SparkFun Forums

[Azimon]

I have the attached circuit as a input protection to a device that runs off a truck battery. After being on a truck for some time, fuse F1 had blown. On investigation, it is found that zener D1 is shorting. How can this be? I mean, how/why would a zener fail this way? I thought that the entire purpose of a zener was that if the voltage increased above it's rated voltage it would conduct, but then when the voltage goes down it would again stop conducting.

[jremington]

The maximum power dissipation of the Zener diode was probably exceeded.

[Azimon]

The maximum power dissipation of the Zener diode was probably exceeded.

Hmmm.....

I assume that what happened, was that first the zener failed and then the fuse blew (well, obviously...). "Maximum power dissipation", do you mean that the current was large? Would that mean the voltage was great?

Also, I was thinking of using a resettable fuse and a TVS instead of the zeners. Would that be better?

Actually, I admit, though I think the circuit is OK, I don't know how to rate the components properly. Of course, the circuit might also be better....

Thanks for your continued suggestions

[jremington]

The circuit is not OK. There should be a resistor to limit the current through the 32V Zener to a safe value, which is the function of the 10 ohm resistor before the 24V Zener. Evidently in your circuit there are or have been large voltage spikes (substantially exceeding the 32V Zener voltage) and the Zener shorted. You have to know the ratings of the Zener diode and the magnitude of the voltage spikes to work out what that resistor should be.

Such spikes are unusual in a 12V vehicle circuit, so something is probably wrong. It might be worthwhile to try to locate the source of the problem. A faulty or intermittent regulator on the alternator is one possibility, but there are many other possibilities, including a nearby lightning strike!

Cheers, Jim

[waltr]

An automotive electrical system is a harsh environment for electronics. Just a wide temperature range can cause issues with parts like Zener diodes. Also the voltage can spike and rapidly fluctuate from ignition and load dumps.

Protection in an automotive system is difficult. Here are some starting links:
http://www.motor.com/newsletters/201010 ... ction.html
http://electronicsbus.com/resettable-ov ... tomotives/
http://electronics.stackexchange.com/qu ... -load-dump
http://www.eetimes.com/design/automotiv ... uppressors

From these develop some additional Google Search terms for more info.

[Azimon]

Thank you I will have a look.
What is your view on:
- TVS diodes (also vs. zeners)
- Resettable fuses

[waltr]

TVS's have a much faster response than Zeners and that is good for ESD events and other fast transits.
This keeps the peak voltage lower.

[jremington]

Are you certain that the circuit you posted is correct? It doesn't do much in the best of circumstances (just clips some of the more serious voltage spikes) and in the worst case (i.e. where the device you are powering draws more than about 100 milliamperes), the 10 ohm resistor will drop a significant portion of the the battery voltage.

You might look into commercial automotive electrical noise/surge suppressors.

[colinb]

Another thing to look at is the Iz vs. Vz curve on your Zener diode data sheet. Some Zener diodes have a lesser slope that means significant reverse current flows even when the reverse voltage is a little less than the Zener voltage Vz.