I’ve been doing a lot of prototyping and research for this project, and have solved lots of my questions - however I’m still having trouble with a couple of them - the main one at the moment being:
It was trying to understand why people were using zener diodes in their piezo projects, I got the answer that it was about ‘over-voltage’ but no information on why some people use it and others don’t, how to calculate the values etc and how it actually works.
I’ve since found some great information about why people might use it:
Pins configured as inputs with either INPUT or INPUT_PULLUP can be damaged or destroyed if they are connected to voltages below ground (negative voltages) or above the positive power rail (5V or 3V).
So that’s a great start, the I found some information that the Arduino boards have internal pin diodes that already protect them somewhat like this.
So now I’m guessing that why I’ve seen people use them with multiplexer setups, but often not when connecting piezos directly into the arduino, is because the multiplexer has no internal protection. Is that correct?
I’ve seen some people using schottky diodes and some using Zener diodes, from what I’ve read, I can see why a Zener is used for this exact purpose.
Voltage clamps: Limiting signals with Zener diodes
A varying analog signal can be constrained to a fairly narrow range of voltages with a single Zener diode. If you have a voltage that swings between + 7 V and – 7 V, you could use a single 4 V Zener, connected to ground, to ensure that the signal does not exceed 4 V or go below -0.7 V (where the diode conducts forward to ground).
If you wanted to constrain the signal to never go negative– e.g., for input to an analog-to-digital converter that accepts signals in the 0 – 5 V range, you could connect the anode of the Zener diode to a power rail at 1 V, instead of ground. Then, the output signal range would be constrained to the range of 0.3 V – 5 V.
Basics: Introduction to Zener Diodes | Evil Mad Scientist Laboratories
So that sounds good. Since there’s only a 4.7 V Zener and a 5.1V Zener, I’m guessing that the 4.7 is better to use? However I don’t understand how you would connect the diode to a power rail at 1V to constrain the values to never go negative.
Additionally there’s this:
Zener diodes are manufactured with a wide range of voltages and can be used to give different voltage references on each half cycle, the same as above. Zener diodes are available with zener breakdown voltages, VZ ranging from 2.4 to 33 volts, with a typical tolerance of 1 or 5%. Note that once conducting in the reverse breakdown region, full current will flow through the zener diode so a suitable current limiting resistor, R1 must be chosen.
This is suggesting needing a current limiting resistor in series. The piezo circuits I’m building already feature a pull down resistor in parallel, do I then need both resistors in the circuit or will the parallel resistor achieve the same function?
I’ve seen some people just using 5.1 V Zener Diodes in parallel with a 1 mOhm parallel resistor as well, but I don’t understand why that would work in terms of the question above.
Would love some help if possible!