[sdiy] Protecting microcontroller inputs from - voltage?

[harrybissell]

Hah... a trick question

Putting the diode right AT the chip will probably not help you, unless you
use shottky diodes.  A diode at the chip is a race... will the external diode
conduct before the internal diode ?

I'd even suggest using two resistors in series... with the diodes connected at
the
center of the resistors.. It would protect in both directions and not bother
either
the chip or the external input/output with too much current draw in a fault.

Chip protection should be in proportion to the chances that someone will connect

something unusual / stupid to it... almost a sure bet in a modular :^P

H^) harry

Ken Stone wrote:

> >>   Use a diode clamp.  That is, a diode from the I/O pin to ground, with
> >> the cathode on the I/O pin.  Check the microcontroller electrical spec,
> >> but most will tolerate -1V or so.  If even less drop is needed, use a
> >> Schottky diode.  In practice, a 1N4148 will work fine.  Caveat: put a
> >> small resistor in series with the I/O pin and whatever it is sensing.
> >
> >On which side of the resistor would you put the diode ?
>
> The diode goes directly to the IC pin. The resistor goes between the pin and
> whatever you are connecting to. The purpose of the resistor is to limit the
> current flow through the diode in the "protected" condition. If you put the
> resistor on the other side, you'll probably blow the diode up when the input
> goes negative, or at very least, you will be sending whatever is doing the
> driver to it's current limit.
> _______________________________________________________________________
> Ken Stone   sasami at hotkey.net.au
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