Electronic – What are bidirectional Zener diodes used for in DC transient protection


I often see two zeners (in opposition to each other) being placed across the power supply rails. I don't understand why that is necessary.

For example, if I have a 16V Zener diode and a 12V power rail, the 16V Zener will protect the supply from transients above 16V. It will also prevent negative transients below -0.7V – say, if i reversed the battery.

Why would placing two zeners in opposition improve this. Say if I placed two 15V Zeners in the same setup. It will now protect the supply from transients above ~16V (which is fine). but it now will only protect the supply from negative transients below -16V instead of -0.7V. Why would anyone want this? I don't want the supply rail to go to the negative at any stage!

For example: see example circuits in http://cds.linear.com/docs/Datasheet/4359f.pdf

Best Answer

Automotive Design standards are very strict.

Electronics must survive a car being jump started in reverse and boosted with 24V. (Yes people still do that in dead of winter) It must also handle load dump with a battery under load being disconnected.

If your design shunts the battery at -0.7V it will blow like a fuse then you have no protection.

So be warned automotive tests are much stricter. This particular design of zener is to protect the regulator from failure within its abs. max input. of -40V ~+80V, which might seem enough but these zeners will not blow in the automotive +24/12V test yet protect the device from additional spikes above that.

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