UC3825A based smps resistor burst


Joined Aug 7, 2020
Whatever the mystery value of HVDC is, R35 should be dissipating a thousand time what R34 dissipates. Is there something connected to TP16 that shouldn't be?


Joined Mar 27, 2017
The HVDC voltage could be very high voltage. Those resistors may not be able to handle the voltage rating. Find out the voltage at HVC and search for suitable resistor type to withstand that voltage. Or adding more resistors in series. Good luck


Joined Apr 5, 2019
Since you state the power supply has a 0-180V output, +HVDC maximum could be considered 180VDC. The voltage ratings of the capacitors across the high voltage output is 200V so 180V would be an acceptable safe maximum. The DC current through R34, R35 and R49 would be 180/(100+100k+2.67K)=1.75mA. The power dissipation across R35 would be \( 0.00175^2*100000=0.307W \). R35 is only rated for 0.125 watts. I would say R35 is shorting when it overheats. Replace R35 with a 1/2W resistor as mentioned above. R52 would have the same problem.