Front End Failure for 12V 3 Phase - TNY278 based regulator

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3 Phase 3 wire, 12V Regulator TNY278 based.

I have a failure in front end section. I have used Surge Protect resistors and MOVS to protect the surge when Motor is turned On/Off. Most offen R6, R7, and R8 resistors are burning. I think this is due to surge voltage generated by Motor when it is turning On.

Can any one suggest me how to proetct the front end circuit?

3Phase 12V.png56.77 KB

Do you know these resistors were burnt during start up or sfter some time? I fthey were damaged after some time, would you like to try to reduce the resistor value by half?





They are burning all three after some time. If they run for more than 2 days, there are no fails. All failed resistor are happend within 2 days of deployment. Power up time no issues. It looks like power line may have swicthing surge, and causing these resistors failed.

Resistor are not specialized for surge protections, they are general purpouse wirewoon resisitors. 10Ohm and 2.5 Watts.

But the current drawn by the system for 3 phase supply much less than 25mAmp.

Can you please suggest what could be the issue.


Since your 25mA normal input power is very low, I really agree with you that it is caused by input surge. So you either use bigger resistor to handle the power loss or use a lower value resistor to reduce the power loss.