2 solonoids or just one?

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pequeajim

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I am going to use a Compoiste Design power panel in the Lightning that I am building. This has a split power toggle switch on it (master/alternator). The panel wants to see power from a battery solonoid (like a Cessna), and not wired direct.

My engine is a Jabiru 3300 and most of the aircraft built by the Jabiru USA guys just takes power directly off the battery to a 25-35 amp master breaker switch. Based on this, I have a couple of questions:

Why does Cessna use the extra solonoid?

Why dothe Jabiru guys use a breaker switch and not the extra coil like the Cessna guys? Is there an advantage of one over the other?

Thanks ahead of time for the help...
 

wally

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The Cessna way comes from years ago. It is a convenient way to just have one cockpit switch disconnect all the battery power to the airplane. Of course, back then there was just a generator so no alternator switch was needed. The alternator switch came about because of the quirks of alternators and it is nice to be able to turn it off separately when the voltage regulator fails and it starts putting out max voltage, sometimes 30V or higer!

This is most likely because way back when, wire insulation wasn't as good as now. A lot of it was rubber wraped with cotton thread. The rubber would age and get brittle after a few years and shorted wiring happened a lot more than today. The MIL-W-22759/16 wire, and other types, now in common use are wonderfully tough and dependable stuff.
Wally
 
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Joe Fisher

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The 135 hp Tri Pacers have a switch under the pilots seat that faces forward center off up and down connect 30 amp fuses to the bus bar in the panel. Under the seat is a big push butten that connects the battery derectely to the starter. Its an FAA certfyed airplane and there are thousands of them.
 

Lucrum

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Anyone have a diagram or link to a diagram on wiring a 3 terminal Master relay/solenoid like this one? MASTER RELAY from Aircraft Spruce

One terminal is labeled battery, I assume that goes to the battery positive terminal. What I'm not sure about is the panel switch to solenoid wiring.
 

Joe Fisher

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Since the solenoid is labeled battery terminal I would expect that the master switch would apply ground to turn it on. If they did not mark the battery side then I would expect you would need to apply power to turn it on.
 

Lucrum

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Since the solenoid is labeled battery terminal I would expect that the master switch would apply ground to turn it on. If they did not mark the battery side then I would expect you would need to apply power to turn it on.
I'll give that a try, applying power didn't work.
 

djschwartz

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The panel mounted switches have limited current capability. Cessna designed their electrical system with the intention of being able to handle a full panel of older avionics, lights, etc. That current would have well beyond the rating of a panel switch. They also wanted enough margin that if a short occurred or something failed and started drawing excessive current the switch wouldn't get welded "on". Thus, the solenoid is is used. The two together make up what is essentially a reliable high current switch.

In order to determine if you need a solenoid, add up the peak current drawn by every part of the electrical system that will be switched. Then look very carefully at the ratings of the switch. Many commonly available switches have both an AC and a DC current rating and the DC rating is usually lower. Sometimes much lower. This is because the arc that occurs when a switch is opened is self extinguishing with AC but not with DC. The switch must have margin in its rating over the expected peak current. 2:1 is a good goal for a switch, 1.5: 1 is a practical minimum. That is to say, it the total peak current of the switched portion of your electrical system is 30 Amps, you should have a switch, or switch plus solenoid, rated for 60 amps DC.
 

Dan Thomas

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Cessna did it, and still does, for safety. In the event of a forced landing or accident, you don't want a huge cable capable of delivering a lot of amps to the cockpit. Electrical fires can generate enough smoke to incapacitate the occupants, and sparks can ignite spilled fuel. The master contactor is located right next to the battery to minimize the unfused cable length, and is switched on by grounding it. That keeps the "live" bit of wiring into the cockpit down to a small 18-gauge wire that already has a load on it (the contactor's coil) so that it isn't going to smoke if it shorts; it'll just turn the master back on.

Any large switch or breaker in the cabin will have a long, unfused length of cable involved that cannot be shut off if it shorts somewhere along its length, and any fuse or breaker at the battery can't be reset in flight if it pops.

Dan
 
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