Avionics Ground ??

One of the gizmos (Shurecheck traffic spotter thing)I am putting in my plane said not to connect the ground to the firewall (airframe) gound but to connect it to the avionics ground. Where is the avionics groung connected to? If I read the Aero Connection book correctly Bob K said to make a D-sub avionics ground connector and then run it to the firewall ground. I am confused. Please educate me.

Thanks
Jeff Vaughan
7A

ground

Hey Jeff,

Your airframe ground will be typically brought up to the instrument sub panel and the terminations for your avionics, etc will terminate at this point. The goal is to prevent ground loops which are little electronic gremlins that will drive you and your avionics nuts for the rest of time if you don't terminate at a common point. This ground loop thing is close to impossable to troubleshoot, and if you bring in a professional later he or she will not usually be looking for this condition. Most shops look at problems as if they are a new occurance, not installation errors. Basically, you could end up with a noisy airframe electrically if you are not careful with the grounding points. Mooneys are famous for this and sometimes the airframes have to be degaussed to get rid of magnetic fields in the airframes. Our planes should never have this problem since they are missing the steel tubular frame of the Mooneys, but when you equiptment is on it will cause noisy places that can potentially disrupt your gear.

Regards,

The way I read the book all the grounds from avionics simply go to a central ground block with flat blade connectors on the inside of the firewall. A brass bolt goes through the firewall and attaches to another ground block for all firewall forward grounds. This basically grounds everything at the same centralized location.

I have a related question. In the AeroElectric book it down plays the need for shielded wires for most applications but my wiring diagram for my PS Engineering intercom says to use shielded wires on everything but only terminating the shield at the intercom. The book says you can have more problems by improperly connecting the shield than not using one at all. I already have unshielded wire for this application. What did others use for most of the avionics wiring.

For the Audio Panel....

.....I'd do it the way PS Enginering says! My audio system has been perfect (done with shielded wires per their manual), with no whine or hum at all. Perfect, that is, until this weekend when I plugged in my MP3 Jukebox for the first time. You guessed it - got a hum! I can plug the stereo cable into the airframe jack, and no problem, but when I plug the other end into the Jukebox - Alternator whine (if I kill the alternator, the whine is gone)! It's not really that bad if I turn the music up, but that's not good for the ears!

Anyway, I talked to PS Engineering yesterday, to see if there is a "simple fix" (A filter or something), and alas - no dice. It's all magic, and probably has to do with a floating ground somewhere. Bottom line - do the very best you can to follow the installation manual to mimimize the chance of noise in the system. You're not going to know you have a problme until you are flying (you can't test it "all-up" until the wiring is finished), and at that point, fixing a wiring problem is WAY too hard....

I have much respect for Bob Nuckoll's and learned a lot from his book, but in the end, he doesn't have to fly your airplane - and you do!

I am going to check the easy stuff - the ground potential at the airframe jack - and then live with it (at least until I decide to upgrade to a stereo intercom...there's always more money to spend! ).

Paul

rv9aviator said:

but only terminating the shield at the intercom. The book says you can have more problems by improperly connecting the shield than not using one at all.

Click to expand...

All they're trying to say here is don't connect the shield to ground at both ends...you'll make a hairball for yourself. There are reasons connect both sides of the sheild on occasion (for high frequency stuff, for example) but it's a black art that even the "authorities" don't quite understand.

I've had some similiar problems. ALL my grounds terminate at the ground block on the firewall. My intercom was working perfectly until I upgraded my LightSpeed headsets from the 25 XL to the Thirty 3G. Now, I've got a slight alternator whine in the headsets. I know what's causing it though. My headset wires run very close to my alternator field CB wire. I think I'm getting some radiated noise as a result of that proximity.

At least it solved my problem with connecting my MP/3 player. I could never get the audio input jack to work on the PS Engineering PM3000 stereo intercom. The music volume was barely audible. I even sent the intercom back to PS for a field service change internally. Still no joy. With the new headsets, I just plug the MP/3 player into the headset and bingo, works great.

I think the key to the "avionics ground" issue, is to terminate all your avionic grounds at the same location. That can be on your common ground block or a separate ground block behind your panel that is then tied to your gound bus. Just as long as they all terminate together so there's no ground loops between different avionics gear.

Intercom ground

This discussion got me thinking.

I am prepairing to install my avionics, including a PS3000 intercom. What I don't understand is how to connect all of the ground and shield wires at the intercom connection and then take them to my common ground block on the firewall. As I read the instructions PS Engineering recommends that all the grounds be tied together at the plug and then taken to ground.

Do you connect 3 pr 4 together in a butt splice then run a single wire from there into another group of 3 or 4 wires until you have one ground to go to the ground block?

'lectric Bob to teh rescue!

Check out this article on Bob Nuckoll's site - I have learned LOT of neat fabrication techniques from his pictures....


http://www.aeroelectric.com/articles/pigtail/pigtail.html

Paul

Avionics grounding

There are some other cool pics on Bob's site that deal with grounding here:

http://www.aeroelectric.com/articles/Grounding

I really thought someone sold a kit that makes this pretty easy, but I can't find that information.

Hi Paul,
I know very little about wiring but did you use fiber washers to insulate the mike and headphone jacks from the panel and run a seperate ground wire from each jack back to the central ground? I'll bet 90 percent of all metal airplanes are grounded right at the jack to the panel. According to what I've read this is not good. I'm just curious how others did the grounding in that area.

Well - sort of...

Jim -

Your question is exactly why I have a glimmer of hope that I can fix my problem! I carefully used insulating washers for the Mic and Phone jacks in both front and back seats, because I knew that they shouldn't be grounded - BUT...I am pretty sure I didn't do that for my AuxAudio jacks (where I am gettign teh hum). I am goign to pull the jack away from the airframe and see fi the problem goes away...at least I can hope!

Paul

Noise from MP3 Jukebox

Dear Paul:

My guess is that you are powering the Jukebox from the aircraft power. If that is the case, what you are experiencing is Ground Loop noise. The ground potential at the audio panel's ground and the ground potential at the Jukebox ground are different. What you are hearing in the headset is that difference, amplified by the audio amplifiers needed to take the lower level signals from the Jukebox and make it large enough to drive your aviation headset.

To confirm, power your Jukebox by batteries and the noise should go away. If it does, you might want to take the power from your Jukebox at the exact same spot as the audio panel.

Let's see if we can solve this noise issue together.

Sincerely,
Mark Scheuer
PS Engineering, Inc.

Ironflight said:

.....I'd do it the way PS Enginering says! My audio system has been perfect (done with shielded wires per their manual), with no whine or hum at all. Perfect, that is, until this weekend when I plugged in my MP3 Jukebox for the first time. You guessed it - got a hum! I can plug the stereo cable into the airframe jack, and no problem, but when I plug the other end into the Jukebox - Alternator whine (if I kill the alternator, the whine is gone)! It's not really that bad if I turn the music up, but that's not good for the ears!

Anyway, I talked to PS Engineering yesterday, to see if there is a "simple fix" (A filter or something), and alas - no dice. It's all magic, and probably has to do with a floating ground somewhere. Bottom line - do the very best you can to follow the installation manual to mimimize the chance of noise in the system. You're not going to know you have a problme until you are flying (you can't test it "all-up" until the wiring is finished), and at that point, fixing a wiring problem is WAY too hard....

I have much respect for Bob Nuckoll's and learned a lot from his book, but in the end, he doesn't have to fly your airplane - and you do!

I am going to check the easy stuff - the ground potential at the airframe jack - and then live with it (at least until I decide to upgrade to a stereo intercom...there's always more money to spend! ).

Paul

Click to expand...

Power Source...

Thanks very much for chiming in Mark! It's really great to know that we have experts in the business like you and Mahlon (for engines) reading and contributing to the boards!

Good suggestion, but unfortunately, I've already got this square on the troubleshooting checked off - I used two diffenrt MP3 players (hoping that the first one was the problem) - a jukebox and a memory-card one, and both of them only on their battery power. Got the same whine each way. I also tried grounding the case of the players to see if that made a difference, but you know, everything is plastic these days! I can't remember who I talked with at your shop, but the suggestion to remove the jack from the airframe (to remove that ground path) sounded like a reasonable first troubleshooting step, and I haven't had a chance to try it yet. I will do that and report back!

Thanks again for joining in the loop!

(Of course, if I get htis workign, it will give me less incentive to trade up from my 6000 to that new 8000 that I've been trying to convince myself that I just have to have for the cell phone interface..... )

Paul

Mark, I would also like to extend my gratitude for you taking the time to help others in this group. People like you, Mahlon,Stein and numerous others with excellent backgrounds in a particular area can only help us build better, more enjoyable and safer airplanes.
Welcome

May be its...

Ironflight said:

Good suggestion, but unfortunately, I've already got this square on the troubleshooting checked off - Paul

Click to expand...

What alternator are you using? How is it wired. I am just curious? I agree with all the previous post that is sounds like a ground loop, however may be we should also start to look at the alternator itself? You do not get noise with the COM but the music you do? May be the humm was there all the time and the COM just has a better power filter.

NOISE FILTER

What about an aftermarket audio noise suppressor, aka Radio shack. You can filter the power supply to the intercom and portable device. If you are using a power converter for the portable device (14 volts to say 6 volts, they often act as power protection/filter/conditioner.)

Here is an article that Bob posted how to build the RS kit (no longer available but you can buy other kits or buy a pre-fabed filter.)
http://www.aeroelectric.com/articles/filter/RS_Noise_Filters.pdf

ALTERNATOR NOISE:

Most modern alternator are very quite electrically and expect noise free power. However you are taking AC and rectifying it with some diodes. Sometimes a soft failure of the alternator (rectifier/regulator/brushes) can result in greater ripple in the DC, which can be heard as a whine.

Next is the voltage regulator. All modern voltage regulators are electronic linear "negative feed back" type, very quite, making little electrical noise if any. Again soft failure can produce "dirty" DC.

When a semi-conductor starts to break down, there's a poor ground or if the alternator (internal or external) has a poor connection (anywhere), it can cause increased noise.

Just a SWAG (scientific wild ass guess). I know on guy mounted his alternator with some fancy plated/painted brackets, making for a poor ground to the engine. It cause some weird problems until he figured it out. Is your engine grounded well? All guessing but its a thought. The fact is your intercom is taking a very low signal and boosting it to a higher audio level to power the big headset drivers (speakers). It is no doubt taking that little noise and boosting it also. Chances are its coming thru the power/ground path. What kind of intercom do you have?

RFI

My last SWAG is it could be picking up the noise as RFI radio req interference. COM radios audio section have shielding from outside interference. The portable devices are not built to demanding spec. The wire from the portable device, thru the jack, to the intercom could pick up nose, like an antenna. Just moving or shielding these wires relative to the B-lead may help? Remember the voltages in the wires are small and get boosted by you intercom. Any small noise that you feed the intercom gets amplified with the music.

I think the other post are more likely (ground loop) but I give you my guess. To check the above remove the alternator an oscilloscope would be needed. To check the RFI you could move wires around to see if that brings relief. Last might be trying a power filter for the audio equipment.

Keep as updated. George

SSHHH George!!

I'm an Internally regulated ND alternator guy, just like you George ...we dont' want to wake up THAT sleeping giant of a debate!!

I'm going to go troubleshoot this afternoon and see if I get anywhere. To be honest, this is not something I'm going to take the airplane apart to fix - everything else is so perfect that it's not worth it! It would be easier and cheaper (as someone else suggested) to upgrade to a headset with a music interface.

I really screwed up though - it is an absolutely gorgeous day here in Houston, perfect for flying, and I figured I'd go check this out in the air, but I had to go diving this morning, and now the rules don't let me fly for 12 hours.... Guess I'll just have to troubleshoot on the ramp....

Paul

No More Whining!!

Well Mark, you guys at PS-Engineering know what you're talking about! I just got back from a nice sunset flight where I pulled the Aux Audio jack out of it's mounting hole, so it had no contact with ground, plugged in the MP3 player, and got crystal-clear music.

(Of course, then my four-year-old MP3 jukebox started giving me Hard Drive errors...hey, that's an excuse to buy a new iPod, isn't it!)

The books are right...wire it per the drawings and experienced advice, and you'll likely get good results. Now I just have to run by my electronics parts place and find insulating washers for a 3.5 mm plug.

Flying Aerobatics to music as the sun was sinking low - this airplane just keeps getting better and better!

Paul