ATP 737,DC-10
RV-7 slow build
AUO
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05-10-2019, 12:25 PM
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Join Date: Feb 2005
Location: LSGY
Posts: 3,202
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good choice
Quote:
Originally Posted by RV74ME
... I think now I will forgo the switch altogether and just power them on/off individually.
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According to Bob, that's a good choice. Apparently not using a device's on/off switch can allow corrosion to build up on the contacts over time. No idea if all electronics are susceptible to this phenomena, but Bob recommends no avionics master switch for this and a few other reasons.
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05-10-2019, 02:39 PM
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Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 997
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Need clarification from lr172
Why is it inappropriate for two fuses to feed one switch if all the wires and switch are protected by the fuses ?
Similarly, what is wrong with an avionics bus that is fed from adequate wiring and fusing for all loads /wires. ie, if you switch feed a bus from a 10A fuse with wires to all loads sized for 10Amps ?
I currently have all loads separately fused and switched on my flying -3, I am just trying to learn here.
__________________
Larry DeCamp
RV-3B flying w/7:1 0320 / carb / Pmags / Catto 3b / digital steam
RV-4 fastback w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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