Abstract: The invention described here is concerning a power supply device of a thermionic emitting cathode, an electron source in an electron vapor deposition plant. This device is provided with a transmitter whose secondary feeds the thermionic emitting cathode and whose primary is supplied with pulse width modulated and controlled heater current (I.sub.HC). The control of this heater current (I.sub.HC) is provided directly by introducing the heater current nominal value (I.sub.HC nominal) from the outside or in a manner that the current emitted (I.sub.EC) from the thermionic emitting cathode is monitored and fed galvanically decoupled to the controller. Here the galvanic decoupled connection is achieved by a pulse width modulated, optical signal.