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switch current limits

bobmarkert

bobmarkert

Well Known Member
I have convinced myself that a 20 amp switch mean it can withstand 20 amps flow during the on-off cycle. Once contact is made can a 20 amp switch pass through more than 20 amps? What's driving this question is my desire to use a switch, without a relay, to power/de-power my avionics buss. The current is less than 20 amps when turning the switched ON, but once ON the the transponder and VHF radio may occasionally take the load above 20 amps during simultaneous transmissions. Will this be a problem as long as the load is less than 20 amps during the switching cycle?
Thank You
 
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Yes, that is going to be a problem. Put in dual switches and split the avionics bus. This might be a good time to make them basically redundant (comm1 on one bus, comm2 on the other, etc) so that if you lose one bus you don't lose all functionality.
 
airguy said:
Yes, that is going to be a problem. Put in dual switches and split the avionics bus. This might be a good time to make them basically redundant (comm1 on one bus, comm2 on the other, etc) so that if you lose one bus you don't lose all functionality.
Click to expand...

Ok so that got me thinking,..... Will a DPST 20 amp switch handle 20 amps per pole (side?) for a total of 40amps? If anyone can point me to basic "switch 101" class I'll gladly do the research
 
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Yes, wire two poles in parallel.

It might not make 40 amps but it will be better than one pole.


Check that the 20 amp rating you are using is a 12 V DC rating.
 
Just a reminder

I would like to add a reminder also that switches are rated in ac and dc currents, please make sure you buy a switch, for your airplane, that is rated in dc amperes.
The difference is very important. Happy building.

Bird
 
6 Gun said:
Thats a lot of amps for two radios and a transponder what are you using.
Bob
Click to expand...

2 x G3X screens
GMC305 AP controller
Dual ADAHARS
Dual magnetometer's
Dual TCW backup battery
GTN 650
PAR 100EX
GDL 39
GTX 23 ES transponder
proton torpedo generator

i have yet to do the math with a sharp pencil but the GTN Comm and GTX23 in reply mode eats up some amps. there is another thread running right now about an EFIS switch which has me considering moving the screens to the main buss
 
bobmarkert said:
2 x G3X screens
GMC305 AP controller
Dual ADAHARS
Dual magnetometer's
Dual TCW backup battery
GTN 650
PAR 100EX
GDL 39
GTX 23 ES transponder
proton torpedo generator
Click to expand...

A VP-X would complement all that awesome equipment very nicely. Use any switch you'd like to turn equipment on/off.

They don't pay me, I'm just a big fan. :D
 
thanks Jeff,
I thought long and hard about VP-X and decided against it, so that decision is behind me and in my mind irrevociable. I know the cost will be similar for VP-X and traditional CB's. I'm starting to pull wires for a conventianal electrical system.
 
Personally, I'd put the EFIS/EMS/ADAHARS/etc on an "instruments" breaker and switch circuit that is separate from the radios/xponder breaker and switch. Having it all on one single "avionics" switched circuit is putting a lot of eggs into one basket. Just my personal opinion anyway.
 
why use an avionics master at all?

Most equipment has its own on/off switch, can can be powered through a traditional breaker from whatever buss you choose. Those items without integral power switches (e.g. EFIS, AHRS, Gemini MFD) can be powered from whatever buss via a switch type breaker.

what is the purpose of an avionics master other than to introduce additional single points of failure?
 
switch

Ok I see thanks for reply Bob I just finished my panel last night lot more work than I thought it would be . I went with a seperate switch for EfIS and I put my GPS and EIS on another switch and then my radio stack audio and trans on radio master also went with Approach Fast Stack for premade wiring for radio, trans ,G-240 auido,and Wiring for GRT HX Efis Im really pleased with there work and Hub.
Bob
 
Recalculating....

I didn't look up any of your equipment to see the actual load, but I doubt you have 20 amps even during transmit. The only wild card might be the TCW backup, if it / they are discharged they might load it up some for a while.
You may be able to move the TCW charge over to the main bus.
FWIW, as a reality check, my panel/plane, 2 GRTs, 1 Icom, 1 430W, 327 Xpndr, TT AP, AHRS, magnetometer and everything normally on during day flight draws 8-9 amps once the batteries have charged a while. Add perhaps 4-6 amps during Comm Xmit, all running through a B&C Spec Carling switch.
I suspect your equipment load would be pretty close to the same.
Tim

2013 dues paid.
 
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walkman said:
what is the purpose of an avionics master other than to introduce additional single points of failure?
Click to expand...

To isolate expensive avionics from voltage swings on startup, without having to turn them all on/off individually.
 
airguy said:
To isolate expensive avionics from voltage swings on startup, without having to turn them all on/off individually.
Click to expand...

I will admit to having an avionics master... kind of violates the simplicity rule, but here is why I chose to do so:

1) I consider it the 'non-essential' bus. Turning the switch off reduces load on the main battery for start-up (engine cranking) or emergency (electrical fire) situations. Saves the battery during those hours of debugging and programming while building.

2) Allows me to use a two-pole switch, with one pole dedicated to a starter enable/disable function. I am using an Infinity stick grip, and the starter push-button would be live for the entire flight without the avionics switch. To start the engine, the Avionics Master must be OFF. This means that essential instruments (EFIS included) are not wired to the avionics bus.

The failure scenario (engine quits during taxi or flight) would require shutting down the avionics bus for restart... but isn't that the point?

In my case, the load is a COM, GPS (AERA), MONROY ATD, Remote Transponder and a spare. Total load is typically less than 5 amps, well within the specs of a normal power toggle switch, even for 'funny' loads (defined as non-linear loads like lights or strobes which burn up switches).
 
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