ID of this 3-pin SMD semi?

[EHT]

In process of debugging this PSU I'm wondering what this IC designated A4 in the attached pic is.

It is a small SMD with 3-pins and a tab. It is labelled "4A" above the line to the tab and "7J3" below the line. I'm not sure what this case style is called - maybe SOT89 - about 4mm wide.

The tab is connected to the centre pin, and in the circuit this is connected to GND. The label suggests it is an IC (D, ZD, Q etc for diodes and transistors). I thought it might be a voltage reg but it doesn't have stable DC on either pin (could be a fault). I've been able to identify most other ICs and larger semiconductors in this unit and they're not that unusual. It dates back to about 2009.

[gamalot]

TL431ACPK

https://www.ti.com/store/ti/en/p/product/?p=TL431ACPK

[EHT]

Excellent, thank you very much! It makes sense given where it is too.
 
I wonder also if its possible to identify small SMD diodes and transistors that just have a 2 character marking, maybe there's a reference that helps, or is it just experience!!??

[gamalot]

Identifying an SMD is often easy, but sometimes you may need the right resources, experience, and a little luck. I usually use google and those SMD code look up sites (eg https://smd.yooneed.one), and I also use taobao.com to search because I speak Chinese.

Just upload pictures of devices you can't identify, there's always someone willing to help.

[abdulbadii]

prefix is got on
https://www.electronics-notes.com/articles/electronic_components/transistor/transistor-codes-numbering.php

[fzabkar]

https://www.alldatasheet.com/view_marking.jsp?Searchword=4A