# What is the function of the zener diodes used in this setup?

I have been studying some electronics schematics to get background information on a power generator we use in my work. In one of these schematics I ran into the schematic I drew below. As seen here the Zener diodes are in opposite direction of each other.

simulate this circuit – Schematic created using CircuitLab

My first impression was that I was looking at a (1)Transient-voltage-suppression diode that would catch voltage spikes.
(1) https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

But in electronics this is indicated with the below symbol:

simulate this circuit

As far as my knowledge goes on electronics (I am not an experienced electrician) zener diodes (or diodes in general) only conducts current in one direction. So I am curious to know what the function is of the diodes in the schematic from my work. Cause in my opinion they could be removed since they do not add any value to the circuit. Since if there is a current spike, it wont be caught.

None of my college knew the function of these and I haven’t been able to find any information or example on google or this site. I am hoping someone can explain it to me.

So my question is: What is the function of the zener diodes used in this setup?

• What is the purpose of the circuit? Jan 27, 2017 at 1:09
• By convention, schematics are usually drawn with inputs on the left and output(s) on the right. Yours is backwards. Jan 27, 2017 at 2:13
• @tcrosley I know, but I coppy pasted this schematic out of a much larger one, hence it is missing any sense of direction :).
– Nick
Jan 27, 2017 at 7:38

zener diodes (or diodes in general) only conducts current in one direction

That is only partly true, Zener diodes will also conduct when the reverse voltage exceeds a certain value. Here that value is 2.4 V (they are 2.4 V zeners). Add to that the forward voltage of the other Zener (about 0.7 V) and the two Zeners in series will start to conduct when the voltage across both exceeds 3.1 V. When that voltage is negative (-3.1 V) the same will happen.

These diodes limit the input voltage difference to the opamp. Too much voltage difference will destroy the opamp.

– Nick
Jan 26, 2017 at 16:28
• All diodes will conduct when the reverse voltage exceeds a certain value :D Jan 26, 2017 at 19:55
• @NickT That is true but that value is more controlled and guaranteed in Zener diodes making ordinary (non-Zener) diodes pretty useless as zener diode. You could buy a 1N4001 which is rated for at least 50 V, it can be that the manufacturer made a very good batch of 1N4004 which can handle at least 400 V. Since 400 V is more than 50 V so these 1N4004 can also be sold as 1N4001, the rest of the specs. are identical ('m looking at the datasheet from DIODES). i.e. you never know what you will get. Jan 26, 2017 at 21:03
• when both diodes are reversed, so they are facing cathode to cathode, would that make any difference? Jan 27, 2017 at 8:47

zener diodes ... only conducts current in one direction.

On the contrary, the defining behaviour of Zener diodes is that they conduct in both directions, albeit with different forward voltages. That is, forward biased, they typically behave like a regular silicon diode, and have a forward voltage of ~0.7 V. However, when reverse biased, they have a well defined breakdown voltage (2.4 V in your schematic above), and a reverse bias above their breakdown voltage causes current to flow.

Therefore, if the voltage across the D3/D4 pair exceeds approximately ±3.1 V (2.4 V + 0.7 V), they will "clamp" and prevent the voltage from increasing further. So yes, this acts as a transient voltage suppression technique.