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ID a zenor diode

KevinW

Member
I have a zenor from a 3vdc wall adapter, it has a 7 on one side and a c2 on the other.
Can someone help me identify this, thanks.
 

ci139

Active Member
Assuming it's a surface-mount device
how exactly does this help anything ↑↑ you do realize polluting the information space by such
has a 7 on one side and a c2 on the other
if you seek web for glass zener markings you might be lucky to find it

. . . just a moment . . .
i have 3.7V (@2mA) glass zener marked :
B
3
·
9
assumes 3.9V nominal @ unspec. reverse current
? yours 2.7 assuming from 3V adapter (which the d/s exctract also hints but NOT confirms for your case)
 

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Pommie

Well-Known Member
Most Helpful Member
If you can work out a schematic we should be able to suggest a replacment.

Mike.
 

ci139

Active Member
the zener has(/ is biased to (by other components on the circuit)) it's operating range and in normal conditions won't fail
so if it has not failed by input transients OR wrong polarity to I/O OR long term high temperature -- that is the causes that would get the circuit to operate outside it's design limits -- then it might have failed due some other failed components -- in which case replacing it would not have expected effect
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Assuming is IS a zener, and the circuit is similar to the second image, then it's only a crowbar - and would almost certainly have failed because of a high ESR electrolytic.
It looks like zenor and marked on the board as Z5 along with a diode symbol.
It's got a diode symbol, but NOT a zener symbol, and it looks like a diode/zener - they don't really look any different.

Why don't you draw the circuit out?, that should make it obvious what it is, and also suggest why it failed. It's possible it's a crowbar zener?, in which case it most probably did it's job and went S/C due to a high ESR electrolytic.
 

KevinW

Member
It looks similar to this schematic upload_2017-7-4_8-26-58.png


which uses the power transistor and opti- coupler.
Universal US Plug DC 3V 1A Power Supply Adaptor Adapter 100-240 AC Charger
 

KevinW

Member
I have a 10 volt 220uF cap in that location, it was really hot to the touch when I first started looking at this so I pulled it and it was fine, turned out to be the z diode that was hot and it was heat sinked to the cap transferring the heat to the capacitor.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I have a 10 volt 220uF cap in that location, it was really hot to the touch when I first started looking at this so I pulled it and it was fine, turned out to be the z diode that was hot and it was heat sinked to the cap transferring the heat to the capacitor.
How do you know it's fine?, have you checked it with an ESR meter? - that's the ONLY way to test it. If you checked in in any other way, then you've not checked it at all - failure mode is high-ESR, which can only be checked with an ESR meter (or a scope, if it's in a working circuit).

It's a component which commonly fails in these PSU's (because they use a cheap lo-spec capacitor), and failure of that capacitor will blow the crowbar zener.
 

ci139

Active Member
It looks similar to this
but there might be differences
i improved your resolution -- sorry!
adapter2x.png -- the pin-1 dot for opto is top-right so this is the Low side and the BJT at Hi (there was adoubt)
  • if you reverse polarity connected the battery to the output then that might have blown the zener and in that case your transformer is ok
  • if your switching transistor has failed switching then the TF might be done -- and it is (a lot) less trouble to buy a new supply
  • also possible that the sw'g-bjt failed first with some more components from line input side . . . but the TF might have survived -- then again testing it all out requires some specific knowledge/experience and maybe specialized el. lab. equipment
    -- e.g. it takes time and you have to proceed systematically - possibly logging the relevant data on the go
 
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