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From: Richard Goodwin (no email)
Date: Thu Nov 01 2001 - 13:05:29 EST
> I don't see where all the fuss is necessary about starter motor noise.
> It runs for less than .0001% of the time.
No "fuss" -- just an example of a noise source.
> The only concern is if the noise disrupts your electronics.
A starter could do that, but if the disruption lasts only a few seconds,
perhaps that is no problem. Depends a lot on what electronics you are
talking about and how hardened they are to mobile power conditions. The
degree of such hardening varies considerably.
> The spike noise
> from brushes and switching are very unlikely to disrupt any electronics,
> except perhaps momentarily while they are actually occurring.
Switch noise is indeed momentary, but can be very large in magnitude,
e.g., from the starter solenoid or an anchor winch switch or a blower or
pump switch, which can actually damage unprotected electronics
permanently. Brush noise on the other hand, like from a small electric
brush type fan or water pump could go on for some time and cause
instrument malfunction if not damage.
> 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.
True. That could be roughly the equivalent of turning units off for a
few seconds. In the case of a computer running windows that could
result in corrupted files, etc. Not a good thing, but as you point out,
not filterable either.
> 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).
Or just have a separate starting battery, as most larger boats do. That
isolates everything else from the starting circuit, which eliminates
that one source of power noise. But then once you start charging and
running all other equipment in the ship, you still have all the rest of
the noise sources to deal with.
A little bit of intermittent noise may in fact not be worth worring
about. But some types of noise can go on for long periods of time,
which could cause data errors in some equipment. Large spike noise,
while it doesn't last long, could very well put enough energy onto the
12v line to permanently damage electronics that are not protected well
enough. The old ignition points would put nice fat 300v DC spikes on
the 12v line that lasted for several milliseconds, if the condenser
failed. I'm sure an anchor winch and possibly a blower, fan or pump
motor could do the same when it is switched off. Mobile electronics
really need good protection from dirty power supply. Even the lowly old
car radio generally has an RC circuit on the 12v input. Fortunately
most mobile equipment is designed with pretty good protection, but it is
something to keep in mind, especially if you bring a non-mobile piece of
electronics on board, and wire it to 12v. It may not have been designed
ruggedly enough for all real-life mobile power conditions.
Dick
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