triac half burn

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Ario87

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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.
 

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Are you sure it's not when the SCR is energized as opposed to just being plugged in?

Otherwise it could be the surge current when the TRIAC is turned on at the peak of the AC waveform, which is random in occurrence.
Try testing by generating a turn-on signal at the peak of the AC voltage, and see if that causes a failure.

You may need to add a current surge suppressor if that's the case.
 

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I think you in some ways give it away when you tell that the fault happens when it is plugged in....and then you also say its the one with the capacitive , or RC load , that blows it.......it looks like it could be inrush into this capacitor when it gets started up at mains peak....what is the size of the said capacitor?
....aaah...i see Crutschow was typing his own kind answer as i typed mine...
 

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1) You say the load can be RC ? if the C part is too large - you can damage the triac from current rising too fast at turn on - and being too great in magnitude overall - you need a bigger triac - also faster di/dt at turn on demands higher gate current (20mA - see data sheet ).

1a) The I^2t for this device is only 0.78, so for an 10mS half cycle of mains this is 8.8A - it seems likely you are exceeding this in combination with 1) above - triggering near mains peak would give very high di/dt in a capacitive load - as already mentioned by a contributor above.

2) Also you cannot have " half-burned (triac is short-circuited in one direction) " if a triac is shorted in one direction - it is, by definition, shorted in both directions - MT1 to MT2.

3) check the drive to the BC817, it seems odd the base is pulled to -5V and the link from the emitter to its driving ckt is not clear.

4) the 100nF cap on the base does not seem entirely helpful - 4n7 max - just for RFI ?
 
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We have applied 4KV surge on angles 0, 90, 180 and 270 of input AC waveform in both positive and negative polarities but TRIAC burning didn't happen. Also there's VARISTOR and EMI filter at the input of main board with the attached schematic.
 

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We have tested different conditions at lab but no TRIAC burning happened. The load is a wax motor that pushes out a shaft when connected to AC supply and warmed up. The equivalent circuit is a parallel RC with 2.2nF capacitor and 2.2K resistor.
 

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If what you have stated is indeed the case - then it would appear the wax valve is going faulty and frying the triac

or the wiring loom is chafing causing same effect.
 

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