A Novel Approach to Implement a Closed Loop Controlled Power Converter in Thermal Vacuum Tubes

Abstract

In electronics, vacuum tube or electron tube is a device that controls electric current between electrodes in an evacuated container. Vacuum tubes mostly rely on thermionic emission of electrons from a hot filament or a cathode heated by the filament. One of the critical issues of these vacuum tubes is, maintaining the powered filament tube for a long time. Filament gradually becomes weak due to irregularities in supply voltage and higher heat generation. Weak filament can be easily identified by observing the reduced output power of the tube. Sudden filament supply failures and irregularities in supply cause the filament to be deteriorated. In this approach, we consider a self-controllable switching power supply which produces a ramp voltage output by varying firing angle of thyristors with respect to time. To generate the ramp voltage, the power converter is introduced. The ramp voltage is set as the input power supply to the filament. Such power supply helps minimize possibilities to occur sudden and huge current spikes which are capable to harm filament. Proposed power converter consist of a pulse generator to generate PWM pulse, a pulse amplifier to step up the input voltage, an initiator to create basic ramp signal, a synchronizer to match triggering pulses with ac line input, a supervision circuit to detect faults and a communication unit for dealing with plant’s main control unit. Here, a controllable AC output is used for the purpose of pulse generation and supervision. Constant negative ramp voltage generated through the initiator circuit is fed to a differential amplifier to obtain an inverted output which is decreasing with time. The pulse generator is built with a RC circuit of which charging time is proportional to the output voltage of the differential amplifier. The RC circuit fires a PNPN diode to generate a low voltage basic pulse. It is recognized that the generated pulse is not sufficient enough to fire two back to back thyristors configured in the thyristor bank. A pulse transformer and amplifier circuits are occupied to step up the generated pulses. Finally amplified pulses are fed to a gate of the three terminal thyristors to fire up and obtain the desired output voltage of the converter. This approach makes the filament supply voltage a reliable input regardless of the magnitude or a specific application.