RECTIFICATION

RECTIFICATION

Rectification is the process of changing alternating current into direct current. The device that produces the change is called a rectifier.A rectifier allows an electrical current to flow in one direction but does not allow current to flow in the other direction. Rectifiers are connected into the X-ray circuit in series. They are mainly divided into half wave and full wave rectifiers. If alternating voltage is applied directly to the X-ray tube, the anode will emit electrons, whenever it is negative with respect to cathode.These electrons will travel towards the cathode and bombard the filament and destroy the filament. This is called back projection which is avoided by the supply of rectified DC voltage. Thus, rectifiers play an important role in X-ray production.

HALF WAVE RECTIFIER

Vacuum tube diodes or solid state (semiconductor) diodes can be used for rectification. In a half wave rectifier, a single diode is used. An alternating voltage is applied to the

diode as input. The output is obtained across the resistance R. When the plate is positive, the diode will allow the current to flow. When the plate is negative, the diode will not allow the current. Therefore, the diode will allow the current only during those half cycles when the plate is positive. Hence, the output current is always in one direction. This circuit is known as half wave rectifier and it is mainly used in mobile and dental X-ray units. A single solid state diode cannot prevent reverse current at higher voltages. Hence, many diodes are placed in series in a stick to do rectification.

FULL WAVE RECTIFIER

In the half wave rectifier, the input voltage is used only in one half of the cycle. The other half of the cycle is not used. Therefore, there is a need for a rectifier, which will use the full cycle of the input. This is possible by having two or more number of diodes. The alternating voltage is applied between A and The output is obtained across the resistance R.When end A is positive, D1 and D4 will conduct and a current flows through R. During the next half of the cycle end A is negative, and end B is positive. Now, the diodes D2 and D3 will conduct and a current flows through R. Thus, the current flows through the resistance R during full cycle of the input voltage, in the same direction. X-rays are produced in two pulses per cycle, irrespective of the polarity of the transformer. Three phase generator employ multiple rectifiers in the secondary circuit. Full wave rectifiers are used in high end B. X-ray tubes which employ rotating anode X-ray tubes.