Valve Tubes and Rectification

5/21/2018 Valve Tubes and Rectification

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Valve Tubes and Rectification

Paul Frame, Oak Ridge Associated Universities

The transformers used to provide the high voltage for x-ray systems require alternating current (AC). Sinthe induced current on the secondary side of the transformer is also AC, its polarity is constantly changinfrom positive to negative and ac! again - the frequency of the current is "# cycles per second and theduration of a complete cycle is $%"# of a second. &n a given cycle, the potential is positive for $%$'# of asecond and negative for $%$'# of a second.

This poses a potential (pun intended) prolem for x-ray tues since the latter are designed to operate ithspecific polarity a negative cathode and a positive anode (target). *lectrons are supposed to go from theheated filament of the cathode to the anode (target) here the x-rays are produced. &f the alternating currcauses the potential across the tue to change so that the +cathode+ is positive and the +anode+ negative, t

is the possiility that the electron stream in the tue could reverse so that x-rays ere eing produced yelectrons stri!ing the cathode. The result ould e that the x-rays ould no longer e focused since theycoming from to parts of the tue. &n addition, the intensity and energy of the x-rays ould vary.

The folloing diagram shos the to high voltage circuits for an x-ray system one to provide the potentacross the tue (ith full ave rectification), and the other to provide the current to the filament. The forcircuit controls the energy of the x-rays hile the latter circuit controls the intensity of the x-rays.


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A step don transformer is used for the filament circuit to reduce the high voltage from $$#%$'# volts tovolts.

Since these Coolidge x-ray tues employ a vacuum, electrons ill not flo in the rong direction from anegatively charged anode to a positive cathode unless the anode is hot enough to emit electrons. The ano

(targets) of radiator tues, for example, never get hot enough for electrons to e eected. As such, electroncan only flo in one direction and these tues are said to e +self-rectifying.+ The folloing diagram shothe proper flo of electrons through a self-rectifying tue during the half of the AC cycle hen the anode

positive and the heated cathode is negative. -rays are emitted from the target.

The folloing diagram shos that the electrons do not flo through a self-rectifying tue during the halfthe AC cycle hen the anode is negative and the heated cathode is positive. -rays ould not e emittedfrom the tue.


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The folloing diagram shos the potential high voltage across a self-rectifying tue and the current throthe tue during to complete cycles. This is half ave rectification.

Some tues, the +niversal+ tue for example, are not self-rectifying. To achieve the half-ave rectificatshon aove, a circuit employing to valve tues (or solid state diodes) ould e used.

/alve tues are essentially self-rectifying x-ray tues that are designed to produce as lo an intensity of x


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rays as possile. The cathode is a central heated filament that is surrounded y a large annular anode. Theelectrons from the cathode are not focused and the stri!e the entire surface area of the anode. Since the lanever gets hot enough to emit electrons, current can only flo in one direction. &n other ords, it is a oneay valve0

1y using a ridge ith four valve tues in it, e can achieve full-ave rectification and doule the tue2sray output. Such a full ave rectification circuit ould or! as indicated in the folloing to diagrams.


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The folloing diagram shos the potential high voltage across a tue and the current through the tue duto complete cycles hen a full ave rectification circuit is employed. &t also shos ho the range (ripof the tue current can e smoothed out y employing a capacitor in the tue circuit. An even smoothercurrent ith less ripple can e achieved y employing a three phase system. State of the art systems do e

etter and produce hat is called a constant potential y employing high and medium frequency generatothat rectify, smooth, chop and invert the high voltage on the primary side of the high voltage transformer.


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Coolidge X-ray Tubes

Paul Frame, Oak Ridge Associated UniversitiesIntroduction

3ithout a dout, the single most important event in the progress of radiology as the invention y 3illiaCoolidge in $4$5 of hat came to e !non as the Coolidge x-ray tue. 6evertheless, despite its clearsuperiority, the Coolidge tue did not immediately replace cold cathode tues - the latter continued to emanufactured into the $4'#s and sa routine use into the $45#s. &n fact, there ere instances of cold cathtues eing employed in radiology as late as the $4"#s0

&f you are interested in the details of Cooludge tue production, the $4'# paperManufacture of theCoolidge X-ray Tubey 7oinson and 8oore is orth a read.

The characteristic features of the Coolidge tue are its high vacuum and its use of a heated filament as thsource of electrons. There is so little gas inside the tue that it is not involved in the production of x-rays,unli!e the situation ith cold cathode gas discharge tues.

The operation of the Coolidge tue is as follos. As the cathode filament is heated, it emits electrons. Thhotter the filament gets, the greater the emission of electrons. These electrons are accelerated toards the

positively charged anode and hen the electrons stri!e the anode, they change direction and emitremsstrahlung, i.e., x-rays ith a continuous range of energies. The maximum energy of the x-rays is thsame as the !inetic energy of the electrons stri!ing the anode. &n addition to the x-rays produced at the fo

spot of the anode, some undesirale x-rays (stray radiation) are produced y electrons stri!ing other tuecomponents.

The !ey advantages of the Coolidge tue are its staility, and the fact that the intensity and energy of therays can e controlled independently. &ncreasing the current to the cathode increases its temperature. Thiincreases the numer of electrons emitted y the cathode, and as a result, the intensity of the x-rays.&ncreasing the high voltage potential difference eteen the anode and the cathode increases the velocitythe electrons stri!ing the anode, and this increases the energy of the emitted x-rays. 9ecreasing the currethe high voltage ould have the opposite effects. The high degree of control over the tue output meant tthe early radiologists could do ith one Coolidge tue hat efore had required a stale of finic!y coldcathode tues. As a onus, the Coolidge tue could function almost indefinitely unless ro!en or adly

aused.
http://www.orau.org/ptp/Library/X-ray/manufacture.pdfhttp://www.orau.org/ptp/Library/X-ray/manufacture.pdfhttp://www.orau.org/ptp/Library/X-ray/manufacture.pdfhttp://www.orau.org/ptp/Library/X-ray/manufacture.pdfhttp://www.orau.org/ptp/Library/X-ray/manufacture.pdf


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The Shape of the Tube

The asic shape of the original Coolidge tue consisted of a spherical ul ith to cylindrical armsextending out on opposite sides the cathode arm and the anode arm. The arms increase the dimensions otue so as to avoid electrical arcing from one end to the other. The length of the arms also helps prevent selectrons on the inside of the tue from getting close to the tue ends.

The ul is large so as to increase the surface area through hich the tue radiates heat (to limit the glasstemperature). &n addition, its large si:e reduces the amount of tungsten that can e deposited per unit areait !eeps the glass aay from the strongest portion of the electrostatic field - the longitudinal electrostaticstresses in tues ith a conductive layer of tungsten deposited on the inner surface could negatively affectue;s operation. &f the tue incorporates another mechanism y hich heat can e dissipated (other thansimple radiation through the glass envelope), the ul can e made smaller.

The hard glasses (e.g.,


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high x-ray output is required, a larger focal spot ould e needed to mitigate the temperature increase.

&n the early tues the angle of the target as usually >? degrees (see figure elo left). @ater tues oftenemployed the so-called line-focus principle in hich the target angle as closer to '# degrees (see figure

elo right) . This reduced the effective area of the focal spot (as vieed from the perspective of the oeeing x-rayed permitted) ithout significantly affecting the area of the anode omarded y the electroneam from the cathode. &n other ords, it permitted high loading (x-ray intensity) ithout having to sacimage detail.

Some degree of crac!ing and pitting of the focal spot is normal, especially in tues ith a large focal spotortunately this doesn2t affect the tue2s performance unless the damage is so severe that the aility of theanode to dissipate heat is compromised. At least in some cases, a tungsten-rhenium alloy has een used inconstruction of the anode to reduce the crac!ing and pitting, as ell as reduce the evaporation of tungstenonto the tue envelope.

The Cathode

The cathode of the Coolidge tue incorporates a ound tungsten filament that emits electrons hen heateThere are to general configurations of the filament. *ither the spiral tungsten ire ta!es a circular%conic

form (typical of the +niversal+ tues) or it is shaped li!e an elongate coil (the so-called 1enson design).


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The filament is located in a cylindrical chamer or slot machined into in the cathode. As a rule, the focalis similar in si:e and shape (either round or elongate) to this opening.

The si:e and shape of the focal spot is partly determined y the filament2s position relative to the othercomponents of the cathode. The focal spot si:e can e reduced y positioning the filament deeper in the s(or opening) in the cathode, or increased y moving the filament closer to the top of the opening.

9ual focus tues employ cathodes ith to filaments one small, the other large. *ach filament is positioin its on slot. The smaller filament produces a small focal spot for fine focus or!. The larger filament,hich produces a roader focal spot, is employed hen faster exposures (higher intensities) are required.These dual focus tues usually have a sitch at the end of the cathode arm that is used to select the desirefocal spot.

A step don transformer is used to reduce the line voltage from $$# volts to the $' volts required for thefilament. The current to the filament is adusted ith either a 7heostat (resistance) or &nductance typeregulator. An ammeter is employed to measure the current used to heat the filament - the current servesmeasure of the intensity of the x-rays that are eing produced.

To help prevent cold cathode discharges that can result in erratic performance of the tue, the electrodessmooth and rounded. Such discharges are more li!ely hen very high voltages are used ith tues in hithe anode and cathode are closely spaced.

The Glass

Although the envelopes of x-ray tues can e made of metal, glass is more popular ecause it is a goodinsulator, it is vacuum tight, it has a relatively high melting point, it does not seriously attenuate the x-rayand it can e faricated into a ide range of shapes. The early tues (e.g., pre $45#) ere made from sof

glass containing sodium or cerium. @ater tues tended to use hard orosilicate glass even though it as mdifficult to or! ith. 1orosilicate glass has the advantages of a greater dielectric strength and higher mepoint. The latter means that the tues could e more effectively degassed and evacuated. 1ecause orosili


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glass alls ere thic!er than those made from softer glass, a indo had to e created y grinding donregion of the glass all that the x-rays needed to penetrate.

3ith use, the clear colorless glass of an x-ray tue can ecome colored. Such a color change is either due

the asorption of radiation energy, or the deposition of tungsten on the inside surface of the glass. Theasorption of x-ray energy y electrons in the glass promotes them to a higher energy state (the conductio

and). The no moile electrons move to positively charged impurities in the glass here they ecometrapped. Since these trapped electrons are at a higher energy level than they ere prior to the exposure toradiation, the asorption spectrum and the color of the glass is affected. The resulting color depends on ttype of the impurities the softer glass of the older tues (e.g., $4'#s vintage) tended to turn purple, hileharder glass of the more modern tues (e.g., post $45#) tended to ta!e on an amer color.

Bust as tungsten can e deposited on the inside surface of an incandescent light ul, tungsten can edeposited on the inside surface of the envelope of an x-ray tue. &f this deposition is significant, the tue

proaly operated eyond its rated capacity. &n extreme cases, the deposition can e sufficiently great thgives the glass a mirrored appearance. A prolem ith an excessive accumulation of tungsten is that theinside of the glass ecomes conductive and allos electrons to lea! across the glass surface. This ma!estue more susceptile to puncture.

&n addition, the accumulation of electrons on the inside of the glass can distort the flo of electrons fromcathode toards the anode. &t can also produce a static charge that attracts dust onto the outer surface of ttue. &f this dust asors moisture from the air, the outer surface of the tue ecomes conductive and thisincreases the possiility that spar!s ill e produced that can puncture the tue;s envelope. Tues designfor therapy required thic!er glass than diagnostic tues ecause the higher voltages at hich they operate

produced a greater potential difference eteen the negatively charged inside of the glass and the outside

the glass. &f the glass as too thin, the charge ould rea! through the glass and puncture it.

3hen operated at lo voltages, the glass envelope might fluoresce. The color of the fluorescence dependthe type of the glass soft and hard glass usually exhiited a green and lue fluorescence respectively.

The ire penetrating the glass seals at the end of the tue is hat as !non as 9umet ire, a copper-coalloy of nic!el and iron that had the same coefficient of expansion as the glass.

Shoc-!roof Syste"s

The electrical cales providing high voltage and current to the early (pre $45# or so) x-ray systems used

uninsulated ire. The reason for this as that insulated ire could duplicate a cho!e-coil and limit thefilament current. As a result, early radiologists had to e very careful to avoid electrocution. The firstcompletely shoc!-proof x-ray system seems to have een a dental x-ray unit developed in $4'$ y 3illiaCoolidge - the high voltage system and tue as immersed in a grounded metal tan! filled ith oil.

&mmersing the tue in oil not only improves cooling, it reduces the potential for lea!age current across thtue surface, and this means that the tue can e made shorter. &t also reduced the effect of altitude andhumidity on tue performance (altitude and humidity affect the cooling capaility of air).

References

*. C. Berman. 8odern -7ay technic. 1ruce


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7.@. *isenerg. 7adiology D An &llustrated Eistory. 8osy. St. @ouis. $44$.

3.9. Coolidge. The development of modern roentgen-ray generating apparatus. Am. B. 7oentgenology. '"#?-"'#. $45#.

8.B. Fross, G.B. Atlee.


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#$niversal# Coolidge X-ray Tube


The niversal Tue as so named ecause it could e used in almost any application, e.g., diagnostic x-r(roentenography), fluoroscopy and therapy as ell as various non-medical uses. &n essence, it is the origiCoolidge tue ith a fe minor modifications.

9uring operation, the heat from the anode is radiated out through the ul of the tue. So that the glasson;t overheat, the ul has to e !ept fairly large ca. " D =I in diameter.

The threaded scre-in connector at the end of the cathode arm is the same as that employed ith a light

ul. This reflects the fact that 3illiam Coolidge;s inspiration for the hot cathode x-ray tue as his earlor! developing tungsten filaments for incandescent lights.

The anode is solid tungsten and it is attached to amolydenum stem. The latter, in turn, is connectea split metal tue that helps provide support.

There ere three variations on the niversal tuethose ith a fine focal spot, those ith a mediumfocal spot, and those ith a road focal spot. Thecathode filament of the fine focus version forms a

spiral near the end of a short molydenum tue, that of the medium and road focal spot tues forslightly conical spiral.

The folloing tale indicates the ratings for these tues.

Tube TypeMa%i"u"

Voltage

&'uivalent

Spar Gap

Ma%i"u"

Current

Continuous

use li"it

ine ocus $##,### "I '? mA '# seconds

8edium ocus $##,### "I ># mA $? seconds1road ocus $##,### "I # mA ? seconds

1road ocus $>#,### $#I ? mA continuous

The fine and medium focus tues ere etter for general radiographic or!. The road focus niversal tas est suited for therapeutic use ecause of its greater potential output. 3hen the road focus tue asoperated at very high voltages, it ould have een particularly important to !eep the tue clean and farenough aay from metal oects to prevent spar!ing.

The niversal tue needed to e operated ith a rectified current. &f unrectified AC current ere used,electron emission from the tungsten target ould e possile during the half of the cycle hen the target

(+anode+) had a negative charge and the JcathodeI a positive charge. This could only occur if the target h


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een heated to the point that it emitted electrons.

References



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Radiator Coolidge X-ray Tubes

Paul Frame, Oak Ridge Associated UniversitiesTo learn aout Coolidge 7adiator tues, you couldn2t do much etter than read 3illiam Coolidge2s $4$4

paper The R adiator Type of Tube- 7adiator tues (and niversal tues) ere first produced y the Fen*lectric Company. to $#%"> inch), fine image detail as possile. The other common type o7adiator tue had a 5# milliamps rating ith a ?I spar! gap. &t could e operated continuously for up to 'seconds. This tue had a relatively large focal spot ($$%"> to $?%"> inch), hich sacrificed image detail

permitted shorter exposures. A less common radiator tue, rated for a $## milliamp current ith a ?I spagap, had a large focal spot ($>%"> to '#%"> inch) and could e operated continuously for up to ? seconds athe rated current.

The type of tue as sometimes indicated according to the spar! gap and the maximum rated current asfollos ?-$#, ?-5# and ?-$##.

An advantage of the 7adiator tue as that it could e operated ithout a rectified current ecause it is srectifying. &f unrectified AC current ere used, electron emission from the tungsten target is not possileduring the half of the cycle hen the target (anode) has a negative charge and the JcathodeI has a positivcharge. This is ecause the temperature of the tungsten target does not get hot enough for it to emit electr

References



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Rotating Anode X-ray Tubes


9uring operation, a large dis!-shaped tungsten anode is rotated at high speed (5### to 4### revolutions pminute). The motive force for the rotation is provided y an induction motor the indings of hich arehoused outside the tue. Although the focal spot of the electrons impinging on the anode is no larger thanthat in a tue ith a stationary anode, the effective area of the anode exposed to the eam is much larger.this means, the heating of the anode is reduced and the tue loading can e increased (e.g., up to ?## mAith a ' mm x ' mm focal spot).

Kne of the tric!y prolems that had to e overcome in uilding these tues as ho to luricate the tue2

moving components - et luricants could not e used ecause of the vacuum.

According to 7ichard 8ould, the concept of the rotating anode x-ray tue originated ith *lihu Thomso$4". Kthers have suggested that it as 7oert 3ood in $4=. *ither ay, it as some time efore aor!ing model could e constructed. The first practical or!ing version seems to have een descried Feneral *lectric2s 3illiam Coolidge in $4$?. Luoting the latter2s paper in the 9ecemer $4$? issue of theAmerican Bournal of 7oentgenology, the tue had a +target rotation of =?# revolutions per second ith thfocal spot descriing a circle #.=?+ ($4 mm) in diameter. '.? times as much energy for the si:e of the focspot is otained hen compared ith the stationary target.+


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General &lectric (R-) (enotron Tube *ca+ ,).s-,/.s0

Luoting Frigg2s Trail of the Invisible ight +&n $4$>,at F*2s 7esearch @a in Schenectady, MSaulN9ushman developed the hot cathode valve tue. ThisMreferring to a photo of the O7-5N is the firstcommercial model (O7-5). &ts long life stemmed, in

part, from the fact that the anode surrounded thecathode, thus preventing erratic voltage drops fromcharges developed on the inner surface of the glass.+

The lael on the stem identifies it as a +Coolidge+tue, ut it came to e !non as a +!enotron.+

The photo to the right sho the spiral cathode insidethe tuular, open-ended anode.

The O7-5 as air cooled (the O7-> as oil cooled).

Si1e2 '#+ long, ?+ in diameter (ul)

!rice2P'> in $4?5-$4?> F* catalog

References

*.7.6. Frigg. Trail of the &nvisile @ight. Charles C Thomas. $4"?.

Feneral *lectric -ray Corporation. !oolidge #$ra" Tubes $ 'enotrons& 1ulletin 6o. '45. $45>.

Feneral *lectric Catalog. -ray Supplies and Accessories. $4?5-$4?>.


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(onated b" the Universit" of 'ansas courtes" of %ichael emon

Valve Tubes and Rectification

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