which capacity of zener diode i should use for...

Discussion in 'General Electronics Chat' started by Kishore P, Aug 21, 2016.

  1. Kishore P

    Thread Starter New Member

    Jul 23, 2015
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    Hi i'm trying to repair Cisco EA4500 wifi device which was not powering up, I found that an SMD DIODE with the code Z16 is short. I understood it is being used as overvoltage protection circuit and it is 16 volt zener diode. My problem is i'm not sure which capacity of watt of diode should i use because i replaced with 15 volt - 1 watt (1N4744) diode which should work but it is overheating and short circuiting the board. can someone please suggest the right capacity of diode that i should use.
    for info:
    1). Adapter rating is 12 volts 1 Amps.
    2). Device works well without the diode. ( still I would like to have that protection diode).
    3). I'm not finding SMD diode Z16 in market.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,281
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    If your description is correct and it is for over voltage protection it should not do any work (heat up) unless there is an over voltage condition. So probably something else is also wrong or it's function is not what you think it is.
    Any chance of a schematic?
     
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,778
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    If the replacement diode is overheating (applied as described) at least one of the following is true regardless of power handling capability:


    1) The replacement diode's 'Zener EMF' of is too low...
    2) The replacement diode is installed incorrectly (such that it is under forward bias)...
    3) A persistent 'over voltage' condition is present (across the diode)...
    4) The replacement diode is defective....

    Please note that, applied as an OVP device, the power handling need only be sufficient to 'pull' the over current interrupter in the event of an OV condition -- Diode conduction should remain insignificant under 'normal' conditions...

    Best regards
    HP
     
  4. hp1729

    Well-Known Member

    Nov 23, 2015
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    Hmmm. Z16 crosses to a 3.9 V Zener. BZX84-C3V9. 300 mW, 5% SOT-23 case???
    Suitable for a 3.3 V supply or such.
    Search on "smd cross" + "z16" and see what you find.
     
    Last edited: Aug 22, 2016
  5. Kishore P

    Thread Starter New Member

    Jul 23, 2015
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    Hi thanks for response, I checked the circuit once again and there is around 13.20 V constant volts, hence no over voltage found., also I mentioned the device works well without that diode, today i powered it up for half an hour did not find any other problem.

    the diode is mounted in reversed bias so i assuming its for protection, here is diagram that gives you idea how the diode is placed.
    download.png My Board.jpg
     
  6. Kishore P

    Thread Starter New Member

    Jul 23, 2015
    5
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    Hi thanks for response, I checked the circuit once again and there is around 13.20 V constant volts, hence no over voltage found., also I mentioned the device works well without that diode, today i powered it up for half an hour did not find any other problem.

    the diode is mounted in reversed bias so i assuming its for protection, here is diagram that gives you idea how the diode is placed.
     
  7. Kishore P

    Thread Starter New Member

    Jul 23, 2015
    5
    0

    Hi thanks for response, i need to check on 1st point u mention... there is symbol on pcb showing how should we install the diode so i can not make mistake and i tried replacing new diode.
     
    Last edited: Aug 22, 2016
  8. Kishore P

    Thread Starter New Member

    Jul 23, 2015
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    0
    I googled it before and I found it is acually 16v zener, however i could not find any other rating related watt or current on it. since the adaptor is of 12v,1A first i tried with 15v, 1 Watt zener normally available in market, Not SOT-23.
     
  9. hp1729

    Well-Known Member

    Nov 23, 2015
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    Can anybody come up with a better cross for a Z16? Mine doesn't make sense as a 3.9 V zener.
     
  10. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,778
    1,215
    Two points:

    1) Re: Zener diodes: Please be advised that reverse breakdown begins slightly below V{r}

    2) Inasmuch as (unregulated) power adapters (a.k.a. 'wall warts') offer negligible 'load regulation', accurate determination of output excursion (which being 'modulated' by ripple and load variations) requires observation via TDO.

    Note: Should a TDO be unavailable, one may, of course, 'kludge' a peak reading DC probe via a 'diode fed capacitor' arrangement...

    Bottom line: Based upon the provided information, you require a Zener specified at a greater V{r} figure (specifically ≈ 2 volts greater than the peak OC PSU output excursion) -- Should you wish to 'cut to the chase' sans further examination, I recommend a Zener Specified at a V{r} of ≈17V:)

    Best regards and good luck!
    HP:cool:
     
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