X-Ray Tube Symbol
Definition: The X-Ray Tube symbol represents a vacuum tube that generates ionising X-radiation by accelerating electrons from a heated cathode filament toward a tungsten anode target, as used in medical, industrial, and analytical radiography schematics; the component is designated XV or XT in circuit documentation and is referenced under IEC 60617 and ANSI/IEEE 315 tube symbols.
Also known as: roentgen tube, X-ray generator tube, Coolidge tube, rotating-anode tube, X-ray source symbol.
What the X-Ray Tube symbol means
The X-Ray Tube symbol denotes a thermionic vacuum tube in which a high-voltage potential (typically 20 kV–150 kV) accelerates a beam of electrons from the cathode toward the anode. When electrons strike the anode target material (usually tungsten), they decelerate rapidly and emit X-radiation (bremsstrahlung) as well as characteristic X-rays specific to the anode material.
In schematic diagrams, the symbol appears in medical imaging equipment (CT scanners, fluoroscopy, dental X-ray units), industrial non-destructive testing (NDT) systems, and analytical instruments such as X-ray diffractometers (XRD) and X-ray fluorescence (XRF) spectrometers. The symbol communicates the presence of high-voltage hazard and ionising radiation in the circuit.
How to identify the X-Ray Tube symbol
The X-Ray Tube symbol is drawn as a large ellipse or elongated oval (representing the evacuated glass or metal envelope) with two main internal elements: on the left, a small heating coil or line near the cathode end indicating the electron-emitting filament, and on the right, a diagonal or angled solid line or wedge representing the anode target. A horizontal arrow or line between cathode and anode indicates the electron beam path. The cathode terminal exits the left side of the envelope and the anode terminal exits the right side; a third terminal on top represents the filament heater supply. The symbol clearly distinguishes it from a generic diode or triode by the elongated envelope shape and the explicit anode-target wedge.
Function in a circuit
An X-Ray Tube converts electrical energy into X-radiation. A low-voltage supply (3–12 V AC) heats the tungsten filament cathode, releasing electrons by thermionic emission; a high-voltage DC supply (kVp) between anode and cathode then accelerates those electrons across the vacuum gap. When the electron beam strikes the anode (rotating or stationary tungsten target), approximately 1% of the electron kinetic energy is converted to X-rays (bremsstrahlung and characteristic radiation) and 99% becomes heat, which the anode dissipates through rotation, oil cooling, or radiative cooling.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 provides graphical symbols for electron tubes including X-ray tubes; the envelope is drawn as an oval or circle with internal cathode (heated filament) and anode (target) elements. IEC 60617-05 covers electron tube symbols. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 defines the X-ray tube symbol similarly: an oval envelope with a diagonally placed anode target and a filament symbol near the cathode; the symbol is consistent with IEC practice for thermionic tube diagrams. |
| Key difference | IEC and ANSI/IEEE representations are functionally identical for the X-ray tube; both use an oval envelope with internal cathode-filament and anode-target elements. Minor stylistic differences may exist in the exact angle of the anode target wedge but the topology is the same. |
Terminals / pins
| Pin | Name |
|---|---|
| cathode | Cathode |
| anode | Anode |
| filament | Filament |
Typical values
Anode voltage (kVp): 20 kV–150 kV (medical), up to 450 kV (industrial NDT); tube current (mA): 0.1 mA–1200 mA; filament voltage: 3–12 V AC; focal spot size: 0.1 mm–2 mm; anode heat capacity: 100 kHU–2000 kHU (kilo-heat-units); anode rotation speed: 3000–10 000 rpm for rotating-anode designs.
Where the X-Ray Tube symbol is used
- Medical diagnostic imaging: radiography, fluoroscopy, CT scanners, dental panoramic and periapical units
- Industrial non-destructive testing (NDT): weld inspection, casting flaw detection, pipeline inspection
- Security screening: airport baggage X-ray systems and cargo inspection portals
- Analytical instrumentation: X-ray diffractometers (XRD), X-ray fluorescence (XRF) spectrometers, electron microprobes
- Radiation therapy: orthovoltage X-ray therapy units for treating superficial tumours
- Scientific research: synchrotron beamline insertion devices, laboratory micro-CT scanners
Example
In a portable industrial NDT radiography unit, the X-Ray Tube symbol is connected between a high-voltage transformer secondary (rated 160 kVp) and a mains-referenced common, with the filament heater supply shown as a separate low-voltage winding; the schematic also shows the exposure timer relay (K1) that gates the high-voltage contactor, illustrating how a short 2 s pulse of 5 mA at 100 kVp is used to radiograph a steel weld for internal defects.
Key facts
- The X-Ray Tube symbol represents a thermionic vacuum tube in which high-voltage electron bombardment of a tungsten anode generates ionising X-radiation (bremsstrahlung); it is designated XV or XT in circuit schematics.
- X-ray tubes operate with anode-to-cathode voltages from 20 kV (dental units) to 450 kV (industrial NDT), making them among the highest-voltage components found in electronic schematics.
- The symbol has three terminals: Cathode (electron-emitting filament end), Anode (target end), and Filament (heater supply), corresponding to the physical x-ray tube's three electrical connections.
- X-ray tube efficiency is approximately 1%: only 1% of electron kinetic energy is converted to X-rays; the remaining 99% becomes heat at the anode target, requiring active cooling in high-power designs.
- Rotating-anode X-ray tubes spread the heat load over a larger track area (up to 50 mm diameter), allowing tube currents up to 1200 mA; stationary-anode tubes are limited to low-power applications such as dental units.
- The symbol appears in IEC 60617-05 (electron tube symbols) and ANSI/IEEE 315-1975; both standards depict an oval envelope with internal cathode and angled anode-target elements.
- X-ray tube schematics require radiation safety and high-voltage interlock annotations per IEC 60601-1 (medical electrical equipment) or EN 61010-1 (laboratory equipment) to warn of dual hazards: ionising radiation and lethal voltage.
Frequently asked questions
What does the X-Ray Tube symbol look like in a circuit diagram?
The X-Ray Tube symbol is drawn as a large oval or elongated ellipse (representing the vacuum envelope) with a small filament coil near the left (cathode) end and a diagonal wedge or angled line near the right (anode) end representing the tungsten target. Three terminals exit the symbol: Cathode on the left, Anode on the right, and Filament on the top or bottom.
What does the X-Ray Tube symbol mean in a schematic?
The X-Ray Tube symbol means the circuit contains a high-voltage vacuum tube that generates X-radiation. It signals both a lethal high-voltage hazard (20 kV–450 kV at the anode) and an ionising radiation hazard. The symbol tells a schematic reader that a specialised high-voltage power supply, protective enclosure, and radiation-safety interlocks are required.
What are the pin names on the X-Ray Tube symbol?
The X-Ray Tube symbol has three pins: Cathode (the heated filament end that emits electrons), Anode (the tungsten target that receives the electron beam and emits X-rays), and Filament (the low-voltage heater supply terminal, typically 3–12 V AC). The high voltage (kVp) is applied between the Anode and Cathode terminals.
What is the IEC standard for the X-Ray Tube symbol?
The X-Ray Tube symbol is covered by IEC 60617-05, which specifies graphical symbols for electron tubes including thermionic devices. ANSI/IEEE 315-1975 also defines the symbol with a consistent oval-envelope and anode-target representation. Both standards treat the X-ray tube as a specialised thermionic diode with a separate filament heater circuit.
How does an X-Ray Tube differ from a regular diode symbol?
A regular diode symbol is a triangle pointing to a vertical bar, representing a two-terminal semiconductor device. The X-Ray Tube symbol uses a large oval envelope (indicating a vacuum tube) with an internal cathode-filament and angled anode-target, plus an explicit third terminal for the filament heater. The oval envelope distinguishes all tube symbols from semiconductor symbols.
What voltage and current values are typical for an X-Ray Tube?
Medical diagnostic X-ray tubes operate at 40 kV–150 kV (kVp) with tube currents of 0.5 mA–1200 mA. Dental units typically run at 60–70 kVp and 7 mA. Industrial NDT tubes can reach 450 kVp at lower currents. Filament heater circuits operate at 3–12 V AC, drawing 3–6 A to produce the required thermionic emission current.
Where is the X-Ray Tube symbol used in circuit diagrams?
The X-Ray Tube symbol appears in schematics for medical imaging equipment (radiography, CT, fluoroscopy, dental X-ray), industrial non-destructive testing systems, airport security baggage screeners, X-ray fluorescence (XRF) and X-ray diffractometry (XRD) instruments, and orthovoltage radiation therapy units. It is always accompanied by high-voltage and radiation-hazard warning annotations.
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