CHEATER CIRCUITS FOR THE TESTING OF THYRATRONS

Transcription

1 . AUGUST CHEATER CIRCUITS FOR THE TESTING OF THYRATRONS I. MEASUREMENT OF GRID CURRENT hy M. W. BROOKER *) and D. G. WARE **) : The range of hyrarons now being produced commercially includes cerain ypes which operae a high anode volages and draw high currens. The convenional ways of horoughly esing hese valves under heir full-load operaing condiions would make excessive demands on he power supply." Aemps have herefore heen made o devise "cheaer circuis" which simulae full-lood operaion a only afracion of he power load and cos of energy. In Par I ofheir aricle he auhors describe a cheaer circui which allows he measuremen of grid curren jus before he vllive srikes. The pre-sriking grid curren is an imporan check on valve qualiy. Par II will deal ioih anoher form of cheaer circui, designedfor he lifeesing of hyrarons. In a high-vacuum riode wih unsauraed emission he flow of anode curren is limied mainly by a space charge of elecrons. This in urn can he conrolled a will hy variaion of he negaive poenial ofhe grid. In a gas-filled riode (hyraron) here is no such conrol and he grid acs only as a kind of rigger which deermines when he valve shall fire. This, oo, means a cerain conrollahiliy, hough of a slighly differen naure from ha in a vacuum uhe. When he volage applied o he grid is made less negaive and ulimaely reaches a cerain criical level, anode curren commen~es o flow; he posiive ions produced neuralize he elecronic space charge and he anode curren rises rapidly o he value prescribed hy he circui parameers. This process is known as he firing or sriking of he valve. Once a gas-filled valve has sruck, he grid has no conrolover he curren, as he effec of is poenial is masked hy a sheah of posiive ions. When a gas-filled valve is in he non-conducing, sae wih he grid hiassed negaively; a grid curren will flow. This is due o a numher of effecs, which will he explained laer. The pre-sriking grid curren flows hrough he grid resisor and may disurh he accuracy of conrol of he valve. Indeed, if he grid curren hecomes relaively large and he grid resisance is high, i may he quie impossihle o conrol he valve. I is herefore necessary o -have an accurae mehod of measuring he presriking grid curren, hoh as an aid in design and as a check on qualiy. This aricle firs reviews some known mehods of -measuring grid curren in hyrarons and hen *) Mullard Radio Valve Co., Ld., Micham, England. *"') Formerly wih Mullard Radio Valve Co. Ld. describes heer mehods, which are simple o use and which measure grid curren under, condiions idenical wih he full-load valve operaing condiions. Convenional mehods of measuring grid curren The grid curren is a hyraron hefore i srikes is acually he sum of several currens flowing' in he. same sense. I has componens due o: 1) elecrical leakage beween he elecrodes, 2) he flow o he grid of posiive ions produced hy elecrons escaping conrol by he grid ("unconrolled cahode.emission"), 3) primary hermionic emission from he grid iself, and perhaps 4) phoo-emission from he grid. I is possihle o measure some of hese com'pnens individually. Thus he leakage curren alone can he found by normal insulaion measuring mehods. The curren due o unconrolled cahode emission can he measured by puing a micromeer in he grid circui and applying he raed anode volage; he grid curren-grid volage may hen he ploed, (The. microammeer should he proeced agains overload, as he grid curren rises o a high value when he valve srikes.) Bu he real prohlem is o measure i under acual operaing condiions. I is in securing hese condiions.ha he main difficuly lies. The hasic idea employed hy mos of he known mehods of measuring he pre-sriking grid curren of gas-filled valves are given below, The swich mehod This mehod is very simple o se up and simple o use, hu i only 'gives an approximae idea of he grid curren under working condiions.

2 44. PHILIPS TECHNICAL REVIEW VOL. 16, No. 2. The procedure is o operae he valve wih a resisive load a is raed mean anode curren for a ime sufficien o raise all he elecrodes o heir working emperaures. A swich is hen operaed which disconnecs he load and connecs anode o cahode and a he same ime connecs a negaive volage of, say, 1 V o he grid via a micro ammeer. The curren read on his meer is called "he" grid curren. The advanages of his mehod can be lised as follows: 1) The mehod can be applied very easily o any ype of hyraron, 2) The equipmen is cheap. The disadvanages are equally obvious, hus: 1) The curren recorded decreases due o he valve elecredes cooling. 2,) Leakage curren (if presen) is incorrecly measured owing o anode and grid volages being abnormal. 3) Unconrolled cahode emission is absen. 4) Ionizaion due o emission from oher elecredes such as screens and anode is absen. The cu-off mehod A mehod generally used in he Unied Saes, known as he cu-off mehod, consiss in' operaing he valve from an A.C. power supply, which may he he 22 vol mains, hrough a resisor which is adjused o give he maximum raed mean anode curren when he valve conducs for complee half cycles. Afer running like his for a long enough period for he valve o reach emperaure equilibrium,.he grid volage is made more negaive unil he valve jus fails o re-srike, and he volage V gl a he inpu side of he grid resisor is noed a he cu-off poin. This is done wih a low and a high grid resisance (R gl, R g2 ). If he criical grid curren is represened by I geri, he criical volage a he grid iself is given by Vgl-IgeriRgl = Vg2-IgeriRg2' Vgl and Vg2 being he volmeer readings. From his w~ find: I _ Vg2- Vgl --<c:»: g2 - gl This mehod has' a number of advanages over he previous one, viz.: 1) The grid volage a he poin a which he measuremen is made is he normal criical grid volage. Thus he elecrical leakage curren included in he measuring resuls is correc. 2) Any unconrolled emission will be measured, bu as his' depends upon he applied anode volage, i is necessary o perform he es a (1) he full raed anode volage if any accurae resul is o be obained. 3) Grid emission is also measured a he propel' volage. However, he mehod does no allow he full raed peak anode curren o be drawn, as for a half-sine waveform, he peak-o-mean anode curren raio hardly exceeds 3, while modern hyrarons are raed for peak-o-mean raios of 1 or even more. This disadvanage is imporan, because in general he valve volage drop is higher when high peak currens are drawn; herefore he valve runs a a higher J;em.peraure, and he grid curren is greaer under such condiions. Anoher disadvanage is ha as he grid volage is made more negaive oward he cu-off poin, he original half-sine waves diminish o quarer sine waves and he mean anode curren correspondingly decreases o half is original value. Now in order o obain an accurae reading of cu-off volage, i is necessary o raise he negaive grid volage raher slowly, wih he resul ha he valve cools appreciably during he ime aken for he measuremen. This of course.means ha he measured grid curren is lower han he acual working grid curren. On he oher hand, on swiching from a low grid resisance o a high one, sriking will occur earlier in he cycle, so he mean anode curren will increase, making he valve hoer and his in urn increases he grid curren. I was found ha in some cases he grid curren changed so rapidly ha i was no possible o obain any seady operaing condiion wih a high grid resisance. This, of course, occurred wih defecive valves where he oal grid currerr which flows is high, bu for es purposes i is essenial o be able o measure grid currens up o exremely high values. The modified cu-off mehod wih feedback The drawbacks of he sandard cu-off mehod have lèd o he design of a modified mehod. Here, he difficulies menioned have been avoided by means of negaive feedback. This mehod, which was he firs o be developed by he auhors, proved of considerable value in early hyraron developmen work by Mullard. The basic mehod is shown in fig. 1. The anode volage of he es valve is supplied from 22 V A.C. mains. The grid circui includes a sabilized D.C. supply adjusable by a poeniomeer supplying a posiive volage o he grid. A variable load resisor RI is in series wih he cahode, and a group

3 AUGUST 1954 CHEATER CIRCUITS FOR THYRATRON TESTING 45 of resisors Rg can be swiched, one by one, ino he grid circui. When an anode curren flows, a recified halfwave volage appears across he load resisor Rl' This volage is smoohed by a filer L 1 -C 1 -L 2 -C 2 and appears as a negaive volage a he grid of he valve 22V'V + b J. Fig. 1. Cu-off mehod for measuring he grid curren of a hyraron (Thi)' wih negaive feedback. The D.C. volage developed across he load resisor RI is smoohed by a filer L I -C I -L 2 -C 2 and fed back wih negaive polariy o he grid of Th!, in opposiion o an adjusable D.C. volage from a sabilized supply (85 V). The resuling grid volage Vg is read on a volmeer, he mean anode curren lam on an ammeer. By means of a swich, various resisors Rg can be insered ino he grid circui..' This negaive volage is parially counerbalanced by he sabilized supply, which acs in he opposie sense. Thus he sriking of he valve is conrolled by a (negaive) grid volage, which increases linearly wih he mean anode curren.. The self-compensaing acion of he circui is bes undersood by reference o fig. 2. Consider he effec of a rise in grid vólage from levelio level 2 caused by a change in curren flowing hrough he grid resisor. The valve will srike earlier in he cycle, he "firing angle" being reduced from al o a 2. The mean anode curren and he mean volage across Rl will herefore increase. A" greaer negaive D.C. volage is herefore fed back o he grid of he valve, hus ending o keep he anode curren seady. This self-compensaing effec ensures sable running a a seady anode curren, providing no aemp is made o fire he valve near he peak of he anode volage curve. Wih firing angles greaer han abou 8, insabiliy and oscillaion may se in because of he ime delay in he filer circui. Using his new mehod, he variaions of he mean anode curren are reduced o a much lower level and i is easy o mainain a seady anode curren, which is se by adjusing he load resisor. The resulan grid volage applied o he hyraron is read on he volmeer Vg of fig. 1, and he grid currens may hen he measured wih a firing angle of, say, 45, as described above under he heading The cu-off mehod. The chief advanage of he mehod jus described is ha measuremens of grid curren can be made wihou affecing he working condiions. However, i sill suffers from he disadvanage ha he peak anode curren canno exceed 5 imes he mean anode curren, owing o he firing angle being limied o abou 8 1). I is possible o overcome his disadvanage by superimposing on he D.C. bias an A.C. volage (sinusoidal, or beer sill, pulseshaped), leaving he res of he circui unalered. By phase-shifing he A.C. volage, firing angles up o 18 and smooh anode curren conrol down o zero can hen he obained. In he nex secion a mehod of measuremen will be described which makes use of pulse firing o obain sable eperaion a high peak-o-mean curren raios. Fig. 2. Explanaion of sabilizing effec of feedback in fig. 1. A rise in grid volage V 8 from leveilo level 2 decreases he firing angle (deermined by he poin of inersecion wih he doed eriical grid volage curve, 3) from al o a 2, hereby increasing he mean anode curren lam, i.e. diminishing Vg. This ends o keep lam seady. Val volage across Th!; Vore are volage; ia! anode curren; w = 2:n:X mains frequency; ime. Cheaer circui A disadvanage common o all mehods menioned above - so far as high-power hyrarons are concerned - is ha he esing makes an excessive.demand on he power supply, especially when volages much higher han 22 V r.m.s. are o be used. Aemps have herefore been made o devise special circuis which simulae full-load operaien a only a fracion of he power. I) As can be shown by a simple calculaion, he peak-o-mean anode curren raio for firing angles a up o 9 is given by 2:n:/(I+eos a), and for a beween 9 and 18 by 2:n:sin a/(i+eos a ), see fig. 3. The laer expression ends o infiniy when a approaches 18.

4 4 6 PHILJPS TECHNICAL REVIEW VOL. 16, No ~----~------~~~~------r-----~ o '87 _a Fig. 3. Raio of peak anode curren la p o mean anode curren lam as a funcion of he firing angle a.. A characerisic fundamenal o all hyrarons makes possible such circuis: he fac ha once a valve has sruck, he volage across i drops o a low level, which is almos independen of he anode curren. Alhough he peak anode volage before sriking may be, say, 15 V, ye when carrying is full raed anode curren he anode volage Varc is less han 2 V, jus sufficien o mainain ionizaion (fig. 2). Providing ha he volage applied o a valve does no fall below his mainaining poenial (arc volage), he valve coninues o conduc. If, x 8& Fig. 4. Cheaer circui for measuring grid curren under full operaing condiions. Th! hyraron under es. Th 2 auxiliary hyraron. T 1 1w volage ransformer (1 V). T 2 high volage ransformer, fed from variable auoransformer Ta. RI load resisor. R 2 curren limiing resisor in H.T. circui. Rg bank of grid resisors. Firing pulses are applied a X and also o ransformer T 4 (see fig. 6). hen, a circui can be devised which swiches auomaically from a high o a low volage supply immediaely afer he valves has sruck, only a low surplus volage will have o be aken up by he load, and he power economy will be considerable. Such a circui is.known -- for obvious reasons - as a cheaercircui. To provide a versaile circui, useful in cerain oher ess, a special cheaer circui was devised. Is mode of operaion is based on he same principles as he mehods previously described for he cu-off mehod:,he grid curren is deduced from he. change observed in he criical grid volage a a given anode volage when a resisor is swiched ino he grid circui. Once again a D.C. bias is used for conrol and measuremen, bu he sysem of firing he valve is changed. A pulse riggering circui is inroduced, which ensures reliable firing a any poin in he posiive anode volage half-cycle, and also plays an imporan par in he cheaer acion.. The main circui of he new app~raus is shown is jig. 4. The valve under es Th l is conneced o a 1 V A.C. supply, in series "wiha load resisor RI and an auxiliary hyraron Th 2 I is conneced also, via a large series. resisor R2' o a high volage A.C. supply. The high and low volage supplies are conneced so as o he in phase. In he grid circui of Th l here is he usual bank of resisors Rg used in he acual measuremen of grid curren. The cheaer acion of his circui is bes explained diagrammàically. Figs. Sa, band c show he waveforms of he volages appearing across Th l and Th 2 and he curren hrough Th l. Consider firs T~. In is non-conducing sae, he volage across i is he high one supplied by he high-ension ransformer T 2 If now he valve is caused by a pulse o srike a P, his volage drops o abou 1 V; because of R2' however, he curren which flows hrough he es valve is small, say 1 ma. Considering now Th 2, during he firs par of he highvolage cycle, he cahode of Th 2 is driven posiive, which is, of" course, equivalen o he negaive volage shown in fig. Sc. This negaive volage is much greaer han he 1 V supply o he anode of Th 2, which is herefore non-conducive. When Th l has sruck, however, he large negaive volage on he anode of Th 2 disappears: i has hen a posiive anode volage from he 1 V supply and i oo can be fired by he same pulse hrough Th l and Th 2 This curren can be he full load curren of he es valve Th l, which is hus being fired from a high-volage supply (say 15 V), bu subsequenly drawing is curren from a low-volage source (1 V).

5 AUGUST 1954 CHEATER CIRCUITS FOR THYRATRON TESTING 47 Vaa ' Fig. 5. Explanaion of cheaer circui of fig. ~'. a) Volage Val across valve under es, due o source of high ension (here supposed o be 1 V r.m.s.). b) Curren i al hrough es valve. c) Volage Va2 across auxiliary valve. Immediaely before sriking of es valve (a P), Val is high. Power consumpion is low, owing o curren being mainly drawn from low volage source. (Firing of auxiliary valve is shown slighly laer han sriking of es valve.) To provide a grid volage for Tlu; he circui shown in fig. 6 is employed, which supplies boh he D.C. componen and he riggering pulse. The negaive D.C. bias, which is variable up o -1 V, is drawn from a full-wave recifier (no shown). The rigger pulse, which has an ampliude of 3 V and a duraion of 5 microseconds, is generaed as follows. An A.C. volage of 5 V is applied o he conrol grid of a high-slope penode Pel. This A.C. volage can be phaseshifed by means of a variable resisor Ra. The valve is overloaded by such a volage and herefore produces a square-wave anode curren. This waveform is differeniaed (C 4 -R 4 ) and he resul applied o he grid of a Mullard 2D21 hyraron (Tha), whichforms a low-impedance oupu sage. The valve Th'a is herefore caused o srike and so iniiaes he discharge of a capacior Cs via a choke Ls and a resisor Rs. The peakshaped volage which hen appears across Ls-Rs is fed, via he capacior CG' o he grid of he valve under es. The grid of he auxiliary valve (Th 2, fig. 4) is conneced o a permanen D.C. bias in series wih a ransformer hrough which is fed he rigger pulse generaed by he circui of fig. 6. Measuremen of pre-sriking grid curren Suppose a valve is se up in he above apparaus (fig. 4) and fired by he pulse a some' ime afer he occurrence of he peak anode volage (firing angle a l >9). The pulse is also fed o he auxiliary valve Th 2, so i oo fires and allows fullload curren o flow hrough Th l and Th 2 in he way previously described. If now he negaive grid bias of Th l is gradually reduced, he valve firs coninues o srike a he angle al deermined by he grid pulse, unil he D.e. bias reaches a value equal o he criical grid volage corresponding o he H.T. peak anode volage. In he H.T. circui he firing of Th l is hen suddenly advanced from al (>9 ) o 9. The grid volage Vg,oo a which his happens can be read!1 a volmeer. If he grid resisance is increased and he measuremen repeaed, a higher value of - V g,9 will be noed. From he change in resisance and he change in Vg,oo he pre-sriking grid curren can he deduced wih he help of equaion (1), see above.. Fig. 7 represens he whole operaion in erms of volage and curren wave-forms. I also helps o make clear exacly wha is being measured and o bring ou he disincion beween his mehod and previous ones. Previously, he grid curren measured was ha a he momen when he valve sruck; he valve was a i correc working emperaure, bu i was no subjec o is raed peak anode volage, and also i was impossible o fire he valve a angles greaer han 9. As poined ou earlier, he maximum peak-o-mean anode curren raio occurs when he valve is fired lae in he posiive anode volage half-cycle. I was herefore impos-. sihle, using previous mehods, o es a valve wih boh maximum anode volage and maximum peak +3V o -1V 81 Fig. 6. Circui providing firing pulses and D.C. bias for cheaer circui (fig. 4.).A ransformer conneced o Ra-Ca supplies 5 V A.C., wih variable phase shif, o conrol grid of penode PC l (EF 91). Square-wave anode curren of PC l is differeniaed by C4-R4' producing pulses which fire oupu sage hyraron Tha (Mullard 2 D 21). Tha being fired, capacior Cs discharges hrough Tha, Rs and Ls (choke Ls helping o exinguish Tha). Via coupling capacior CG' volage pulse across Rs-Ls is applied o grids of es valve (a X) and of auxiliary valve (via ransformer T 4, see fig. 3). Variable D.C. bias is obained from poeniomeer P.

6 48 PHJLIPS TECHNICAL REVIEW VOL. 16, No. 2 and mean anode currens, and hese are he very condiions which lead o maximum grid emission, leakage and unconrolled cahode emission - i.e..o maximum grid curren.' The new mehod has no such drawbacks. The grid curren is necessarily measured when he anode volage is a is peak value, whaever he peak-omean anode curren raio. The value obained, is herefore a rue indicaion of he wors possible grid curren likely o flow under any given condiions. val -~ ia./. Vaz 12 J;, afl Fig. 7. Explanaion of pre-sriking grid curren measuremen wih cheaer circui of fig. 4.. A (a), D.C. bias of es valve Th l is shown adjused o op of criical grid volage (curve 3), causing Th o fire a peak of H.T. anode volage (see b). Auxilliary valve Th 2 is fired by a pulse (firing angle a l >9), iniiaing main curren hrough boh valves. Volage Va2 across Th 2 is shown a (c). I will be appreciaed ha he ime available for de-ionizaion of he auxiliary valve Th 2 is very shor. In fac, if I denoes he insan where Th 2 exinguishes (i.e., where hè anode volage of boh valves in series has dropped o, say, 3 V), and 2 he insan where va2' in he subsequen half-cycle, reaches he value of, say, 2 V a which ionizaion becomes possible again, and if he applied A.C. volages are 1 V and 1 V r.m.s., hen 1 1'2 sin co I = 3 and (1-1) 1'2 sin co 2 = 2 (wih co = 2n X 5 radians per second, and he load resisors supposed o be non-inducive). If 7: is he inerval beween I' and he momen where he A.C. volages pass hrough zero, and 7: 2 he inerval beween his momen and 2, hen we find from he above equaions: 7: 1 = 68 (J.sec and.7: 2 = 5 (J.sec.If he high volage is increased o 1 kv, his merely reduces 7: 2 o 4 (J.sec,leaving 7: 1 U:nafreced.Thus i is clearly possible o use his circui a very high volages, provided he low volage does no exceed 1 V r.m.s. and provided he valve de-ionizaion ime does no exceed 5 flsec. A circui of his ype has been successfully used for esing mercury hyrarons a volages up o 3 kv peak. Summing up, his mehod of measuring grid currens has he following advanages: 1) I measures he maximum grid curren flowing under any given condiions of loading for any anode volage up o, say, 3 kv peak. 2).I economizes in power when esing hyrarons drawing high currens a high volages.. The power required o operae his circui for a 6 A hyraron a 1 V, including auxiliaries, amouns o 8 W only. Compa~ng his wih he power required for a sraigh es a, say, 1 V, i is eviden ha he advanages of his circui are very real, and when volages of 2 kv or more are required, i is clear ha a direc es mehod is prohibiive, so ha his circui is no merely useful bu a necessiy. Summary. Par I of his aricle deals wih he measuremen of he pre-sriking grid curren of hyrarons under full operaing condiions. Several convenional circuis are reviewed, and shown o be no enirely saisfacory. One of he drawhacks when large hyrarons are o he esed, is he high power consumpion. A "cheaer circui" has herefore heen devised, which simulaes he desired working condiions wih a grea economy of power. Here he valve is par of a low-curren H.T. circui, unil immediaely afer firing, when i is auomaically changed over o a L.T. circui, in which he full raed anode curren is permied o flow. This sysem allows he measuremen of he pre-sriking grid curren under any condiions of loading..