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Variable laptop power supply

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Mr_Max

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Hi everyone

Kindly I need your help, I have a Compaq PA-1500-051 18.5v 4.9A that I need to use as a variable power supply, I can't find schematic for this
Particular model, but a close one Liteon PA-1500-05 19v 4.47A, I found that a 28.7K and 4.4K resistors form a voltage divider connected to Pin 2 of the (unknown 8 pin IC it might be TSM103) I tried changing the 28.7K resistor with 20K pot in series with a 10K resistor
strangely while I set the 20K Pot combined with 10K to the value of 28.7K the output goes up and down from as low as .75v to 3.7v no matter how I change
the value the output never change accordingly !
I soldered the 28.7K resistor back the output is 18.53v & drops down with no load, and then when I put a 21w 12v car bulb the output voltage is not stable
goes up and down from 11v to 13v

liteon_pa-1900_05_out_power_schematic.pdf_1.png Liteon PA-1500-05.jpg 20170829_190046.jpg
 
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