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

Thread Starter

Kishore P

Joined Jul 23, 2015
5
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.
 

ronv

Joined Nov 12, 2008
3,770
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.
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?
 

Hypatia's Protege

Joined Mar 1, 2015
3,223
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.
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
 

hp1729

Joined Nov 23, 2015
2,304
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.
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:

Thread Starter

Kishore P

Joined Jul 23, 2015
5
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
 

Thread Starter

Kishore P

Joined Jul 23, 2015
5
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?



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.
 

Attachments

Thread Starter

Kishore P

Joined Jul 23, 2015
5
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

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:

Thread Starter

Kishore P

Joined Jul 23, 2015
5
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.
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.
 
I checked the circuit once again and there is around 13.20 V constant volts, hence no over voltage found.,
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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