I want to protect a micro-controller board from voltage spikes generated piezos. The board can only handle 3.3 volts, would just a 3.3v zener diode in parallel work? I'd like to keep it as simple as possible and I read that a resistor in series has an effect on the signal, Also, If there are more than 1 triggered at the same time even though they're on different pins does that add to the over all voltage in the board?

 

Schematic of your plan?

 

A series resistor from the piezo to the ADCpin, with a 3.3V Zener from the pin to Gnd. Cathode (stripe) to the pin.

 

Put the zener on the piezo side of the resistor.

 

Put the zener on the piezo side of the resistor.

That would most probably work ok too in this case as the piezo is high impeadance.
I would do it the other way though.

 

That would most probably work ok too in this case as the piezo is high impeadance.
I would do it the other way though.

The other way round, on the negative excursion of the piezo signal, The forward conducting zener would be directly parallel with the PIC protection diode and it would be a toss up which way the current goes. Having the resistor between the zener and the chip will keep the current through the diode in the chip to an absolute minimum.

 

AVR MCUs have internal protection diodes
Check datasheet
if your mcu doesnt have some, you may add yourself
Here is from ATmega328 datasheet :

 

AVR MCUs have internal protection diodes
Check datasheet
if your mcu doesnt have some, you may add yourself
Here is from ATmega328 datasheet :
View attachment 144126

Right, but it's quite easy to overload them. They're tiny and can't dissipate much power before failing. It's better to add external protection in situations where you expect to encounter out of range voltages.

 

what about a varistor?

 

I want to protect a micro-controller board from voltage spikes generated piezos. The board can only handle 3.3 volts, would just a 3.3v zener diode in parallel work? I'd like to keep it as simple as possible and I read that a resistor in series has an effect on the signal, Also, If there are more than 1 triggered at the same time even though they're on different pins does that add to the over all voltage in the board?

The piezo has capacitance, so a sensible precaution is to diode clamp that input to Vdd so switching off cant collapse Vdd faster than that input.

The input probably already has Vdd and vss clamp diodes, they can probably only handle about 10mA so external ones are worthwhile. A hefty piezo could bump Vdd up, so you need either Vdd voltage protection or an input clamp.