The system of the electronic transformer is described. Two types of optical transmission systems are compared. The optical data transmission scheme with synchronous clock imitating asynchronous transmission is presented. The data transmission through single optical fiber is realized with 16-bits serial output A/D converter and single chip computer. Demodulation of synchronous clock and converting serial to parallel transmission of data are done with CPLD at secondary side. The linearity characteristic of the electronic transformer reaches level 0.2.