Chinese TL431 super capacitor balancer

[wictor]

Hi,

I wanted to share my experience of Chinese TL431 super capacitor balance and protection board design and it's flaws. There are many of them available from various sellers.

This is photo of the board:


The designer wanted probably do something like this:


And circuit would work so, that it limits the capacitor voltage to 2.65 as seen in this plot:


However the board had some design flaws and it actually was more like following circuit:


Which was drawing current as soon as voltage was above 0.7V.


So, there was two problems. TL431 was connected incorrectly, the layout would work with TL432, since it has ref and cathode swapped. And another problem was that SOT23 with marking Y2 was actually 12V zener(I verified this by desoldering it and measuring the breakdown voltage) and not SS8550, which would work and has also Y2 marking...

Cheers,
Wictor

[exe]

Wow, that's nuts! So, basically, they used random sot-23 components and it didn't work at all...

[magic]

Haha, that's what happens when you buy your parts on those AliExpress auctions where SMD components are only described by their markings 

Speaking of TL431 fails, I bought an adjustable voltage wall wart once whose lowest setting is 3.3V.
The problem is, regulation is done by a TL431 sinking current through an optocoupler LED. The LED has a forward voltage of 1.2V and the TL431 barely sinks anything until its cathode reaches 2.5V.
This PSU will never, ever regulate less than 3.7V

[wictor]

There are many versions of similar design sold from China. I think they have not tested the this layout at all, since TL431 cannot work with swapped ref and cathode in this kind of purpose...

Heh, but anyway this design is not going to keep caps charged for too long