Backup Zener

Thread Starter


Joined Feb 25, 2008
well, i really didnt know if this should go in this forum or the "general" forum, but since this is a project of mine i figured lets try here.

i am monitoring a inductive load via a 0.05ohm sensing resistor. to combat the flyback voltage i use a single 33v 5watt zener to ground (there is a reason for 33v, but beyond the scope of this post). without the zener the flyback voltage generated could destroy several components in my cicuitry. the inherent concern is zener failure.

to take care of this issue would it be better just to use two 33v 5watt zeners in parallel, or perhaps use a 33v and 40v so if the 33v dies i still get voltage protection from the 40v? any suggestions which is a better design?


Joined Jul 17, 2007
You might consider paralleling the Zener with a cap, the size of which is difficult to estimate without seeing the circuit. A small cap will tend to absorb the peak transients, giving the Zener time to conduct at a lower max voltage.

Try a starting value between 300pF and 3nF. Use a ceramic or tantalum cap.

The larger the cap, the greater the phase shift (delay) you'll see in the response across the sensing resistor.