Avionics Wiring Harness: A Tutorial

[Eric Page]

I have 2 Ray Allen control sticks with trims switches. I need to bring them together with the panel mounted trim switch and then back to the GA28. What is the best way to bring these wires together? Do I need to utilize a mixer?

I presume you mean GSA28, Garmin's autopilot servo. The GA28 appears to be a marine GPS antenna...

Just to clarify, are we talking about flaperon trim via the GSA28 servo, or elevator trim via the Kitfox-supplied DC linear actuator?

If you mean elevator trim, then how/why is the GSA28 servo involved?

Set me straight on those questions and I'm sure we can get your trim sorted out!

[Eric Page]

OK, after sleeping on this, I've decided that I might be misunderstanding the way Garmin handles trim.

Does the Garmin autopilot accomplish pitch trim using only the elevator servo (i.e. no control of the Kitfox stabilizer actuator)?

Or, does the Garmin pitch servo do two things: 1. control the elevator; and, 2. pass trim commands to the stabilizer actuator from either the instrument panel control head or the trim switches?

I guess I'm just thoroughly confused!

What I really need to know is, where are the signals from your trim switches going, and does the receiver want to see 12V or ground from the switches to command the trim motor to run?

Also, what are the current limits of whatever Garmin device is directly driving the Kitfox actuator?

[tdldedcd]

Hi Eric,

After re-reading my post, I realize it was clear as mud. The flow of the system is trim switch (pilot/co-pilot Ray Allen G4 control sticks) to the GSA28 to the Safety Trim and out to the Kitfox trim actuator.

I've attached the schematic from Safety Trim.

Safety.jpg

I'm comfortable with the all of this. My confusion is in having multiple trim switches. Both G4 grips will have trim switches. Do the switch wires need diodes in place before they are brought together?I think I might have found the answer in a different Safety Trim product manual.
Screenshot 2023-08-06 at 11.23.41 AM.jpg
My switch wiring will go to the GSA28. If I'm reading this right, the wires from each grip trim switch can be combined and then sent to the GSA28 without the use of diodes. Do you see it the same?

Thanks for your time with this.

[Eric Page]

OK, that first TCW diagram (page 4, here) explains a lot. I was unaware that the GSA28 handled pitch trim through a separate motor, in addition to direct pitch control via its own servo. Strange arrangement, but I guess it works. It's really not that much different from the Dynon system, except that Dynon routes the switch signals through the autopilot module on the instrument panel.

WRT your question, no, you don't need any diodes or mixer in the circuit. Just parallel all of the switches together as shown in the second diagram (TCW App Note #1). There's no practical limit on the number of switches that can be paralleled this way.

[tdldedcd]

Perfect. Thanks Eric!!

[alexM]

WRT your question, no, you don't need any diodes or mixer in the circuit. Just parallel all of the switches together as shown in the second diagram (TCW App Note #1). There's no practical limit on the number of switches that can be paralleled this way.

Okay, type real slow for me here so I can hopefully understand. Why do they show diodes used in this installation where I'll have the two hat switches and the rocker switch on the center console?
Screenshot 2023-08-08 at 08-24-29 Visio-information single pages.vsd - Safety-Trim-Application-N.png

[Eric Page]

Why do they show diodes used in this installation where I'll have the two hat switches and the rocker switch on the center console?

That's an excellent question. All the switches are doing is pulling the input on the trim controller (Garmin GSA28, Dynon A/P Panel, TCW Safety-Trim, whatever MGL uses) to ground. There's no current steering or blocking required to do that, and no switch should care what the others are doing.

Unless there's something strange about the way the Ray Allen RS-2 switch works, the diodes shouldn't be necessary. That said, a SPDT switch has three terminals; what's the unconnected white/black wire on the RS-2 for? I guess if you're using the RA switch, install the diodes. Or call TCW or RA and ask if they're really needed.

For a bog-standard SPDT rocker, or ten of them in parallel, no diodes.

Incidentally, the Kitfox-supplied rocker switch that came with the Series 5 (and presumably later models) is DPDT, because it was used to switch and reverse motor power directly. If you use that switch with a modern EFIS setup that controls trim for autopilot functions, it's only necessary to use three of the six switch contacts, all on one side of the switch (i.e. just treat it as a SPDT switch). Center terminal to ground, the other two to "up" and "down".

If one of the switches makes the motor run the wrong direction, swap the "up" and "down" wires on the switch. If all switches make the motor go the wrong direction, swap the wires on the motor.

[Geek]

Okay, type real slow for me here so I can hopefully understand. Why do they show diodes used in this installation where I'll have the two hat switches and the rocker switch on the center console?
Screenshot 2023-08-08 at 08-24-29 Visio-information single pages.vsd - Safety-Trim-Application-N.png

OK Alex. Just for you. Sloooooooowwwwww. Eric got it. The diodes are required in this use because of the internal wiring of the Ray Allen RS2 trim switch. Specifically that switch. Here's why right from Ray Allen: (the bold/underline is from me)

"The RS2 rocker switch provided with your RayAllen trim system is designed to electrically short the servo motor to ground (-) when released. This stops the output shaft without any coasting, allowing precise positioning when trimming your aircraft. The RS2 rocker switch is rated at 1 amp.
NOTE: If you want to use this rocker switch along with a stick grip mounted switch, or any other switch, to simultaneously control a servo, you must use a different wiring diagram (available through our web site at www.rayallencompany.com)."

Here's a link to the Ray Allen site that specifically addresses that switch. http://www.rayallencompany.com/RACme...S2withmods.pdf

The Garmin system has two ways of addressing pitch trim. One is adding a second GSA28 that is trim only (but this doesn't work for Kitfox trim). The other is to identify to the GSA28 that you are using a separate motor to control the trim (like we do with the Kitfox) and wire the commands to the motor through pins 13 and 14. BUT those commands are limited to 1Amp which is why the Safety Trim system is added in series between the GSA and the trim motor; it supplies the current for the trim motor when it gets the trim input from the autopilot through the GSA28.

Hope this helps. I can provide you a schematic of how I did mine for the stick grip trim but I didn't use the console switch. As Eric pointed out - you can put in any number of switches without an issue.

Geek

[Eric Page]

"The RS2 rocker switch provided with your RayAllen trim system is designed to electrically short the servo motor to ground (-) when released.

OK, after reading through the RS-2 document that Gary linked to, it makes sense. Thanks, Gary.

What RA have done is to build a DPDT switch by mounting two separate SPDT switches inside a custom enclosure, then wiring them together to make a motor reversing circuit (see Diagram 1; this is just like what Kitfox instructs us to do with their factory-supplied switch). RA have taken care of the confusing crossover links at the factory and simply brought out four wires: 12V, ground and two motor wires.

When it's used in the application we're discussing (i.e. signaling to a controller), that wiring scheme won't work. Because the two normally closed (NC) contacts are shorted together inside the enclosure, the diodes are needed so that the other two switches (on the sticks) can't pull the "up" and "down" lines low simultaneously through the RS-2's shorted NC contacts.

Notice also that when it's used this way, the wire that normally carries 12V to the switch (and is connected to both normally open contacts), is instead wired to ground; thus, the "motor" wires only connect to ground when the normally open contacts are closed by switch activation. The wire that normally ties to ground (and shorts both NC contacts) is left unconnected.

The other wiring option (and the one I would choose, if I were installing an RS-2) is to open the back of the switch case and re-wire it to function as a SPDT switch (see Diagram 2 on the last page). Much less fiddly than trying to add diodes to a wire bundle.

All clear as mud now?

[alexM]

Dog-Tilting-Head-While-Listening-741764752.jpg
Yep, totally clear now. I obtained the RA switch as a package deal with the RA LED trim indicator. I had read that the Kitfox toggle switch was, uh, "less than durable" over the long run, so I had intended to go with the RA and TCW Safety Trim.