PURPOSE: To enhance response by conducting an inductance coil cyclically thereby charging a capacitor with magnetic energy.

CONSTITUTION: Upon transition of a transistor 3 to nonconduction, magnetic energy stored in an inductance coil 1 is fed through diodes 4a, 4b to a capacitor 5 thus charging the capacitor 5. At that time, an SCR 7 is held in nonconductive state. Upon drop of conducting current by a predetermined value, the transistor 3 is conducted but since the SCR 7 is also conducted simultaneously, voltage of the capacitor 5 causes quick rising of conducting current. When the cycle is repeated, conduction control of the inductance coil 1 can be performed accurately according to a signal voltage curve with no time lag.