Abstract: Electrical energy being supplied to an electrical heater, typically a glow plug within an internal combustion engine (ICE), is easily and simply controlled by controlling the duty cycle or ON/OFF duration of a switch (10) serially connected between the glow plug (11) and a supply voltage source, for example a vehicular battery (U.sub.Batt). An operational amplifier (15) controls the duty cycle of the switch (10) by comparing a periodically varying voltage (37a) derived from a stabilized frequency or pulse generator (30) with battery voltage (U.sub.Batt) and, at the cross-over points, respectively changing the switching condition of the switch. Upon drop in battery voltage, the switch will be longer in an ON position than at a higher battery voltage, so that the average voltage, over a plurality of cycles, across the glow plug (11), will have a value (U.sub.KEM) which will result in a predetermined temperature thereof.