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P-leads and grounding of shields

 
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gilalex(at)earthlink.net
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PostPosted: Sun Feb 26, 2006 3:49 pm    Post subject: P-leads and grounding of shields Reply with quote

posted on the other list too....

I'm installing a new Horizon electronic tach. and replacing my p--leads at
the same time, since the switch is out, and the cowling off for the
Mitchell instrument installation - the old "while we are it" - and the
magneto end does look ratty...

However, I can't find the correct wiring for the shields of the p-leads at
the ignition switch.

My p-leads have been changed in the past (actually had a log book
entry...:
) so I no longer know what the correct original configuration
should be...

Do the p-lead shields connect to the ground terminal at the ignition switch?
This would seem to be a good way of connecting, and no extra ground would
be needed at the switch.

However, Gary's schematic shows a separate ground wire at the switch (going
to the airframe - called 51JA3), and NO shield connections at the
switch.... this would also seem correct.

But, if I attach the P-leads shield to the ignition switch ground, and also
take it to the airframe, I have made a large ground loop in a noisy circuit
- I think. This does not seem correct, but that is how the present switch
is wired.

Does anyone know the correct wiring of shields at the ignition switch?

...thanks gil in Tucson

77 Tiger


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GrummanDude



Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

PostPosted: Sun Feb 26, 2006 9:08 pm    Post subject: P-leads and grounding of shields Reply with quote

the shield on the P-lead doesn't ground at the magneto switch.

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Gary
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gilalex(at)earthlink.net
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PostPosted: Sun Feb 26, 2006 9:34 pm    Post subject: P-leads and grounding of shields Reply with quote

Thanks Gary... that makes sense from a "no ground loop" point of view.

And it makes the wiring easier.... less shields to dress on the wire.

I presume 18 gauge shielded wire from Aircraft Spruce is OK.... MIL-C-27500

...thanks gil in Tucson
At 10:07 PM 2/26/2006, you wrote:
Quote:


the shield on the P-lead doesn't ground at the magneto switch.




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FLYaDIVE(at)aol.com
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PostPosted: Mon Feb 27, 2006 11:36 am    Post subject: P-leads and grounding of shields Reply with quote

In a message dated 2/26/2006 6:49:55 PM Eastern Standard Time,
gilalex(at)earthlink.net writes:


posted on the other list too....

I'm installing a new Horizon electronic tach. and replacing my p--leads at
the same time, since the switch is out, and the cowling off for the
Mitchell instrument installation - the old "while we are it" - and the
magneto end does look ratty...

However, I can't find the correct wiring for the shields of the p-leads at
the ignition switch.

My p-leads have been changed in the past (actually had a log book
entry...:
) so I no longer know what the correct original configuration
should be...

Do the p-lead shields connect to the ground terminal at the ignition switch?
This would seem to be a good way of connecting, and no extra ground would
be needed at the switch.

However, Gary's schematic shows a separate ground wire at the switch (going
to the airframe - called 51JA3), and NO shield connections at the
switch.... this would also seem correct.

But, if I attach the P-leads shield to the ignition switch ground, and also
take it to the airframe, I have made a large ground loop in a noisy circuit
- I think. This does not seem correct, but that is how the present switch
is wired.

Does anyone know the correct wiring of shields at the ignition switch?

...thanks gil in Tucson

77 Tiger
====================================================
Gil:

I usually agree with Gary but in this case the BASIC rules of electronics
would over ride what Gary said.
The basic rule of:
On RF (Radio Frequencies) the Shield is grounded at BOTH ends and on AF
(Audio Frequencies) the grounding is done at ONLY the Source.
In a Mag situation there could be some discrepancy and here is why. The
rotation speed of the Mag and the opening and closing of the contact points as
well as the speed that the spark is sent down the ignition wires IS at Audio
Frequencies. And that would indicate that the Shield should be grounded at the
Mag (Source).
YET! If the noise is being transmitted to free air and then to the radios
via the ignition wires then the noise is RF and should be grounded at both ends.
I would start by grounding at both ends, fly the plane and see what happens.
IF you want to try shield grounding at one end then all you have to do is
snip the shield ground at the far end. Maybe even install a temporary switch and
see what happens. BUT REMEMBER you are breaking the ground at the far end
away from the Mag.

Ground loops do not happen at RF. They are a DC and AF problem. If there is
a noise problem involving RF it is just an RF problem. There are three rules
when dealing with RF problems:
1 - GROUND
2 - BLOCK &
3 - SHIELD
Barry
"Chop'd Liver"


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GrummanDude



Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA

PostPosted: Mon Feb 27, 2006 1:07 pm    Post subject: P-leads and grounding of shields Reply with quote

In a message dated 2/26/06 9:34:55 PM, gilalex(at)earthlink.net writes:
Quote:
I presume 18 gauge shielded wire from Aircraft Spruce is OK.... MIL-C-27500


I think I used 20, but, 18 is ok.


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