Avionics master solenoid

[Dave Cole]

Fuses (or circuit breakers) are used to protect wiring, not devices. If a device, or the wire supplying it, becomes shorted to ground, the fuse will prevent smoke and fire. If you connect a device directly to the main power bus with out a fuse, then a shorted condition in the device would cause the fuse that feeds the bus to take the main buss off line.

Dave Cole
RV-7 Wings

[Will Womack]

IMHO, an avionics master switch is required. Shutting off a radio does not necessarily remove power from its sensitive components, it just disables its internal power supply.

I plan to design my RV7 DC power distribution exactly like (although on a smaller scale) I design DC power distribution on G-IVs: by using a main and essential bus structure that is sheddable via masters.

Note that in the certificated world you can't use a breaker as a switch. A relay (not a solenoid) should be used for a master. A switch is installed to switch the ground of the relay. Voltage is provided directly to the positive side of the relay (and its control) from the bus feeder breaker (you do have a feeder breaker right?)

Because this is how power distribution is designed on just about every type of certificated aircraft I have ever done an avionics retrofit design on, this will be the way I design my aircraft.

Will,

Avionics Engineer
Custom Engineering
Gulfstream Aerospace Corp.

RV-7A Emp

[jcoloccia]

Originally Posted by Will Womack

Note that in the certificated world you can't use a breaker as a switch.

The certified world is built around 50 year old technology, tradition and lore. In the military world (built around reliability and ruggedness) we use switchable breakers all over the place.

Now, I've given a pretty good explanation of why I think an avionics master is a bad idea, and why I think if you use one anyway a switchable breaker (or at least a simple switch) is the way to go.

No one has explained to me yet what the advantages of a relay is in terms of function and reliability. It appears to be weight, an unescessary current draw, an additional component that can fail, and more complex than a simple switched circuit. I would really like to understand this.

[Will Womack]

The standard breaker like the Klixon 7274 series (MS 26574) is not designed to be used as a switch. It has a finite number of cycles before it will fail. It is designed to open internally from too much current. Manually opening the breaker reduces its service life.

A switchable breaker is an entirely different animal. It is designed to be opened both manually or internally. I have little experience with these however because you just don?t see them in the commercial world too often. I believe it is because you will get a longer MTBF with a switch rather than using a switchable breaker (the switch need only be designed to switch, where as the breaker must perform the circuit protection function as well)

Now, the reason I would use a switch to activate a relay instead of just using a switch alone is simple. When you are trying to switch a relatively large amount of current (like an avionics bus) you use a remotely mounted relay because it can handle the current. That way you can use a nice little switch in the cockpit instead of a big one that has to handle the current flow of the entire bus.

Whether you use a relay or a switchable breaker, I highly recommend an avionics master as a means of protection for your high $$$$ equipment. Shutting off the individual radio isn?t always good enough.

[L.Adamson]

Originally Posted by jcoloccia

No one has explained to me yet what the advantages of a relay is in terms of function and reliability.

That would all depend on current draw of the avionics, and not wanting to install a massive switch along with wiring to handle the load. For my plane, a simple switch is just fine.

[jcoloccia]

just fyi: The Klixon breaker is rated at 2500 cycles. Now, at one cycle per flight (one on/off cycle), that's close to flying once a day for 7 years. At 2 flights a day (once there, once back), that's 3.5 years. Incidentally, I've used enough Klixons that I've come to interpret the 2500 cycle rating as a bare minimum used to plan maintenance. Those things are built very well and last more than their "rating". In our application, I doubt you'd see a failure due to service life. Just my opinion, though.

In practice, though, those numbers are meaningless. I can't think of the last time I've had a switch simply wear out and fail. I take that back....I've been in older houses where the light switches are actually starting to fail due to simply wearing out from use. In real life, though, switches typically fail because of disuse, misuse or mechanical breakage.

I've see switches in the aircraft spruce catalog that are rated at 25,000 cycles. That's 2 flights a day for 35 years. If you fly 2 flights a day for 35 years, maybe the switch will actually last that long . Most of us fly once a week, if that. Theoretically, that switch should then last 240 years. Hmmmm.

Once again...this is all just my opinion. I think it's valuble to share ideas with others, and then they can make up their own mind.

What really determines how "big" a switch needs to be is the power being drawn when you switch it. Even small switches can handle significant amounts of current...they can't switch them without frying, though. If everything's off before you hit the avionics master, you can use a reasonably sized switch.

In terms of protecting things in the airplane: my feeling is that the only thing worth protecting in that cockpit is ME . Breakers aren't simply there to keep wires from burning....they're there to keep wires from burning ME. Given a choice between "extra" protection for my avionics (or pretty switches for that matter), or increased SYSTEM reliablity that facilitates safe completion of my flight....well....the avionics can just fry, I guess. I'll depend on my checklists to protect the avionics, and on my system architecture to protect me.

Once again, though, this is all just my opinion. I don't have a flying airplane, have never built an airplane, have never owned an airplane and am years away from installing avionics. My opinion is in the minority and is certainly against the conventional wisdom of what's worked for the last 50 years. Just consider it food for thought

[Will Womack]

I supose that by building an experimental aircraft, you can try various experiments with the electrical system. Just remember that it is very difficult to get the magic smoke back in box once you let it out (I've yet to find a way).

One more point though:

Switches are rated by the current thier contacts can handle. Switches must be sized using the worst case (maximum posible current available at ANY time) senerio. Relying on the operator not to overload a circuit is not permissable. Of course, this is the rule for certificated aircraft.

[szicree]

Originally Posted by jcoloccia

switches in the aircraft spruce catalog that are rated at 25,000 cycles. That's 2 flights a day for 35 years. If you fly 2 flights a day for 35 years, maybe the switch will actually last that long . Most of us fly once a week, if that. Theoretically, that switch should then last 240 years. Hmmmm.

That's not counting all the times I've switched things back and forth while making airplane sounds in my garage.

Steve Zicree
RV4 finishing

[jcoloccia]

Originally Posted by szicree

That's not counting all the times I've switched things back and forth while making airplane sounds in my garage.

Steve Zicree
RV4 finishing

Lol...I thought I was the only one.

[gmcjetpilot]

Originally Posted by rv72004

Thanks for the reply guys, but still no one has told me if the solenoid must be protected. Yes a combination c/b switch is a good idea . Its just that I am using really nice lighted rocker switches and would like to use one of these instead.

As you know most would recommend to avoid a relay because its a failure point, but if you want to use a relay and a Rocker switch that's OK. The thing is rocker switches in general have a high rating and likely could handle the avionics buss with out a relay. Note: DC rating and AC rating are TWO different things in switches.
Here is Bob's article: http://www.aeroelectric.com/articles/swtchrat.pdf

Rocker switches are known for taking high amps 10,15, 20 and even 30 amps DC:
(typical: small- 0.5"w x 1"h; large- .8" x 1.45"h rectangle, even round)
http://www.action-electronics.com/gcswrot.htm#Marine
http://www.steinair.com/switches.htm

The avionics you state sound reasonable. I guess you are under 10 amps max (even while transmitting/autopilot servo on). Your rocker switch capability should take this load, unless you are using a mini switch (you should use heavy duty switches of at least 10 amps). My typical avionics drain is less than 7 amps and standby initial power-up around 4 amps (not transmitting or auto pilot servo engaged).

To answer you question, yes you should provide a C/B or fuse for the relay (solenoid as you called it, also called contactor by others), unless you don't need it. Let me see if I can explain. (Wire protection is a little bit art, science and paranoia.)

Ok, Where should have circuit protection on the switch and or relay, if at all.

Depends on the length of the wire. Fuses/CB's/Fusible links provide wire protection of LONG lengths of wire. If two components are next to each other, essentially co-located you don't need to protect the wire between the components. Many guys set the master and avionics buss next to each other so the jump wire is short. You could squeeze a relay in between the busses and use short jumpers. If you a Rocker switch in the panel (handling the positive current) you will no doubt have a length of wire to/from the panel needing protection. So you need a fuse/CB maybe.

OK the wire run is long and not adjacent. Where to put the fuse/CB? As close to the source of power as possible, the main buss as you mentioned. Here is a crude dwg to show what I mean.




Lets say an electrical "short" of the long feed wire to the avionics buss. Assume the "short" occurs near the avionics buss, so the whole length of the feed wire will smoke and glow red, all the way back thru the relay, to the source, the main buss, which is also not protected, direct to the battery (which is common). You will have a problem. A single fuse (one close to the main buss) will save the day. With out it you are..... Since the switch (relay activation) is a ground switch it's not critical and does not need a fuse. I would put a fuse / CB device in at least adjacent to the main buss, which you are trying to protect. Also consider a second electrical path to power the avionics buss in the event of a relay or fuse/CB failure. Depends on your mission (IFR/VFR), Examples below.

Examples:
Below is a basic (but very well done) pwr diagram showing an avionics buss with a avionics master switch from a RV-8a builder. He uses a switch with NO fuse/CB. I don't agree unless the busses and switch are close / co-located. This diagram is based on Bob N's recommend diagrams. Bob N does not use (believe in) avionic switches but does recommend an avionics buss. Bob N. likes automotive blade fuses in plastic multi fuse holders. You would use one for the main and another fuse block for the avionics items, placing them next to each other, close. The two busses are direct connect with a short wire/jumper direct or thru a diode bridge (see next example). Of course where the short jumper is, is where a switch or relay would go if you wanted an avionics master switch. Again proximity plays a part in this. If you run a long wire to a switch, relay or buss away from the main buss, protect it. May be a fusible link would be a good thing?

http://www.rv8a.com/wiring/dwg/rev6/n8wv_wire_r6.pdf

Below is a fancy (very nice) diagram that shows the avionics buss, driven by both the main and also an emergency hot batt buss. So there are two switches, main and emergency (from hot batt buss). You see he has two fuses on the main feed (and emergency feed) to the buss and a diode bridge. The diode keeps the emergency feed from back feeding the main buss from the hot batt buss when the emergency switch is on. I am not sure why he uses two fuses (four total) but he probably had room in his "Fuse Block" and used them, it works. Where he shows switch [A7] is where your relay goes, right. Emergency buss feed is not a bad idea in case the relay dies or you want to shut down the the main buss (thru the master relay). This way you can still power the avionics direct from the battery, thru the hot battery buss, even with the master off. What if the master relay fails in flight IFR? (right) Is it worth the extra wire, CB's switch and diodes? What is the chance the master relay will fail? (small) Can you live with it if it happens in flight? What is the back up? Is a back up needed. These are the question you need to ask. There is a balance and than there is over kill. People are putting in two alternators, two batteries and all kind of switches and alternate power sources in their plane. Sounds like a lot for a little plane. Be realistic and not to paranoid. What ever floats your boat, but you have to float (fly) your airplane and WEIGHT is critical. Build is simple, straight and as light as possible. Remember most Cessna's and Piper have very simple one battery, one alternator, one or two buss wiring and have worked fine for decades.


http://www.pflanzer-aviation.com/PDF%20Files/Power.pdf


Wire protection and electrical systems are designed by good practice, common sense and trade-offs , safety (redundancy) vs. complication. You don't want any wire big or small, of any length, grounding or getting real hot. You have the choice of fuse, CB, fusible link or nothing. When you figure out what you want to do, post it. Make a diagram for yourself (which is a key tool) , look at builders web sites who post their wiring diagram. It will become more clear. I hope that helps. You are the Captain and you get to choose.

One thing to keep in mind, can you afford to loose ALL your avionics buss items with a single relay, single fuse. Are you VFR/IFR? Either the answer is yes you are OK without the avionics or you need to think of multipaths to assure a single fault will not leave you in the dark.

You are learning. Bob's book is good, may be over kill. Key to a good electrical system is a good central ground. I would get some more info, aviation book store, web and check back. Check Bob's web site out and look for articles, lots of free advice, diagrams and info (http://www.aeroelectric.com/ and http://www.aeroelectric.com/articles.html).

George

PS some other stuff to read

http://www.zenithair.com/kit-data/ra/electric1.html
http://www.zenithair.com/kit-data/ra/electric2.html
http://www.bluemountainavionics.com/...cember2004.pdf