I would like to be able to charge my laptop via a USB-PD power source as seen in this article. I need to find out if my laptop input, which is rated at 19V 3.42A can handle the 20V provided by the usb c board. Is there any way to find out without having a chance of frying my laptop?
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1\$\begingroup\$ If you read the description of the project, it states that the criteria is "it doesn't really matter if you supply it with 12, 15, 19 or 20V as long as the MOS-FETs can handle it". Without details regarding the MOSFETs and other elements of the switching regulator presumably connected to your charging port, we cannot reasonably answer this question. \$\endgroup\$– nanofaradCommented Aug 13, 2020 at 21:09
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\$\begingroup\$ That article looks like it is taking shortcuts and making things look easier than they are. USB-PD needs some negotiation to get the 20 V, that's to protect devices that can only handle 5 V (the device has to ask for 20 V). If you're not exactly sure what you're doing you run the risk of damaging things. Is having this really worth the risk? Personally I would take that article with a large pinch of salt, there is no proof that this "hack" is actually working well and will still be working after some months. \$\endgroup\$– BimpelrekkieCommented Aug 13, 2020 at 21:09
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2 Answers
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If you cautiously follow the Hackaday article you mentioned, then yes, the approximately 20V PD output will match-up okay with the specified 19V input required your computer. Give or take a volt (+/- 1V) here will likely not hurt anything here -- yet, it can be argued.
Notice carefully in the article, the hacker used an off-the-shelf USB PD "trigger" board to request 20V from the PD supply and that he skillfully wired it in. In my findings, 20V comes out of PD supplies at something less... typically 19.9 or 19.8 or so.
Just a word of caution, be smart that your computer would be an expensive loss if you were unsuccessful. Maybe try this experiment on a breadboard circuit, first.
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There are no such thing as Absolutely Exact Voltage. Everything have it's tolerances. Check how your charger works on low and high loads, it's probably 18..21V, depending on the actual load.
If your USB charger have smaller tolerances, it's 20V is OK.
But remember low voltage is dangerous, too! Having 15V on the 19V input makes the DC/DC converter keep very, very long duty cycles, overheating the MOSFETs by high amperage, not high voltage. I've got some troubles by having 18V on a 20V input.
You can probably use a 18V diac, which does not allow to run the charger if the negotiation ended at 15V instead of 20V.
Also, you can't protect the USB-C charger from current overload. Laptop will take 3.42A even if the negotiation ended on 2A.
