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  #1  
Old 05-10-2019, 10:39 AM
RV74ME RV74ME is offline
 
Join Date: Dec 2015
Location: Auburn, AL
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Default 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.
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  #2  
Old 05-10-2019, 11:04 AM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by RV74ME View Post
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.
If you must, at least use a DPST switch....
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  #3  
Old 05-10-2019, 11:12 AM
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Jvon811 Jvon811 is offline
 
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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.
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  #4  
Old 05-10-2019, 11:18 AM
jabarr jabarr is offline
 
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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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  #5  
Old 05-10-2019, 11:25 AM
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dwrichey dwrichey is offline
 
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Location: Reedley, California
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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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  #6  
Old 05-10-2019, 11:28 AM
Bavafa Bavafa is offline
 
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Location: Sacramento, CA
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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
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  #7  
Old 05-10-2019, 11:48 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by RV74ME View Post
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.
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
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Last edited by lr172 : 05-10-2019 at 03:11 PM.
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  #8  
Old 05-10-2019, 12:19 PM
RV74ME RV74ME is offline
 
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Location: Auburn, AL
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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!
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  #9  
Old 05-10-2019, 12:25 PM
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rv8ch rv8ch is online now
 
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Default good choice

Quote:
Originally Posted by RV74ME View Post
... I think now I will forgo the switch altogether and just power them on/off individually.
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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  #10  
Old 05-10-2019, 02:39 PM
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Larry DeCamp Larry DeCamp is offline
 
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Location: Clinton, Indiana
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Default 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.
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