Hi,
I would use a serial connected Schottky.
Regards,
Imre


On Mon, 8 Nov 1999, Wagner Lipnharski wrote:

> Ok, use a reversed diode to the PIC with a low value resistor in series
> to the cells. It means, that if reversing cells polarity it will be
> drained by the resistor+diode. In real you would be short circuiting the
> cells, but not in a dangerous current (limited by the resistor).
> Calculate the resistor to drop only 50mV on normal operation, but when
> cells are inverted it will dissipate heat, leaving only the diode
> voltage drop to the PIC.
>
> For example, if the normal operation current is around 10mA, the
> resistor should be 50mV/10mA = 5 Ohms.  If cells are assembled reversed,
> the diode will conduct directly over the resistor, so it will be
> 3V-0.6V(diode) / 5 Ohms = 480mA, enough to keep the reversed voltage
> over the PIC at 0.6V without exploding the cells.
>
> For better protection, you could use a Schottky diode or even an "old
> germanium diode" that drops only 0.2Vdc.
> If your circuit drain pulses of high current, bigger than 10mA, you can
> add an electrolytic capacitor in parallel to the 5 Ohms diode, as a
> bypass to those high surge periods.
>
> For sure, if you can afford to reduce 0.2Vdc from the PIC, install only
> the germanium diode in series with the cells... :)
>
> Wagner.
>
>
>
>
> Jason Langenauer wrote:
> >
> > On Mon, 08 Nov 1999, you wrote:
> > > Hello List,
> > >
> > > I have realized an alarm for my bycicle using a 12C508. It is powered by t
wo AA
> > > size alkaline battery.
> > >
> > > I would like to know if there is a simple (and economic!!) solution which
> > > I can use to protect my circuit against polarity inversion during battery
change.
> > > (I know it's a remote possibility, but it exists)
> > >
> > > I thought to use a serie diode, but the voltage drop bring the PIC power v
oltage
> > > near the power supply limit of the MCU.
> > >
> > I can't think of it right now, but there is an arrangement using a single
> > MOSFET which will provide this. Possibly the gate is connected to the drain,
> > possibly it is an N-channel MOSFET, and possibly the circuit is in Horowitz
and
> > Hill. It's a very simple circuit, but as I said, I can't think of it now (su
ch
> > is the effect of a few beers..). Play around with this, and see what you can
> > come up with. I'll try again in the morning.
> >
> > Jason Langenauer
> > Mechanical and Space Enginnering, The University of Queensland
> > --
> > Lady Astor: If you were my husband, I'd poison you drink.
> > Winston Churchill : If I was your husband, I'd drink it.
>
>