Thyristol - Determine RMS Current Rating

A thryistol is used a half- wave controlled rectifier in series with a variable load resistor across a 240v,50hz supply. If constant mean current of 10A is to be maintained in the load, whose resistance may vary between 0.5ohms and 5 ohms, determine the required rms current rating for the thryristor, and the range of firing angles which the thyristor trigger circuit must provide.