zener
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?
A regulator would be the best solution, but a zener (one 'n') is ok, at least if you don't want to draw too much power from the regulated voltage. You don't place the zener directly on the -12V, but use a series resistor to limit the current. It's this series resistor which dictates how much current you can draw. The -12V will still be available.
To calculate the resistor value, you have to know how much current your load will draw. Suppose this is 1 mA. Also suppose the zener diode needs 10 mA. That's 11 mA through the resistor. Voltage drop is 12V - 5V = 7V. Then R = 7V / 11 mA = 640\$\Omega\$.
edit
You may think that in my example I'm exaggerating a bit to have 10mA for a zener if the circuit would require only 1/10th of that. But you'll find that zeners are often specified at much higher currents, like 50mA. Certain newer zeners are specified at much lower currents, these ones only need 50\$\mu\$A.
YAE (Yet Another Edit)
The reason why you don't want to use the zener in series to get the voltage drop is that especially at low currents the reverse voltage may be much lower than the rated value. This diode for instance is specified at 5.1V @ 50mA, but only drops 1V at 10\$\mu\$A.
Those are not Zeners. They are Schottky diodes.
Shottkies have a lower voltage drop than regular Si junction diodes, about 0.3V instead of 0.7V. They are also generally faster.
Their purpose is to keep the inputs within the supply rails.
Best 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.