Re: [EE] PIC Input protection. by Matt Pobursky .

On Wed, 21 Mar 2007 20:57:20 -0700, John Dammeyer wrote: > Hi all, > > Quick question here. > > http://www.pacificsun.ca/~john/ELS/photos/IO_Circuit.jpg > > The link to the above circuit shows how I plan on changing the Electronic > Lead Screw circuit to protect the processor inputs from transients. I > can see someone accidentally putting 12V onto an input pin with switch > that should close to ground instead closing to 12V. > > The circuit using R45 and R43 limit the current through the diode but a > spike might still trash the processor. > > The circuit using R44 and R46 forces the power supply to absorb the input > and perhaps can trash the diode with excess current but R46 limits the > current into the processor. > > Suggestions or alternatives? I always hate *any* circuit that possibly exposes a PIC pin to the outside world or has any chance of causing current flow through the pin protection diodes. By adding a transistor buffer to your circuit and reworking it a bit you can make a nearly bulletproof input circuit to the outside world. You also will never see a voltage higher than VCC at the PIC input. The following circuit is a suggestion. CR2 is optional and it protects against inputs below ground. http://www.mps-design.com/misc-images/buffer1.gif The following circuit is a possible improvement on the first, I think, as it uses one less component (which may or may not be a concern). The positive input voltage limit will be about 60V (limited by the reverse breakdown of the diode - 75V). Input voltages below ground are OK, provided R1 is sized so you don't exceed the maximum base current of Q1. http://www.mps-design.com/misc-images/buffer2.gif I've used both these circuits (or variations of them) many many times in industrial and automative products that operated in some pretty nasty environments with totally reliable operation and nary a blown PIC input. Matt Pobursky Maximum Performance Systems