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Old 09-14-2018, 08:35 AM   #1
Dannyx
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Join Date: Aug 2016
City & State: Constanta
My Country: Romania
Line Voltage: 230VAC 50Hz
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Posts: 2,105
Question Modifying laptop adapter for 12v

Good day folks. Out of pure boredom and also to learn stuff, I'm trying to hack a laptop power brick to output 12v instead of 19v. Now before you people start yelling at your screens for not going with an off the shelf 12v supply, that's not the point - the point is to hack stuff and also to put something something unused to good...use

So, I know how SMPS feeback works and I DID this once before with great results, but this one is a bit different: I was expecting to see a TL431 on there with a resistor divider on its REF pin to drive the feedback optoisolator, but this one uses an IC (this DAS001 which is TWO comparators in one. I put together a rough schematic and also some shots of the secondary of the board with the IC removed so you can see the traces beneath.

I BELIEVE I need to tweak those resistors on the left side of the schematic (just barely encircled there) which correspond to the ones to the left of the IC in the pictures, since I tried to draw it close to the original layout. The way I see it, by lowering the value of the one going to VCC, the inverting input will be biased towards VCC more, thus turning its output more "negative" and make the LED inside the optocoupler shine brighter, telling the switching IC in the primary (not pictured) to reduce the switching frequency and lower the output....at least that's the theory and how I did it with the TL431 version. Now I don't really understand why there's TWO comparators used here, for what purpose ?
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