The transition from the pulsed state to the steady state in a tokamak is numerically studied using a model circuit for the current drive operations. The problem of shutting off the primary circuit without reducing plasma current is analysed for the cases of air core and iron core transformers in a medium-sized tokamak. It is found that the air core transformer is more suitable for the current drive operations because of smaller primary-plasma coupling and the absence of saturation problems. Iron core transformers require careful adjustment in the vertical field in order to compensate for the change due to core saturation. The iron core transformer may be shut off only when plasma equilibrium in the negative saturation phase is maintained by a supplementary vertical field.