piclist 2000\03\22\213506a >
Thread: 14V - 12V
face BY : Mark Willis email (remove spam text)

Paul B. Webster VK2BZC wrote:
> Mark Willis wrote:
> > A second fuse is redundant, really, IMO (so far.)  Maybe you can
> > convince me, though?  <G>
>   I thought the battery was there as a glorified Zener?  If the fuse or
> Polyswitch opens because it happens to be subjected to a ridiculous
> over-voltage (60V or 100V) and you have the regulator connected to the
> source side of the fuse, does not the regulator and possibly your
> circuit-to-be-protected begin to develop smoke enuresis ever-so-shortly
> afterward?
> --
>   Cheers,
>         Paul B.

"smoke enuresis"?  Wazzat?  =)

Aah.  I omitted my reasoning (or something) there I guess?!

I size that PolyFuse at the maximum capacity allowed for the battery -
say 7A, for a 4500mAh battery.

Most likely cause of it's opening when set up for a 1A digital camera,
let's look:

We size the series resistor for 1 ohm as that drops one amp / one volt,
so it's a say 2W resistor.  Make it 5W, for luck <G>

We use a 5 Amp or so diode, probably.  And thus size the fuse in the
auto power connector at 5A (can install a polyswitch there, maybe!)  If
we use one.  If not and we get a spike, what happens, though?

We start off with the small battery basically charged fully, and the
connections are Auto power - 5A fuse - wires with inductance and
resistance - blocking diode - far terminal of the battery polyswitch, a
7A unit (RGE700) - wires with inductance and resistance - regulator with
bypass capacitor.

Say we get a 100V spike on the car power bus;  the battery charges like
this, assuming that we have a half ohm of wire & joint resistance (in
practice, the auto power connector tends to be a little higher
resistance unless you buy a higher quality one <G>:

E = IR so (100V-14V-0.7V) = I (1.5) so I = about 56.866 A, for how long,
though?  The Polyswitch will take 100A maximum, and trips at somewhere
between 7A and 11.9A, *sustained* - it has a Rmin of 0.006 ohms.  If
tripped, it'll still let 7A through, max.  It has to melt, remember, to
trip - that phase change, takes TIME, and some energy.

Under those conditions, the Polyswitch will open in about 1/2 second
(give or take);  If you have 100V on a car power bus, for over 1/100
second or so, something is **WRONG**!  <G>  (Yep, we see quick tiny
spikes all the time, if those were Battery Killers, what's happening to
your car battery right now, as it's sitting there without any fuse?  <G>

Also, within less than a quarter second at 56.866A (or so), I'd expect
the auto power connector, the series diode, and the wires from the auto
connector to the small battery terminal, so do the "smoke enuresis"
thing <G>

Read http://circuitprotection.raychem.com/pdf/p17_28.pdf and
http://circuitprotection.raychem.com/pdf/p129_139.pdf, for good info <G>


I re-ship for small US & overseas businesses, world-wide.
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See also: www.piclist.com/techref/power/batterys.htm?key=12v
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