Trim servo wiring

[Bill Wightman]

Yeah, the empennage deck is good also for accessability, with the fairing being right on top of it. The elevator control weldments are there, but don't adversely affect things (at least in our 8). The other area for the mag flux sensor is, like you said, inside the fuselage behind the baggage bulkhead. Either way, it requires wire back there. If you're gonna run anything other than steam gauges, you're going to need to plan for the flux sensor, I think.

I'm doing well, thx. Hope to have our 8 in the air before OSH. I'll be down to visit sometime later this year, so see ya then if not sooner!

[Don Patrick]

Folks,

Just running wires to the rear of the plane and I have on hand the recommended for Ray Allen installations WC2 5-WIRE TEFLON? INSTALLATION CABLE from ACS. I also have 24 gauge shielded 5-wire as well.

Is it necessary to use shielded wire here or is the "recommended" good enough?

Thanks


Don

[Mike S]

I used unshielded, for both elevator, and rudder trims-----no problems with it.

[vlittle]

It's only necessary to shield or twist the three position sensor wires. The motor wires can be run discretely. Connect the shield (if using shielded wire) at one end to ground and the three wires as recommended by RAC. Even though one of these wires is eventually connected to ground, treat it as a signal.

Cheers

[Don Patrick]

Thanks folks! Much appreciated!

Don

[az_gila]

Originally Posted by vlittle

It's only necessary to shield or twist the three position sensor wires. The motor wires can be run discretely. Connect the shield (if using shielded wire) at one end to ground and the three wires as recommended by RAC. Even though one of these wires is eventually connected to ground, treat it as a signal.

Cheers

Why?

The Ray Allen instructions don't seem to mention shielding...

http://www.rayallencompany.com/RACme...onsT2andT3.pdf


Are there other instructions around?

[vlittle]

Originally Posted by az_gila

Why?

The Ray Allen instructions don't seem to mention shielding...

http://www.rayallencompany.com/RACme...onsT2andT3.pdf


Are there other instructions around?

Enough people have had problems with RACs trim displays acting up during radio transmissions that shielding the sensor wires makes sense. The underlying problem is that the position sensor is high impedance (up to 5K ohms.. the center of a 10K potentiometer).

This high impedance source, when connected to an unshielded wire is susceptible to picking up interference from noisy sources (RS-232 lines, antennas and so on). Normally, the electronics at the instrument end will filter the signal and reject the noise, but not always.

Early in my career, I designed SLICs (Subscriber Line Interface Circuits) on thick-film hybrid substrates. Despite careful layout, these devices were really good at demodulating the local A.M. radio stations so that we could hear the interference. The characteristic impedance of these lines was 600 ohms. The trim application is not the same, but the underlying principles are. The fix, in this case, was two 5pF capacitors fabricated with metal and dielectric on the hybrid circuit.

A similar case was found in my HR-II audio circuit. I have a Garmin Aera that uses a cable where the audio wires are bundled with RS-232 wires... all unshielded. The resulting audio noise was tolerable when the Aera was powered on, but when it was off the noise was much worse. When the Aera was off, the audio and data wires were unterminated (high impedance) and the RS-232 noise swamped the audio inputs. The fix? I used 10 ohm termination resistors to ground on the audio wires at the intercom end. The Aera can drive 8 ohm speakers, so this worked well.

It's just good practice for long runs of sensitive wiring to shield it. It's easy to do at installation, and a lot harder to do after the aircraft is complete and you have trouble.

[az_gila]

Originally Posted by vlittle

Enough people have had problems with RACs trim displays acting up during radio transmissions that shielding the sensor wires makes sense. The underlying problem is that the position sensor is high impedance (up to 5K ohms.. the center of a 10K potentiometer).

This high impedance source, when connected to an unshielded wire is susceptible to picking up interference from noisy sources (RS-232 lines, antennas and so on). Normally, the electronics at the instrument end will filter the signal and reject the noise, but not always.

Early in my career, I designed SLICs (Subscriber Line Interface Circuits) on thick-film hybrid substrates. Despite careful layout, these devices were really good at demodulating the local A.M. radio stations so that we could hear the interference. The characteristic impedance of these lines was 600 ohms. The trim application is not the same, but the underlying principles are. The fix, in this case, was two 5pF capacitors fabricated with metal and dielectric on the hybrid circuit.

A similar case was found in my HR-II audio circuit. I have a Garmin Aera that uses a cable where the audio wires are bundled with RS-232 wires... all unshielded. The resulting audio noise was tolerable when the Aera was powered on, but when it was off the noise was much worse. When the Aera was off, the audio and data wires were unterminated (high impedance) and the RS-232 noise swamped the audio inputs. The fix? I used 10 ohm termination resistors to ground on the audio wires at the intercom end. The Aera can drive 8 ohm speakers, so this worked well.

It's just good practice for long runs of sensitive wiring to shield it. It's easy to do at installation, and a lot harder to do after the aircraft is complete and you have trouble.

Perhaps this Ironflight post on this thread from 4 years back is relevant?

http://www.vansairforce.com/communit...4&postcount=10