week 8 Flashcards
(11 cards)
how does direct current electricity work?
battery supplies V
resistor controls current
current (I)
I=V/R
power
P=I^2 x R
lots of energy lost with the resistor
how does alternating current electricity work?
mechanical work to energy (turbine with cable in magnetic field)
how do transformers work and what are the two types?
low current maximises voltage, low energy loss for AC
step-up transformer
- more primary turns
- voltage down
- current up
step-down transformer
- more secondary turns
- voltage up
- current down
describe isolating transformers and autotransformers
isolating transformer
- no direct contact between primary and secondary circuits
- linked magnetically
- more for safety
autotransformer
- one single coil
- adjusts voltage
what are the problems with AC current in x-ray
x-ray is pulsed because of rapid direction change, when voltage is negative no x-rays are produced
what is rectification and the two types?
diode- current in one direction (negative voltage eliminated)
half-wave rectification uses 2 diodes (no current half the time=waste power, double exposure for patient)
full-wave rectification uses 4 diodes (still varying voltage)
what is smoothing?
uses a capacitor to store and release charge (reduces variation)
describe the three phase power supply system
3 turbines at angles 120 spin, current induced in each turbine with phase difference, little variation with rectification and smoothing
how is current used to control x-ray tube output?
resistors control current in primary side
filament transformer controls secondary side
this controls heating of filament
filament temp controls rate of thermionic emission and tube current
how is voltage used to control x-ray tube output?
V is set in low voltage part of circuit, stepped up in high voltage part (500:1)
how is time used to control x-ray tube output?
operator chooses time
mAs timer- ends when desired charge passed
automatic exposure control- ends when required radiation incident on image receptor
