Protecting PIC I/O Lines

Jul 12, 2005 3 Replies
G

Hi,



I was just wondering what the best way to protect a PIC I/O in an automotive application. I want to be able to detect a positive input ranging from



5-40V. The PIC is running at 5V. I have thought of a zener regulator on the input but at over such a wide voltage range means large parts to dissipate the power which I am trying to avoid. Is it safe to trust the clamp diodes in the PIC or will this lead to damage over the long term? I have been told to avoid this method.

Any feedback appreciated,


Geoff


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A

Microchip have an Application Note for a lamp dimmer using a 12C508. It features a zero crossing detector with the mains (120V) connected to an I/O pin via nothing more than a 20M resistor (well, OK, there is also a varistor across the mains, and a bit of a disclaimer about transients). If they can do that I'm sure you don't need anything elaborate to handle +40V.

Andy Wood snipped-for-privacy@trap.ozemail.com.au

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T

All depends on what is is connected to and what speed you want from the pins. A simple resistor + zener may do the trick. If you a real worried, a TVS is good, but beware of the capacitance if its a high speed application.

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C

There's some good advice in this:

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