OT: I/O protection

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

Hi all,
I have to feed some output lines out of a product. What I would really like is a little IC which will take care of protecting the I/O lines of my PIC. I don't want to go to the expense of opto-isolation as power is also leaving the main board and the GNDs are linked so it's not really needed. What do you guys use in this situation? A sacrificial IC to protect the pic is what is needed?
Keep well,

Will

Chas · Guest

The design of the protection depends on what you are driving. As a generic output you could add 2 diodes, one between the output and VDD and one between the output and VSS. This will protect the output from being back driven above or below ground. Then add a series resistor between the output of the PIC and the real world. This could be something on the order of 100 to 1k Ohms or more. If you are driving some sort of load, like a relay, then a driver would be required. This could be a single transistor configured as a pull up or pull down, or an IC driver like the ULN2003. TI has some app notes on driving inductive loads. Microchip may also.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

You might also use Transient Voltage Suppressors (TVS, transorbs) instead of diodes. This will give you some ESD protection. 1N5908 is an example of TVS.

RHA · Joined: 25 Apr 2006 Posts: 31 Location: Germany

What´s with ULN2003 or ULN2803 ?

The only thing is that you switch GND then.

greets
RHA

Charlie U · Posted: Wed Apr 26, 2006 9:48 pm

There are a couple of nifty features of devices like the ULN2003. First is that you drive it with a standard CMOS level like that of a PIC's output. The second is that you can drive a load that is powered from a voltage up to 50V and requires up to 0.5A!! Not only that, but the load can be inductive, like a relay coil, and the inductive kickback is absorbed by an integral diode (if you connect the +V pin of the ULN2003 to the supply). One ULN2003 is the equivalent of at least 21 discrete parts.