15 Replies Latest reply on Dec 26, 2019 4:16 AM by three-phase

    Why are the Triac Snubber Resistors Failing in this circuit.

    jw0752

      I have recently been repairing power supply / control circuit boards for a small machine that cuts tape to a specified length. Here is a picture of the circuit board in question along with my notes so far:

       

       

      Here is the schematic that I have reverse engineered from the circuit board:

       

       

      I have been working my way through a box of circuit boards, that were replaced in the past as black boxes, and repairing them. I have noticed that on almost every board the 150 Ohm 1/8 Watt snubber resistors are open or showing meg ohms of resistance. I have done the math for the circuit and with our local 120 volt 60 Hz line voltage the .1 uF cap in series with the 150 Ohm resistor should have 4.5 mA of current and a corresponding 3 mW of power delivered to the resistor. Since the resistor appears to be an eighth Watt there should be plenty of room for it to handle this voltage, current and wattage.

       

      Unfortunately I am finding almost 100% of these 150 Ohm snubber resistors failed. There are more of the same 150 Ohm 1/8 Watt resistors used on this board in other parts of the circuit but the only ones that are failing are in the snubber circuit.

       

      I am hoping that someone with more experience than me can tell me what is going on in this circuit that is taking out the snubber resistors. The Triacs drive a Variable direction motor and a large solenoid so the loads are definitely inductive. The little resistors do not look heat damaged. They also do not look to be of high quality if that may be a contributing factor.

       

      Any insights you may have will be appreciated.

       

      John