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From: Richard Goodwin (no email)
Date: Thu Nov 01 2001 - 10:50:10 EST
There are two sources of spikes from a starter (at least): Brush noise,
and a big spike when you let off the starter button and the solenoid
breaks contact. Brush noise can be transmitted through the air as
well. Other sources of spikes or "noise" are: turning on or off any
electrical switch, especially a switch to an inductive load like a
motor, motor brushes, generator brushes, alternator or other charger
pulsating DC voltage output, ignition point break if you have a gasoline
engine with mechanical distributor.
The above are soruces of noise directly on the power line itself. Noise
can also be induced in the power lines from outside sources, such as
spark plug wires, brushes, switch sparks, any sparks, lightning in the
vicinity, 120v power lines, ignition from passing vehicles, etc.
Most noise can be minimized by putting a suitably sized capacitor
between +12 and ground, and/or by putting an inductor in series with the
+12 line. The size of the capacitor or inductor depends on the
frequency and amplitude of noise. To absorb a really big inductive
voltage spike, such as from point break or starter solenoid break would
require a larger capacitor than small motor brush noise.
A typical filter for electronic devices such as radios can be installed
close to the radio itself, and consists of a capacitor between +12 and
ground, plus an inductor in series with the +12 line. The inductor
resists passing higher frequency noise, and the capacitor shorts the
higher frequency noise to ground. Many mobile electronics units have
such filtering built in to the unit itself or into their power line
connection.
Noise generated by a starter or other motor can be filtered at the
source, since that will protect the rest of the +12 system. Generators
and points, e.g., have capacitors to ground from their +12 connections.
Noise elimination can be as much of an art as a science in some cases.
Success depends on discovering the source(s) of the noise(s), and
putting in the right kind of filtering. To protect against large
inductive spikes such as from a starter you need a large capacitor to
absorb the energy of the spike before it gets into your electronics. It
might be easier and safer to just disconnect the sensitive loads first,
as has been suggested, since starting can involve dramatic variations in
battery voltage that cannot be filtered out.
Dick
Gary Hawkins wrote:
>
> > the checklists? Make SURE all avionics are switched off before starting. The
> > voltage kick from the starter can destroy them. Same for VHF radios,
> > fathometers, autopilots, GPS, etc. I rewired my engine lead directly to the
> > starting battery terminal of the switch, so I can leave everything on to
> > crank up.
>
> At an aircraft parts supply I noticed they sell a .05 1000V capacitor. Was
> told that it was to eliminate engine noise in the radio. But, would it also
> reliably absorb such a spike from the starter? (Simply wired from positive
> battery wire connection on the starter, to cap, then engine block).
>
> /g
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