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Please help identify this SOT23 diode that is fitted across the rectified mains

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treez

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Hi,
We have an offline LED lighting product from an old company. It had been running on the mains for two years.
It has a high voltage linear regulator just downstream of a mains diode bridge (there is no smoothing capacitor after the diode bridge) . The regulator output is 2.5V. It just supplies a microcontroller. Connected from output to input of this regulator, is a SOT23 diode. This diode obviously stands off the rectified mains haversines (339Vpk).
An ex member of staff assures us that this diode is a BAS16……but BAS16 is only rated to 100V. The diode itself is still functional, and is pinned out like a BAS16. But surely such a low voltage diode wouldn’t have been able to survive going across the mains?

The diode has markings on it…….”CJB” ( or it might actually be “CJ8”)…and then perpendicular to this, it is marked “CN”.
Does anyone know what actual diode this might be?

BAS16 datasheet
https://www.vishay.com/docs/85539/bas16.pdf
 

betwixt

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If it's a BAS16 it's a very lucky one. They are coded 'A6' anyway.
The obvious component to place there is a voltage clamp of some kind but one in a SOT23 package doesn't make sense. Does it only have two connections to it?

Brian.
 
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treez

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Sorry, Ive a massive apology to make....it is marked "KJ8" (or "KJB") and not "CJ8"
...I was in the dept till late last night and had to leave before id got a good look.
Im pretty sure no SOT23 diode in the world is rated for mains voltage?

The obvious component to place there is a voltage clamp of some kind but one in a SOT23 package doesn't make sense. Does it only have two connections to it?
Thanks Yes it does only have two tracks going into it.

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i dont know if i explained myself well....
I am sure you know of the connection of a diode from output back to input of a linear regulator....thats what i mean.
The linear regulator is a LR8 type

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The board was manufactured in early 2016, i wonder if BAS16's were marked A6 back then....i suspect they still were.
In which case i suspect the "CN" stands for "China", and this is possibly some kind of high voltage Chinese diode?

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The MMBD3004 is the highest voltage SOT23 diode on digikey, at 350V.
It however, is not marked "KJ..."

MMBD3004
https://www.taiwansemi.com/products/datasheet/MMBD3004 SERIES_B15.pdf

- - - Updated - - -

Aaah...solved it, its a 400v rated MMBD5004S, and it was made in Nov 2015
 

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The manufacturer advises a diode is connected across the LR8 whenever the output voltage may be higher than the input voltage, essentially for reverse polarity protection. A lower voltage diode would be suitable as long as the input - regulated output voltage was less than it's PIV but what I don't understand is what is ahead of the regulator. You imply there is a mains AC bridge feeding he regulator with no reservoir capacitor. That doesn't sound right somehow.

Brian.
 
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treez

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Thanks, yes i admit its a strange setup!
I'd never seen anything like this before.
I must admit that same thing looked bad to me...but this product ran for two years on the mains.
 

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