Avionics Switch Circuit Breaker Sizing

AirWolf · Post by **AirWolf** » Thu Nov 25, 2004 1:44 am

I've got a question for everyone. I am planning on running my Dynon D10A (whenever it gets here), my Garmin GNC 300XL GPS/COM, and my Garmin GTX 320A Transponder on my 'Avionics' Switch Breaker. Dynon recommends a 2A Breaker, and Garmin recommends a 10A for the GPS and a 3A for the Transponder. I was looking at Aircraft Spruce and I see that they carry both 15A and 20A SCB's. From an electrical standpoint, which SCB would I be better off with? Is there any potential damage in using a 20A Breaker when only 15A is needed?

Thanks for the help on this electrical stuff!

Ron
RV-9A
90965 Electrical/Finishing

captain_john · Post by **captain_john** » Thu Nov 25, 2004 7:25 am

...actually, 15 IS NOT needed. What is needed is a 2, 10 & a 3.

What I mean by that is if the Xponder (with a value of 3a) goes belly up it would take 15 to trip the breaker!

That is not what the mfg. has spec'ed.

If the goal is ONE SWITCH to activate them all, then install ONE SWITCH that controls 3 seperate fuses feeding each of the instruments. This will provide each instrument with it's intended PROPER overcurrent protection.

NOTE: I said FUSE, not circuit breaker. Bob Nuckolls from www.aeroelectric.com says that fuses are the preferred method of protection... and I agree with him!

Fuses are virtually failsafe. Anything that trips in MY plane is going to the radio shop to be diagnosed, not in flight. This means NO PILOT ability to reset.

Hope this helps.

CJ

Heli-Wrench · Post by **Heli-Wrench** » Thu Nov 25, 2004 7:52 pm

I was always told that circuit breakers and fuses are there to protect the wires, not the radio. Meaning that if the radio (or transponder, or trim servo, or what ever) goes for a dive and starts to draw 15 amps, accept the fact it's going to shop. But if that poor little 20 or 22 gauge wire under the panel gets 15amps, it's not only going to let out the magic smoke, you may be risking a fire.

Any of your new toys (sorry, Aircraft Accessoires) will come with drawings, and these should have a recomended wire size and circuit breaker rating. I would suggest to stick with what the manufacture recommends.

As for Fuse VS Circuit Breaker. That's it's own topic. Used both. Been thankful for both. Swore at both. Thats a question up there with Sliding Canopy vs Tip up; Tail wheel Vs Nose wheel, and 7 vs 9.

captain_john · Post by **captain_john** » Thu Nov 25, 2004 10:35 pm

No lies there!

The hardest part is getting the magic smoke back INTO the device!

CJ

svanarts · Post by **svanarts** » Fri Nov 26, 2004 12:02 am

I went with a single master switch and skipped the avionics master switch altogether. Every device in my panel has it's own dedicated fuse of whatever rating the manufacturer wanted. The fuses I uses...uh...used are the automotive blade type fuses and I used the fuse block that Knuckolls sells. Worked out very well for me.

captain_john · Post by **captain_john** » Fri Nov 26, 2004 9:25 am

I never even THOUGHT of usung blade type! I will do just that! They are MUCH more reliable, in my mind than the glass ones. BRILLIANT! They are more difficult to examine, in flight though. Hmmmmmm, perhaps a small PC board that would light up a corresponding LED when a fuse would blow. Furthurmore, a test button would be SLICK! Well, that is food for thought.

Regarding the single master switch idea. A good one! My flight instructors always told me that the power irregularities while starting the engine could wreak havoc on the delicate circuitry in the avionics equipment, hence the avionics master switch. Now, if you turn OFF all the avionics one at a time (assuming my instructor was right) prior to start up this would be a suitable alternative. Do you do that?

CJ

jim_geo · Post by **jim_geo** » Fri Nov 26, 2004 3:24 pm

Some blade type auto fuses come with a small LED built in that lights when the fuse blows. I've used em, they work.

captain_john · Post by **captain_john** » Fri Nov 26, 2004 5:50 pm

Hmmm, never seen 'em!

I will keep my eyes peeled!

CJ

jim_geo · Post by **jim_geo** » Fri Nov 26, 2004 6:20 pm

Go to any auto parts dealer and ask. Or for that mater just look they're on the shelf. Failing that if you can't for some reason find them send me an email and I'll send em U.S. mail to ya.

captain_john · Post by **captain_john** » Fri Nov 26, 2004 6:46 pm

Cool! I will!

CJ

AirWolf · Post by **AirWolf** » Sat Nov 27, 2004 3:46 pm

Thanks for the info. From all of the information, I think that I am going to go for individual fuses for the Instruments, and control them off of one swtich. Thanks guys!

captain_john · Post by **captain_john** » Sat Nov 27, 2004 4:29 pm

I just thought of this one. In the electrical trade, we have "switch rated" circuit breakers. This means the CB will handle the rigors of daily use. Perhaps one rated at 15a could serve as your avionics master and have the fuses downstream of that? I am sure there must be one made for avaion needs.

CJ

Post by **Spike** » Sat Nov 27, 2004 4:48 pm

Socata makes use of circuit breakers in their TB line of piston aircraft that also act as switches. Ive flown with them before and kind of like their functionality.

-- John

captain_john · Post by **captain_john** » Sat Nov 27, 2004 4:51 pm

Spike, are they "toggle" style or push/pull?

CJ

captain_john · Post by **captain_john** » Sat Nov 27, 2004 4:56 pm

Here are some from Aircraft Spruce:

See page:

http://www.aircraftspruce.com/catalog/e ... itbrkr.php

They have them in 15a too!

CJ

AirWolf · Post by **AirWolf** » Mon Nov 29, 2004 3:40 pm

Those (W31) Series SCB's are exactly what Van's uses for their wiring harness kits. So with a 15A SCB, I would then need to have the (10,3,2A) fuses connected to the respective components power lead. In essence, the 15A SCB would provide an avionics master (with a breaker), and the fuses would provide the correct individual protection.

Do I have this concept correct?

Thanks!
Ron

spikescopilot · Post by **spikescopilot** » Mon Nov 29, 2004 4:47 pm

Heli-Wrench wrote:it's not only going to let out the magic smoke, you may be risking a fire.

Wow... that's the first "magic smoke" reference that I've ever seen on a messageboard other than Slashdot. I salute you!

Carry on.....

4kilo · Post by **4kilo** » Mon Nov 29, 2004 9:06 pm

Cyphergirl,

One of my (many) other hobbies is electric radio controlled helicopters. We talk about letting the magic smoke out of electronics all the time. If you want, I can send you a link to one of those boards...

Pat