We considered the Martin-Taylor model (1984) for a magnetic limiter of a tokamak. Calculating the Lyapunov exponent and diffusion coefficient we analysed the dependence of the length of the chaotic region at the edge of the tokamak on the magnetic limiter current. Our results indicate that for a large tokamak, operating in a certain range of the shear values, this current can be lowered by as much as one order of magnitude, with respect to the value calculated by Martin and Taylor.