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  #11  
Old 09-04-2017, 09:09 PM
Aluminum Aluminum is offline
 
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Quote:
Originally Posted by SabreFlyr View Post
I'm a few months from needing to worry about this and haven't studied it yet but this confuses me a bit. You DON'T want the ground connected to either the pitot tube or its mount?!?
Correct, wire the ground all the way back to the bus with same size wire as the power wire.

This applies more generally: all your high-amp circuits should go into a "star" where all the return current comes back to a single bolt on the bus--your reference ground. If you return the ten amps through aircraft skin, then connect another sensitive instrument's ground to the skin halfway to where the high-amp current originates, you will see that ground voltage fluctuate from reference zero as the skin has a small but finite resistance. If the high-amp current is intermittent, your readings on the other instrument end up jumping all over the place.
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  #12  
Old 09-04-2017, 09:54 PM
JDA_BTR JDA_BTR is offline
 
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The problem with the pitot heat controller is that the chassis itself is grounded, and this is the same ground as the line that normally bolts to the wing structure. This is done for a few reasons, and is part of the way the MOSFET in the system is grounded to carry the heater current and the heat sink design to the case. So I don't think that it is possible to completely return the ground current via the ground lead. Most might go that way but a very significant portion will return via the chassis mount. The controller case should not be isolated from the mount with fiber washers; it is designed to be grounded. The ground return line exists to ensure there is a good ground return separate from the chassis mounting, and because the resistance to ground via the case is sometimes a little higher because the mounting hardware isn't as reliable for electrical connections. But I haven't checked the differences because I haven't cracked the case to see. A lot of circuits like this have a ground lead on the same connection inside that bolts to the case - the screw that holds the MOSFET down.

I think that putting a larger diameter wire cuts down on impedance as well as reducing overall resistance but is probably a trivial difference for the magnetic field of the power feed.

If it wasn't for the proximity to the magnetometer it would be moot. Nuckoll's book says wire to the chassis don't worry about it and that is correct in general.
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N1463 seems a good number....
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Last edited by JDA_BTR : 09-04-2017 at 09:57 PM.
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  #13  
Old 09-05-2017, 06:54 AM
BobTurner BobTurner is online now
 
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Location: Livermore, CA
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For JDA's case I would try to mount the box to a heat sink, and keep the heat sink electrically insulated from the airframe. Any decent electronics store sells very thin mica sheets, along with dielectric grease. These are electrically insulating but provide pretty good heat transfer.
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  #14  
Old 09-05-2017, 09:52 AM
SabreFlyr SabreFlyr is offline
 
Join Date: Sep 2015
Location: Marion, IN
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I think I was a bit sleepy when I wrote my response last night. It registers with me now that connecting the ground to the mounting hardware or the tube would route ground through the airframe. I guess I must have been thinking "where is ground coming FROM?," if that makes sense. But, at the same time, I was thinking that the tube was grounded through the chassis as Dudley (JDA) says. I can see mounting with mica and dielectric grease if that provides an adequate heat sink but I'm wondering if the mounting bolts wouldn't still ground to the wing. I guess you would also use the dielectric grease in the bolt holes and use a fiber washer but then I wonder if you wouldn't end up with some grease on the bolt/nut interface.
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