A transformer is a passive electrical device used to transfer electrical energy from one circuit to another, via the phenomenon of electromagnetic induction.
It is fundamental in the modulation of voltage and current in the x-ray generator.
On this page:
Components
- primary coil
- a set of insulated wires attached to the input current
- iron core
- the conduit for the changing magnetic field
- secondary coil
- a set of insulated wires attached to the output current
The primary and secondary coils both have winding turns which will vary in number with respect to each other depending on the type of transformer (see below):
Types
- step-up transformer
- secondary coil has more turns than the primary coil (Ns > Np)
- step-down transformer
- secondary coil has less turns than the primary coil (Ns < Np)
Laws
First law of transformers
The secondary voltage (Vs) is related to the primary voltage (VP) by the number of ratio of the number of turns (N) in each coil, such that:
Vs/Vp = Ns/Np
Second law of transformers
The product of voltage (V) and current (I) in the two circuits are equal (thereby meeting the Conservation of Energy principle).
VpIp = VsIs
This means that a voltage increase in one circuit must result in a proportional decrease in current (and vice versa).
Applications in radiology
In the high-voltage circuit of the X-ray generator, step-up transformers are used to convert the mains voltage (120 - 240 V) to the high voltages (20,000 - 150,000 V) required for x-ray production.
In the filament circuit of the X-ray generator, step-down transformers are used to increase the heating current applied to the filaments of the cathode for thermionic emission.
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