Socket lock

Overview~

A socket lock mechanically retains the plug in the charger's socket for the duration of a charging session. It is used on chargers with a socket outlet (detachable cable): when a vehicle is plugged in the lock engages so the plug cannot be pulled out while current may be flowing, and it releases when the session ends. A charger with a permanently attached cable does not use a socket lock.

The lock is a small motorized actuator of the type built into Type 2 sockets. The firmware drives the actuator in two directions to lock and unlock, and reads a feedback contact to confirm the mechanism actually reached the commanded position.

How it works~

Drive~

The actuator motor can be driven through an H-bridge by two control lines, A and B. Their relative polarity sets the direction:

Lock – drive A high and B low for the operating time, then release both lines (the motor coasts and the mechanism holds).
Unlock – drive A low and B high for the operating time, then release both lines.

Driving the lines for a fixed time, rather than until a hard stop, is what limits the motor current; the actuator is designed to reach its end position within that window.

Detection~

To check the current position the firmware drives both control lines to the same level. With no voltage difference across it the motor does not move, while a position contact (a microswitch inside the actuator) can be read on a separate detection input. After a configurable detection delay the input is read and compared against the expected level for "locked".

Because actuators differ in which way their contact reads, a detection polarity setting selects whether the locked position corresponds to a logic high or a logic low on the detection input.

Sequence and timing~

A lock or unlock operation runs as its own task so it never blocks the main control loop:

On a lock command the firmware waits a short settle delay (about 500 ms) before the first attempt, giving the plug time to seat fully.
It drives the actuator for the operating time.
After the detection delay it reads the feedback contact.
If the position is not yet reached it retries, up to the configured retry count.
A minimum break time is enforced between consecutive operations so the motor is not driven again too soon.

If, after all retries, the actuator has not reached the locked position the status becomes a locking fault; the equivalent for release is an unlocking fault.

Interaction with charging~

The lock is operated by the state machine:

it engages when a session begins (entering state B1, i.e. when the vehicle is plugged in),
it releases when the machine returns to state A (vehicle unplugged) or goes to E / F.

Charging will not start until the lock has settled to its idle (successfully locked) state – the charging allowed check requires it. A lock fault or unlock fault raises the corresponding error and puts the charger in state E. These faults latch: rather than repeatedly driving a jammed mechanism, the firmware stops actuating the lock until the problem is addressed and the device is restarted.

Settings~

The timing and detection behavior are adjustable at runtime (web UI, Modbus, REST, scripts):

Setting	Default	Notes
Operating time	300 ms	How long the motor is driven (range 100–1000 ms)
Break time	1000 ms	Minimum rest between operations (not below the board's minimum)
Retry count	5	Attempts before declaring a fault
Detection polarity	–	Whether the locked position reads high or low

Configuration~

The lock hardware is declared in board.yaml:

socketLock:
  gpios: [20, 19]      # control lines A and B (H-bridge)
  detectionGpio: 34    # feedback contact input
  detectionDelay: 1000 # delay before reading the contact, in ms
  minBreakTime: 1000   # lower bound for the break time, in ms

gpios are the two motor control lines (A then B). detectionGpio is the position feedback input. detectionDelay is how long to wait after commanding a move before reading the contact, and minBreakTime is the floor for the user-configurable break time.

For how these terminals are wired on a real board, see the socket lock section of the ESP32-S2 EVSE DIY Alpha as an example.

If you're building an EVSE without a socket-outlet (having the cable directly attached) omit this configuration from board.yaml.

Warning

The socket lock is a safety feature: it prevents the plug being removed under load. Verify both locking and unlocking, and the fault behavior, before putting a socket-outlet charger into service. Make sure the actuator releases on power loss or fault so a vehicle can never be trapped.

CAUTION

Working with high voltage is dangerous. Always follow local laws and regulations regarding high voltage work. If you are unsure about the rules in your country, consult a licensed electrician for more information.