I'm trying to understand the purpose of these two Zener diodes in a single package (DZ2S180C) on an audio mixer circuit. I suppose it's to protect the circuit from voltage spikes possibly coming from phantom power. Could anyone explain this in details? One of them broke along with the fuse and I had a distorted wave output. I replaced the components and it's all good now, but I'm curious to understand the design.
Best Answer
As you suspected, it's for over-voltage protection.
A Zener diode has a (good as) normal diode action when forward biased and a predictable breakdown voltage when reverse biased.
So connecting two Zener diodes back-to-back in series means that one is always ready to be forward biased and the other to have its reverse breakdown.
This means it can clamp AC voltages that it's connected across. If the voltage goes above the sum of the two voltages (a forward drop plus a reverse breakdown voltage) in either direction then the diodes conduct. With your part, that's spec'd at 17.5..20 V in total.
At that point, the diodes will try to conduct as much current as the voltage source can supply, limited a little by their own resistance. That can drop quote a lot of power in the diodes so the source current has to be limited, either by the source's design or with something like a series resistor before the clamp diodes.
In your circuit, one diode pair are probably expected to blow the fuse in the HOT side. The COLD side doesn't have a fuse.