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  #21  
Old 03-26-2018, 07:30 AM
rv7charlie rv7charlie is online now
 
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One thing to keep in mind is that fuses/breakers are intended to protect the wire; not the device. (Contrary to cheesy mfgrs that expect you to 'protect' their device instead of building it into their device.) For your P mags: If the mfgr insists on you providing 'protection' for the mag itself, then fuse the line at the source. The switch provides the means to shut the mag down if there's an operational malfunction, so you shouldn't need a fuse + a breaker. Only variable is if they specifically spec a breaker because they need the longer time constant (high inrush currents), but that seems doubtful on that device.

On fusible links: size the wire for the load, then size the fusible link 4 sizes under the wire size. Since 22 is the smallest practical size that's physically rugged enough to work with, 22 would protect 18, 20 would protect 16, etc. The link should be ~5-6" long, be on the 'supply' end, and have some sort of flame proof protection. B&C sells a kit with fiberglass tubing. I've experimented with thick wall silicone tubing (a lot cheaper for a virtually unlimited supply of different sizes), and with a catastrophic fault (which is what you're really worried about when using a fusible link), the link vaporizes so quickly that the silicone tubing has no trouble containing the event.

Charlie
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  #22  
Old 03-26-2018, 07:41 AM
YvesCH YvesCH is offline
 
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Quote:
Originally Posted by Nihon_Ni View Post
The reason for all of my CBs is because I want a way to disable individual appliances. In the G3X, the trim wires through the servo, so if I get a runaway trim situation, I'll disable the trim motor by pulling the auto pilot servo CB.
But if the trim servo is the problem who creates the runaway you won`t stop the trim servo by pulling the CB for the GSA-28 Servos as they have a relais inside. Unless you have the trim and autopilot servos on the same CB:

Quote:
Originally Posted by g3xpert View Post
We answered Mike's question offline, but wanted to post here as well in case others were waiting on an answer.

As shown on the drawing on page C-6 in the Rev. K G3X Installation Manual, the servo is installed inline with whatever you would normally use to drive your trim motors.

When the autopilot is engaged, the autopilot immediately takes control and trims up the airplane to zero the forces on the servos and keeps doing this as long as the autopilot is engaged.

When you disconnect the autopilot, you restore manual electric trim, but the servos still drive the trim motors and do speed scheduling per your configuration mode setup.

If you remove power from the servos (not normal) a relay drops, and you can continue to run the trim motors from your trim switch, albeit without speed scheduling.

Thanks,
Steve
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  #23  
Old 03-26-2018, 07:43 AM
Nihon_Ni's Avatar
Nihon_Ni Nihon_Ni is offline
 
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Location: Fredericksburg, VA
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Quote:
Originally Posted by rv7charlie View Post
One thing to keep in mind is that fuses/breakers are intended to protect the wire; not the device. (Contrary to cheesy mfgrs that expect you to 'protect' their device instead of building it into their device.) For your P mags: If the mfgr insists on you providing 'protection' for the mag itself, then fuse the line at the source. The switch provides the means to shut the mag down if there's an operational malfunction, so you shouldn't need a fuse + a breaker. Only variable is if they specifically spec a breaker because they need the longer time constant (high inrush currents), but that seems doubtful on that device.
Thanks Charlie, that's really helpful.

Quote:
On fusible links: size the wire for the load, then size the fusible link 4 sizes under the wire size. Since 22 is the smallest practical size that's physically rugged enough to work with, 22 would protect 18, 20 would protect 16, etc. The link should be ~5-6" long, be on the 'supply' end, and have some sort of flame proof protection. B&C sells a kit with fiberglass tubing. I've experimented with thick wall silicone tubing (a lot cheaper for a virtually unlimited supply of different sizes), and with a catastrophic fault (which is what you're really worried about when using a fusible link), the link vaporizes so quickly that the silicone tubing has no trouble containing the event.

Charlie
So this is what is confusing me about fusible links. So if I started with the wire sized at 20 AWG for the load, and stepped it down 4 sized for the fuselink, that would give me 24 AWG fuselink. Bob's book and website both say a 24 AWG fuselink should be limited to 3 Amps, whereas I need 8.2 Amps continuous for E-Bus 2. Am I looking at this wrong?

Thanks again,
Rob
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  #24  
Old 03-26-2018, 07:53 AM
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Nihon_Ni Nihon_Ni is offline
 
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Quote:
Originally Posted by YvesCH View Post
But if the trim servo is the problem who creates the runaway you won`t stop the trim servo by pulling the CB for the GSA-28 Servos as they have a relais inside. Unless you have the trim and autopilot servos on the same CB:
Thanks for the info, I didn't realize that was the case. I do have autopilot and servo CBs next to each other, so I guess in a runaway trim situation I'd need to pull both of them. I'll have to think about this one some more to decide if I'm happy with a multi-step immediate action response against a likelihood/severity analysis. I suppose I could run the auto pilot servo to electric trim motor wire back to the CB Console and put a switch in it and then run the wire back to the tail to meet the electric trim motor, but that's more complexity and more potential points of failure. Hmmmm....
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  #25  
Old 03-26-2018, 08:00 AM
YvesCH YvesCH is offline
 
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Quote:
Originally Posted by Nihon_Ni View Post
Thanks for the info, I didn't realize that was the case. I do have autopilot and servo CBs next to each other, so I guess in a runaway trim situation I'd need to pull both of them. I'll have to think about this one some more to decide if I'm happy with a multi-step immediate action response against a likelihood/severity analysis. I suppose I could run the auto pilot servo to electric trim motor wire back to the CB Console and put a switch in it and then run the wire back to the tail to meet the electric trim motor, but that's more complexity and more potential points of failure. Hmmmm....
Thats why i placed my trim CB on top right so I can reach it blind:
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  #26  
Old 03-26-2018, 08:39 AM
rv7charlie rv7charlie is online now
 
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Quote:
Originally Posted by Nihon_Ni View Post
Thanks Charlie, that's really helpful.



So this is what is confusing me about fusible links. So if I started with the wire sized at 20 AWG for the load, and stepped it down 4 sized for the fuselink, that would give me 24 AWG fuselink. Bob's book and website both say a 24 AWG fuselink should be limited to 3 Amps, whereas I need 8.2 Amps continuous for E-Bus 2. Am I looking at this wrong?

Thanks again,
Rob
I went back and reviewed what he wrote about links, and you're right. But...sometimes reading his book is a little like reading FAA regs; you sometimes have to look in multiple places to get a complete picture.

If you look at the link ratings in that section, they are assuming a 10*C temperature rise (found in the 'Wire' section of the book). This is a *very* conservative rating, especially for a 4"-6" length of wire that will almost certainly be in free air (not buried in a bundle).

From the 'wire' section, I offer this quote:

Now that we've laid out the "rules" we can discuss how
and why they are sometimes broken. If you were to
run 10 Amps through a length of 22 gauge wire, does
this imply that you are going to come spinning out of
the sky trailing black smoke like the victim of a dogfight?
No, specially if the wire is short for low energy
loss in spite of overload, well ventilated to control
temperature rise, or is subject to an overload for only
short durations.


There's more in that section, addressing intermittent loads, etc.

So if you want to be *super* conservative, pick your link for that 10*C temp rise at continuous load and go 4 sizes larger for the wire. If your circuit will have intermittent loads, or you're willing to accept slightly higher temps in the link (remember, it's only 4" long so not much resistance), you could lean toward sizing the *wire* to the load and go 4 sizes smaller for the link.

Charlie
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  #27  
Old 03-26-2018, 09:07 AM
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CubedRoot CubedRoot is offline
 
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Rob and folks with diagrams,
I don't want to derail the discussion, or change the topic, but can you share what tools you used to come up with your electrical diagrams? They all look great! I am about to start doing electrical (just for the wings) but I'd like to create some diagrams like you guys have done.
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  #28  
Old 03-26-2018, 09:29 AM
YvesCH YvesCH is offline
 
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Quote:
Originally Posted by CubedRoot View Post
Rob and folks with diagrams,
I don't want to derail the discussion, or change the topic, but can you share what tools you used to come up with your electrical diagrams? They all look great! I am about to start doing electrical (just for the wings) but I'd like to create some diagrams like you guys have done.
Quote:
Originally Posted by Nihon_Ni View Post

For those who are wondering, I used PowerPoint to produce this drawing as well as the individual component drawings. I experimented with a couple of the CAD programs others have used, but I had trouble getting those programs to do what I wanted. I'm familiar with pptx and use it nearly daily at work, so I just switched to it for my electrical drawings.
Looks like Rob is using pptx as well. I did the same Thing as I did not have access to a CAD software at the time I did mine.. It works not to bad!
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  #29  
Old 03-26-2018, 10:06 AM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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Rob,
The attached drawing shows the correct way to connect the diode. The way
that the diode is connected in your drawing does no good at all. In fact, it
adds an unnecessary failure point. Capacitors resist any change in voltage.
Inductors (contactor coil) oppose any change in current. When the current to
an inductor is shut off, the inductor opposes any change in current and tries
to keep the current flowing in the SAME direction. Therefore, the diode
connected as shown in your diagram does nothing to prevent current from
flowing in the same direction that it was flowing. The induced current could
have a high voltage that will arc across the opening switch contacts,
eventually causing the switch to fail. Since your starter contactor already
has a diode across the coil, another diode is not required. But it is OK to
have a second diode as long as it is connected correctly, which is in parallel
with the contactor coil with the diode arrow pointing towards positive.
Below is a picture showing the correct way to protect the switch.

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  #30  
Old 03-26-2018, 10:08 AM
Mich48041 Mich48041 is offline
 
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The above drawing was made using a very simple CAD program called ExpressPCB.
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Last edited by Mich48041 : 03-26-2018 at 10:13 AM.
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