The power inlets
Servo power comes in through a USB-C socket or a two-way screw terminal. The two join, then pass through one P-channel MOSFET that blocks a supply connected backwards, and land on a 1000 µF capacitor. The USB-C socket carries power only, and it is for an adapter, never a computer.
Two inlets, one rail
The screw terminal takes two wires, 5.0 mm apart. It is printed − on the left and + on the right. Use it for a bench supply or a battery pack.
The USB-C socket is a six-pin, power-only socket: VBUS, GND, and the two CC pins, each with 5.1 kΩ to ground. Those two resistors are what a USB-C charger looks for before it turns on 5 V, so any USB-C phone charger works. No data pins are connected.
Both inlets join before the protection, so they are one rail. Use one at a time: two supplies plugged in at once are simply connected to each other.
The transistor that blocks a reversed supply
Straight after the inlets sits Q1, an AO3401A P-channel MOSFET, with its gate tied to GND. Connected the right way round, current first flows through the MOSFET's built-in diode, which raises its source above its gate and switches the MOSFET fully on. From then on the supply passes through a closed switch rather than a diode, and loses very little voltage on the way.
Connected backwards, the source never rises, the MOSFET stays off, and its diode points the wrong way to conduct. Nothing downstream sees the reversed supply. This covers the terminal and the USB-C socket alike.
What it does not cover: a supply over 6 V, and anything fed in through the header's V+ pin, which is after the MOSFET.
The capacitor
Behind the MOSFET, a 1000 µF, 10 V electrolytic capacitor, C1, sits across V+. It is the tall green part at the top left. It supplies the short bursts when servos start moving, before the adapter and its cable can respond.
Why not your computer
The underside prints it in capitals: DO NOT PLUG TYPE-C INTO YOUR COMPUTER. USE A DEDICATED POWER ADAPTER. The socket is wired straight to V+, so a computer on that cable would be feeding servo motors. A computer's USB port is built to power small devices, and when a servo stalls it can ask for more than that. The board has no data connection, so there is nothing to gain from a computer port either.
The same goes for the microcontroller's 5V pin: servos on it pull the microcontroller's own supply down with them. Give V+ an adapter of its own.
When it does not work
Try the adapter with a phone to prove it gives power. A USB-C charger only turns on 5 V when it sees the board's 5.1 kΩ resistors on CC1 and CC2, which this board has, so a working charger and cable should light it. Also try another cable.
The MOSFET will have blocked it: nothing lit, nothing reached the servos. Swap the wires so + goes to the terminal's right-hand screw, marked +. The protection covers both inlets, but not a supply over 6 V.
Yes. The socket connects only VBUS, CC1, CC2 and GND: there are no data lines on this board to use. Any cable that carries power is enough.
It works electrically, but that pin is after the MOSFET, so a reversed supply there is not blocked. Use the screw terminal or USB-C for the supply and treat the header's V+ pin as an output.
How many amps sixteen servos can ask for, and why the answer is set by a stall, not by a sweep.
Sizing the supply →Edit this page — content/books/pca9685/the-power-inlets.mdx
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PCA9685 16-Channel Servo Driver Board, USB-C Powered
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