Filament Controls Flashcards
In a mobile unit, how is filament current controlled ?
Using tube current. Higher tube current means more filaments are emitted which means there is more filament current which in turn increases the tube current.
How does a filament circuit work ?
Supplied by a step down transformer from 230V to lower voltage( 10V) in the high voltage tank.
Step down transformer increases current.
Filament current is present at the cathode and when heated causes thermionic emission of electrons at the cathode.
this allows the free electrodes to be accelerated across the tube potential and produce x rays.
What is the voltage of the primary and secondary of the filament circuit transformer ?
Primary – 230V A.C
Secondary 10V A.C
Why is a transformer used ?
The tungsten cathode does not get damaged at voltage around 10V but does at 230 V.
What current value is the minimum required to drive the filament in the secondary of the transformer ?
1- 1.5 Amps A.C is needed. (3-5 A a.c is common filament current value)
Does filament current impact the beam quality ?
No, it purely impacts x ray quantity.
What modes are present on a filament circuit ?
OFF, STANDBY, FLUORO, EXPOSURE.
standby still provides some current but not enough for thermionic emission.
fluoro uses an A.C current which will only cause slight emission in the filament over a longer period of time.
Higher current and voltages are used in exposure mode and there can sometimes be a cool down mode to extend the lifespan of the cathode.
How are temperature and tube current related ?
a higher filament current will increase the temperature of the filament causing more electrons to be released therefore increasing tube current .
What is electron flow compared to tube current ?
Tube current is the mA from anode to cathode. Electron flow is the thermionic emission from the filament in the cathode to the anode.
How does the filament control circuit work ?
It uses feedback from the middle of the tube (where current is measured) which helps monitor filament current.
The driving control circuit is indirectly connected to the tube midpoint so the actual mA (tube current) can be used to trim the primary of the filament current driver.
Also connected with the kV monitoring to ensure the correct filament current is being given using a voltage comparator where the current is converted to voltage and compared to be changed if too high.
How is the filament of an x ray tested ?
generators now have filament fault technology.
requires the use of an old HT cable end to do manually and the safety exposure switch must be turned on to isolate the primary of the filament.
What is tube current and what is filament current ?
tube –> current between the cathode and the anode. measured in mA. can control the intensity of the x ray beam, high current = high intensity.
filament –> current which is used to heat the cathode and cause thermionic emission. Used to control tube current.
