"Charlie Morgan" wrote in message
...
On Thu, 12 Apr 2007 09:59:39 -0400, Wayne.B
wrote:

On Thu, 12 Apr 2007 05:34:42 -0400, "Roger Long"
wrote:

Isn't there something that could be inserted in the feed to the
instruments that would restrict voltage spikes?

Yes, it's called a zener diode and they come in different voltage
ratings.


You may have trouble finding a zener diode rated at enough amps,
though. You might do better by wiring in an LM7812 voltage regulator
at each instrument you want to protect. They are rated at 1 amp output
if you mount them on a heat sink.

The 7812 will lose a significant amount of voltage across it. The 7812
drops about 2V if I recall correctly, so if your battery is putting out
12.5V, for example, the regulator output will only be around 10.5V. If the
battery is above 14V, the output would be held at 12V (the drop may only be
1.5V, but the principle is the same).

There are other regulators with much lower dropout voltages, but you will
still lose something, which will cause your electronics to flake out sooner
as the battery discharges.

There are high-power clamping devices called MOVs (Metal Oxide Varistors)
and SADs (Silicon Avalanche Diodes), that could be used, but the problem
with any clamp (Zener, MOV, SAD) is that the current levels you get in a
starter-motor inductive kick will be quite high, so you need to introduce
some series impedance -- a large choke, for example. This will limit the
current the clamp has to handle. It gets complicated pretty quickly, if you
are going for bulletproof. MOVs also have pretty gradual clmaping
thresholds, so they probably will only be good for clamping 18V+ levels
anyway. SADs have tighter specs, but the other problems remain.

I recommend a separate battery for the electronics, if you can figure out
how to charge it. Better still is having a house battery and a starting
battery.

-Paul