The regulator shown in the diagram enables the speed of a d.c. motor to be set over a wide range. Although the most frequently used method for achieving this is pulse-width modulation, in the present design pulse position modulation (also called pulse time modulation) is used. That is, rectangular modulator IC1 always switches the motor on for 0.5 ms (= R1C4 ) For most motors that is sufficient to start. The speed at which it will finally rotate is influenced by the pulse rate. The pulse spacing, which is the discharge time of C4, may be set between 1 Us and 14 ms with R2 and P1. T1 can switch up to 6 A, provided it is fitted with a heat sink of not less than 20 K W.