How to protect triac ? - Page 1
Forum section: electronics / projects

author: k2teknik
author_rank: Regular Contributor
timestamp: May 24, 2012, 10:29:20 am
content: I have some problems with triacs, a very small part of them just die (short circuit, half-wave short circuit and open circuit). It is less than 2%.The workload is very small, 230 volt AC motor from 15 to 30 watts, and it is only on-off, no "speed regulation".I started with to use BTA08-600 snubberless, and is now ended up with to use BTA12-600 snubberless, but I still have a few who simply die.There is a varistor above A1-A2 and a transil between A1-G as protection.I would like to be able to reproduce the killing of these triacs, but I cant do that, not even with a motor that is 100% blocked for running.

author: T4P
author_rank: Super Contributor
timestamp: May 24, 2012, 10:43:00 am
content: I would figure ... i do not work on triacs so take my words with a pinch of saltinductance spikes killing the triac ?

author: Psi
author_rank: Super Contributor
timestamp: May 24, 2012, 11:03:39 am
content: When they do fail, do any of your protection devices die as well?

author: k2teknik
author_rank: Regular Contributor
timestamp: May 24, 2012, 01:53:19 pm
content: Quote from: DaveXRQ on May 24, 2012, 10:43:00 amMy varistor and transil should take care of that part?And if it is spikes, why do it not kill more of my triacs?Has any one been able to find a MTBF value for triacs?I had been working with triacs for more than 25 years, and my experience is that they just die now and then, and that is more often than other components, without any plausibel/convincing reason.

author: k2teknik
author_rank: Regular Contributor
timestamp: May 24, 2012, 01:56:10 pm
content: Quote from: Psi on May 24, 2012, 11:03:39 amNo

author: SeanB
author_rank: Super Contributor
timestamp: May 24, 2012, 04:10:52 pm
content: Add the snubber. Snubberless devices are as vulnerable as normal devices to spikes that are just too short for the transient supression to react to. Snubberless devices are really best with resistive loads only.

author: k2teknik
author_rank: Regular Contributor
timestamp: May 24, 2012, 04:36:13 pm
content: Quote from: SeanB on May 24, 2012, 04:10:52 pmVery interesting, do you have some further information/sources on this?I am using snubberless triacs because I will like to save the cost (and the space) of the snubber

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 24, 2012, 05:32:07 pm
content: In addition to adding the snubber you might look into switching only at zero-crossing.

author: k2teknik
author_rank: Regular Contributor
timestamp: May 24, 2012, 07:03:41 pm
content: Quote from: TerminalJack505 on May 24, 2012, 05:32:07 pmI do so allready, I was not clear with that, but that was the point with*only on-off, no "speed regulation"*

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 24, 2012, 07:39:54 pm
content: Quote from: k2teknik on May 24, 2012, 07:03:41 pmI see. I saw your note about not using the TRIAC for phase control but didn't want to infer that meant you were switching at 0V.That's good since you would be introducing unnecessary stress otherwise.

author: SeanB
author_rank: Super Contributor
timestamp: May 24, 2012, 08:32:40 pm
content: Snubberless just means that they take a really fast edge to turn on from transients, not that they never turn on from transients. A resistive load will never generate a large enough transient, and will not have any method where it can conduct enough current to blow the device.An inductive load can generate these, and if the spike occurs at the peak of the mains waveform, the initial current can be big enough to damage the device over time. This will probably only affect the devices that were between the "typical" and "minimum" end of the spec range, most being nearer the mid part, only those that just passed test failing over time.Thus you probably want a snubber at least, as well as the overvoltage protection, which will provide protection against spikes as well as against fast transients turning the device on.