SMPS Question: Resistor across the primary winding of the chopper transformer

Thread Starter

John Czerwinski

Joined Jun 19, 2017
62
I'm repairing this SMPS and had a question of the function of the R104 resistor (10K Ohm at 7W) across the primary winding of the transformer. I'm guessing it's for the flywheel circuit to control the current, but not 100%.

Reason I ask is, besides my edification, is this resistor is a bit out of spec (~11K instead of 10K) and wonder if this could be why the power supply is trying to start up (not a dead, but looking like it's in protection mode).

Thank you for any insight or references in advance!


Question SMPS - Resistor across primary winding of chopper transformer - k7500 Power Supply Ci...png
 

Dodgydave

Joined Jun 22, 2012
11,239
Possibly, check the two diodes D107/ D108, is the chip supply on pin 7 correct for starting up, also it might be in overcurrent mode due to loading on the output side, check the output for shorts , diodes D117/118 and D106ab.
 
Last edited:

Thread Starter

John Czerwinski

Joined Jun 19, 2017
62
Possibly, check the two diodes D107/ D108, is the chip supply on pin 8 correct for starting up, also it might be in overcurrent mode due to loading on the output side, check the output for shorts , diodes D117/118 and D106ab.
Thank you for the response. I did verify that pin 7 is the Vcc on the chip. I'll give those diodes a look and see if they're causing the issue. I do have the 10K 7W resistor on order, so I'll give that a try when it comes in
 

Ian0

Joined Aug 7, 2020
9,502
R104 is there to discharge C106. C106 clamps the flybaack voltage, or, more specifically, the overshoot of the flyback voltage due to the leakage inductance of the transformer.
 
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