triac half burn

[Ario87]

Hi,
In a dishwasher machine main board we have used 107NN0 TRIAC for controlling ac loads such as valves, pumps, dispenser, etc. In about 2 to 3 percent of products sold to customers, one of TRIACs that has a parallel RC equivalent load is half-burned (TRIAC is short-circuited in one direction) when the machine gets plugged in. Here's the schematic of a single TRIAC on mainboard. All 8 TRIACs have the same configuration. One TRIAC has RC load and others have inductive load. Surprising point is that we couldn't reproduce the fault in lab under different test conditions such as Surge or EFT noise, input voltage connection in different angles of sine wave. We're glad if anyone can help us about this problem.

[KlausKragelund]

There’s a 100nf cap from BC817 emitter to Null… Perhaps the 100nF cap is there to slow the turn-on down?

But, any noise, surge, burst or notches on Null will have an easy path through the 100nF to emitter of BC817, and possibly also gate of triac. You would need to have a varistor or eq across phase/null

Also 1k for gate is a quite high value. Normally that is 10 times lower to avoid spurious turn-on.
The drive is very slow, could case turn on losses or failures

[temperance]

Modern household machines are plagued with bad electronics and mechanical engineering problems. The best solution when buying a new machine is to request all schematic and go to the store with some tools and take the machine apart in order to find out where it will fail. If they allow, also take a an EFT generator with you to torture the machine a little bit such that constructions like the one shown will fail at once.

[Poroit]

G'day Ario87,

What item is the RC load on this triac?

Have you contacted WeEN for technical assistance?

[bdunham7]

What exactly is the RC/RL load in this setup and how much current goes through the device in normal usage? 

If the failure occurs when plugged in but the triac is not turned on, perhaps you are exceeding the dV/dt(off) spec.  If it occurs when the triac turns on as power is applied, then perhaps dI/dt is the issue.  Either way, the fix may be an inductor, perhaps with some resistance, in series with the load.  Being more generous with the RC snubber might help too. 

As for why it happens in the field but not in your lab, have you tried actually plugging them in manually?  Does the device draw any power as it is plugged in?  If so, there might be a bit of arcing that makes things worse.  Also, once a device has been plugged in without a failure, does that device then work properly indefinitely even when moved or unplugged/plugged?  That might mean that some of the devices are a bit more fragile than others, which would be expected I suppose.