DIY high power USB car charger

For some time I felt the need of an USB charger in my car, but all the commercial ones I’ve seen haven’t convinced me they are reliable. So I proposed myself to build one which would have to be “virtually indestructible”, stable and safe for the devices I connect to it.

I remembered that I had a LM340KC-5.0 (5V positive regulator in TO-3 case), laying around somewhere, and told myself that this is perfect for the job. After I looked over the datasheet, I did a test circuit, and it performed perfectly.

I bought a telephone outlet to use it as a case, and I mounted two USB receptacles (which I dismembered from an old computer motherboard) instead of the telephone jack, and the LM340 on top of the case.

The USB pins configuration you can find on Wikipedia.

I have also added a green LED to know if is working.

Have fun.


  1. written by: Mike Kelley on April 19, 2020 at 2:27 am - Reply

    I know this is awhile back, but looking through your pictures, I don’t see the 22 ohm resistor installed in the project box. Is it really required? If so, what wattage would you recommend? I don’t see it called out in the lm340 datasheet.

    • written by: Mihai on April 30, 2020 at 1:25 pm - Reply

      It can be omitted. But it is recommended.
      I’ve put a 10 Ohm resistor after I installed the LED.
      It is soldered directly to pin 1 of the TO-3 case.
      I forgot to take pictures of the inside after that.

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