Paul B. Webster VK2BZC wrote: > Mark Willis wrote: > > A second fuse is redundant, really, IMO (so far.) Maybe you can > > convince me, though? > > 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 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 : 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**! (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? 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 Read http://circuitprotection.raychem.com/pdf/p17_28.pdf and http://circuitprotection.raychem.com/pdf/p129_139.pdf, for good info Mark -- I re-ship for small US & overseas businesses, world-wide. (For private individuals at cost; ask.)