Zener Diode Protection
Figure 10-3 shows an input protected with a zener diode. A zener diode acts like a regular diode with one exception. If the reverse voltage reaches a certain point, the diode will start to conduct. In combination with R to limit the current and drop the excess voltage, the voltage seen by the input will never exceed the zener voltage, Vz. This of course assumes the input signal does not exceed the ratings of the zener diode or resistor.
Select a zener diode with a Vz that is higher than the Vcc powering the microcontroller, but less than the voltage that will cause damage. For 5V systems, 5.1V zeners such as the 1N5231 and the 1N4733 are commonly used.
Transient Suppressor Protection
Special electronic components specifically designed for ESD protection are available. The go by the name of TVS, MOV, and a large number of trade names. They are easy to use, and have many advantages over the zener protection including faster response times and higher energy capacity. Some are also available as arrays, handling a number of signal lines in a single package. Figure 10-4 shows a microcontroller input protected by one of these devices. The schematic symbol may vary depending on the technology of the device.
In normal operation these look like a capacitor. If the applied voltage reaches the Varistor Voltage, the varistor acts like a switch shorting the signal to ground. Most types are bidirectional, meaning they will switch on if the varistor voltage is reached regardless of polarity.
The first data sheet parameter to look for is the Maximum Allowable Voltage. This must be higher than the normal operating signal voltages. Otherwise, the normal signal will trip it, possibly preventing the signal from reaching the threshold voltage. Another key specification is the capacitance. You will want to use low capacitance suppressors for high frequency signals. Otherwise, the signal will get filtered out as in the RC filter described earlier. Another specification is the maximum surge current. The higher this is, the more abuse the circuit can take, but often at a higher cost, size and capacitance.