A defibrillation circuit comprising a gas discharge tube and a light source arranged to pre-energize the gas discharge tube in order to provide predictable breakdown conditions of the gas discharge tube. The gas discharge tube may be used as an overvoltage protection device for the defibrillation circuit or for certain parts of the defibrillation circuit. An overvoltage protection device for medical devices is also described. The overvoltage protection device comprises a gas discharge tube and a light source arranged to pre-energize the gas discharge tube in order to provide predictable breakdown conditions of the gas discharge tube.