USB-C Legacy modification - Page 1
Forum section: electronics / beginners

author: IsmAvatar
author_rank: Contributor
timestamp: January 10, 2023, 01:18:53 am
content: tl;dr: I have a device with a 6-pin USB-c connector but the inner 2 pins (presumably CC) are left floating. Can I stick a resistor on it to make it charge 5V (Legacy)? What value resistor, between what pins?I have a rechargeable device that seems to only support USB-C legacy (USB-A to USB-C cable) charging. When I plug a compliant USB-C to USB-C cable into it from a compliant power supply, the device does not charge - likewise with any other USB-C to USB-C cable.My vague understanding is that Legacy cables provide 5V regardless, while USB-C cables require some sort of pull-up or pull-down resistor of some value (5.1k or 56k) from the CC pins in order to provide legacy 5V charging with some low amperage (some claim 500mA, some claim 1.5A, some places say 3A, whatever), otherwise they will not be able to negotiate and thus will not supply current. Or maybe a resistor between the Data pins?So I cracked open the device, and observed that it uses a 6-pin USB-C connector (with 4 additional standoffs built into the shell). Examination of the traces indicate that the outermost traces are connected to the standoffs, the shell of the connector, and thus ground (meter confirmed this). The next pins (pins 2 and 5) were connected together and went off somewhere, so presumably voltage (VBus). The inner 2 pins were simply soldered to the board but left floating. A quick google indicated that 6-pin connectors typically supply CC as the center pins.I then confirmed with my microscope by looking inside the connector, on each side (balanced) are 3 pins, with one on the rightmost side (GND pin 12), roughly 2 empty spaces to the left, a pin (VBus pin 9), a slightly larger gap of about 3 spaces to the left, and the third pin (CC pin 5), and more space to the other end of the board of maybe 4 spaces. (flip the connector over, see the same thing).0 0 0 0 1 0 0 0 1 0 0 1-------------------------1 0 0 1 0 0 0 1 0 0 0 0So correct me if I'm wrong, I should be able to modify this by sticking 1 or 2 resistors of some specified value between specified pins to force the power supply to supply a sufficient (500 or more mA) charging current at 5V. So what values and which pins?Will this affect legacy cables (USB-A to USB-C)? (I've no idea what they do with their CC pins, not use them? Dunno if I'd step on their toes, but I'm guessing not). My goal is to use more USB-C to USB-C cables, rather than hanging on to these Legacy USB-A to USB-C cables and having to guess/memorize which devices are naughty and only use Legacy cables, so upgrading those devices would be really nice.I assume full responsibility for any modifications I do, and don't hold any of y'all accountable if you get it wrong (nor if I get it wrong). I'm doing it on a low-stakes device (a flashlight) with a small battery and all sorts of precautions (safety squints, bottle of alcohol to extinguish any electrical fires, etc). Although it's a nice flashlight, so I am really hoping to make a small quality of life upgrade and ideally not destroy it.

author: Berni
author_rank: Super Contributor
timestamp: January 10, 2023, 07:46:32 am
content: Yep this is an issue when using a USB-C to USB-C cable plugged directly into a USB-C charger, since those sense the CC pins. While anything with a USB-A plug doesn't have the pins so it can't sense them, hence why it works.You should be perfectly fine adding in the resistors.The only case where it might be an issue is if you have a device that changes with a lot of current >1A since the device is supposed to sense the resistance on the host side to determine how much power is available. Tho pretty much all modern USB-C chargers can supply 3A anyway.

author: IsmAvatar
author_rank: Contributor
timestamp: January 10, 2023, 09:01:39 pm
content: Awesome, I'm a little confused though, what value resistors, is it 5.1k or 56k? And should they be pull-down?

author: Berni
author_rank: Super Contributor
timestamp: January 11, 2023, 06:14:35 am
content: The device side (the one consuming power) should always have 5.1k pulldown resistors on both CC pins.