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From: Colin Foster (no email)
Date: Thu Nov 01 2001 - 09:49:45 EST
> > 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).
>
A 0.05 1000V capacitor is just about useless for stopping engine noise in a
radio, and absolutely useless for stopping the starter motor "spike".
There are no capacitors large enough to absorb the starter motor spike. In
fact it is not a spike. What happens is when you apply a heavy load such as
a starter motor to the battery, the output voltage of the battery drops due
to the internal resistance of the battery multiplied by the current flowing
through it. It is this drop in voltage, often to as low as 6 volts, that
upsets older electronics when the voltage gets too low to keep the internal
computer or memory functioning. So if the battery couldn't keep the voltage
up, there is no way a little capacitor is going to keep it up.
When it comes to removing high frequency noise from the radio, you need a
much larger capacitor than 0.05. I would start at 0.1 for the high
frequency component, in parallel with a 1000 uF electrolytic for the lower
frequency stuff. But most modern radios already have these installed inside
so adding them at the radio power input will usually provide no improvement.
A major source of noise on many boats is the alternator. In the days of
LORAN receivers which were very sensitive to it, we found the best way to
suppress the noise was a large good quality electrolytic - minimum 1000 ufd,
rated at least 50 volts, mounted right on the alternator, with the positive
lead on the alternator positive output and the negative on the frame. It is
important to keep those leads as SHORT as possible - less than one inch if
possible.
Colin Foster,
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