Ignition noise suppressing

ian field

Joined Oct 27, 2012
6,539
This presumes that the problem is RFI; alternator whine isn't. It is at audio frequencies.

I have tried "filtering" power going to audio and two-way radio devices with simple bypasses (nF to 10,000uF), and LC filters. Never resulted in an improvement, except in a couple of cases.
I never said anything about filtering the power lead at the audio gear - they usually have an LC filter just inside the metal case.

The whine probably isn't getting in by the power lead anyway.
 

nsaspook

Joined Aug 27, 2009
8,157
Shunts the start of the wire antenna whose other end terminates at the battery.

And points capacitors are usually around 220 - 250nf - not 100nf. The units sold for suppression are probably bigger still - but I don't do much car electrics these days.
It's usually not very effective in most cases of current loops as the noise source is a very low impedance mainly magnetic field from the battery charging current loop that induces small amounts of audio frequency current (and voltage on input resistance) into any other loop circuit whose conductors cut across the field. Single point grounding with careful routing of audio cables cures most problems but sometimes you can't lift the ground on something so a isolation device has to be used.
 
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ian field

Joined Oct 27, 2012
6,539
It's usually not very effective in most cases of current loops as the noise source is a very low impedance mainly magnetic field from the battery charging current loop that induces small amounts of audio frequency current (and voltage on input resistance) into any other loop circuit whose conductors cut across the field.
That's why you have to fit the suppression capacitors as close as possible to the alternator.
 

nsaspook

Joined Aug 27, 2009
8,157
That's why you have to fit the suppression capacitors as close as possible to the alternator.
With an effective impedance in the milliohm range for the desired frequency to be suppressed more than a couple of db. Sometimes is just an old battery, dirty battery terminals/connections, a bad diode in the alt or even the fuel injector system causing the noise.
 

Thread Starter

R!f@@

Joined Apr 2, 2009
9,747
OKAY.
What if I eliminate the ground wire of the capacitor which is mounted at the back and ground the cap negative to the chassis from the back.
The Power leads are there before, I did not route those. The power lines are routed from battery to cap using two wires. One for +ve and one for -ve
 

ian field

Joined Oct 27, 2012
6,539
With an effective impedance in the milliohm range for the desired frequency to be suppressed more than a couple of db. Sometimes is just an old battery, dirty battery terminals/connections, a bad diode in the alt or even the fuel injector system causing the noise.
Maybe I've been lucky - I've never experienced alternator whine on any car I've owned, and most had a knackered battery at some point.

If they ever had whine, it was probably taken care of by a previous owner.

Some were Reliant vans with entirely fibreglass body shell, so not a lot in the way of grounding or screening.
 
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