Abstract: A supply device (FD1, FD2) for power supply to an electronic unit (EU1, EU2) for a controllable semiconductor element (T1, T2) in a semiconductor valve in a shunt-connected thyristor-switched capacitor (CA). The capacitor being intended to carry an alternating current with a known period (T), the semiconductor valve comprising a snubber circuit (SC) with a first and a second terminal (CS1 and CS2, respectively) only. The supply device has an energy storage (C1, C2) for storing electrical energy, a valve terminal (J13, J23), a snubber terminal (J12, J22), a supply terminal (J11, J21) connected to the energy storage and a first current path from the snubber terminal to the supply terminal. The energy storage is designed to store an amount of energy which is larger than the energy requirement of the electronic unit during one cycle of the alternating current but smaller than its energy requirement during two cycles.