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Dear StackExchange users,

I want to add redundancy to a flyback diode system that is protecting an N channel MOSFET from flyback of an inductive load but I don't know how to accomplish this!

I am using a Zener Diode + normal diode. I think when the Zener is going bad, then it will conduct the current without any resistance, So adding a parallel diodes is not going to work.

What shall I do?

Thank you

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arudino.tyro
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  • You have redundancy already. Normally the relay clamps at Vz+0.7. If the zener fails short circuit it still clamps but at 0.7v. If the fail mode of the zener is open circuit then put two zeners in parallel. If D2 fails short circuit put two in series. If D2 fails open circuit have independent zener/diode circuits. – Andy aka Jun 01 '15 at 14:03
  • What's the point of the redundancy? The most likely parts to fail are the transistor and the relay. – Spehro Pefhany Jun 01 '15 at 14:19
  • @Andyaka: Thank you very much. How can I find out if the failures of diodes are open or short? Should I deliberately try to fail them to find out? Or is there a better way to find out? – arudino.tyro Jun 01 '15 at 14:19
  • @SpehroPefhany: Yes. That's true. My testing platform is extremely expensive and any failure in MOSFET can cause huge damage. – arudino.tyro Jun 01 '15 at 14:21
  • In that case I suggest two independent drivers and relays with the contacts in series. And a detection circuit to tell you when one has failed 'on' (assuming that's the state to be avoided). – Spehro Pefhany Jun 01 '15 at 14:25
  • @SpehroPefhany could you tell me more about it? I don't much about electronics. Especially about the detection circuit!! I'd be thankful if you could give a through explanation. I was able to test the platform successfully with your help (from my other questions). Thanks. – arudino.tyro Jun 01 '15 at 14:35
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    http://electronics.stackexchange.com/questions/77712/can-a-diode-fail-to-open-position – Andy aka Jun 01 '15 at 14:40
  • @Andyaka so I think I should count for both short and open failures as it is not predictable which mode of failuer will happen? Is it true? – arudino.tyro Jun 01 '15 at 16:13
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    Here is another thread worth reading: http://electronics.stackexchange.com/questions/157032/are-normally-open-contactors-virtually-safe-from-failing-closed – user57037 Jun 01 '15 at 16:59
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    I think you should consider using beefier components as a means to improve reliability. You should also review the stresses on all components that are relevant to the functioning of the whole circuit especially power supply diodes as an example. On the limited circuit you provided this is the only advice anyone could give. I think it's called MIL-HDBK-417F. This is an old document by todays standards but I still use it for determining reliability. It's online too – Andy aka Jun 01 '15 at 17:13
  • Thank you, Vcc is 12V, I have solenoid valves (0.015H, 20ohm,12V,7Watt), 5KP70A Zener (70V,8watt) and 1N5408 diode. The only part that I am worrying about is 5KP70A. I don't know if it can endure 0.6A current or not !!! – arudino.tyro Jun 01 '15 at 17:20
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    MIL-HDBK-217F: http://www.sre.org/pubs/Mil-Hdbk-217F.pdf. Why are you using a 70V zener diode for flyback protection? – Andy aka Jun 01 '15 at 17:25
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    Because turn off speed of the valves are crucial for me. Higher zener Voltage = Higher turn off speed. I am worried about its amperage rating. Oh by the way, I am using an IRFP260N MOSFET – arudino.tyro Jun 01 '15 at 17:48
  • https://books.google.com/books?id=rnKmmCqLTW4C&pg=PA158&lpg=PA158&dq=failure+modes+of+zener+diodes&source=bl&ots=xff9f4bkYS&sig=OfHASqTStIv5CkKGW36npEMBEIE&hl=en&sa=X&ei=bP6cVd_lGoO_ygPM9Iu4Cg&ved=0CCkQ6AEwAg#v=onepage&q=failure%20modes%20of%20zener%20diodes&f=false – arudino.tyro Jul 08 '15 at 10:52
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    5KP70A isn't a zener -- it's a TVS. These are not very precise, and while it seems the specs can handle the instantaneous power, you need to determine if it can handle the energy, and the average power (how frequently do you activate the solenoid ?) – jp314 Dec 29 '15 at 04:40

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