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

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

Two ziner diodes opposing each other, is working as a voltage/current regulator. ziner dioxde A will regulators current and Voltage that goes through B. The same goes for B to A. The Aim of this to gives a regulated voltage/current to the Capacitor to Avoid break.