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Exam category: Image Production → Equipment Operation & QA
Why it matters: This module explains why the tube can even work — where its voltage comes from, and why an x-ray tube would destroy itself on ordinary household AC. Skip the transformer math here; the registry wants the relationships, not the calculations.
Three components hand voltage off to each other in sequence: the autotransformer selects how much voltage to send, the step-up transformer multiplies it to the tens of thousands of volts the tube needs, and rectifiers force the current to flow in one direction only — because the tube can't survive it flowing the wrong way.
Incoming wall power goes to the autotransformer first. It has a single winding with multiple tap connections along its length — tapping closer to one end increases voltage, tapping closer to the other decreases it. This is also what supplies the filament circuit.
This is a true two-winding transformer: secondary windings outnumber primary windings, so voltage goes up (turns ratio typically 500:1 to 1000:1). Current drops proportionately as voltage rises — power in ≈ power out.
| Primary side | Secondary side | |
|---|---|---|
| Voltage | Volts (V) | Kilovolts (kV) |
| Current | Amperes (A) | Milliamperes (mA) |
A separate step-down transformer, also fed from the autotransformer, that lowers voltage to whatever the filament circuit needs to heat the filament and control tube current (mA).
Memory cue: the transformer that talks to the tube steps UP. The transformer that talks to the filament steps DOWN.
Wall current is AC, reversing direction 120 times a second (60 Hz). X-rays are only produced by electrons flowing cathode → anode — reverse flow would be catastrophic for the tube (the cathode assembly can't handle electrons arriving instead of leaving). Rectifiers force current through in one direction only, converting the secondary AC into usable DC.
Rectified single-phase power still isn't constant — it pulses from zero up to peak voltage and back, 120 times a second, which means every pulse briefly drops to zero and produces useless low-energy x-rays along the way. Three generator designs handle this differently:
| Generator type | Voltage ripple | What it means |
|---|---|---|
| Single-phase | 100% | Voltage swings all the way from 0 to peak, every pulse |
| Three-phase, 6-pulse | ~14% | Voltage never drops below ~86% of peak |
| Three-phase, 12-pulse | ~4% | Voltage never drops below ~96% of peak |
| High-frequency | ~1% | Converts 60 Hz to 500-25,000 Hz before stepping up — smallest, most efficient, nearly constant potential |
Where those percentages come from — and why you'll see 13% elsewhere. The ripple isn't arbitrary, it's geometry. In a six-pulse generator the voltage dips to cos 30° = 0.866 of peak before the next pulse takes over, so the ripple is 1 − 0.866 = 13.4% — which is also exactly why the floor is ~86%. Twelve-pulse dips only to cos 15° = 0.966, giving 3.4% and a ~96% floor. Textbooks round that one number different ways: Bushong reports 14% and 4%, Fauber reports 13% and 4%. They aren't disagreeing. If a question forces a single value, ~13–14% for six-pulse and ~3–4% for twelve-pulse are both defensible.
Practical effect: because there's no "wasted" low-voltage portion of the waveform generating low-energy photons, three-phase and high-frequency generators produce the same image receptor exposure as single-phase using less kVp at the same mAs — three-phase may need up to a 10-kVp reduction, high-frequency up to a 12-kVp reduction, to match single-phase output.
Questions try to swap which transformer is which — "which one steps up, which steps down." The tube-facing transformer always steps up (kVp is large); the filament-facing transformer always steps down (filament heating needs low voltage, controlled current). A second common trap: assuming lower ripple changes what x-rays are produced — it doesn't change the maximum energy (still equals kVp), it just produces more of them more efficiently.
Source: Bushong, Radiologic Science for Technologists, 12th ed., Chs. 5 and 6 (transformer principles in Ch. 5; x-ray circuit and generators in Ch. 6). Ripple percentages are Bushong Ch. 6 (100% / ~14% / 4% / ~1%); Fauber, Radiographic Imaging and Exposure, 6th ed., Ch. 2 prints 13% for six-pulse (re-verified against the 6th ed. text 2026-08-02). Externally verified and reviewed (2026-07-29): both are roundings of the derivable value — six-pulse ripple = 1 − cos 30° = 13.4%, twelve-pulse = 1 − cos 15° = 3.4% — so this is a rounding difference, not a source disagreement. Confirmed against published rectifier ripple-factor figures (~13.4–13.5% for a six-pulse circuit).
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