I decided to tap off the negative and positive wires for a 12V cigarette lighter and wire in a USB C charger to discretely provide permanent power to a Bluetooth receiver in my car. I have a 4A inline fuse which then leads to the USB C cable. I'm confident I've done the wiring correctly, the cigarette lighter socket still works, heat shrink solder joins look good everything should be connected but both my phone and Bluetooth receiver won't charge using the cable.

USB C power loom

I have a friend who's suggested this is because I haven't stepped down the 12V power to 5V before it gets to the USB C cable and my devices aren't recognising the out of spec 12V signal.

I just wanted to check if this sounds correct and a 12V - 5V regulator should solve the problem?



Fitting a 5V regulator resolved the issue!

New loom

End result

  • 3
    \$\begingroup\$ Depending on what is inside the USB connector molding, it may not look like a charger to the device it is plugged in. And directly applying 12V from a car to an USB-C plug sounds absolutely wrong thing to do and if the 12V itself has not fried any devices yet, all the positive and negative spikes on it when different loads are turned on or off will. Simply get a USB-C cigarette socker charger and wire it permanently in - although it will also add a permanent extra load on your car battery so car battery will drain faster if it is not used for a while, so I would not recommend that either. \$\endgroup\$
    – Justme
    Commented Jan 31, 2021 at 10:47
  • 1
    \$\begingroup\$ Hardwiring a USB charger in is fine, you just have to use a wiring diagram or a multimeter to make sure the circuit you add it to cuts power when the ignition shuts off. Many cars are wired with one or more cigarette sockets that do and one or more that don't. \$\endgroup\$
    – K H
    Commented Jan 31, 2021 at 10:53
  • \$\begingroup\$ The wires I'm tapping off are ignition live, so I'm not worried about draining the battery. \$\endgroup\$ Commented Jan 31, 2021 at 16:43

1 Answer 1


USB C is supposed to deliver 5V unless the device being charged says it wants a higher voltage. To implement that would require a USB controller fitted to the car, so that it can negotiate the correct voltage.

If you are lucky, you won't have fried anything yet by supplying 12V to devices that only expect 5V.

  • \$\begingroup\$ I definitely haven't fried anything. My phone and Bluetooth receiver still work fine. I've ordered a 12V to 5V regulator off eBay designed for cars. Will that work? Or does it need a microcontroller to regulate it down to a specific USB C format? My backup plan if the regulator doesn't work is to take a USB C hardwire kit designed for dashcams and plug it into my wire taps. But it's more expensive. \$\endgroup\$ Commented Jan 31, 2021 at 16:46
  • 2
    \$\begingroup\$ @EM-Creations It may depend on how you wire the data pins. Most devices will assume that the USB port can only deliver 5V 500mA unless the USB power source tells them otherwise. That's only a trickle charge for modern devices. \$\endgroup\$
    – Simon B
    Commented Jan 31, 2021 at 19:07
  • 1
    \$\begingroup\$ Nice idea, but the implementation needs room. That says a lot for the quality and ruggedness of the devices you plugged in. You can buy a 12V usb adapter for maybe a buck, you have more then that in your solution. I would suggest you read the USB specifications from 1.0 through at least 3. After that you will understand enough to realize how lucky you are. \$\endgroup\$
    – Gil
    Commented Jan 31, 2021 at 19:14
  • \$\begingroup\$ @SimonB the wire came from a USB C to DC wire I got off eBay, only 2 wires (positive and negative) are there wired into the USB socket, so I assume that is wired correctly. Data pins will be dummy I guess. \$\endgroup\$ Commented Jan 31, 2021 at 20:06
  • \$\begingroup\$ @SimonB 5V regulator costing £3.95 solved the problem, thanks everyone! I learnt something :) \$\endgroup\$ Commented Feb 7, 2021 at 19:27

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