Why is my replacement triac not working? Hakko fx 888d

[mluna]

I have a Hakko FX-888D soldering station which stopped heating the iron. I found out that the triac (which is a BTA06-600b) went bad so I went ahead and replaced it with a BTA12-600bw. It didnt work. The triac was off all the time even though it was getting its triggering pulses (I know because I saw it in the oscilloscope and compared it with another working soldering station of the same model and brand).

The replacement triac was ok, it was knew and I also tested it outside the circuit.

I know the problem was just that because later I soldered another triac of the same the station came with and it worked.

If you check the datasheets of the two triacs you can see they are really close in specs, so, why didnt it work?

[fzabkar]

AFAICT, the BW version is snubberless and has a higher dV/dt rating, but is otherwise the same.

https://www.st.com/resource/en/datasheet/bta12.pdf

[David Hess]

The BTA12-600bw is less sensitive than the BTA06-600b under some conditions.  (1) Supporting a higher dV/dt for snubberless operation is done by using a lower internal shunt resistance between the gate and anode which lowers sensitivity.  So maybe the gate driver circuit is not quite powerful enough.

This would also mean that the gate driver was marginal for the original TRIAC which might possibly explain failure of the original TRIAC.  Marginal gate drive increases turn-on time and causes localized heating in the gate structure which can lead to poor reliability.

(1) 70 versus 50 milliamps under certain conditions.

[mluna]

Thank you for the answers.

I didn't really dig too much into the driving circuit; all I saw was the triac being driven by a transistor.

Makes sense about the sensitivity. I'll take a look more in deep and keep this updated.

Can't believe they designed that so tightly it can't work with such a close replacement.