WEEK 5 X-ray tubes I

LO

Describe the three main circuits of an x-ray tube: the current, voltage and timing circuits.

• Explain the role and function of the transformer in producing x-rays in an x-ray tube.

• Explain the problem with using AC electricity to generate x-rays in an x-ray tube.

• Describe and explain how rectification and smoothing can be used to achieve a constant x-ray output

AC

Alternating current - electricity is generated by converting mechanical work into electrical energy (opposite of a motor), usually by turbine.

AC current induced in the coil i.e. direction of current flow changes or alternates at a rate proportional to the rate of rotation

Transformers

• Can increase voltage of AC

• Can decrease voltage too

X-ray tubes and AC current

As current and voltage are rapidly changing directions 50 times per second x-ray production output would be reduced and x-ray beam would be pulsed.

Ideally, a steady output of x-rays (DC) is preferred.

Rectification

Converts AC to DC through a process known as rectification

Diode will only allow current flow in one direction

Half - Wave Rectification

2 diodes that allows 0 current flow for half the time, waste half the power and doubles exposure time

Full wave rectification

Output still pulsed

One pulse every 1/100s

Voltage always positive

Half the exposure time of half wave rectification

Smoothing

achieved with capacitor, which stores and releases charge.

Rectified 3 phase power

Rectifying circuits can be used with the 3 phase power supply

There are now 6 pulses per cycle and variation in amplitude is much less