BT138 triac fail

[cosminnci]

Hello,

I have the circuit attached, with an small 220v motor, it draws 300mA. after 2 months and about 500 cycles of 40 sec one of the BT138 started to fail ( the channel started to go on like a flicker, when I put a light bulb on it flashes from slow to faster).
control is done from a microcontroller.
I read it might be a snubber problem, but this one works for 2 months or so..

and the bad part is that when it fails it stays on and the motor runs forever and the microcontroller doesn`t sense that and can`t stop it either.

regards,
Cosmin

[bdivi]

Looks like overvoltage from inductive load turn-off that gradually degraded the triac and then it finally failed.

Put a RC snubber of 470R/22n across the triac and you should be fine - just make sure the resistor and capacitor ratings are suitable for 230V AC.
More info here: http://www.st.com/web/en/resource/technical/document/application_note/CD00004096.pdf

Cheers

[cosminnci]

I have a kill switch on the line too. Which is the worse case, being turned off by the switch or through the optocoupler?
the loads I plan to use are variable should that  change the value of the snubber?
could I use a snubberless triac instead or would it have the same problem ?

[bktemp]

A snubberless triac does not solve all problems: The optocoupler is only rated for 400V, therefore you should use either a snubber or a varistor or a TVS diode to prevent it from retriggering due to the voltage spike.

[oldway]

With inductive load, snubberless triac is a good improvement because it has an higher dV/dt rating but you have to limit dV/dt on the optocoupler too.
Use a RC snubber anyway.

[Zero999]

It could be overvoltage.

Add a MOV across the TRIAC or a bidirectional TVS diode between MT2 and the gate.
https://www.fairchildsemi.com/datasheets/MO/MOC3052M.pdf
http://www.onsemi.com/pub_link/Collateral/AND8012-D.PDF

[cosminnci]

Thank you for your replies,
I plan on replacing the triacs with BTA12 snubberless and add a MOV
should the mov be across the triac or across the load?

SR PASSIVES VAR20-250
Varistor: oxid de metal; THT; 250VAC; 320VDC; 390V; ±10%; 6,5kA; 1W
should this one be ok ?

[Zero999]

It doesn't matter if the snubber or MOV is across the load or TRIAC but more often than not it's placed across the TRIAC for convenience.

Refer to the links in my previous post for more information.