Madey, Richard.

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Lawrence Berkeley National LaboratoryRecent WorkTitleTRANSISTOR CIRCUIT THEORY AND APPLICATIONSPermalinkhttps://escholarship.org/uc/item/24k5j755AuthorMadey, Richard.Publication Date1953-06-01eScholarship.org Powered by the California Digital LibraryUniversity of California.~~t"' ._;:··~ f~ '·~ <},.\ ,, ... •, ' >-w .J w ~ 0:: w m I ~ z 0:: 0 lL .J ~ u lL 0 >-J--(/) 0:: w > -z :::> UCRL-1_895 UNCLASSIFIED · TWO-WEEK LOAN COPY This is a library Circulating Copy which may be borrowed for two weeks. for a personal retention copy, call Tech. Info. Division, Ext. 5545 RADIATION LABORATORY DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. '•t\ INTRODUCTION -1-DESIGN .. OF A TRIGGERE[),,QAP :.·~,-·:, Leonard E. Gibson. and Mark Harris II Radiation Laboratory~ Department of Physics, Universitr of California, Berkeley, California July 15, 1952 UCRL-1895 . ·"'f In many ~pplica.tions there is a need for high. energy, high current pulses. Of fundamental importance in.such problems is the switch which transfers the energy from the energy storage device to the load. Where high currents are desired, it is necessa·ry that the impedance of the switch during discharge remah:i low.· Both thyratrons and spark gaps serve '-satisfactorily as high current switohs. ~here exceptionally high currents · :r,n'ust be· handled, the ·inducta.nc.e of the leaqs in a thyratron may prove a limiting factor. Since a spark gap does not suffer from this limitation, work was inaugurated to ~eve1o~e a triggered spark gap having low inductance, high current carrying ca.paoi ty and a minimum time jitter, (i.e., the variation in the time delay after the start of the trigger pulse should be minimum• ) The specific requirements were as follows: 1. A pulse current of approximately 10,000 amperes. 2. An operating voltage runge of from 10 to 15 kilovolts. 3. A time jitter not to exceed 25 x 10-9 seconds. 4. A unit ~1ich could be operated immersed in oil (i.e., a. sealed unit.) 5. A unit .which would be simple to construct, rugged. and compact. In view of its simpli:aity, a coaxial trigger was selected. Such a. gap has a trigger· pin inserted in the face of one of the gap surfaces, (in this -2-UCRL-1395 case the one at gro\Uld potential), and is fired by initiating a. discharge between the pin ·and the gap surface in which it is located. DESIGN OF GAP . Figures 1 through 3 are the fabrication drawings for the ball gap . electrodes and spacer. Figure 4 shows an exploded view of the various parts before assembly. As shown in Fig. 4, a small .steatite tube is used to insulate -the trigger pin from the ball gap surface. In add~tion, the steatite serves to stabilize the trigger arc since rafter several operation~ a path is etched on its surface ins-qring operation at the same point in subseq.uent .:' . operations. As de~cribedtbelow. additional steps were :taken to make certain the trigger aro. occurred in the se.me position during each operation. The operating voltage of. th,e ge.p determined rouf1lly the spacing required. : . ,i .·. It. haa been previously ascertained· that minimum jitter could. be obtained ·when the ratio of main gap spacing to the trigger gap spacing was approxi-mately 4 to 1. Thus 'the trigge~ spacing was more or less defined by the operating voltageo In setting up·the final dimensions, a standerds.teatite ,tub.e WSI.S selected. v:hose wall thicimass corresponded closely to the el:ltimated trigger ga·p. The .spacer used ·was a ;.093" I.D. by .187" O.D. stee.ti te tube manufactured ·i,y the ~arican. Lava:· Co • ., Chattanooga, Tenn. · Thus, the. trigger gap spacing was .047 inches which indicated (using the above ratio). the main gap spacing should be .188 inches. With this spacing and 2 inch. radius spherical segments,· the· static breakdown in air (760 min, 25°c:) is 16 kv',* or approximately 1 kv e.bove the hie;hest,'operating voltage • • Handbook of Chemistry and-Physics, 32nd edition, pg. 2106. ,.,, ... -3-UC!tL-1895 The gap surfaces and end plates Vfere made as a single unit of dural metal:, and required no special machining. The ball gap faces were brought to a high finish using a commercial metal polish and were carefully cleaned prior to assembly. Bo~h plates were carefully finished to remove all burrs and sharp corners. The luoite spacer is grooved to carry "011 rings so that sealed off. oil immersed operation is possible. The width of the spacer determines the static breakdown voltage. On the fabrication draw~ng no dimension is given for th~ width of the spacer, since this value depends on the operating voltage of the gap. The gap used for the investigation required a spacer having a width o£ .68811 • The trigger pin consisted of a 2 inch long piece of 80 mil tungsten rod. The end of this rod was ground to ·a wedge shape and relieved as shown in Figs. 4 and 5. This process. (end bne other described below), was .instituted to stabilize the trigger a.rc. Before the final assembly, the gap without trigger (Fig. 6) was subjected to a 'voltage ~lightly above static breakdown and allo.wed to break rlovm approximately·. 25 times. The purpose of this. process was to mark the gap surfaces at the npreferred spots. 11 The gap was then disassembled. and the surfaces exwmined. £igures 7 and 8 show the appearance of the surfaces following this operationo In the final assembly of the gap. the trigger pin is oriented so that the pointed end faces the 11 preferred spot." In the final assembly, a Kovar seal (.105" I.D. by .281" H.D. by 11/16" o. L., Stupakoff Ceramic and _Mfg. co., Latrobe, Pa., No. 952008) was fastened to the gap·stem, the trigger inserted and oorrectly oriented and fastened to the Kovar seal. In this case, both the Kovar seal and the trigger pin were affixed with araldyte, although other methods are equally adaptable. UG!lL-18')5 After assembly, the gap was once again subjected to high voltage and allowed to~rk over until the static breakdown voltage had stabilized. It was obserired that during the "seasoning" process the voltage required for static breakdown would increase slightly during the first twenty to thirty operations, and then fall to a value very slightly below that required at the start of the process. Approximately 150 to 200 operations were required before the static breakdown voltage stabilized. TRIGGER PULSER The ma.rmer ·in which the trigger gap is fired determines, to a large ~xtent, the jitter of the system. ·it is believed that the trigger operates through a combination of the following effects: 1. The production of ions and ionizing electrons. 2. The production of ultra-violet radiation. 3. The increase in voltage gradient across the main gap. In order that item (3),apove, be effective, it is important that the polarity of the pulse applied to the trigger pin be the same as the polarity of the ball in which it is.centered. For example.i in the circuit used 1 ·' the trigger was placed in the negative (in this case, grounded) bali and ·the trigger was fired with a negative pulse. All of the above items indicate that the trigger gap should be fired from a lOW impedance SOUrce Ca?able Of supplyine high energy to the arc. In order that the trigger gap fire with a minimum of time delay and time jitter, a large overvoltage should be applied a.nd this voltage should have as fast a rise time as possible. In addition, the circuit supplying the trigger pulse should have negligible time jitter. ., ,! .\< -5- UC.::\L-1895 In the test gap the voltage applied to the trigger gap was nearly four times that required for static breakdown. Thus the voltage applied to the trigger gap was equal to the main gap voltage, and in several cases exceeded it. It was subsequently found that th,is produced a large .margin of safety, since a change of 50% in the trigger firing voltage failed to affect the operation of the.gap. The oircui t for the trigger pulsar unit is shoV¥n in l''ig. s.· The 3C45 operated with a plate voltage of 3 kv1 and used with a. four to one step-up pulse transformer, developed sufficient voltage and power to reduce the trigger g~p jitter to a negligible amount. The grid of the 3C45 was driven with a pulse of 1 kv rising to peak value in 1 billisecond (1 x 10..;9 sec.}. The impedance of the source was 50 ohms. The pulse transformer was wound on a standard 1/2" x 1-13/16111 hypersil core. The primary, consisting of five turns of 1/211 x .00511 copper strap, was_ strap wound. The secondary consisted of 20 turns of No. 23 formvar wire. Another pulse transformer, (built for use in a slightly different circuit), using the s,a.me core and. same number of primary and secondary turns but having interleaved, strap' wound primary and secondary, allowed a. lower impedance circuit to.be obtained. but time did not permit complete tests on this unit. . . F. To test the gap under actual conditions~ a high voltage circuit was constructed as shown in Figs. 10 e.nd 11. Ten General Electric. pyranol pulse capacitors were arranged in a circle and oonneoted to two heavy, circular, copper sheets. A 5" diameter section was removed from the upper plate. The gap was located at the center of the bottom plate, and connected to the upper plate by six copper straps. A 5-15 kv, 1 ma power supply ' (Fig. 12} was used to charge the. capacitor bank through a 5 megohm charging resistor. . '- .. ., ) The jitter measurements were made ~sing a. scope having a sweep speed of 2.5 x lo-9 seconds/em. The scope was triggered by the same pulse that fired the grid of the· 3C45 and the scope signal derived .from either a loop in the vicinity of the gap or by-measuring the voltage drop between two closely spaced points along the current path. In order to meet the requirement o.f less than 25 billiseconds jitter. it ~as sufficient that the signal indicating the discharge of the gap remain on the 10 em screen. RESULTS The equipment as described above was set up and operated by an automatic ,. cycler and counter. The gap was discharged at 30 second intervals over a period of several day.s. Over 5000 firings were counted, und of these more than half were observed and many groups were photographed. In all of the monitered firings, the observed jitter was less than 25 x lo-9 seconds. Photographs (Figs. 13 and 14) show the gap surfaces at the completion of the tests. Despite the severe erosion of the surfaces the time jitter at the completion of the test was still within the specified requirements. CONCLUSIONS Due to the limitations of time, it was not possible to persue many interesting modifications of the basic design. Originally it was hoped that the investigation could include operation in inert atmospheres, pressurized operati-on, variation o'f trigger pin and gap surface material, and various configurations of the gap surfaces. However, the brief investigation conducted disclosed a number of interesting points. It was observed that the time jitter could be < I , -7- UCRL-1 S95 reduced by the following operations: 1. Shaping the trigger pin. 2. Stabilizing the trigger gap by firinc; over a ceramic insulator. 3. Allow·ing the firing point to occur· at a "preferred position. 11 4. Operating the ~;ep at a voltage near static breakdown voltage. 5. Allowing the gap to operate under sts.tic breakdo;,vn conditions previous to actual triggered operation. 6. Providing low impedance circuits throughout the entire tr~gger chain. 7. Providing high amplitude. fast r~s~g pulses to the thyratron grid and, especially. to the trir;(;er pin itself. Extreme precautions arenecessary in connecting the measuring equipment to minimize the disturbing effects caused b:r the fields resulting from the . main, gap discharge. A common copper ground plane should be provided for all components especially mea.surint; equipment. and e.ll shields and grounded wires should be bonded to the. ground plane •. SHOWN ON ')' JOB NO. SER. NO. DATE ISSUED DATe REQ'D ~ '2:;/,t;/VJ<~· r----: .6 /7CJL~5. '/) .( / <::;J'..L ., -71/./1 '-n c· , .. C//.V_~.._)~+·~. V/rTt .a, ___ ~"-' ', / ./Cf --..-·--/ ·. ~------r- . i '•, - '.--1-"-, __ r.:A '0, ~ NO. REQ'D MATERIAL 6/.,)- T6 /lL.t/M, ,-9.L .. L.O y , , . z.zc--A£<' ,/;-L~ ctJ;r?A/L) rra ~' '''·:;e.?z?, i!if-• _, __ ,_, I * SAWCUT, , UNIESSOTKElWISE- SCAlE ~u I f flAME CUT, SPECifiED J , ,u /-J :if4LL- 6~JO Chono•j Dt-n I ChKk let!.. &y !y Date SHEAR 1. TOLERANCES + DESIGN ,1 , _k_ ROUGH ONDIMENSIONS- JOII'"'l-IO."tL}0 -03 DATE . ·I ' I ~ MACHINE' 2. &R£Ak' EDGES l/6o1AWC. 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TRANSISTOR CIRCUIT THEORY AND APPLICATIONS

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TRANSISTOR CIRCUIT THEORY AND APPLICATIONS

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