Arduino I/O Protection

When your project moves to a level where you start making a dedicated case for it to protect it from the environmental influences, what do you do to protect the exposed signal lines from the harsh world outside?

Absolute Maximum Ratings

The ATmega on the Arduino can handle only Voltage between the the supply rails, so in most cases 0-5V, sometimes 0-3.3V.

When Looking at the Datasheet from the ATmega328 used in the Arduino Uno and Arduino Duemilanove we see in the Absolute Maximum Ratings that on every Pin only -0.5V to Vcc+0.5V.

That means that a broken wire from the 9V Battery touching a Pin from the Arduino will fry your Project!

In the Datasheet they call it: "Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device." And: "Exposure to absolute maximum rating conditions for extended periods may affect device reliability."

So How to Protect the Arduino from this exposure?

Thankfully there are already a lot of articles on the internet giving ideas, the most reliable information i take from Application Note´s given out by big IC manufacturers like ST Microelectronic.

Let's start with a example, a Potentiometer is used by the User to adjust something:

Now we want to protect the Analog input on the Arduino from Voltage outside the 0-Vcc Range!

So we add 2 Diodes, Limiting the Signal between the Rails:

The Diode on top will start to conduct if the Signal goes above Vcc and the Diode on the Bottom as soon as the Signal goes below Ground.

But just like this there will be a shortcut resulting in a big current most probably destroying the Diode.

So lets add an Resistor in Line!

With a small Resistor (like 22R) in front of the Diodes the current will be limited and the Diodes will survive a Signal outside of the Supply Rails.

To absorb sudden transients also a small capacitor can be added:

100nF would be great, but for less signal distortion 100pF can be used, with 100pf and 22R only 2.2ns delay are added to the Signal. Wha should be ok for having a safe input!

Of course there is a lot more possible, and you need to have a clear understanding of what your worst case could be to prepare your circuit for it. This should give you some ideas, but is far from complete!

If you have additional information, please don’t hesitate and contact me!

References:

www.opencircuits.com Input Protection

www.industrologic.com Autoelectronics

Fairchild Application Note AN248 Electrostatic Discharge Prevention-Input Protection Circuits and Handling Guide for CMOS Devices

ST Microelectronics Application Note AN820 INPUT/OUTPUT PROTECTION FOR AUTOMOTIVE COMPUTER