Please help identify a SOT23 diode that goes across the rectified mains?

[Flyback]

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…….”KJB” ( or it might actually be “KJ8”)…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

 

[dknguyen]

If the circuit really is as you described, wouldn't the negative AC half cycle fry the load on it's way through the diode? And then explode since it would be a short-circuit.

 

[Flyback]

Thanks, but the regulator is after the diode bridge...its pulsating DC.

 

[rjenkinsgb]

Re-reading the question, are you sure the diode is between input & output of the regulator and not in series?

I could understand a reverse diode on a higher output voltage reg where the input supply may be shorted by another load, but it makes no sense in a circuit as you describe?

 

[Flyback]

thanks, well the input to the reg does go to zero every 10ms.
(its the rectified mains haversines)
The diode has 339v peak across it every 10ms.

 

[Pommie]

Can you provide a picture or a schematic?

Mike.

 

[Flyback]

Thanks i will provide.
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.

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

 

[Pommie]

Can anyone explain its purpose?

Mike.

 

[Flyback]

its in case the vout is higher than the vin of the regulator (its an LR8 regulator)......a diode must be used in case the reg is damaged by such an event

 

[Pommie]

How will the diode ever conduct if it's reverse biased across the DC supply?

Mike.

 

[JonSea]

Say the input voltage to the voltage regulator is lost, and there's a big cap on the output. The normally reverse-biased diode conducts from output to input, so the voltage on the output is never more than 0.7V above the input voltage.

This is a good trick for a circuit that may get regulated voltage from multiple sources, like a board powered either from USB or a non-regulated wall wart through a regulator.

Most 3 terminal regulators don't like their output voltage greater than input.