# UC3825A based smps resistor burst

#### aksnis

Joined Oct 6, 2020
11
Hello
I am trying to repair a smps power supply based on UC3825A
Problem is the resistor R49 & R34 get burst as soon as i start the smps
I am not able to find out the issue
SYS_ECON is a 0-5v given from MCU to control 0-180v supply of smps

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#### Ian0

Joined Aug 7, 2020
4,119
I'd have to guess at a problem with R35

#### aksnis

Joined Oct 6, 2020
11
R35 is good

#### Dodgydave

Joined Jun 22, 2012
10,139
What is the Voltage on TP 3 and TP 16 ?

I would suggest using 1/2 W resistors.

#### Ian0

Joined Aug 7, 2020
4,119
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?

#### aksnis

Joined Oct 6, 2020
11
No sir nothing is connected to tp16

#### aksnis

Joined Oct 6, 2020
11
This is the hv side

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#### jtn

Joined Mar 27, 2017
22
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

#### Jettman

Joined Apr 5, 2019
3
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.