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Old 05-28-2020, 03:19 AM   #1
xbdwk
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Default Modifying Laptop PSU 19.5V to 24.0V

I have a 19.5V 11.8A 230W laptop/touchscreen desktop PSU. It's for HP machines but is branded Bestec, and seems to be the same one as Dell PA-19.

I'd like to use it for a project that requires 24.0V, so I'd like to modify the PSU's voltage divider according to https://polonai.se/smps/tosh/ and https://hackaday.io/project/3469-mod...k-power-supply

The output caps are rated at 25V, so I'll be close, but not over their limit.

I've identified the opamp/comparator and voltage reference as ST's TSM103 in red, and pin 2 is the inverting input and pin 3 is the 2.5V reference. I don't have a variable PSU with me right now, so I can't identify when the optocouplers turn off and the voltage divider to the output voltage.

1. Can someone help me identifying the voltage divider?

2. The PSU has two optocouplers separating mains from the secondary. Why two, not one?

3. There's a 50k trimpot in the area highlighted in yellow on the other side of the board. I've tweaked it, and the output voltage is constant at exactly 20.00V, regardless of the trimpot position. Since it's not affecting voltage, could it be used to set the OCP current limit?

Thanks!
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File Type: jpg HP PSU Marked.jpg (818.7 KB, 24 views)
File Type: jpg TSM103 Pinout.JPG (31.7 KB, 13 views)
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Old 06-01-2020, 12:16 PM   #2
cpt.charlie
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Default Re: Modifying Laptop PSU 19.5V to 24.0V

One optocoupler is for controlling the voltage, and maybe the other is used for overcurrent protection (the trimpot is likely part of the OCP circuit), if you need to locate the voltage divider, start looking for the voltage reference (normally TL341) in a sot23 package.
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