Triac drive

[hfleming]

Hi all,
 Have recently attempted to repair a PCB used in an oven. I noticed that they drive the triacs switching the lamps, fans directly from the microcontroller, without any isolation. Of course, the one triac failed, direct short circuit between the gate and the other 2 terminals, thereby taking out the micro, and everything else on that port of the micro. Is it normal nowadays to drive a triac directly? I have always learnt to drive a triac via an optocoupler or pulse transformer to avoid exactly this very situation. I guess it is a cost-cutting excercise, but is it normal to do drive triacs directly from a micro nowadays without isolation of the gate?

[wraper]

Isolation generally is used not to prevent control circuit failure but to isolate control circuit from mains voltage. Also optotriacs fail more often than triacs themselves, so their use only decreases reliability.  If isolation is not needed, it's better to drive triacs directly.

[hfleming]

…leading to a very expensive repair.

[wraper]

…leading to a very expensive repair.

As far as manufacturer cares, it's cheaper and more reliable overall. Also if manufacturer does component level repair to begin with, MCU firmware is not an issue. If it does not, it doesn't care how many components fail at once as board will be thrown out anyway.

[hfleming]

I can see that approach in all sorts of that domestic devices. Replace the whole PCB, if it is still available, but at half the price of a whole new device… the throw away and replace mentality…