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One switch to power two devices

RV74ME

Well Known Member
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.
 
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....:)
 
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.
 
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.
 
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.
 
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
 
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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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!
 
good choice

... 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.
 
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.
 
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.

to be clear, I said don't splice two fused circuits into one single pole switch. (I apparently didn't state SP switch in my post) It is fine and normal to run them into a double pole switch. Don't have time to address the details, but if you want 10 amps coming out of the switch, run one 10 amp fused feed into it, not two 5 amp fused feeds. It's all downside with no upside. It is also fine to run multiple devices from a single pole switch/single feed, assuming switch, wire and fuse can handle the summed load. I never stated otherwise.

I should have been a bit clearer in my guidance.

Larry
 
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2 fused sources complicate the decision tree. Every wire would need the capacity to handle the sum current of the 2 fuses, and total load on the circuit should be less than the rating on 1 fuse.
 
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,

The other Larry may give a better answer, but if you have two fused leads going to a single-pole switch, and if you pull enough current to blow one of the fuses, the other fuse has not blown and so power is not interrupted. I suppose it might not be long before the second fuse blew too.

Now, on the other hand, if you were to have two fused circuits go FROM a single-pole switch to two separate devices, that would be better, but then those fuses do not protect the wire upstream from the switch if something should go wrong at the switch. You could use a toggle switch/breaker for the switch, rated at a little over the combined demands of the two devices, and a fuse for each device downstream of the switch.
 
Fuses should not be connected in parallel because current will not be divided equally between the two fuses due to resistance variations in the wire and terminals and in the fuses themselves. The fuse carrying the most current is likely to blow. When it does, the other fuse will be carrying all of the current and it will also blow.
If two loads are connected to two fuses in parallel, an overload or short in one load will blow both fuses. Now neither load has power.
 
DPDT switch, keeping the two fused loads separate, or use a relay (available at Steinair), which allows modifications later by you or a future owner when you/he upgrades the panel - which WILL happen. Solid state surface mounted switches on many of our new avionics don?t corrode with disuse.
 
I have a four pole single throw switch as my avionics master. Each device is individually fused and wired, intercom, radio 1, radio 2 and transponder. The transponder and radio come from the endurance buss and the others from the normal buss.

This places a lot of trust in a switch, however good quality switches should have a very long life. Relays will have lower reliability. I got it from Digi Key. It is much wider than the normal SPST and has a bigger handle.

Makes wiring easier. All of my power ships have the same arrangement.
 
Probably "safer" to use a multi pole switch than to trust a single set of relay contacts powering the entire electronics buss...
 
If you think of electrical wire like they are water hoses, it can sometimes make it easier to understand.

If you have two water hoses connected to hose bibs on your house, and then spliced together to a single sprinkler for the garden, the water will flow through both hoses, if you run over one hose with your car and block it off, all of that water will flow through the other hose, Now the sprinkler has half the flow(current), and cant supply enough pressure(voltage) the garden doesn't get watered like it should and will die. Or more likely in this case, blow the fuse on the other side.
 
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