I'm planning to clamp an AC 12V volt signal with two 8.2V zeners (1N5344B).
After the zener couple the signal will be fed to an opAmp as in the figure below.
Do I need a resistor (Rz in the figure) before or after these zeners? What value would be suitable?
Answer
Yes you do need Rz to limit the current through the zeners. you have it in the correct place.
First find what voltage you need to drop across Rz. 12VAC has a peak voltage of 17V. You will drop 8.2V across the reverse biased zener and 0.7V across the forward biased zener. So you will need to drop a peak of 8.1V across Rz.
What current do you need through the Zeners? Unfortunately you have chosen a 5W zener. Its zener voltage of 8.2V is given at a current of 150mA which seems to be excessively high for this application. If you chose a zener current of 5mA then you can easily calculate value of Rz (8.1/5mA gives 1620 Ohm). Use either a 1k5 or 1k8 resistor.
With a current of 5mA the 1N5344B has a typical zener voltage of 8V (from the datasheet), so the 2 zeners together will give clip the AC voltage at about 8.7V. I would use a much smaller zener such as a BZX83C.
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