# Choosing series resistors for zener diodes

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?

• Do some ballpark calculations of the current through the zenerdiodes when Vin = +12 V and Vin = -12 V with 1) no resistor 2) a resistor before the zeners 3) a resistor after the zeners. – Bimpelrekkie Feb 29 '16 at 11:27
• Please any rule of thumb? – user16307 Feb 29 '16 at 11:43
• I can't read that schematic. Everything is too small, and the text and some of the lines are dropping out. You have done this before. Draw your schematics to fit to 620 pixels wide, which is what SE shrink larger images to. Make the symbols and text large enough to read. Pack the parts closer to fit within the image better. For example, the long line coming out of the opamp adds nothing, but makes the image take more space. – Olin Lathrop Feb 29 '16 at 12:19
• Click and open in a new tab and when you zoom it is very clear – user16307 Feb 29 '16 at 13:07

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.

• Good answer, but note that the voltage he has is 12V peak, not RMS. Possibly an error. – Spehro Pefhany Feb 29 '16 at 13:18
• yes 12V is peak amplitude – user16307 Feb 29 '16 at 13:57
• What is 150mA and 5mA? How do you know the currents to calculate the resistors? – user16307 Feb 29 '16 at 14:01
• @user16307 A zener diode has a zener voltage Vz which is specified at a certain current Izt by the manufacturer in the datasheet. In the case of the 1N5344B Vz is 8.2 at 150mA. If you pass less than 150mA through the zener then the voltage across it will be less than 8.2V, you need to look at the datasheet to find the approximate voltage (it is not well specified). – Steve G Feb 29 '16 at 14:24
• @user16307 if you say "AC 12V volt signal" then you imply an RMS voltage, not a peak voltage. I hadn't noticed that your schematic had a 12V peak generator. – Steve G Feb 29 '16 at 14:28