Electronic lock cylinder with: - a blocking member reversibly movable between: - a locking position in which it locks the electronic cylinder, and - a retracted position in which the electronic cylinder is unlocked, - an electrical actuator adapted to move the locking member from its locked position to its retracted position in response to receiving an unlock command, - first and second electrical contacts (101, 102) adapted to come directly into electrical and mechanical contact with, respectively, first and second electrical contacts of a key inserted inside the electronic cylinder, to power the electronic cylinder from a battery housed inside the key, - a microcontroller configured to determine whether the key inserted in said electronic cylinder is a key authorised to unlock said electronic cylinder and, only if so, to transmit the unlocking command to the electrical actuator, said microcontroller comprising a first and a second power supply port (204, 208) electrically connected, respectively, to the first and the second electrical contacts of the electronic cylinder, - a circuit for protecting the microcontroller against an overvoltage applied between the first and second electrical contacts (101, 102), characterised in that the protection circuit comprises: - a first switch comprising a control terminal and first and second power terminals (114, 116), the first and second power terminals being electrically connected, respectively, to the second electrical contact of the cylinder and to the second power supply port of the microcontroller, this first switch being able to switch in response to the reception of an isolation command on its control terminal: - from a closed state in which it electrically connects the second electrical contact to the second power port of the microcontroller, - to an open state in which it electrically isolates the second electrical contact from the second power port of the microcontroller, and - an isolation command generator adapted to generate and transmit the isolation command to the control terminal of the first switch when the voltage between the first and second electrical contacts exceeds a first predetermined threshold. - a blocking member reversibly movable between: - a locking position in which it locks the electronic cylinder, and - a retracted position in which the electronic cylinder is unlocked, - an electrical actuator adapted to move the locking member from its locked position to its retracted position in response to receiving an unlock command, - first and second electrical contacts (101, 102) adapted to come directly into electrical and mechanical contact with, respectively, first and second electrical contacts of a key inserted inside the electronic cylinder, to power the electronic cylinder from a battery housed inside the key, - a microcontroller configured to determine whether the key inserted in said electronic cylinder is a key authorised to unlock said electronic cylinder and, only if so, to transmit the unlocking command to the electrical actuator, said microcontroller comprising a first and a second power supply port (204, 208) electrically connected, respectively, to the first and the second electrical contacts of the electronic cylinder, - a circuit for protecting the microcontroller against an overvoltage applied between the first and second electrical contacts (101, 102), - a first switch comprising a control terminal and first and second power terminals (114, 116), the first and second power terminals being electrically connected, respectively, to the second electrical contact of the cylinder and to the second power supply port of the microcontroller, this first switch being able to switch in response to the reception of an isolation command on its control terminal: - from a closed state in which it electrically connects the second electrical contact to the second power port of the microcontroller, - to an open state in which it electrically isolates the second electrical contact from the second power port of the microcontroller, and - an isolation command generator adapted to generate and transmit the isolation command to the control terminal of the first switch when the voltage between the first and second electrical contacts exceeds a first predetermined threshold. - from a closed state in which it electrically connects the second electrical contact to the second power port of the microcontroller, - to an open state in which it electrically isolates the second electrical contact from the second power port of the microcontroller, and - an isolation command generator adapted to generate and transmit the isolation command to the control terminal of the first switch when the voltage between the first and second electrical contacts exceeds a first predetermined threshold.