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Power supply help

Discussion in 'Power Electronics' started by Lincoln Hooley, Jul 21, 2019.

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  1. Lincoln Hooley

    Lincoln Hooley

    Feb 11, 2018

    I have a question about a power supply I am trying to set up.
    I have a need for a 12v supply and wanted to use a buck converter to step down the voltage of a laptop power supply from 19v to 12v. I have set this up and while the laptop psu works by itself and the buck converter works by itself they do not work when paired together.

    I assume this is a loading issue and wondered whether there was some sort of buffer I could put between to remedy this, or something else that I'm not thinking of?

    Thank you for your help.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Try a big capacitor at the input of the buck converter. E.g. an electrolytic capacitor rated 35 V, 1000µF per 1A of current (a rule of thumb).
  3. davenn

    davenn Moderator

    Sep 5, 2009

    Not likely

    Lappy PSU's usually have a 3rd wire that indicates the lappy connection to the PSU .... call it a feedback line

    with it not wired correctly, you wont use it as a general PSU
    Harald Kapp likes this.
  4. dave9


    Mar 5, 2017
    Depends on the laptop PSU, can you link to this make/model?

    On some, the 3rd wire is to signal to the laptop that it has the genuine PSU plugged in instead of a some random aftermarket/etc, causing the laptop to run at reduced speed or not charge the battery if it does not get this signal, but the PSU itself is still fully capable of outputting spec'd voltage, and current if the laptop were to produce that load.

    If by loading issue you mean that the PSU senses lack of (adequate) load and goes into a power conserving sleep mode, because some people leave their PSU plugged into the wall when not using it, it could be that you need to put a load on the output of the buck converter or have you already tried this?

    I am not saying it isn't possible that the PSU goes into this sleep mode if it doesn't detect the laptop over the 3rd wire, that too is possible. What exactly does "laptop psu works by itself" mean, to power the laptop, or some other unintended load, or it just measured 19V unloaded or ??
  5. davenn

    davenn Moderator

    Sep 5, 2009

    Yes, that's a better way of putting it
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