On Monday 31 March 2003 12:46 am, Jinx wrote:
> I'm using this circuit to protect the PIC's input against voltage
> spikes from relays and a motor. With a 1N5818 the input is
> successfully stopped from going too far below 0V wrt the PIC's
> internal diodes. Is the 1N5818 a bit OTT and could a smaller,
> cheaper diode like a BAT be used ?
>
> 1N5818, 1A, Vf 0.55V
> BAT81, 30mA, Vf 0.40V
> BAT46, 150mA, Vf 0.45V
> BAT43/BAT86, 200mA, Vf 0.33V

Depends on the voltages and currents you're likely to be seeing.

What I do is even simpler: I use a series-connected dual Schottky in a
SOT-23 case (like the Fairchild BAT54S
http://rocky.digikey.com/WebLib/Fairchild/Web Data/BAT54_A_C_S.pdf),
running the cathode to V+, anode to GND, and common to the input pin.
Then I use a resistor/capacitor to limit the current and fast pulses.

The BAT54 is a 200mA 30V part (though of course the voltage rating
doesn't matter here, as you're never going to see more than 5V
reverse). You could use a much larger resistor than a 4.7K. If you're
using this as an analog input, your external capacitor, if large
enough, can provide the current to charge the sampling cap.

--
Ned Konz
http://bike-nomad.com
GPG key ID: BEEA7EFE

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