Provide high voltage (typically 40–150 kVp) across the X-ray tube.
Control tube current (mA) and exposure time accurately.
Stabilise voltage output to maintain consistent beam intensity.
Rectify and smooth AC to minimise voltage ripple.
Synchronise exposure timing with automatic exposure control (AEC) and other system functions.

Essentially we want a constant voltage across the tube to accelerate electrons in a predictable flow from cathode to anode.

We’re going to briefly look at three types of generators. each with increasing utility and efficiency.

Types of generators

In the following summaries I will be referring to the term voltage ripple, this is discussed directly after we’ve gone through the types of generators. Feel free to read that first before studying the generator types.

Single phase, half-wave rectified

Uses one half of the AC cycle.
X-rays produced only during positive half-cycles.
Output is pulsed with 100% voltage ripple.
Very inefficient and now obsolete (found in basic dental or portable units).

Single phase, full-wave rectified

Uses both halves of the AC cycle.
Two pulses of radiation per cycle → 100% ripple.
Exposure times are halved compared to half-wave systems.
Doubles tube efficiency compared with half-wave.
Beam intensity still fluctuates from zero to maximum; poor consistency.

Three phase generators (6 pulse and 12 pulse)

Use three out-of-phase AC inputs, each separated by 120°.
Overlapping waveforms provide more constant voltage.

Type	Ripple	Comment
6-pulse	~13%	Moderate smoothing; significant improvement over single-phase.
12-pulse	~4%	Nearly constant potential; high efficiency and short exposure times.

Advantages:

Higher mean tube voltage → higher X-ray output.
Reduced patient dose for same receptor exposure.
Shorter exposure times minimise motion artefacts.

High frequency (HF) generators

Convert mains AC to DC, then to high-frequency (~20–100 kHz) AC using electronic inverters before transforming voltage.
Compact, efficient, and now standard in all modern X-ray systems.
Voltage ripple < 1%.
Very stable output → consistent image quality and reproducible exposures.
Shorter exposure times and improved tube loading capacity.

Advantages over three-phase:

Higher mean photon energy at same kVp.
Reduced patient dose (10–20% lower).
Improved reproducibility and reliability.

So what exactly is voltage ripple?

Voltage ripple = percentage variation in tube voltage during exposure.

Ripple determines the uniformity of photon energies in the beam.
Lower ripple → higher effective photon energy → more efficient beam.

Generator Type	Ripple (%)	Mean Photon Energy (% of kVp)
Single-phase	100%	70%
3-phase 6-pulse	13%	91%
3-phase 12-pulse	4%	96%
High-frequency	<1%	~99%

Using generator constants to calculate heat units

We discussed this in the previous lesson. Let’s quickly review how heat units are calculated. This is a favourite exam question because changing any variable in the following equation will affect the heat units (HUs).

HU = kVp x mA x time x generator constant

As a reminder the generator constants are as follows:

Generator Type	Constant
Single-phase	1.0
3-phase 6-pulse	1.35
3-phase 12-pulse	1.41
High-frequency	1.4

Key takeaways and exam tips:

The generator converts AC to stable high-voltage DC for X-ray production.
Beam intensity ∝ (kVp)² meaning a small voltage increase produces a large output increase.
Voltage ripple ↓ → beam quality ↑.
High-frequency generators: most efficient, <1% ripple, lowest dose.
Heat unit constants are frequently examined.
Ripple effect alters effective photon energy; for the same kVp, HF generators produce a more penetrating beam.

Up next:

I’m glad that section is behind us. Bit of a snooze fest if you ask me. But we do what we need for the exam. Next, we’ll cover the focal spot and beam geometry.