nuckolls.bob(at)aeroelect
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 Posted: Fri Dec 05, 2008 8:03 am Post subject: Help debugging an alternator noise problem |
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At 03:16 PM 12/2/2008, you wrote:
| Quote: | I submitted this before, but am now including much more information, per Bob's request.
Problem - I am getting a lot of clearly alternator (or voltage regulator noise) on the receive side of my radio. There is no noise on only battery power, or with the alternator disconnect relay unpowered. The noise is very low at idle, and increases in both volume and frequency as RPM increases, At high RPM makes it very difficult to understand transmissions. Radio is a Terra 760D fed from the main bus. I have checked to be sure that the problem persists even with no other equipment powered up (not coming somehow from lights or avionics).
System - The system is pretty similar to Z-17, except that I have two batteries (PC-680) connected through a marine selector switch (A-B-Both-Off) and no master relay.
There is a secondary EFIS bus which can be powered through a DC-DC regulator, but the problem does not depend on whether this bus is powered up or not.
In place of the SD-8, I have an 18 amp John Deere permanent magnet alternator feeding through a 4-wire motorcycle voltage regulator (Crane Fireball, installation doc here http://tinyurl.com/17a), connected to the main bus using the B&C relay/overvoltage protection kit as shown in the diagram, which includes a big filter capacitor.
Bus voltage is fine with the alternator connected (about 14V) and the charging portion is working OK.
Questions:
[1] Should not the big capacitor be filtering out this noise? |
Not necessarily
| Quote: | [2] Could the capacitor be bad? Can I test it?
[3] What would happen if I wired the cap backwards?
(I will check ASAP.) |
It makes smoke . . . or even loud noises!
| Quote: | [4] Is there other or additional filtering I could put on the
feed? If so, what?
[5] Are there other things I could check/test? |
Yes, you need to play the Noise Game . . . it's
sorta like Clue. You have a victim, a probable
source . . . now you need a propagation mode. This
process is described in the 'Connection chapter on
Noise.
You may have a ground loop that injects noise into
an audio system . . . this is a very common condition.
Operate the victim from a 12v lantern battery independent
of the aircraft bus to see if the noise goes away.
This tells you if the noise is coming in from the bus
or other pathway(s).
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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[quote][b]
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