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Keith Williams

Well Known Member
I have been installing a Skyview system in my RV6 and came across the need for a diode to protect the Skyview from ?flashback? from the Master solenoid (did not use one 22 years ago when I wired the RV6).

That got me to thinking about why I even have a Master solenoid in the RV6, and more to the point here, whether one is really needed in the RV12. With the higher loads on the main buss (pitot heat, high amp landing lights, etc.) I sort of see the reason on the RV6.

So I thought I would ask the question here. On the RV12 weight and electric load of the solenoid are more of an issue. I expect there are simple switches rated to handle the normal (non-starter) electrical loads of the RV12 (especially mine which has no lights). If that is the case, why is the Master solenoid needed?

The only reason I can think of is to provide the ability to shut off the starter if it were stuck ?on?. That leads to questions about starters. Do they get stuck ?on? on RV12s? If that happens how do you know, and what is the result?
 
1. Total shut down of electrical system for inflight emergency such as fire.

2. Total electrical shutdown in case of potential emergency such as off field landing/crash.

3. Keep battery from draining from any parasite loads on a vehicle that sits for days/weeks without use.
 
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Master or Starter Solenoids

It could be done but if solenoids were not used you would have to run large heavy battery size cables to panel mounted master and starter switches which would have to be large, heavy and expensive. Much better to keep the high draw cables short and remotely controlling those big "switches" via small wires and little panel switches.

Don Broussard

RV 9 Rebuild in Progress
 
Thanks for the reply, Mike. I was thinking the master switch could do those three things without the solenoid. What am I missing?
 
Don, I am just questioning the master solenoid. I understand the need for a solenoid for the starter loads. But the battery feed for the master would handle just 10 amps or so in the RV12. No heavy cable involved.
 
Keith Williams said:
What am I missing?
Click to expand...

Tradition ????

I do not have a solenoid in my Aux power circuit (which has its own battery and alternator)------just a DPST switch rated at 30A, and I paralleled the contacts to more-or-less double the amperage of the switch. But then, I am only running a #8 wire forward and it is protected with a 30A fuse back at the battery. RV 10, dual batteries in the back. Standard solenoid for the main battery.

Working fine for the last 180+ hours.
 
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Hard to justify it based on the facts presented. Frankly with solid state relays coming down in price I might just go with one of those versus the mechanical clunkers.

If it could easily be made accessible from the cockpit I might put in sort sort of battery switch like those found in boats, instead. I can't recall a car with a "master" circuit so the need for one seems more rooted in tradition than any logical basis, IMHO.

The need for a starter relay is obvious; master, not so much.
 
rbibb said:
I can't recall a car with a "master" circuit so the need for one seems more rooted in tradition than any logical basis, IMHO.
Click to expand...

See post 2, item 3.

Most of the fire engines/trucks and other heavy equipment I have driven over the years had a battery disconnect switch.

Your typical automobile is driven often enough to keep the battery fully charged.
 
Agree with need for shutoff that's why I suggested disconnect like found on boats.
 
They do make battery disconnect switches that will carry the full load of a starter. They are manual so you must have a way to turn them on and off from your seat as they would be mounted NEXT to the battery for fire protection.

Search "summit racing" for battery disconnect.
 
The biggest reason is to keep the length of the high-current main cable short. If you run it to the panel and back out it might only add a couple of feet of length, but there is a substantial voltage drop due to that run and the switch also has to be much larger as a result. There are also mechanical reasons to use a solenoid for improved reliability. A relay might do the job as well but it is the mechanical reliability of the solenoid that gives it the advantage. A lot of simple relays aren't as robust in the spring department.
 
On top if the reasons mentioned earlier there is a big safety issue.

It is very unwise to have an unswitched, unregulated direct connection from the battery into the crew cabin/instrument panel.

If that wire chafes at best you burn a hole in the metal around it. At worst the battery goes boom. I also would want nothing to do with working on a panel with a direct always live connection to it while undergoing maintenance. Imagine disconnecting that hot live wire while removing the panel only to have it arc to any grounded structure

Using a solenoid keeps the dangerous currents out of the instrument/crew cabin
 
Batt Disconnect

If a guy was going to do this the battery disconnect switch could be mounted thru the firewall. The wires and terminals stay on the engine side and a shaft could run aft from the switch to the panel for remote operation of switch. I've done similar setup before on a homebuilt that had fuel tank aft of the firewall and fuel valve on bottom of tank. 3/16" steel rod worked fine as a driveshaft to turn valve. Made a little alum plate that attached to bottom of inst panel with a rubber grommet to support shaft and to put ON-OFF labels on. Stainless tubing would make good shaft too.


Don Broussard

RV 9 Rebuild in Progress
 
I've worked on a couple of older aircraft which used a firewall-mounted mechanical battery switch with either a "cable drive" or an actuator rod going back to the instrument panel. These things are not a lot lighter than a solenoid but they do provide very solid switching, including the capacity to carry the starter load.

The only down-side is that you might not be able to easily find a replacement part if yours goes T/U while away from home. Having already baled out a visiting aircraft with the loan of a battery master contactor, the availability of spare parts isn't something to be ignored.
 
Keith, your missing the point of how a master solenoid works. By using a solenoid when the power is disconnected (master off) no power is passing from the battery beyond the master. If you were to use a standard switch on the panel as a disconnect, that switch would have live power all the time on one side. In an accident , with fuel spilling all around you that would not be a good thing, correct?
The way a master solenoid works is that it is the ground (no power) side of the coil that actuates the solenoid that runs out from the solenoid that enters the cabin. The master switch is only breaking the grounding of the solenoid therefore opening the coil and disconnecting the solenoid switch. When the master is open , or closed no current is flowing thru the master switch. So now with your aircraft inverted in an accident with fuel flowing (and the master turned off before hand hopefully) if the panel was distorted in the crash and these wires are now touching something there will be no sparks flying to ignite that fuel as these wires are only ground.
Hope this helps explain that this is not just old school. Now there are other types of devices newer school that will work the same way as mentioned above.
Ron
 
Good explanation, Ron and thanks to all who replied. I did know how the master solenoid works and understood that aspect of the design.

My thinking was that there would be a short high amp connection from the battery to the starter solenoid (similar to the normal connection to the master solenoid). The line from the battery solenoid to the panel (10 amps max in an RV12) would be fused close to the battery solenoid. In an accident with the master switch turned off, my thinking was that if a short circuit happened in that "hot" 30 inches of wire it would blow the fuse, removing the problem.

But I guess that spark as the fuse blows could be the difference.

The many (too many) hours I have spent working on old cars wired the way I described make me question some the airplane approaches I keep coming across.

Again, thanks to all who replied.
 
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