How to protect an op-amp against high inrush current following a high input voltage pulse?

Question

I have a circuit where I'm amplifying the output of a piezo transducer with an op-amp. By taking some readings, i've found that the piezo can output fairly high voltages when struck hard (~20V), and so I want to protect the op-amp against high currents.

To that end I thought about including a resister in series with the op-amp's non-inverting input, but I'm unsure on a few things. Namely, my questions are:

1. What value to use for resistor (R1 below), and how to calculate it?
2. How much current will be flowing through the op-amp due to the high voltage spike?
3. What's the maximum current the op-amp can handle on the non-inverting input?

Below I've drawn up a simple schematic to represent my circuit - I've used a 1Ω resitor to represent the load, but in actuality the output signal will be fed into an op-amp comparator and then into a microcontroller subsequently:

^{simulate this circuit – Schematic created using CircuitLab}

Thanks for your help!

Answer 1

What value to use for resistor (R1 below), and how to calculate it?

Depends on input bias current, an opamp with very low bias current can have a R1 very high without affecting the measurement, several Mohms. Higher resistor also means higher noise.

How much current will be flowing through the op-amp due to the high voltage spike?

if the opamp has input protection diodes, then I=V/R1. If not, then current is almost zero and the whole voltage is applied to opamp input.

What's the maximum current the op-amp can handle on the non-inverting input?

It's in datasheet. Read absolute maximum ratings.

Use an opamp with ESD protection diodes; low bias current- usually a JFET or CMOS opamp comes into a play, then calculate a minimal resistance.

^{simulate this circuit – Schematic created using CircuitLab}

EDIT:

You won't find a max current for LM358, because it has no protection diode at the inputs.

I have added two diodes, which they should be low leakage diodes, for example BAS116, BAV199, BAS21, BAS45A (covered from light source). The output load resistor can't be 1 ohm, it is too low. Added feedack resistors to set the gain, else it will work as comparator. Added R2, C1 to act as low pass filter (with R1), this will prevent a voltage spike while diode is not conducting due to switching delay.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

If it generates more than 30V , consider this simple solution.

Op Amps make slow comparators without gain reduction. So be careful in your specs/choices. You probably want a comparator with a pull up R for open collector/drain types.