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From: Colin Foster (no email)
Date: Thu Nov 01 2001 - 12:13:32 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.
>
I don't see where all the fuss is necessary about starter motor noise.
It runs for less than .0001% of the time.
The only concern is if the noise disrupts your electronics. The spike noise
from brushes and switching are very unlikely to disrupt any electronics,
except perhaps momentarily while they are actually occurring.
The big concern would be from the sudden drop in the 12 volt supply to the
electronics and no filtering is going to stop this. If you want to protect
your electronics from starter motor droop, this is where you should have a
separate battery just to run the electronics. It doesn't need to be very
large - one of the motorbike style will do (depending on your total
instrument requirements).
HOW DO YOU CHARGE THE INSTRUMENT SUPPLY BATTERY?
I'm glad you asked - that was an intelligent and interesting question.
Do not use a combiner, they are bi-directional. A combiner may still be
closed from a shore power charging so if the contact is still closed it will
droop the instrument battery along with the starting battery.
You could use an isolator here because the diode will not droop the
instrument battery with it. But isolator diodes have a built in voltage
drop so the instrument battery is not getting fully charged and diodes can
rupture if the batteries are connected together when one is fully charged
and the other is discharged.
Here is an interesting solution.
Connect the starting/house battery to the instrument battery through an
automobile headlamp using both filaments in parallel. The resistance at
instrument loads is negligible and the battery will get a full voltage
charge. If the source voltage droops by 6 volts during a starting load, the
headlamp will glow but not pass the droop along to the instrument battery.
You may have to match the headlamp to the load - if you have some high power
radar etc., on the instrument battery it may take two headlamps in parallel.
The big advantage of using headlamps is their resistance is non linear -
very low when cold, higher when hot. So under normal circumstances there
will be very little drop across them but if there is a larger voltage, they
heat up, increase significantly in resistance, and limit the amount of
current that flows.
Colin Foster,
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