Simple question: Why relays?

[Mich48041]

Whatever turns an avionics bus on, mechanical switch, electro-mechanical relay or solid state relay, is a single failure point. There should be an alternate current path. Bob Nuckolls' E-Bus has two current paths, one through a DIODE and one through a switch or relay. The diode is highly unlikely to fail if properly installed to a heat-sink. And even if the normal feed path fails, there is an alternate feed path. There is no need to carry spare parts.

[maniago]

15A switches - oh yeah! Now I can get behind that in a hurry. Yeehaw! http://www.texasindustrialelectric.com/switches.asp

Pretty cool how many ways there are to skin this cat. Makes for a very rich read.

[RVDan]

I have often wondered why the auto industry still uses relays. One would think that if FET's were cheaper and more reliable, the auto industry would have adopted the technology for all cars.

[Walt]

I use Honeywell switches in my own plane and prefer them for the panels I mod, they have proven to have superior reliability in the airline world I came from. They have literally millions of flight hours over decades of use and I don't ever remember seeing one fail. For circuits like the avioincs bus I use a single DPDT with both sides tied together for a little extra load capacity/redundancy.

Honeywell swithches and Klixon breakers make for a bullet proof electriical system, old school perhaps, but its simple and it works.

[maniago]

Originally Posted by Walt

I use Honeywell micro switches in my own plane and prefer them for the panels I mod, they have proven to have superior reliability in the airline world I came from. .

All I get on a search are squat switches. Got a model number or some such on the Honeywell toggle switches?

[humptybump]

Originally Posted by maniago

All I get on a search are squat switches. Got a model number or some such on the Honeywell toggle switches?

Mani, I had the same problem. Take at look at the link, about half way down the page ...

Originally Posted by maniago

http://www.texasindustrialelectric.com/switches.asp

It doesn't include all the variations but with those part numbers, it's a start.

[Walt]

Originally Posted by maniago

All I get on a search are squat switches. Got a model number or some such on the Honeywell toggle switches?

Allied Electronics has the best prices I've found on some popular ones:

http://www.alliedelec.com/search/res...932-4294846770

[Flyingleap]

Thanks Glen, now those are the ones I have been looking for also. I have a new retract project and gear switches must be of the proper quality for me!! I have often thought a Hall effect switch might be the way to go for being foolproof but Honeywell is not far from being perfect. Also found a bunch of relays in this bird with a questionable mounting structure, quality, and wiring to boot. Its not fun to be flying at night over water, in thunderstorm conditions, with a load of ice, having declared an emergency AND have to worry about the type of electrics around you. John

[lrgabriel]

Originally Posted by Flyingleap

I have often thought a Hall effect switch

After more microswitch failures than I care to count, I'm now using proximity sensors in my gear indication system with great success. I highly recommend pursuing a solid state solution if you have the spare energy to do so! Commercial jets use prox sensors just about everywhere.

[vlittle]

Last century, I was employed at a nuclear research facility to design high power switch-mode power supplies before they became a commodity.

The key factor in using transistors (bipolar or FET) is what is called SOA or Safe Operating Area protection. This is very much dependent on the load (resistive, capacitive, inductive, combinations thereof). Operating outside of the SOA could lead to disaster, as my garbage can full of transistors would attest to.

Modern automotive solid-state power switches are very robust as long as the load behaves itself. Under certain fault conditions, however, it is possible to let the smoke out. Electromechanical devices (relays) are more tolerant and fool proof, but can degrade with the number of operations.

In my experience, the most hostile loads we have in an aircraft are motors (flaps, gear), gas-discharge strobe power supplies and incandescent lights. With these disappearing due to advances in LED technology, we can see the latter two being a non-issue. With motors, prudent design will avoid any problems.

Nothing, however, beats the convenience of unplugging a relay and plugging in a spare. A few years ago, I designed a circuit board that duplicates an automotive relay footprint with fast-on tabs. It used FET switches and a SPDT configuration. Problem was, it was expensive and polarity dependent... it was not a drop in for every application. Hard to compete with $2.00 relays.

Vertical Power is the best implementation that I have seen, if you want to be locked into a particular achitecture. I would never, however, put high currents through a toggle switch no mater who makes it. My experience is that this is very stressful on the switches and even more stressful on the pilot when the switch fails in flight.

Relays or FETs, both are used in cars for different reasons. Same with aircraft... after all a master contractor or starter contractor is just a relay.