The conventional analog-based firing circuit can be implemented by comparing a linearly decreasing periodic sawtooth waveform synchronized to the ac supply, with a control signal corresponding to the desired converter delay angle. This circuit requires a large number of passive components (resistance and capacitor) and careful adjustment of the synchronization circuity. In this paper a novel firing circuit is proposed for thyristor switch. The proposed circuit is implemented by using digital components(FPGA, A/D, and DSP etc.) on the basis of the analog cosine method.