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Thread: 14V - 12V
www.piclist.com/techref/power/batterys.htm?key=12v
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face BY : Mark Willis email (remove spam text)



Hmmm.  IIRC, 78xx series regulators have a 3V or so Dropout voltage;
You cannot use one to regulate 14V to 12V.  (Also do want to watch the
upper voltage limit!)

If you put a carefully picked power resistor from car power, to a small
local (motorcycle or gel cell) battery, then regulate down off that with
an LM2941 or other fairly Low Dropout regulator, most any battery
problems become sort of irrelevant (Can anyone tell I have quite a few
gel cells here?  <G>)  A diode in series with that is a GOOD idea,
unless you cart the whole deal off when not using it (You don't want to
drain the small cell when starting the car.)

The small battery will charge on high car power system voltages, acting
like one heck of a power Zener diode, pick the series resistor to have a
1 volt drop or so at the desired load current into the camera.  (BGMicro
sells 4500mAh Gel Cells for $6.95, might have 17000mAh cells for the
same price still.)  Learn about Gel Cell charging first, of course, if
you use one <G>

For safety:  fuse the battery, close to the + terminal as you can manage
that - Under the 1/4" terminal if you can manage it.  Duct tape the lot
for safety, afterwards.  (Digikey sells polyswitches, made by RayChem,
86 cents or so, pp 536-537 in their current catalog.  I'm migrating to
those from regular fuses.)  If you expect to recharge that battery
harder than the fuse will like, parallel a power diode across the
polyswitch, it'll at least charge slowly then while the polyswitch is
open.  Maybe use a Schottky, due to lower voltage drop <G>)

If you want to get "fancy", a switcher is probably a "better" answer, so
long as you somehow handle the possibility of power supply disconnection
- that's why I tend to use a battery.  I've done some field research
where we drove around with 8 or so devices, all sucking power off the
12V accessory power socket.  We found the power connector'd fall part
way out quite regularly, thus the battery - AND, the electrical tape we
used to get it down to just falling out every few days, not daily <G>

Or, just use a "spare" small deep-cycle battery, recharge it as needed
etc.  So many "good", working answers...

 Mark

Larry G. Nelson Sr. wrote:
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