On 12/17/2011 3:09 AM, Mike Harrison wrote:
> Schottkys get very leaky at higher temperatures, so probably not a=20
> good choice. Is the current really so high that you can't use a more=20
> chunky sensing resistor? If you really want to bypass the shunt, a=20
> MOSFET is probably a better bet - arranged such that it turns on=20
> quickly when the measured current exceeds a threshold, witha=20
> monostable to hold it on for a while, limiting the current duty cycle=20
> through the resistor to the monostable time / the turn-on time.=20
Just to be clear what we're typically talking about here...

The normal load is around 250mA.  Inrush currents can be extremely high,=20
in the range of amps, but not close to high enough to exceed the=20
resistor rating.

The issue comes where our customer is using a large battery bank with=20
virtually unlimited current available, and decides to dead short the=20
output.  I think we've actually got the firmware in the PIC doing the=20
overcurrent protection tweaked so failures just aren't likely - It's a=20
fine line between being slow enough that the inrush doesn't trip the=20
protection, and a short or long-term overcurrent will.   With some=20
creative integration (in the mathematical sense), things seem to be=20
working well - but I still hate not protecting something I know is close=20
to the edge.

At this point, I think I might look at using a mosfet on the input of=20
the whole device to limit current to something reasonable - dozens of=20
amps for instance - probably a more reasonable solution.

And yes, we're already using about as chunky of a sense resistor as is=20
practicable.

-forrest
--=20
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