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Apr 03, 2017, 01:20 PM
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Changing laptop power supply output voltage

The answer is yes, you can change the voltage of a laptop power supply, within limits. It was long dreamed of changing a 12 year old, 20.5V, 6A deal into a more useful voltage. 1st was discovering that it could be opened. 2nd was discovering there was a common circuit in most power supplies. It could be probed for by providing DC to the output instead of connecting the manes voltage. With the output powered, look for 2 inputs of an op-amp. 1 input should be around 2.5V, the other input should be a ratio of the output voltage.

The ratio is determined by a resistor divider. In this case, it was an irrational number around 15 for the high side & a complicated network for the low side. Decreasing the high side would raise the op-amp input & lower the output voltage.

Whacked in a parallel 47k which reduced the output to 16V, but what the internet didn't say was on a scope, it was a sawtooth. With load, it was 8-16V. Without load, it was 14-16V. Whacked in a 100k & 1 meg in parallel to finally yield a voltage of 17.9, right on the edge of where it became unstable but .1V low enough to power the Accucel 8150. Another 1 meg in parallel threw it off.

The problem is it needs a minimum voltage to power itself. It tries to bootstrap itself when it turns on, but if the minimum voltage isn't reached, it indefinitely tries to bootstrap itself. That's what generates the sawtooth waveform.

The 2.5V reference is generated by a Xena diode labeled 1273. It's biased by the output voltage, but it's just 1 component which could be thrown off. Figuring out how to lower the bootstrap voltage is beyond the internet's ability. There's no more interest in hacking laptop power supplies, as the world has migrated to 5V USB.

Increasing the output voltage isn't much more flexible, as there are caps rated for 25V & many marginally rated components running on the output voltage.
Last edited by Jack Crossfire; Apr 03, 2017 at 11:17 PM.
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