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USB charger? Answered

I want to make a usb charger with 3 ports in which I can plug my laptop adaptor (16v, 2.5A)and charge my phone and other usb devices. I am planning to use a Lm-317 to regulate the voltage. Will it be fine or I need to consider something else also? And please don't tell me I can buy one!


Jack A Lopez

Best Answer 5 years ago

There exist inexpensive DC-to-DC converters made to charge USB devices from input power from a car's cigarette lighter receptacle. You've probably seen these before, e.g.


The output, 16 VDC, from your laptop brick should be comfortably in the input range acceptable to one of these converters. Essentially this adapter is a switching style DC-to-DC converter with a regulated output of approximately 5.0 volts DC. There is a pretty wide range of DC input voltage over which the converter will work.

Actually, I've got a picture of one of these things in my image library, um.... here,


and the label printed on it says it has input voltage range of 12 to 24 VDC, and I think that's typical for a gizmo like this.

Using a linear regulator, like the LM317, is usually a bad idea when the input and output voltage are very different, since this voltage difference, multiplied by the current draw, must necessarily be dissipated as heat. E.g. for 16V in, 5V out, the efficiency is 5/16 = 0.3125, and 11/16=0.6875 of the input power is wasted as heat.

ThisIsSteveJack A Lopez

Answer 5 years ago

Even I got the cigarette lighter and converted to be powered by a laptop battery pack (15V) and it turned out to work just fine.



5 years ago

The 317 isn't capable of handling 2.5A. If I were you, I'd find a cheap 16v-5V buck switcher.