We are using BTA41-600BRG triacs on our temperature control boards to turn on 240V tank heaters. The tank heaters are 1500W, 3000W, or 4500W depending on the unit model and operate at 240VAC. The circuit we are using is attached. We use a FOD4108 driver to activate the triac.
We have seen at least 2 cases recently where the triac continues to conduct even after the trigger for the driver goes off. In both cases, when we move the PCB to a different unit with the reduced load of 1500W heater, the triac works normally and turns off when we turn off the trigger. In another simulation with the failed triac on the larger size heater, if we turn off the unit while near the setpoint temperature, then turn the unit back on, the PID is working and we pulse the heaters; and in this case it is working okay also (triac shuts off when trigger shuts off). But if we turn on the unit when the temperature is far from the setpoint and the heater is on 100%, it never turns off when its gets close and the trigger pulses or turns off completely once we pass the setpoint.
The triacs that fail are brand new and the problem is observed before we send it to our customer.
In the first case we replaced the triac and the problem went away. In the second case we have not made any change yet.
Do you have any suggestions as to why this might be happening? We did not make any changes on the seconds PCB with this problem yet because we would like to troubleshoot to figure out what is going on.
